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Advanced Concepts of CDMA

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1 Advanced Concepts of CDMA
Although there are many types of spread spectrum communications systems, this presentation gives an overview of the proposed CDMA cellular system defined by the TIA 45.5 sub-committee. Largely based on the CDMA system developed by Qualcomm, the TIA system uses direct sequence modulation with digital codes to spread its spectrum. Other types of spread spectrum systems use frequency hopping techniques or a combination of frequency hopping and modulation with digital codes (direct sequence) . The intent of this paper is to provide insight into the technology of CDMA and to describe some of the operating features of the now standardized TIA CDMA system. The TIA standard documents that define this system are EIA/TIA-95-B (the common air interface), EIA/TIA-98-B (mobile minimum performance standard), and EIA/TIA-97-B (base station minimum performance standard). For PCS systems in the United States, the CDMA air interface standard is defined in ANSI J-STD-008. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Advanced Training Version - 6th Edition Copyright © Hewlett-Packard Company 1999

2 Cellular Access Methods
Advanced Concepts of CDMA Cellular Access Methods Power Time Time Power FDMA Frequency Power Time The Problem of Access: The personal communication industry is faced with the problem of an ever increasing number of users sharing the same limited frequency bands. To expand the user base, the industry must find methods to increase capacity without degrading the quality of service. The current analog cellular system uses a complex system of channelization with 30 kHz channels, commonly called FDMA (Frequency Division Multiple Access). To maximize system capacity, analog FDMA cellular uses directive antennas (cell sectoring) and complex frequency reuse planning. To further increase system capacity, a digital access method is being implemented called TDMA (Time Division Multiple Access). This system uses the same frequency channelization as FDMA analog and adds a time sharing element. Each channel is shared in time by three users to effectively triple system capacity. CDMA stands for Code Division Multiple Access and uses correlative codes to distinguish one user from another. Frequency divisions are still used, but in a much larger bandwidth (1.25 MHz). In CDMA, a single user's channel consists of a specific frequency combined with a unique code. CDMA also uses sectored cells to increase capacity. One of the major differences in access is that any CDMA frequency can be used in all sectors of all cells. The correlative codes allow each user to operate in the presence of substantial interference. An analogy to this is a crowded cocktail party. Many people are talking at the same time, but you are able to listen to and understand one person at a time. This is because your brain can sort out the voice characteristics and differentiate them from the other talkers. As the party grows larger, each person must talk louder, and the size of the talk zone grows smaller. Thus, the number of conversations is limited by the overall noise interference in the room. CDMA is similar to this cocktail party analogy, but the recognition is based on digital codes. The interference is the sum of all other users on the same CDMA frequency, both from within and outside the home cell and from delayed versions of these signals. It also includes the usual thermal noise and atmospheric disturbances. Delayed signals caused by multipath are separately received and combined in CDMA. This will be discussed in greater detail later in this presentation. Notes: ____________________________________________________________________________________________________________________________________________________________________________________ Frequency CDMA TDMA Frequency Copyright © Hewlett-Packard Company 1999

3 CDMA is Also Full Duplex
Advanced Concepts of CDMA CDMA is Also Full Duplex US Cellular Channel 384 Amplitude Reverse Link Forward Link AMPS 45 MHz Frequency MHz MHz Amplitude Reverse Link Forward Link Traditional cellular system are known as full duplex systems since two channels are used at the same time. This allows completely independent transmission to and from the mobile at the same time. In the North American Cellular system, the forward and reverse link channels are separated by 45 MHz. The EIA/TIA-95-B system is also full duplex and uses 1.25 MHz wide channels for both the forward and reverse directions. For cellular applications in North America, EIA/TIA-95-B CDMA uses the same 45 MHz separation between forward and reverse links that AMPS uses. For PCS applications, J-STD-008 CDMA uses a separation between forward and reverse links of 80 MHz. Other systems throughout the world may use other spacings. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ CDMA 45 MHz Frequency MHz MHz Copyright © Hewlett-Packard Company 1999

4 Cellular Frequency Reuse Patterns
Advanced Concepts of CDMA Cellular Frequency Reuse Patterns 1 1 2 3 1 1 6 4 2 1 1 1 5 6 1 1 One of the major capacity gains with CDMA is due to its frequency reuse efficiency. In order to eliminate interference from adjacent cells, narrow-band FM systems must physically separate cells using the same frequency. Complex frequency reuse planning must be done in such a system to maximize capacity while eliminating interference. A typical reuse pattern for analog and TDMA systems employs only one seventh of the available frequencies in any given cell. This could really be called frequency non-reuse. With CDMA, the same frequencies are used in all cells. If using sectored cells, the same frequencies can be used in all sectors of all cells. This is possible because CDMA is designed to decode the proper signal in the presence of high interference. Adjacent cells using the same frequency in CDMA simply cause an apparent increase in the channel background noise. By allowing the use of the same frequencies in every cell, CDMA has six times the capacity of existing analog cellular systems. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 7 1 FDMA Reuse CDMA Reuse Copyright © Hewlett-Packard Company 1999

5 Advanced Concepts of CDMA
CDMA Capacity Gains (Chan BW) (1) (1) Capacity = × × × (Fr) Processing Gain (Data Rate) (S/N) (Vaf) Processing Gain (1,230,000) (1) (1) CDMA = × × × (0.67) (9,600) (5.01) (.40) CDMA = 42 calls (Using 1.5 MHz BW) To see how CDMA offers greater capacity, we need to look at its potential in a given bandwidth. Remember that for any cellular system, the capacity can be made arbitrarily large by adding more and more cells. A more realistic approach for a capacity comparison is the number of calls per used bandwidth. Installing CDMA in an existing AMPS analog cellular system requires that a minimum of 1.5 MHz of bandwidth be removed from analog service. While the actual spreading bandwidth of a single CDMA frequency is 1.23 MHz, a total of 1.5 MHz is required to provide guardbands to reduce potential interfere with adjacent analog channels. Additional CDMA frequencies added to the system will only require 1.23 MHz of bandwidth. In this configuration, a single CDMA cell will support 42 telephone calls. This is derived from the equation shown. The processing gain is the ratio of the CDMA final bandwidth divided by the encoded voice data rate. The signal-to-noise ratio required for good voice quality varies greatly with propagation conditions. On average, typical transmission conditions require a signal-to-noise ratio of about 7 dB to provide adequate voice quality. Translated into a ratio, the signal must be 5 times stronger than the noise. The parameter Vaf is the voice activity factor. Since CDMA uses a variable rate voice encoder, Vaf for CDMA is 0.4. Fr is the frequency reuse efficiency and Sg is the sectorization gain. For CDMA, Fr is (in other words almost 70% reuse efficiency). The frequency reuse efficiency is not 1 since the additional interference produced by surrounding cells causes a reduction in capacity. If the CDMA cells use 3-way sectored antennas, Sg is about 2.6 (almost 3 times the capacity when using sectorization). Again, the sectorization gain is not 3 due to the increased interference from the surrounding sectored cells. Given the same amount of bandwidth, an AMPS system has a capacity per cell of only about 7 calls. This is because although AMPS would have 50 channels in 1.5 MHz of bandwidth, only one-seventh can be used in any given cell because of interference. Using sectors in analog does not improve the capacity per MHz since interference from adjacent sector still requires a complex frequency reuse plan. Sectorization in analog simply results in physically smaller cells. AMPS = 1.5 MHz ÷ 30 kHz = 50 Channels Capacity = 50 Channels ÷ 7 (1/7 Frequency Reuse) AMPS = 7 calls (Using 1.5 MHz BW) Copyright © Hewlett-Packard Company 1999

6 Advanced Concepts of CDMA
The CDMA Concept 10 kHz BW 1.23 MHz BW 1.23 MHz BW 10 kHz BW f c f c CDMA Transmitter CDMA Receiver Baseband Data Encoding & Interleaving Walsh Code Spreading Walsh Code Correlator Decode & De- Interleaving Baseband Data 9.6 kbps 19.2 kbps kbps kbps 19.2 kbps 9.6 kbps -113 dBm/1.23 MHz Spurious Signals 1.23 MHz BW 1.23 MHz BW CDMA starts with a narrow-band signal, shown here at the full speech data rate of 9600 bps. This is spread with the use of specialized codes to a bandwidth of 1.23 MHz. The ratio of the spread data rate to the initial data rate is called the processing gain. For EIA/TIA-95-B standard CDMA, the processing gain is 21 dB (10 log ( /9600)). When transmitted, a CDMA signal experiences high levels of interference, dominated by the coded signals of other CDMA users. This takes two forms, interference from other users in the same cell and interference from adjacent cells. The total interference also includes background noise and other spurious signals. When the signal is received, the correlator recovers the desired signal and rejects the interference. The correlators use the processing gain to pull the desired signal out of the noise. Since a signal to noise ratio of about 7 dB is required for acceptable voice quality, this leaves 14 dB of extra processing gain to extract the desired signal from the noise. This is possible because the interference sources are uncorrelated (orthogonal in the case of the forward link) to the desired signal. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ f c f c f c f c Background Noise External Interference Other Cell Interference Other User Noise Interference Sources Copyright © Hewlett-Packard Company 1999

7 Advanced Concepts of CDMA
What is Correlation ? Is a Measure of How Well a Given Signal Matches a Desired Code The Desired Code is Compared to the Given Signal at Various Test Times Received Signal Correlation = 1 Correlation = 0 Time Correlation = 0 Correlation is key enabling concept for direct sequence CDMA systems. Correlation is a measure of how well a given signal spread with a digital code matches a desired code. In the above example, a digital sequence is received and then compared to the desired code. This comparison takes place over a range of different times. When time aligned, the correlation is 1 indicating that an exact match occurred between the received signal and the desired signal. At other times the correlation is near zero, especially if the digital codes used to spread the waveform are designed properly. It is the fact that we can correlate to signals that enables direct spread CDMA to function. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Correlation = 0 Copyright © Hewlett-Packard Company 1999

8 Advanced Concepts of CDMA
CDMA Paradigm Shift Multiple Users on One Frequency Analog/TDMA Try to Prevent Multiple Users Interference Channel is Defined by Code Analog Systems Defined Channels by Frequency Capacity Limit is Soft Allows Degrading Voice Quality to Temporarily Increase Capacity Reduce Surrounding Cell Capacity to Increase a Cell's Capacity Analog CDMA It should be clear at this point that CDMA technology is not intuitive. For most people familiar with FM communication systems, a paradigm shift is needed to properly discuss CDMA. Here are some of the key differences between CDMA and analog FM: * Multiple users are on one frequency simultaneously. For a long time, RF engineers have spent a lot of effort trying to keep other people off the same channel. This is a critical issue for Analog FDMA and TDMA systems. Now, with CDMA, we are trying to put many conversations on the same channel. * A Channel is defined by the correlative code in addition to the frequency. For a long time, people have thought of channels in terms of their frequency. With CDMA, channels are defined by various digital codes as well as by the frequency. * The capacity limit is soft. In analog systems, when all of the available channels are in use, no further calls can be added. Capacity in CDMA can be increased with some degradation of the error rate or voice quality or can be increased in a given cell at the expense of reduced capacity in surrounding cells. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

9 Advanced Concepts of CDMA
CDMA Diversity Spatial Diversity Making Use of Differences in Position Frequency Diversity Making Use of Differences in Frequency Time Diversity Making Use of Differences in Time Another aspect of CDMA is its use of diversity. CDMA uses three types of diversity: Spatial diversity, Frequency diversity, and Time diversity. CDMA uses the diverse nature of these three properties to enhance its system performance. The next slides will examine in detail how CDMA makes uses of the diverse nature of these three properties to enhance performance. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

10 CDMA Spatial Diversity
Advanced Concepts of CDMA CDMA Spatial Diversity Diversity Reception: Multiple Antennas at Base Station Each Antenna Is Affected by Multipath Differently Due to Their Different Location Allows Selection of the Signal Least Affected by Multipath Fading If Diversity Antennas Are Good, Why Not Use Base Stations as a Diversity Network? Soft Handoff The concept of diversity reception has been well known for some time. A diversity receiver uses multiple antennas at one reception site. Since these antenna are placed to be a non-integral number of wavelength apart, when one antenna is experiencing a multipath fade it is likely that the other antennas will not be in a fading condition. This leads to receiver designs where the antenna with the best signal is selected to be processed by the receiver. AMPS analog cellular base stations use this type of diversity for improved fading resistance. CDMA also employs diversity reception for base stations. One of the most problematic locations for a cellular phone is in between cells where handoffs occur. If the mobile experiences a deep fade during handoff, a dropped call can result. If diversity reception is useful at a single receiver location, then can using multiple base station be used in a diversity network to help phone during handoffs? The answer is yes: CDMA seeks to overcome the handoff problem by using two or three base stations as a giant diversity system. Using multiple base stations simultaneously talk to the mobile during a handoff is known as a "soft handoff". Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

11 Spatial Diversity During Soft Handoff
Advanced Concepts of CDMA Spatial Diversity During Soft Handoff MTSO Land Link Vocoder / Selector CDMA extends the idea of diversity reception with the concept of soft handoff. In the slide, a mobile CDMA phone has established a call with base station one. As the mobile moves away from base station one and approaches base station two, a device in the phone known as the searcher identifies base station one as a good candidate for soft handoff. The searcher identifies other base stations as good candidates for soft handoff when the received level exceeds the T_add (Threshold for adding a candidate cell for soft handoff) parameter of the system . Once a candidate exceeds the threshold, the phone sends the candidate information to the Mobile Telephone Switching Office (MTSO) via base station one. If the network has available capacity, the MTSO then directs the base stations and mobile to perform a soft handoff. During soft handoff, the mobile listens to the two cells on different codes while the base stations each listen to the same transmission from the mobile. The signals from the base to mobile are treated as multipath signals and are coherently combined at the mobile unit. Each base station sends its received signal via the network to the (MTSO), where a quality decision is made on a frame-by-frame basis, every 20 msec. The MTSO selects the better frame from the two signals returned from the base stations. Thus the two base stations act like a giant antenna diversity system. This helps to overcome the fading problem that occurs between cells where handoffs must take place. As the mobile moves further away from base station one, the searcher in the phone will determine that its power has dropped below the system parameter T_drop. The T_drop information is sent to the MTSO, which then directs the soft handoff be terminated. This allows for smooth handoffs between cells that the user is totally unaware of. Of course there is a price to pay for this clever design: the system uses more capacity for each soft handoff made and there is greatly increased network traffic between CDMA cellsites and the MTSO. Notes: ____________________________________________________________________________________________________________________________________________________________________________________ Base Station 1 Base Station 2 Copyright © Hewlett-Packard Company 1999

12 CDMA Frequency Diversity
Advanced Concepts of CDMA CDMA Frequency Diversity Combats Fading, Caused by Multipath Fading Acts like Notch Filter to a Wide Spectrum Signal May Notch only Part of Signal Amplitude 1.23 MHz BW Frequency diversity is inherent in a spread spectrum system. A fade of the entire signal is less likely than with narrow band systems. Fading is caused by reflected images of an RF signal arriving at the receiver such that the phase of the delayed (reflected) signal is 180 degrees out of phase with the direct RF signal. Since the direct signal and delayed signal are out of phase, they cancel each other causing the amplitude seen by the receiver to be greatly reduced. In the frequency domain, a fade appears as a notch filter that moves across a band. As the user moves, the frequency of the notch changes. The width of the notch is on the order of one over the difference in arrival time of two signals. For a 1 usec delay, the notch will be approximately 1 MHz wide. The TIA CDMA system uses a 1.25 MHz bandwidth, so only those multipaths of time less than 1 usec actually cause the signal to experience a deep fade. In many environments, the multipath signals will arrive at the receiver after a much longer delay. This means that only a narrow portion of the signal is lost. In the display shown, the fade is 200 to 300 kHz wide. This results in the complete loss of an analog or TDMA signal but only reduces the power in a portion of a CDMA signal. As the spreading width of a CDMA signal increases, so does its multipath fading resistance. Many spread spectrum systems use a 5 or 10 MHz wide channel to further improve fading resistance. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Frequency Copyright © Hewlett-Packard Company 1999

13 Advanced Concepts of CDMA
CDMA Time Diversity Rake Receiver to Find and Demodulate Multipath Signals Data is Interleaved Spreads Adjacent Data in Time to Improve Error Correction Efficiency Convolutional Encoding Adds Error Correction and Detection Viterbi Decoding Most Likely Path Decoder for Convolutionaly Encoded Data Time diversity is a technique common to most digital transmission systems. The Rake Receiver is used to find and demodulate multipath signals that are time delayed from the main signal. The Rake Receiver will be explained in the next slides. Transmitted signals are spread in time by use of interleaving. Interleaving the data improves the performance of the error correction by spreading errors over time. Errors in the real world during radio transmission usually occur in clumps, so when the data is de-interleaved, the errors are spread over a greater period of time. This allows the error correction to fix the resulting smaller, spread out errors. Forward error correction is also applied to the transmitted data. This is usually done by adding parity bits that allow received errors to be detected and to some extent corrected. Performance of the receiver can be further enhanced by using a maximal likelihood detector. The particular scheme used for CDMA is convolutional encoding in the transmitter with Viterbi decoding using soft decision points in the receiver. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

14 Why Interleaving Works
Advanced Concepts of CDMA Why Interleaving Works Original Data Frame Errors/Time TX 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Errors/Time RX 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Interleaved Data Frame Interleaved Data Frame Errors/Time TX This slide graphically demonstrates why interleaving data improves error correction performance of data transmission systems. In the top example, data is sequentially read out of a buffer than goes by rows. No interleaving is employed. The data is read and transmitted in numerical order. During transmission, data blocks 5 through 8 are corrupted by some interference. When the data is received, blocks 5 through 8 are lost and the error correction is insufficient to recover such a large block of lost data. i In the lower example, the same data is first interleaved using a simple pattern of reading the rows into columns. The interleaved data is then read out by row and transmitted. During transmission the data in the same four blocks is corrupted by interference. However, the blocks that were lost are no longer sequential. Blocks 2, 6, 10, and 14 were lost. When the interleaved signal is received, the receiver reverses the interleaving process to restore the data to its original sequential pattern. Notice what happens after de-interleaving: the lost blocks are now spread in time resulting in small, isolated error locations. Now the limited error correction built into the signal can correct the errors. Interleaving makes the most use of the error correction built into a data transmission system. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 1 5 9 13 2 6 10 14 3 7 11 15 4 8 12 16 Errors/Time RX 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Copyright © Hewlett-Packard Company 1999

15 Advanced Concepts of CDMA
The Rake Receiver Time Frequency Amplitude Instead of trying to overpower or correct multipath problems, CDMA takes advantage of the multipath to improve reception quality in fading conditions. CDMA does this by using multiple correlating receivers and assigning them to the strongest signals. This is possible because the CDMA mobile is synchronized to the serving base station. The mobile's receiver can distinguish direct signals from multipath signals because the reflected multipaths signals arrive later than the direct signals. Special circuits called searchers are also used to look for alternate multipaths and for neighboring base station signals. The searchers slide around in time until they find a strong correlation with their assigned code. Once a strong signal is located at a particular time offset, the searcher assigns a receiver element to demodulate that signal. The mobile receiver uses three receiving elements, and the base station uses four. This multiple correlator system is called a rake receiver. As conditions change the searchers rapidly reassign the rake receivers to handle new reception conditions. While each signal being processed by an individual rake receiver experiences fading, the fades are independent because of the different path lengths experienced by each signal. Thus, the receiver can coherently recombine the outputs of the three rake receivers to reconstruct a much more robust version of the transmitted signal. In this way, CDMA uses multipath signals to create a more robust receiver. The rake receivers also allow soft handoff as one or more receivers can be assigned to another base station. We should note that there are some limitations to this scheme. If strong, short transmission paths are present, such as in a very narrow canyon, the rake receiver system cannot function. If the arrival time of a multipath signal is less than one clock cycle of the CDMA system, the rake receiver cannot tell the difference between a direct signal or a reflected signal. It has been found, however, that in real world situations, longer time delayed signals coexist when very strong short multipath signals are present. This allows the searchers to find these other longer delayed signals under these difficult propagation conditions. Copyright © Hewlett-Packard Company 1999

16 Advanced Concepts of CDMA
Rake Receiver Design Output T W 4 1 2 3 + Delay Taps Tap Weights Antenna The design of a rake receiver can be visualized as a series of time delayed correlator taps fed from a common antenna. If each correlator tap is delayed to match the arrival of a particular transmitted signal, then the outputs of each tap can be recombined in phase. Once an RF signal with a particular travel time is locked onto by the correlator tap, an estimate of the gain or loss experienced by that signal must be made. The weighting of the taps perform this gain normalization function. Once adjusted, the outputs of each finger of the rake can be combined to form a better version of the transmitted signal. Notice that this description visually matches the analogy of a common garden rake with each tap forming a tine of the rake, hence the name rake receiver. Another form of time diversity occurs in the base station when transmitting at reduced data rates. When transmitting at a reduced data rate (more detail will be presented on this later), the base station repeats the data resulting in full rate transmission. The base station also reduces the transmitted power when it operates at reduced data rates. This added redundancy in the transmitted signal results in less interference (power is lowered) and improves the CDMA mobile's station receiver performance during high levels of interference. Notes: ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

17 Advanced Concepts of CDMA
Synchronization All Direct Sequence, Spread Spectrum Systems Should be Accurately Synchronized for Efficient Searching Finding the Desired Code Becomes Difficult Without Synchronization In order for any direct sequence, spread spectrum radio system to operate efficiently, all mobiles and base stations should be precisely synchronized. If they are not synchronized, it becomes difficult to search for the codes used to identify individual radio signals. When a direct sequence system is synchronized, the search algorithms need only search over a narrow time window to locate a desired signal. The length of this search window need only be long enough to accommodate the delays caused by the radio waves traveling from the source to the receiver. Precise synchronization also leads to other benefits. It will allow such future services as precise location reporting for emergency or travel usage. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

18 Reverse Link Power Control
Advanced Concepts of CDMA Reverse Link Power Control Maximum System Capacity is Achieved if: All Mobiles are Powered Controlled to the Minimum Power for Acceptable Signal Quality As a result, all Mobiles are Received at About Equal Power at the Base Station Independent of Their Location There are Two Types of Reverse Control: Open Loop Power Control Closed Loop Power Control Open & Closed Loop Power Control are Always Both Active ! One of the fundamental enabling technologies of CDMA is power control. Since the limiting factor for CDMA system capacity is the total interference, controlling the power of each mobile is critical to achieve maximum capacity. CDMA mobiles are power controlled to the minimum power that provides acceptable quality for the given conditions. As a result, each mobile's signal arrives at the base station at approximately equal levels. In this way, the interference from one unit to another is held to a minimum. Two forms of power control are used for the reverse link: open loop power control, and closed loop power control. Despite what seems logical, both open and closed loop power control are active at the same time once a traffic channel is established. Both are constantly active and controlling the power of the phone according to their respective control algorithms. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

19 Open Loop Power Control
Advanced Concepts of CDMA Open Loop Power Control Assumes Loss is Similar on Forward and Reverse Paths Receive Power+Transmit Power = -73 All powers in dBm Example: For a Received Power of -85 dBm Transmit Power = (-73) - (-85) Transmit Power = +12 dBm Provides an Estimate of Reverse TX Power for Given Propagation Conditions Open loop power control is based on the similarity of the loss in the forward path to the loss in the reverse path (forward refers to the base-to-mobile link, while reverse refers to the mobile-to-base link). Open loop control sets the sum of transmit power and receive power to a constant, nominally -73, if both powers are in dBm. A reduction in signal level at the receive antenna will result in an increase in signal power from the transmitter. For example, assume the received power from the base station is -85 dBm. This is the total energy received in the 1.23 MHz receiver bandwidth. It includes the composite signal from the serving base station as well as from other nearby base stations on the same frequency. The open loop transmit power setting for a received power of -85 dBm would be +12 dBm. Thus open loop power control adjusts the transmit power of the phone to match the propagation conditions that the phone is experiencing at any given time. By the TIA/EIA-98 standard specification, the open loop power control slew rate is limited to roughly match the slew rate of closed loop power control directed by the base station. This eliminates the possibility of open loop power control suddenly transmitting excessive power in response to a receiver signal level dropout. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

20 Closed Loop Power Control
Advanced Concepts of CDMA Closed Loop Power Control Directed by Base Station Updated Every 1.25 msec Commands Mobile to Change TX Power in +/-1 dB Step Size Fine Tunes Open Loop Power Estimate Power Control Bits are "Punctured" over the Encoded Voice Data Puncture Period is two 19.2 kbps Symbol Periods = usec Closed loop power control is used to allow the power from the mobile unit to deviate from the nominal as set by open loop control. This is done with a form of delta modulator. The base station monitors the power received from each mobile station and commands the mobile to either raise power or lower power by a fixed step of 1 dB. This process is repeated 800 times per second, or every 1.25 msec. The power control data sent to the mobile from the base station is added to the data stream by replacing the encoded voice data. This processes in called "puncturing", since the power control data is written into the data stream by over writing the encoded voice data. The power control data occupies micro-seconds of each 1.25 milli-second of data transmitted by the base station. Because the mobile's power is controlled to be no more than is needed to maintain the link at the base station, a CDMA mobile typically transmits much less power than an analog phone. The base station monitors the received signal quality 800 times per second and directs the mobile to raise or lower its power until the received signal quality is just adequate. This operating point varies with propagation conditions, the number of users, and the density and loading of the surrounding cells. Analog cellular phones need to transmit enough power to maintain a link even in the presence of a fade. Most of the time, analog phones transmit excess power. CDMA radios are controlled in real time and kept at a power level to just maintain a quality transmission based on the changing RF environment. This has the benefit of longer battery life and smaller, lower cost amplifier design. If recent health concerns over cellular phone radiation are determined to have some basis in fact, CDMA will be preferred because of its much lower RF output power. Notes: ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

21 CDMA Variable Rate Speech Coder
Advanced Concepts of CDMA CDMA Variable Rate Speech Coder DSP Analyzes 20 Millisecond Blocks of Speech for Activity Selects Encoding Rate Based On Activity: High Activity: Full Data Rate Encoding (9600 bps) Some Activity: Half Data Rate Encoding (4800 bps) Low Activity: Quarter Date Rate Encoding (2400 bps) No Activity: 1/8 Data Rate Encoding (1200 bps) How Does This Improve Capacity? Mobile Transmits in Bursts of 1.25 ms System Capacity Increases by 1/Vaf CDMA takes advantage of quiet times during speech to raise capacity. A variable rate vocoder is used; for the original vocoder the channel is a 9,600 bps when the user is talking. When the user pauses, or is listening, the data rate drops to only 1,200 bps. Data rates of 2,400 and 4,800 bps are also used, though not as often as the other two. The CDG 14.4 kbps vocoder is similar with the four channel rates running at 14,400, 7,200, 3,600, and 1,800 bps. The data rate is based on speech activity and a decision as to the appropriate rate is made every 20 msec. Normal telephone speech has approximately a 40% activity factor. The mobile station lowers its data rate by turning off its transmitter when the vocoder is operating at less than 9,600 bps. Thus CDMA mobiles also operate in a TDMA mode (pulsing) when the vocoder determines that the transmission rate required for a given frame is less than full rate. At 1,200 bps, the duty cycle is only one-eighth of the full data rate. The choice of time for this duty cycling is stochastic based on a pseudo random algorithm. This has the affect of randomizing the transmission times of each mobile. When averaged over many users, the average transmitted power is lowered. Lowering the transmit power at the mobile reduces the level of interference for all other users. This increases the capacity of CDMA by nearly a factor of two. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

22 Advanced Concepts of CDMA
Mobile Power Bursting Each Frame is Divided Into Power Control Groups Each Power Control Group Contains Chips (represents encoded voice bits) Average Power Is Lowered 3dB for Each Lower Data Rate CDMA Frame = 20 ms Full Rate Half Rate Quarter Rate Each 20 millisecond frame in EIA/TIA-95-B CDMA is divided into sixteen "power control groups". When the mobile transmits, each power control group contains 1536 data symbols (chips) at a rate of Mbps. When the voice coder moves to a lower date rate, the CDMA mobile bursts its output by only sending the appropriate number of power control groups. For example, at quarter rate, only four of the sixteen power control groups are transmitted. Remember that the exact location of the transmitted groups is randomized to spread the transmitted power over time. For each lowering of the data rate, the average transmitted power is reduced by 3 dB. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Eighth Rate Copyright © Hewlett-Packard Company 1999

23 Base Station Variable Rate Vocoder
Advanced Concepts of CDMA Base Station Variable Rate Vocoder Base Stations Do Not Pulse TX Channels How Does the Base Station Handle Variable Rate Vocoding ? Repeats Data Bits When Transmitting at Reduced Rates Repeating Data Adds 3 dB Coding Gain Lowers the TX Power 3 dB for Each Lower Rate The base station uses a slightly different scheme when the vocoder moves to lower rates. First, EIA/TIA-95-B CDMA base stations do not pulse their transmissions. Rather, base stations repeat the same bit patterns as many times as needed to get back to the full rate of 9,600 bps. So, if the vocoder selects a frame to be half-rate, the data bits are sent twice to fill the entire frame. The transmit power is then adjusted down by 3 dB, since repeating the data twice adds three dB more processing gain to the signal (21 dB + 3 dB = 24 dB for a half rate frame). Adjusting the gain down maintains the approximate same signal to noise ratio that existed for a full rate frame. Quarter and one-eighth rate frames repeat the data four and eight times to fill each frame, and are lowered in power by 6 and 9 dB respectively. This allows more capacity on the forward link since frames operating below full rate are transmitted with lower power which reduces the total interference. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

24 Forward Link Traffic Channel Physical Layer
Advanced Concepts of CDMA Forward Link Traffic Channel Physical Layer Power Control Puncturing Vocoded Speech data Convolutional Encoder Mbps Interleaver 800 bps Walsh Cover I Short Code 9.6 kbps Long Code Scrambling 19.2 kbps 1/2 rate 1.2288 Mbps P.C. MUX FIR I 3/4 rate 19.2 kbps 19.2 kbps 19.2 kbps 19.2 kbps Short Code Scrambler 14.4 kbps Q The next section will follow the digitally encoded voice data through the encoding process for a forward link traffic channel. Voice data at 9600 bps or bps (full rate) is first passed through a convolutional encoder, which doubles the data rate for the 9600 bps case or increases it by 1.33 times for the bps case. It is then interleaved, a process that has no effect on the rate, but does introduce time delays in the final reconstruction of the signal. The interleaving processes increases the effectiveness of the convolutional encoder. A long code is XOR'ed with the data, which is a voice privacy function and not needed for channelization. The closed loop power control data is then punctured into the data stream using the long code to determine the exact location of the power control bits. CDMA then applies a 64 bit Walsh code which is uniquely assigned to a base to mobile link to form one channel. This sets a physical limit of 64 channels on the forward link. If the coded voice data is a zero, the Walsh sequence is output; if the data is a one, the logical not of the Walsh code is sent. The Walsh coding yields a data rate increase of 64 times. The data is then split into I and Q channels, and scrambled with short codes. The final signals are passed through a low pass filter, and eventually sent to an I/Q modulator. We will now take a closer look at each of the steps the base station takes to create the final transmitted CDMA signal. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ FIR 1.2288 Mbps 20 msec blocks 19.2 kbps 800 bps Long Code Walsh Code Generator Q Short Code Mbps Copyright © Hewlett-Packard Company 1999

25 Advanced Concepts of CDMA
CDMA Vocoders Vocoders Convert Voice to/from Analog Using Data Compression There are Three CDMA Vocoders: IS-96A Variable Rate (8 kbps maximum) CDG Variable Rate (13 kbps maximum) EVRC Variable Rate (improved 8 kbps) Each Has Different Voice Quality: IS-96A - moderate quality EVRC - near toll quality CDG - toll quality All digital communication systems use various processes to convert analog voice signals to and from digital form. Long distance telephone systems have used 8-bit PCM (Pulse Code Modulation) for many years to provide high quality voice transmission. Most PCM systems sample with eight bit resolution to convert voice into a digital data stream of 64 kbps. In recent years, ADPCM (Adaptive Delta PCM) has become a popular alternative to straight PCM since it provides essentially the same voice quality as PCM while using only 32 kbps. This allows more voice channels to be sent on the digital network with quality loss. CDMA now has three vocoder standards for converting voice to digital form while providing a high degree of data compression. The original vocoder as defined in IS-96A is a variable rate vocoder with a maximum rate of approximately 8 kbps. This is quite an improvement over PCM or ADPCM encoders (four to eight times more efficient). However, because of the variable rate nature of this vocoder, the average bit rate is under 4 kbps! The new CDG (CDMA Development Group) vocoder is also a variable rate vocoder but uses a maximum data rate of 13 kbps to provide essentially toll quality voice. The most recent addition is the EVRC (Enhanced Variable Rate Coder) that retains the maximum data rate of 8 kbps but yields voice quality just slight less than the CGD 13 kbps vocoder. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

26 Advanced Concepts of CDMA
CDMA Frame Formats 192 bits in a 20 ms Frame 288 bits in a 20 ms Frame 9600 bps Frame 14400 bps Frame 7200 bps 3600 bps 1800 bps Mixed Mode bit 171 12 8 266 12 8 CRC CRC Information Bits Mixed Mode bit Encoder Tail Bits 1-bit Reserved or Frame Erasure Information Bits Encoder Tail Bits 96 bits in a 20 ms Frame 144 bits in a 20 ms Frame 4800 bps Frame Mixed Mode bit 79 8 8 124 10 8 CRC CRC Information Bits Mixed Mode bit 1-bit Reserved or Frame Erasure Information Bits Encoder Tail Bits Encoder Tail Bits 48 bits in a 20 ms Frame 72 bits in a 20 ms Frame 2400 bps Frame Mixed Mode bit 39 8 54 8 8 Once the analog voice is compressed by one of the vocoder processes, some additional data is added to produce a frame. Each frame in CDMA is 20 milliseconds regardless of the data rate used. This slide shows all of the possible frame configurations for both the 8 kbps and 13 kbps vocoders. In the case of the 8 kbps vocoder running at full rate, each frame consists of a mixed mode bit, 171 vocoder bits, 12 bits of CRC, and 8 encoder tail bits. The result is a frame of 192 bits which produces a continuous date rate of 9,600 bps ( 192 bits x 50 frames/sec = 9,600 bps). The mixed mode bit indicates if the frame is pure channel data or if it contains at least some signaling. The CRC bits allow the mobile to verify that it has correctly decoded a frame. The encoder tail bits provide 8 zeroes in a row to flush the contents of the convolutional coder in preparation for processing the next frame of data. For the CDG 13 kbps coder, the frame is still 20 milliseconds, but a total of 288 bits are sent to produce a data rate of 14,400 bps ( 288 bits x 50 frames/sec = 14,400 bps). The 14,400 bps channel has the mixed mode bit, CRC bits, and encoder tail bits like the 9,600 bps channel. However, the 14,400 bps channel includes a CRC for all data rates while the 9,600 bps channel only provides a CRC check for full and half rate frames. The 14,400 bps channel also includes 1 bit that is reserved in the forward link but is used by the mobile in the reverse link to indicate a frame erasure (the CRC check did not pass). This assists the base station in performing forward link power control efficiently. Notes: ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ CRC Information Bits Encoder Tail Bits Mixed Mode bit 1-bit Reserved or Frame Erasure Information Bits Encoder Tail Bits 24 bits in a 20 ms Frame 36 bits in a 20 ms Frame 1200 bps Frame Mixed Mode bit 15 8 20 6 8 CRC Information Bits Encoder Tail Bits 1-bit Reserved or Frame Erasure Mixed Mode bit Information Bits Encoder Tail Bits Copyright © Hewlett-Packard Company 1999

27 Forward Error Protection
Advanced Concepts of CDMA Forward Error Protection Uses Half-Rate Convolutional Encoder Outputs Two Bits of Encoded Data for Every Input Bit Data Out 9600 bps + Data In 9600 bps D Unlike many digital cellular systems, CDMA provides powerful error correction to all voice data bits. This is desirable in CDMA since the idea is to increase the occupied bandwidth (spread the data). The forward link uses a half-rate convolutional encoder to provide error correction capabilities. This type of encoder accepts incoming serial data and outputs encoded data derived from a series of delay taps and summing nodes. A half-rate encoder produces two output symbols for every symbol input. For the CDMA forward link, the half-rate encoder produces two 9,600 bps serial data streams when driven by a single 9,600 bps data stream. These two 9,600 serial data streams are combined at a higher rate to produce a single 19,200 bps data stream. The resulting redundancy in the digital data after convolutional encoding imparts powerful error correction capability to the TIA CDMA cellular system. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ + Data Out 9600 bps Copyright © Hewlett-Packard Company 1999

28 14.4 TCH Forward Link Modifications
Advanced Concepts of CDMA 14.4 TCH Forward Link Modifications Replaces 8 kbps Vocoder with a 13 kbps Vocoder (both Variable Rate) Effects: Provides Toll Quality Speech Uses a 3/4 Rate Encoder Reduces Processing Gain 1.76 dB Results in Reduced Capacity or Smaller Cell Sizes Vocoded Speech data Convolutional Encoder 3/4 rate 14.4 kbps 19.2 kbps In an effort to provide CDMA with even greater voice quality, the CDG (CDMA Development Group) has proposed and implemented a new vocoder. This new vocoder uses and improved, higher data rate of approximately 13 kbps to digitized voice signals. After adding bits used to support the traffic channel, the final traffic channel data rate with this new vocoder is 14.4 kbps. To accommodate this new vocoder with the least impact to the existing 9.6 kbps traffic channel structure, the CDG simply modified the convolutional encoder rate from a one-half rate to a three-quarter rate encoder. Thus, the output from the convolutional encoder is still the same 19.2 kbps used in the original CDMA system. No other changes are required in the coding structure which simplifies the implementation of this new voice quality mode. Testing has shown that the improvements of the 14.4 vocoder result in voice quality that is the equivalent of good land-line long distance telephony! Obviously, this level of voice quality will be a distinct marketing advantage for CDMA in the highly competitive cellular and PCS markets. However, the voice quality improvement does not come for free. By reducing the level of error correction provided in the convolutional encoder, the overall processing gain is reduced. In this case the overall processing gain is lowered from dB to dB. The result of lower processing gain is that something must give: either the capacity is reduced or the cell sizes must be reduced. The capacity loss for this reduction in processing gain is 1/3 (run the formula on slide #4 for 14,400 bps and you will see the result). Both of these choices have negative effects for operators: the cost to support the same number of uses will increase due to the need to install more cell sites. CDMA network operators will have to balance the benefits of this new vocoder against the costs of implementing it. One possible solution is the EVRC (Enhanced Variable Rate Coder). This new 8 kbps coder promises to produce voice quality equal to the 13 kbps coder without losing processing gain. The TIA committee is in the process of standardizing the 13 kbps coder and is working on selecting the EVRC vocoder design. Notes: ____________________________________________________________________________________________________________________________________________________________________________________ 20 msec blocks Copyright © Hewlett-Packard Company 1999

29 Advanced Concepts of CDMA
CDMA System Time How Does CDMA Achieve Synchronization for Efficient Searching ? Use GPS Satellite System Base Stations Use GPS Time via Satellite Receivers as a Common Time Reference GPS Clock Drives the Long Code Generator 1 12 2 3 4 5 6 7 8 9 10 11 As mentioned earlier, both mobiles and base station in direct sequence CDMA must be synchronized. In the IS-95A system, synchronization is based on the Global Positioning Satellite system time. Each CDMA base station incorporates a GPS receiver to provide exact system timing information for the cell. The base station then sends this information to each mobile via a special channel. In this manner, all radios in the system can maintain near perfect synchronization. Most designs also include atomic clocks to provide a backup timing reference in the event that an insufficient number of GPS satellite can be received. These are capable of maintaining synchronization for up to several hours. The GPS clock used for CDMA system time is then used to drive the long code pseudo-random sequence generator. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

30 Advanced Concepts of CDMA
Long Code Generation Long Code Output Modulo-2 Addition User Assigned Long Code Mask 42 bits The Long Code is generated using a 42 bit linear feedback shift register. The pseudo-random data pattern that it generates repeats every days! This is the master clock and is synchronized in all CDMA radios. The GPS receiver's clock output drives the long code's shift register. In order to provide voice privacy, a user specific 42 bit mask is AND'ed with the output of the long code generator to create a unique long code. The large size of the user mask allows for a very large number of unique codes (about 4.29 billion since only the bottom 32 bits are used for the public mask). This is enough codes to ensure that each user can have their own unique scrambling code. There are two types of masks: the public mask and the private mask. The user's public mask is derived from the CDMA mobile's ESN. This provides nominal voice privacy. To provide truly secure voice scrambling, the private mask is formed from a combination of ESN, a random seed, and cryptological processing algorithms. This cryptological processing is the same authentication process that was developed for the AMPS analog system (using, of course, CDMA channels to transmit the information). CDMA authentication is based on an "A" key that is programmed into the phone and is known by the network. The network can then challenge the phone and then compare the result returned from the phone to verify that the phone is legitimate. The phone uses its internal "A" key to process the challenge and return a valid result. Notes: ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 42 41 5 4 3 2 1 Long Code Generator Copyright © Hewlett-Packard Company 1999

31 Advanced Concepts of CDMA
Long Code Scrambling User's Long Code Mask is Applied to the Long Code Masked Long Code is Decimated Down to 19.2 kbps Decimated Long Code is XOR'ed with Voice Data Bits Scrambles the Data to Provide Voice Security XOR Encoded Voice Data 19.2 kbps 19.2 kbps 19.2 kbps Long Code Decimator Long Code Generator Mbps In the forward link the long code is used to scramble the voice data to provide some measure of security. However, the long code is not used to spread the signal bandwidth in the forward channel. To accomplish the scrambling without increasing the data rate, the long code is decimated down to a lower rate after the user's unique long code mask is applied. The decimation is accomplished by essentially using every sixty-fourth bit out of the long code data stream. A sixty four times decimation reduces the data rate of the long code from Mbps to 19.2 kbps. At this point the long code data rate matches that of the encoded voice data it is exclusive OR'ed with. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

32 Closed Loop Power Control Puncturing
Advanced Concepts of CDMA Closed Loop Power Control Puncturing Long Code is Decimated Down to 800 bps Decimated Long Code Controls the Puncture Location Power Control Bits Replace Voice Data Voice Data is Recovered by the Mobile's Viterbi Decoder Closed Loop Power Control Bits 800 bps Long Code Scrambled Voice Data 19.2 kbps 19.2 kbps P.C. Mux 800 bps Long Code Decimator Once the data has been scrambled with the user specific long code, the closed loop power control data is then punctured into the data stream. Remember that the power control bits are sent every 1.25 milli-seconds - once in every power control group (a CDMA frame is 20 milli-seconds with each frame having milli-second power control groups). In each 1.25 milli-second power control group there are 24 modulation symbols of data (the data stream at this point is 19.2 kbps so each of the 24 symbols has a period of micro-seconds). The power control bits are placed somewhere in the first 16 modulation symbols in each power control group. The exact location of the power control bits are determined by decimating the long code down to a rate of 800 bps and then using the data to point to one of the modulation symbol locations. For a 9.6 kbps voice channel, two modulation symbols are punctured allowing the power control data to be sent twice. For 14.4 kbps voice channels, only a single modulation symbol is punctured with the power control bit. Since the power control bits replace the encoded voice data, holes (missing data) are introduced into the data stream from the receiver's point of view. These holes are accepted and the system uses the Viterbi decoder in the receiver to restore the data lost by puncturing. The recovery of the missing data uses some of the available processing gain in the system. This results in a loss of capacity, but the loss has been accounted for in the system's design. Another way to think of this is that slightly more power is required to maintain the link because of the missing data introduced by the power control puncturing. The power control data is only sent once in the 14.4 case since the reduced processing gain results in higher power being transmitted from the base station to maintain an acceptable signal to noise ratio. The higher power results in a much lower symbol error rate and the need to send the power control data twice is eliminated. Notes: ____________________________________________________________________________________________________________________________________________________________________________________ Long Code Decimated Data 19.2 kbps Copyright © Hewlett-Packard Company 1999

33 Advanced Concepts of CDMA
Walsh Codes W = 0 1 0 0 0 1 W = W W W = n n 2 2n n n W = An important feature of the forward link is the use of Walsh codes. These codes have the desirable characteristic of being orthogonal to each other and to the logical NOT of each other. Walsh code sets are generated by the Hadamard matrix expansion shown below: The variable, n, in this expansion must always be a power of two. This is seeded with the one by one matrix: Each higher order Walsh code set is created by placing the entire set into the first three matrix positions and then by placing an inverted set into the lower right hand matrix position. Do not confuse this matrix with some type of matrix math operation. It is simply a place holder to allow the creation of orthogonal code sets of every increasing size. EIA/TIA-95-B standard CDMA uses Walsh code set 64. This set has 64 unique codes with each code having 64 bits. Notice that for each Walsh code set, the first code is composed entirely of zero data bits. Two functions are said to be orthogonal if the cross- correlation coefficient between the two functions is zero. The cross-correlation coefficient for generalized time variant functions is: 4 Copyright © Hewlett-Packard Company 1999

34 Checking for Orthogonality
Advanced Concepts of CDMA Checking for Orthogonality Cross Correlation N agreements - N disagreements N total_number_of_digits = W = 4 Y Y N N If the cross-correlation coefficient zij is zero, the functions fi and fj are said to be orthogonal. The generalized formula for the cross-correlation coefficient can be reduced to the following formula when the two functions are digital codes: This formula is a measurement of the number of bit positions that match and the number that do not match. Orthogonal codes have equal number of matches and mismatches yielding a cross-correlation coefficient of zero. Looking at Walsh code set 4 in slide number 24, notice that each code (defined by rows) has four bits, and that there are four codes. If you compare any two codes using the binary cross-correlation coefficient formula, zij will be zero since half the bits match and half do not (looking down the columns). Thus these codes meet the test for orthogonality. Thus, correlators in CDMA mobile receivers will yield a cross correlation coefficient of 1 when receiving the desired code and zero for all other Walsh codes. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 2 match - 2 don't =0 Copyright © Hewlett-Packard Company 1999

35 Advanced Concepts of CDMA
Walsh Code Spreading XOR Encoded Voice Data 19.2 kbps Mbps What is the Spreading Rate Increase ? Mbps Walsh Code Generator In the forward channel, the Walsh codes provide a means to uniquely identify each user. A Walsh code generator provides one of the sixty four codes to scramble the encoded voice data. The Walsh code generator runs at a data rate of Mbps. Since the encoded voice data runs at a rate of 19.2 kbps, the ratio of the Walsh rate to the voice rate is exactly 64. When the two data streams are XOR'ed together, the result is that the entire 64 bits of the Walsh code are sent out in an inverted or non- inverted condition, depending on the polarity of the voice data bit. Thus the voice data determines the polarity of the Walsh code that is output. This makes it relatively easy for a CDMA mobile to find and decode its assigned Walsh code (channel). Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

36 Why Spread Again with the Short Sequence ?
Advanced Concepts of CDMA Why Spread Again with the Short Sequence ? Provides a Cover to Hide the 64 Walsh Codes Each Base Station is Assigned A Time Offset in its Short Sequences Time Offsets Allow Mobiles to Distinguish Between Adjacent Cells Also Allows Reuse of All Walsh Codes in Each Cell Mbps I Channel Short Sequence Code Generator Walsh Coded Data at Mbps To I/Q Modulator Q Channel Short Sequence Code Generator After all of the coding used so far in the forward channel, you're probably wondering why the data needs to be scrambled again at the same rate. If all cells used the same 64 Walsh codes without another layer of scrambling, the resulting interference would severely limit system capacity. Since all cells can use the same frequency (frequency domain), and all cells use the same Walsh codes (code domain), the only other means to allow cells to reuse the same Walsh codes is by using time offsets (time domain). This final layer of scrambling uses another code called the short code to allow reuse of the Walsh codes and to provide a unique identifier to each cell. The short sequence is bits long and runs at the Mbps rate (repeats every ms). Since everything in CDMA is synchronized to system time, it is possible to have each cell site identified by using a time offset in the short sequence. These "PN Offsets" are separated by multiples of sixty- four Mbps clock chips. This allows for 512 unique time offsets for cell identification (32768 bits / 64 bits = 512 offsets). By scrambling the Walsh encoded channels with the short code, each base station can reuse all 64 Walsh codes and be uniquely identified from other adjacent cells using the same frequency. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Mbps Copyright © Hewlett-Packard Company 1999

37 Advanced Concepts of CDMA
Auto-Correlation Is a Comparison of a Signal Against Itself Good Pseudo-Random Patterns Have: Strong Correlation at Zero Time Offset Weak Correlation at Other Time Offsets chip 5 10 15 20 25 30 1 Pseudo-Random Sequence -1 Auto-Correlation Versus Time Offset 5 10 15 20 25 30 At this point it is worthwhile to discuss the idea of auto-correlation. Auto-correlation is simply a comparison of a signal against itself. For a digital sequence, such as the short codes used in EIA/TIA- 95-B, this comparison is a measure of the number of bits that match relative to the number of bits that do not match. Good pseudo-random patterns (such as the short codes) are designed to have near perfect auto- correlation when time aligned and have very weak auto-correlation at all other time offsets. In other words, the short codes are designed to match when aligned and have a near zero match (equal number of matches and mismatches) at all other time alignments. This slide shows the auto-correlation of a good pseudo-random digital sequence against itself verses time offset. You can see that there is strong correlation with zero offset, and that at all other offsets, the net correlation is near zero. This property makes finding the short code at a given PN Offset easy. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ chip offset Copyright © Hewlett-Packard Company 1999

38 Short Code Correlation
Advanced Concepts of CDMA Short Code Correlation Short Codes Are Designed to Have: Strong Auto-Correlation at Zero Time Offset Weak Auto-Correlation at Other Offsets Good Auto-Correlation In Very Poor Signal-to Noise Ratio Environments Allows Fast Acquisition in Real World Environment Auto-Correlation Versus Time Offset with 17 dB Noise Added As mentioned in the previous slide, the short codes are designed to have strong auto-correlation at zero time offset and weak auto-correlation at other time offsets. But what happens to the auto- correlation properties when there are many short codes with different PN Offsets present at the same time? This is the condition present in a real, working EIT/TIA-95-B network. This slide shows the auto- correlation of a short codes in the presence of 17 dB of added white noise. White noise can be used since another property of the short codes is that they appear as white noise interference to receivers looking for different PN Offset short codes. Even with this much added noise, the auto-correlation at zero offset is strong. At other offsets, the net auto-correlation is not zero, but is still relatively weak compared to the zero offset auto-correlation. These properties of the short codes allow CDMA receivers to quickly acquire the desired PN Offset signal even in the presence of large amounts of interference. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 5 10 15 20 25 30 chip offset Copyright © Hewlett-Packard Company 1999

39 Forward Link Channel Format
Advanced Concepts of CDMA Forward Link Channel Format Walsh Code 0 I Data All 0's Convert to I/Q & Short Code Spreading kbps FIR LP Filter & D/A Conversion Pilot Channel Q Data Walsh Code 32 I I Data Convert to I/Q & Short Code Spreading 4.8 kbps kbps FIR LP Filter & D/A Conversion Sync Channel Q Data Walsh Codes 1 to 7 I Data Convert to I/Q & Short Code Spreading Paging Channels 19.2 kbps kbps FIR LP Filter & D/A Conversion Up to this point we have examined the coding that takes place on a forward traffic channel. The Base Station transmitter signal is, however, the composite of many channels (with a minimum of four). The Pilot channel is unmodulated (Walsh code 0); it consists of only the final spreading sequence (short sequences). The Pilot channel is used by all mobiles linked to a cell as a coherent phase reference and also provides a means for mobiles to identify cells from each other. The other three channels are the Sync channel, the Paging channel, and the Traffic channel which use the same data flow, but different data are sent on these channels. The Sync channel transmits time of day information. This allows the mobile and the base to align clocks which form the basis of the codes that are needed by both to make a link. Specifically, one message sent by the Sync channel contains the state of the long code feedback shift registers 320 milli-seconds in the future. By reading this channel, CDMA mobile can load the data into its long code generator, and then start the generator at the proper time. Once this has been accomplished, the CDMA mobile has achieved full synchronization. The Sync channel always uses Walsh code channel 32. The Paging channel is the digital control channel for the forward link. Its complement is the access channel which is the reverse link control channel. One base station can have multiple paging channels and access channels if needed. Up to seven Walsh code channels can be allocated for use as paging channels. The first paging channel is always assigned to Walsh code 1. When more paging channels are required, Walsh codes 2 through 7 are used. The Traffic channel is equivalent to the analog voice channel. This is where the actual conversations take place. The remaining Walsh codes are assigned to traffic channels as required. At least 55 Walsh codes are available for use as traffic channels. The actual number that can be used is determined by the total interference levels experienced in any given cell. Nominal full loading would typically be around 30 traffic channels in use for equally loaded cells. Once all of the various channels are Walsh modulated, they are converted into I/Q format, re-spread with the I and Q short sequences, low pass filtered to reduce occupied bandwidth, and converted into analog signals. The resulting analog I and Q signals from all the channels are summed together and then sent to the I/Q modulator for modulation onto an RF carrier. 1 up to 7 Channels Q Data Walsh Codes 8-31, 33-63 Q I Data Convert to I/Q & Short Code Spreading 19.2 kbps kbps FIR LP Filter & D/A Conversion Traffic Channels 1 up to 55 Channels Q Data kbps Copyright © Hewlett-Packard Company 1999

40 Advanced Concepts of CDMA
Walsh Coding Example User A 0 0 0 1 User B -1 +1 - User A -1 +1 For a 0 Input, Use Code 00 W = For a 0 Input, Use Code 01 2 - User B 1 -1 +1 1 -1 +1 For a 1 Input, Use Code 11 For a 1 Input, use code 10 1 +1 Channel A Voice Data +1 Channel B Voice Data + 1 1 1 -1 +1 1 -1 +1 1 Channel A Walsh Encoded Voice Data Channel B Walsh Encoded Voice Data It may seem impossible that once all of these Walsh encoded signals are combined that any data can be recovered. To illustrate some of the concepts discussed so far, lets look at a fictitious example using Walsh code set 2. In this set of Walsh codes there are two unique orthogonal codes, namely "00", and "01". Assume that we assign the Walsh code "00" to User A and Walsh code "01" to User B. Thus when User A's digital voice data is a "0", the Walsh encoded output sent will be "-1-1" (remember the voice data is exclusive OR'ed with the Walsh code). Note that for Walsh code addition to work, we must use bipolar values so that a binary "0" has a value of negative 1. The Walsh code for User A will be "11" if the voice data is a binary "1". User B's voice data is Walsh encoded in a similar manner using Walsh code "01". Next let us assume that User A's digital voice data is "0100" and User B's voice data is "1001". Using Walsh encoding, User A's encoded data will be " " and User B's will be " ". You can see that if we add these two data patterns, the result is a data stream that ranges from -2 to 2. It may not look like it at this point, but the data from the original two users is contained in this waveform in such a manner as to not produce any interference between the two signals. Also notice that the combined signal has a much larger peak to average signal ratio. Imagine what happens when you are working with the actual EIA/TIA-95-B system that uses 64 bit Walsh codes. The peak to average ratio further increases when using 64 bits Walsh codes. The summing of the Walsh encoded signals creates the high crest factor (peak to average ratio) that forces amplifiers in CDMA base stations to have very wide dynamic in order to cleanly reproduce the high peaks of the modulated waveform. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Sum of A & B Walsh Encoded Data Streams +2 +1 -1 -2 Copyright © Hewlett-Packard Company 1999

41 Walsh Decoding Example
Advanced Concepts of CDMA Walsh Decoding Example Original User A Voice Data Original User B Voice Data Correlation Coefficient +1 T +1 1 f i (t) z = ij f (t) j dt 1 1 1 User A + B Walsh Data User A + B Walsh Data T +2 -1 +1 -2 +2 +1 -1 -2 Multiply Summed Data with Desired Walsh Code Multiply Summed Data with Desired Walsh Code +2 To prove this point, let us now try to recover the two user's data from the summed signal. As we saw earlier in the presentation, the correlation coefficient is defined as the integral of the product of two functions over a given period of time. To find our desired signal, we must multiply the combined signal containing the sum of the Walsh codes by the inverse of the desired code and then integrate the result (determine the area beneath the curve). For User A, we multiply the summed waveform by the inverse Walsh code "11". If we look at the first two bits of the combined waveform, the result of the multiplication is 0 and then -2 over two bit periods. If we add up the area (-2) and divide by the period (2) we find that the originally transmitted data was -1 (zero data bit)! That is the correct answer. In a similar manner if we multiply the combined waveform by the inverse Walsh code "10" (bipolar values of 1,-1), we get a result of 0 and then 2. Adding the total area and dividing by the period gives a result of 1 which is also the correct answer. This clearly shows that summing Walsh codes still allows for perfect reconstruction of the original data. The TIA CDMA system simply extends this concept to using 64 bit long Walsh codes. It is also easy to show that there is zero cross talk between Walsh code signals (this should be true for orthogonal signals). Just multiply Walsh code "00" with Walsh code "01". Add the resulting area and divide by the period and you will always get a result of zero. This is the definition of orthogonality. Put in simpler terms, since the codes have equal number of bit matches and mismatches, they are orthogonal. To see the effect of Walsh code timing misalignment, shift the inverse Walsh code in time and repeat the multiplication and integration. The answer you now get is no longer exactly 1 or -1 indicating that interference s now present between coded channels. Notes: ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ +2 +2 +2 - +1 1 +1 +1 +1 = = +1 +1 1 x x = = -1 -1 -1 -1 1 1 -1 -1 1 -2 -2 -2 -2 Copyright © Hewlett-Packard Company 1999

42 What if Walsh Codes are Not Time Aligned ?
Advanced Concepts of CDMA What if Walsh Codes are Not Time Aligned ? Original Time Delayed + -1 +1 1 -1 +1 1 Channel A Walsh Encoded Voice Data Channel B Walsh Encoded Voice Data -1 +2 +1 -2 Sum of A & B Walsh Encoded Data Streams What will happen to our nice example if the Walsh codes are not time aligned in the base station? If the Walsh code for user A is delayed in the base station relative to the Walsh code used for User B, when the two signals are summed together the result is not an even period addition or subtraction of the Walsh encoded waveforms. Instead we get a summed waveform that has intermediate steps that do not change state on a clock boundary. The result is that when the signal for either user is decoded, the answer is not the original data state, but rather a fractional value that is close to the correct state. This indicates that the two Walsh encoded waveforms were NOT orthogonal with the result being an error in the received data value (interference). Thus, if the various channels in use in a CDMA base station are not precisely time aligned, the orthogonality of the channels will be degraded. This reduction in orthogonality will directly reduce the capacity of a CDMA base station. From this we can infer that an important parameter to measure in a CDMA base station is the time alignment accuracy of the various CDMA channels in that base station. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Multiply Summed Data with Desired Walsh Code Original Data Was 0 (-1), We Have Interference Now! +2 +2 - +1 +1 +1 x = = 0.75 -1 -1 -1 1 1 -2 -2 Copyright © Hewlett-Packard Company 1999

43 Pilot Channel Physical Layer
Advanced Concepts of CDMA Pilot Channel Physical Layer Uses Walsh Code 0: All 64 bits are 0 All Data into Walsh Modulator is 0 Output of Walsh Modulator is Therefore all 0's Pilot Channel is just the Short Codes Mbps Walsh Modulator I Short Code 1.2288 Mbps All 0 input FIR I Short Code Scrambler Q FIR 1.2288 Mbps As noted before, the Pilot channel is essentially the short codes operating on their assigned PN offset. This is accomplished by selecting Walsh code 0 which is 64 zeroes ( remember, the first code in any Walsh code set is always composed of all zeroes) as the Walsh code modulation data. In addition, the channel data coming into the Walsh modulator is also all zero which results in an all zero output. In practice, the Walsh modulator can be eliminated as only the short code hardware needs to be implemented for a channel generating a pilot. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Walsh Code Generator Q Short Code Walsh Code Mbps Copyright © Hewlett-Packard Company 1999

44 Sync Channel Physical Layer
Advanced Concepts of CDMA Sync Channel Physical Layer Sync Channel Message Data Mbps Walsh 32 Cover Convolutional Encoder Symbol Repetition I Short Code Interleaver 1.2288 Mbps 1/2 rate 2x FIR I 1.2 kbps 2.4 kbps 4.8 kbps 4.8 kbps Short Code Scrambler FIR Q The Sync channel is coded in manner similar to a traffic channel except that the long code scrambling is not needed. The actual messaging data for the sync channel runs at 1,200 bps. This data is first passed through a half-rate covolutional encoder which doubles the data rate to 2,400 bps. The encoded data is then repeated (also increases the processing gain by 3 dB) to bring the data rate to 4,800 bps. The data is then interleaved and then passed on to the Walsh modulator. Since the data rate at this point is only 4,800 bps, the output of the Walsh modulator is Walsh code 32 (or the logical NOT of Walsh code 32) repeated four times for each 4,800 bps bit. The signal is then scrambled against the short codes. This coding provides a total of 30 dB of processing gain (a full rate traffic channel at 9,600 bps gets 21 dB)! The extra processing gain helps mobiles receive the critical timing information of the Sync channel error free. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 1.2288 Mbps Walsh Code Generator Q Short Code Mbps Copyright © Hewlett-Packard Company 1999

45 Paging Channel Physical Layer
Advanced Concepts of CDMA Paging Channel Physical Layer Paging Channel Message Data Mbps Convolutional Encoder Symbol Repetition Walsh 1 to 7 Cover I Short Code Paging Channel Long Code 19.2 kbps Scrambling Interleaver 1.2288 Mbps 1/2 rate 2x FIR I 4.8 kbps 9.6 kbps 19.2 kbps 19.2 kbps 19.2 kbps Short Code Scrambler Q FIR The paging channel message data runs at either 4,800 or 9,600 bps (called half or full rate operation). Normally half rate is used since it provides an extra 3 dB of processing gain. This makes the paging channel message reception more robust. Like a Traffic channel, the Paging channel uses a half-rate convolutional encoder. However, when operating in the half-rate mode, the output of the convolutional encoder is repeated to provide the extra 3 dB of processing gain (waveform redundancy). Following the interleaving operation, the paging channel data is scrambled against a special long code. The paging channel long code is derived from a pre-defined bit pattern plus the paging channel number and the Pilot channel's PN offset. The long code scrambled data is then Walsh modulated with Walsh code 1 (for the first Paging channel) and then scrambled with the short codes. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 1.2288 Mbps Walsh Code Generator Q Short Code Mbps Copyright © Hewlett-Packard Company 1999

46 Reverse Link Traffic Channel Physical Layer
Advanced Concepts of CDMA Reverse Link Traffic Channel Physical Layer 64-ary Modulator Mbps Convolutional Encoder 1 of 64 Walsh Codes Long Code Modulator I Short Code Vocoded Speech Data Interleaver Walsh Code 63 1.2288 Mbps 1/3 rate 9.6 kbps 28.8 kbps Walsh Code 62 FIR I Walsh Code 61 307.2 kbps Short Code Scrambler t/2 FIR Q 1/2 rate Walsh Code 2 The CDMA reverse link uses a different coding scheme to transmit data. Unlike the forward link, the reverse link cannot support a pilot channel for synchronous demodulation (since each mobile station would need its own pilot channel). The lack of a pilot channel is partially responsible for the reverse link's lower capacity than the forward link. In addition, Walsh Codes cannot be used for channelization since the varying time delays from each mobile to the base station destroys the orthogonality of the Walsh Codes (varying arrival time makes the Walsh Codes non -orthogonal). Since the reverse link does not benefit from non-interfering channels, this reduces the capacity of the reverse link when compared to the forward link (all mobiles transmitting interfere with each other). To aid reverse link performance, the 9600 bps voice data uses a one-third rate convolutional code for more powerful error correction. For the bps vocoder, the convolutional encoder is only a half rate encoder that doubles the data rate. Thus the data rate coming out of the convolutional encoder is the same for either the 9.6 or 14.4 kbps voice channels. Then, six data bits at a time are taken to point at one of the 64 available Walsh codes. The data, which is at kbps, is then XOR'ed with the long code to reach the full Mbps data rate. This unique long code is the channelization for the reverse link. We will now look at the processes in the CDMA reverse link in more detail. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 14.4 kbps 1/2 Chip Delay 28.8 kbps 28.8 kbps Walsh Code 1 20 msec blocks Q Short Code Walsh Code 0 Mbps Mbps Long Code Copyright © Hewlett-Packard Company 1999

47 Reverse Error Protection
Advanced Concepts of CDMA Reverse Error Protection Uses Third-Rate Convolutional Encoder Outputs Three Bits for Every Input Bit Data Out 9600 bps + Data In 9600 bps D + To improve the performance of the reverse link (which is less than the forward link due to the lack of a pilot channel), a more powerful convolution encoder is used than in the forward link. The third-rate encoder used in the reverse link outputs three 9,600 bps data streams when driven with a single 9,600 bps data stream. This provides increased error correction capability, but also increases the data rate to 28,800 bps. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Data Out 9600 bps + Data Out 9600 bps Copyright © Hewlett-Packard Company 1999

48 14.4 TCH Reverse Link Modifications
Advanced Concepts of CDMA 14.4 TCH Reverse Link Modifications Replaces 8 kbps Vocoder with a 13 kbps Vocoder (both Variable Rate) Effects: Provides Toll Quality Speech Uses a 1/2 Rate Encoder Reduces Processing Gain 1.76 dB Results in Reduced Capacity or Smaller Cell Sizes Vocoded Speech data Convolutional Encoder 1/2 rate 14.4 kbps 28.8 kbps The 14.4 traffic channel option proposed by CDG effects the reverse link in a similar manner as the forward link. In the reverse link case, the convolutional encoder is reduced from a one-third rate to a one-half rate encoder. The reduction in processing gain is exactly the same as for the forward link. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 20 msec blocks Copyright © Hewlett-Packard Company 1999

49 Advanced Concepts of CDMA
64-ary Modulation Every 6 Encoded Voice Data Bits Points to One of the 64 Walsh Codes Spreads Data From 28.8 kbps to kbps: (28.8 kbps * 64 bits)/ 6 bits = kbps) Is Not the Channelization for the Reverse Link 307.2 kbps Walsh Code 1 Walsh Code 2 Walsh Code 0 Walsh Code 62 Walsh Code 63 Walsh Code 61 28.8 kbps Walsh codes are not used to provide the channelization in the reverse link (more on this in a moment). In the reverse link they are used to randomize the encoded voice data with a modulation format that is easy to recover. Each six serial data bits output from the convolutional encoder are used to point to one of the 64 available Walsh codes (26=64). This modulation has the affect of increasing the data rate by times to kbps (28,800/6 x 64 = 307,200). As the incoming voice data changes, a different Walsh code is selected. Since this type of modulation can output one of sixty-four possible codes, it is referred to as 64-ary modulation. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

50 Why Aren't Walsh Codes Used for Reverse Channelization ?
Advanced Concepts of CDMA Why Aren't Walsh Codes Used for Reverse Channelization ? All Walsh Codes Arrive Together in Time to All Mobiles From the Base Station However, Transmissions from Mobiles DO NOT Arrive at the Same Time at the Base Station There are two key reasons that Walsh codes are not used for the channelization in the reverse link: Mobile transmission are not time aligned and therefore cannot be orthogonal, and Walsh codes do not provide enough unique channels (causing more network overhead to manage Walsh code handoffs). In the forward link, a point source (the base station), transmits a composite signal containing the Walsh encoded channels for many users. Since the various coded channel are sent together, they arrive at a phone together no matter where the phone is. Phones close to the base station receive all channels simultaneously as do phones far from the base station The only difference is that is that the composite signal arrives later for the phone far from the base station. However, the reverse link does not share this property because each phone sends its own signal back to the base station. Since the phones are different distances from the base station, their signals arrive at different times, thus precluding orthogonality. No attempt is made in EIA/TIA-95-B to time align the phones transmissions since achieving the required accuracy for good orthogonality is cost prohibitive. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

51 Reverse Channel Long Code Spreading
Advanced Concepts of CDMA Reverse Channel Long Code Spreading Long Code Spreading Provides Unique Mobile Channelization Mobiles are Uncorrelated but not Orthogonal with Each Other Walsh Modulated Voice Data 307.2 kbps Mbps XOR Long Code Generator The channelization in the reverse link must provide for unique code assignments for every operational phone. . Since the long code is 42 bits in length, this allows 2^42 (4.3 billion) unique channel assignments. Thus the long code imprinted with the user's unique mask is used to provide the channelization in the reverse link. This allows all mobiles in even very large systems to have unique channel assignments. Of course, since the long codes are simply uncorrelated and not orthogonal to each other, the recovery and demodulation process is more difficult for CDMA base stations. The high speed searcher circuits in the base station allow it to quickly search over the wide range of long codes to lock on a particular user's signal. These more expensive modules represent a good design trade-off, since it is possible to design in more expense and complex hardware/software into a base station than into a mobile phone. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

52 Reverse Channel Short Sequence Spreading
Advanced Concepts of CDMA Reverse Channel Short Sequence Spreading Same PN Short Codes Are Used by Mobiles Short Sequence Spreading Aids Base Station Signal Acquisition Extra 1/2 Chip Delay is Inserted into Q Path to Produce OQPSK Modulation to Simplify Power Amplifier Design t/2 Q FIR I Short Code Q Short Code 1.2288 Mbps I 1/2 Chip Delay Mbps CDMA mobiles use the same PN sequences as the base for final short sequence scrambling. An extra one half period clock delay in the mobile's Q channel produces Offset QPSK modulation rather than straight QPSK modulation. This is done so that mobiles can use a simpler and more efficient power amplifier design. OQPSK modulation prevents the signal from going to zero magnitude (going though the origin of the I/Q diagram) and greatly reduces the dynamic range of the modulated signal. Less costly amplifiers can be used on CDMA mobiles because of the reduced linear dynamic range obtained with OQPSK modulation. The I/Q diagram in the next slide shows this property of OQPSK modulation. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

53 Advanced Concepts of CDMA
OQPSK Modulation I Q QPSK Makes one Symbol Change Every Period OQPSK Makes two Symbol Changes Every Period if both I and Q Data Changes Example Symbol Pattern is: 00, 10, 01,11 00 01 10 11 I Q 00 01 If the modulation data changes, QPSK modulation makes one symbol change each period. If the data is the same, then QPSK and OQPSK do not change state. However, if the data changes, then OQPSK will make at least one symbol change and possibly two changes in a single period. In the above example, the data input to the modulator is: 00, 10, 01, 11. The upper I/Q diagram shows the symbol changes made by QPSK. The I/Q modulator for QPSK moves from state to state, for a total of three transitions (with one transition going through the origin). For the same pattern, OQPSK makes four transition, and it avoids the origin. The change from 00 to 10 results in a single transition since the value of Q did not change. However, the change from 10 to 01 makes two transitions because the value of Q does change. First the modulator returns to the 00 state. Then one-half period later, the modulator changes to the final 01 state. This action, due to the delay placed into the Q branch of the modulator, eliminates the transition through the origin. The change from the 01 to 11 state for OQPSK is a single transition since the value of Q again remained constant. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ 10 11 Copyright © Hewlett-Packard Company 1999

54 CDMA Modulation Formats
Advanced Concepts of CDMA CDMA Modulation Formats Base Station Pilot Channel TX Mobile Station TX Q Q I I I The modulation is Filtered QPSK in the base station, and Filtered Offset QPSK in the mobile station. Note that the I/Q diagram for the base station signal is for only a single channel (such as the pilot channel). In normal operation, many channels are summed together and transmitted on top of each other by the base station. O-QPSK is used in the mobile stations because it avoids going through the origin and makes the design of the output amplifier easier. For the base station, since many channels are summed together, using O-QPSK would not always avoid the origin. This is due to the random nature of adding many signals together. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Filtered QPSK Filtered Offset QPSK Copyright © Hewlett-Packard Company 1999

55 Channelization Summary
Advanced Concepts of CDMA Channelization Summary Function Forward Link {Base to Mobile} Reverse Link {Mobile to Base} 9.6 kbps Convolutional Encoder 1/2 Rate {9600 in out} 1/3 Rate {9600 in out} 14.4 kbps Convolutional Encoder 3/4 Rate {14400 in out} 1/2 Rate {14400 in out} Walsh Coding Channelization 64-ary Modulation Now that we have looked at both the forward and reverse links in detail, it is useful to summarize the key differences between them. The forward link uses a one half rate convolutional encoder, Walsh codes for channelization, long code voice privacy scrambling, and short code time offsets for base station identification. The reverse link uses a one third rate convolutional encoder, 64-ary Walsh code modulation for data spreading, long codes for channelization, and short code modulation for transmission format simplification. Next, a review of the key strengths and weakness' of the forward and reverse links will help to highlight the design trade-offs made in the EIA/TIA-95-B cellular system. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Long Code Spreading Voice Privacy Channelization Short Code Spreading Base Station Identification Aid Base Station Searching Copyright © Hewlett-Packard Company 1999

56 Link Advantages & Disadvantages
Advanced Concepts of CDMA Link Advantages & Disadvantages Forward Link {Base to Mobile} Reverse Link {Mobile to Base} + -High Power Transmitter -Pilot Channel -Added Time Diversity -Orthogonal Code Channels -Wide Range Power Control -Diversity Reception at Base - -Complexity of Soft Handoff -Non-coherent Demodulation -Limited Power -Uncorrelated Code Channels The forward link advantages include high transmission power, a pilot channel to provide a timing reference as well as a coherent reference to aid in demodulation, the added time diversity of data bit repetition when transmitting at lower voice data rates, and orthogonal code channel. On the downside, the forward link must support the added cost and complexity imposed by soft handoff capability. The reverse link advantages are wide dynamic range power control, and diversity reception at the base station. The reverse liabilities are limited transmitter power, non-coherent demodulation by the base station, and uncorrelated rather than orthogonal code channels. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

57 CDMA Multiplex Sublayer
Advanced Concepts of CDMA CDMA Multiplex Sublayer Layer 3 Call Processing and Control Layer 2 Layer 2 Layer 2 Primary Traffic Signaling Link Layer Paging & Access Channels Multiplex Sublayer Traffic Channel Signaling is well structured in CDMA. The full data rate of 9600 bps can be shared between data for the user and signaling data. The channel is effectively a modem that can be used for a variety of services. The Multiplex Sublayer directs what data is sent to it through to the physical channel layer. Future options can be integrated into the system because of the signaling structure. A future option now under consideration at the TIA committee will extend the traffic channel to support a 14.4 kbps data rate. Going to 14.4 kbps yields better voice quality, but reduces the capacity and increase the infrastructure costs for CDMA. Current standards exist for service option 1, the vocoder. Service option 2 is a data loopback mode that enables proper testing of CDMA mobiles. Service option 3, data services, is under discussion at the standards committee. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Layer 1 Physical Layer Channel Data bps or bps Copyright © Hewlett-Packard Company 1999

58 Advanced Concepts of CDMA
CDMA Service Options Service Options Are: 1- Voice Using 9600 bps IS-96-A Vocoder 2- Rate Set 1 Loopback (9600 bps) 3- Voice Using 9600 bps (EVRC) 4- Asynchronous Data Service (circuit switched) 5- Group 3 Fax 6- Short Message Service (9600 bps) 7- Internet Standard PPP Packet Data 8- CDPD Over PPP Packet Data 9- Rate Set 2 Loopback (14400 bps) 14-Short Message Service (14400 bps) 32,768- Voice Using bps (CDG) A number of different service options have been defined for use on the CDMA physical channel. A description of each of the service options follows: Service Option 001 -This is the normal, duplex voice conversation mode that uses the original 8 kbps vocoder defined in the TIA IS-96-A specification. Service Option 002 -This is the test mode used to measure receiver performance for voice channels operating at 9600 bps (also known as Rate Set 1 Data Loopback). Service Option 003 -This duplex voice conversation mode uses the new, improved voice quality 8 kbps vocoder known as the Enhanced Variable Rate Coder (EVRC) Service Option 004 -The options supports circuit switched data transmission. Typical throughput with error correction overhead (re-transmitting lost frames) is about 7 kbps. Service Option 005 -This service option allows normal group 3 fax transmissions. Service Option 006 -This service option provides short message service that allows alphanumeric messages to be sent and displayed on a CDMA mobile at 9600 bps. Service Option 007 -This service option allows the transmission of data using packet transmission rather than a circuit switched approach. Service Option 008 -This is similar to service option 007, except that the CDPD packet protocol is used as an overall control layer over the SO7 PPP packet protocol. Service Option 009 -This is the test mode used to measure receiver performance for voice channels operating at bps (also known as Rate Set 2 Data Loopback). Service Option 014 -This service option provides short message service that allows alphanumeric messages to be sent and displayed on a CDMA mobile at bps. Service Opt 32,769 -This is a proprietary service option owned by the CDG that governs the duplex voice conversation mode that uses the CDG kbps vocoder. Copyright © Hewlett-Packard Company 1999

59 Advanced Concepts of CDMA
CDMA Protocol Stacks EIA/TIA 95-B Combines TSB-74 & J-STD-008 for a Universal Protocol J-STD-008 Not Backwards Compatible, PCS only Protocol TSB-74 ARIB T53 Japan CDMA System Cellular Protocol Cellular Protocol that adds Channel Support The original release of the EIA/TIA-95-B standard defined the protocol messages and formats to be used in the CDMA system. This release is know as Revision 0 of the IS-95 standard. Since the release of Revision 0, a newer standard has been released for cellular CDMA that is called the Revision A standard of IS-95. The IS-95-A standard is backwards compatible with the original Revision ) standard. However, it introduces a number of new messages, service options and new addressing modes. IS-95-A now supports more powerful protocol methods that expand the paging, handoff and system parameter message capabilities of the CDMA system. In addition, a new phone addressing method has been added that is based on the International Mobile Station Identity (IMSI) rather than MIN or ESN. IMSI's are currently used by the GSM system and the adoption of this addressing mode will give CDMA a smoother integration path into international systems. The IS-95A cellular protocol has been further upgraded to include bps traffic channels. This is called the TSB-74 protocol standard. Another standard for CDMA operating in the US PCS bands is J-STD-008. Based largely on the IS-95 Revision A standard, J-STD-008 does not support the original protocol methods used in IS-95 Revision 0. J-STD-008 defines the new channels for PCS in the United States and adds extended nominal power control for PCS phones as well as adding extended neighbor list capabilities to the system. J-STD-008 also includes support for bps traffic channels. In an attempt to combine these different protocols into a single standard, the TIA has adopted a new version of protocol known as EIA/TIA-95-B. This protocol essentially combines the TSB-74 and J-STD-008 protocols into one standard. This will allow interband handoffs while operating in the CDMA mode. Other countries are now in the process of defining CDMA systems that will be based on these standards. Japan has adopted a CDMA protocol standard called ARIB T53. It is essentially a cellular standard largely based on TSB-74 but adds some of the more advanced features found in J-STD-008. Notes: ____________________________________________________________________________________________________________________________________________________________________________________ IS-95 Rev A Backwards Compatible with IS-95. First Deployed Protocol IS-95 Rev 0 Original System- never actually deployed. Copyright © Hewlett-Packard Company 1999

60 Ten Minutes in the Life of a CDMA Mobile Phone
Advanced Concepts of CDMA Ten Minutes in the Life of a CDMA Mobile Phone Turn-on System Access Travel Idle State Hand-Off Initiate Call Continue Travel Initiate Soft Handoff Terminate Soft Handoff End Call The last section of this presentation steps through a typical CDMA call scenario. Ten Minutes in the Life of a CDMA Mobile Station starts with turn-on of the cellular phone and system access. It assumes the car is being driven and that the phone performs an idle state handoff. We will then continue with a soft handoff and the processes involved with initiating and terminating soft handoffs Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

61 Advanced Concepts of CDMA
CDMA Turn On Process Find All Receivable Pilot Signals Choose Strongest One Establish Frequency and PN Time Reference (Base Station I.D.) Demodulate Sync Channel Establish System Time Determine Paging Channel Long Code Mask System Access: When the mobile first turns on, it must find the best base station. This is similar to analog where the phone scans all the control channels and selects the best one. In CDMA, the mobile unit scans for available Pilot signals, which are all on different time offsets. This process is made easier because of the fixed nature of these offsets. The timing of any base station is always an exact multiple of 64 system clock cycles (called chips) offset from any other base station. The mobile selects the strongest pilot tone and establishes a frequency and time reference from this signal. The mobile then demodulates the sync channel which is always on Walsh 32. This channel provides master clock information by sending the state of the 42 bit long code shift register 320 milliseconds in the future. Once the mobile has read the sync channel and established system time, the mobile uses the parameters from the sync channel to determine the long code mask being used by the cell site it is acquiring. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

62 Advanced Concepts of CDMA
Sync Channel Message Contains the Following Data: Base Station Protocol Revision Min Protocol Revision Supported SID, NID of Cellular System Pilot PN Offset of Base Station Long Code State System Time Leap Seconds From Start of System Time Local Time Offset from System Time Daylight Savings Time Flag Paging Channel Data Rate Channel Number SYNC The sync channel messages include the CDMA protocol revision supported by the cell site, the minimum protocol revision supported by a CDMA mobile in order to work with the cell site, the system and network identification numbers for the cell site, the PN offset of the cell site, the paging channel data rate, and all of the timing parameters including such items as local time offset from system time and a flag for indicating if daylight savings time is active in the area. As mentioned before, the Long Code State parameter contains the 42 bits the phone needs to place into its Long Code generator to become synchronized with the base station. This information is valid 320 ms in the future. How then does the phone use this information to get synchronized? Remember that at this point the mobile has decoded the Pilot channel from the base station. In decoding the Pilot, the mobile has synchronized its Short Code generator to match that of the received Pilot channel from the base station. The Pilot consists of the just the Short Codes that repeat every ms. The Long Code State information is valid 320 ms in the future. The phone simply counts the repetitions of the short code to determine the 320 ms period that it needs to wait before beginning to clock its Long Code generator: 320 ms/ ms = 12 repetitions of the Short Code. The Long Code generator is driven by the same MHz clock that drives the Pilot Channel's Short Codes. So the phone simply lodes the 42 bits into its Long Code generator, counts 12 repetitions of the Pilot Short Codes, and then begins to clock its Long Code generator with the MHz clock it derived from the Pilot Channel's Short Codes. Notes: __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

63 Read the Paging Channel
Advanced Concepts of CDMA Read the Paging Channel Demodulate the Paging Channel: Use Long Code Mask Derived from the Pilot PN Offset Given in Sync Channel Message Decode Messages Register, if Required by Base Station Monitor Paging Channel Paging At this point the mobile demodulates the Paging channel and decodes all of the data contained in the various messages supplied on the paging channel. If the parameters on the paging channel require it, the phone will then register with the base station. If the phone is a slotted mode phone, it must first register with the base station before it can be paged. The slotted paging channel mode allows the phone to save power by going to sleep and only awakening when it is time to check for page from the base station. During registration, the timeslot for the phone to wake up and listen to is negotiated between the base and mobile. Once this is completed, the phone is ready to place or receive phone calls. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

64 Paging Channel Messages
Advanced Concepts of CDMA Paging Channel Messages J-STD-008 Paging Messages: Overhead Messages System Parameters Access Parameters CDMA Channel List Extended System Params Extended Neighbor List Other Messages Order Channel Assignment Data Burst More Messages Authentication SSD Update Feature Notification Status Request Service Redirection General Page Global Service Redirection TMSI Assignment The paging channel is the heart of a CDMA base station. All of the parameters and signaling necessary for the proper operation of a CDMA cell site are handled by the paging channel. The paging channel supports a number of distinct messages that provide information and send messages. The messages described here are for a CDMA system operating with the J-STD-008 protocol stack. The system parameters message provides the mobile with system information such as the network, system and base station identification numbers, the number of paging channels supported, registration information, and the soft handoff thresholds. The access parameters message provides information to the mobile that dictates the behavior of access probes when a CDMA mobile initiates a call. The CDMA channel list reports the number of CDMA frequencies supported by the cell site as well as the configuration of surrounding cell sites. The channel assignment message is used to communicate the information needed to get the mobile onto a traffic channel. Other supported messages on the paging channel include Data Burst, Authentication Challenge, Shared Secret Data, and Feature Notification messages. The Extended System Parameters Message sends out the preferred MSID type and the base station's country and network codes. The Extended Neighbor List message tells the mobile the PN Offsets of surrounding cell sites that may become likely candidate for soft handoffs. The Status Request Message allows the base station to retrieve important characteristics of a mobile. The Service Redirection allows the base station to redirect a CDMA mobile to other systems (such as analog). The General page message allows the cell site to page CDMA phones for incoming calls. CDMA mobiles operating in the slotted mode must first register with the cell site before they can be paged. This registration is required to establish which slot will be used by the cell site to transmit the page to the mobile. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

65 CDMA Idle State Handoff
Advanced Concepts of CDMA CDMA Idle State Handoff No Call In Progress Mobile Listens to New Cell Move Registration Location if Entering a New Zone The mobile has searchers scanning for alternative Pilot channels at all times. The mobile listens to the strongest base station that it finds. As stronger cells are located, the mobile will switch and listen to the strongest. At some point, the mobile will cross into a new zone. If the CDMA systems has enabled zone based registration, then mobile will register again when it enters the new zone. This allows the system to know where a phone is in order to limit paging operations to a single zone. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

66 Advanced Concepts of CDMA
CDMA Call Initiation Dial Numbers, Then Press Send Mobile Transmits on a Special Channel Called the Access Channel The Access Probe Uses a Long Code Mask Based On: Access & Paging Channel Numbers Base Station ID Pilot PN Offset The user then decides to make a call. The number is keyed in and the send key is hit. This initiates an Access Probe. The mobile uses a special code channel called the Access Channel to make contact with the cell site. CDMA mobiles can transmit two types of channels on the single physical channel provided by the reverse link. These two channel are distinguished by the types of coding that are used. The Access Channel is used by the mobile to initiate calls. The other possible channel is the traffic channel that is used once a call is established. The long code mask used for access probes is determined from parameters obtained from the Sync and Paging channels: the access channel number, the paging channel number, the base station ID, and the Pilot PN offset used by the base station. As no link is yet established, closed-loop power control is not active. The mobile uses open-loop control to guess an initial level. Multiple tries are allowed with random times between the tries to avoid collisions that can occur on the Access Channel. This is necessary since there is no mechanism to prevent multiple users attempting to access the system at the same time. For each cell site there is also a limited number of access channels that are supported. Because of the limited number of access channel receivers in the base station, the odds of collisions occurring is increased. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

67 Reverse Link Access Channel Physical Layer
Advanced Concepts of CDMA Reverse Link Access Channel Physical Layer 64-ary Modulator Mbps 1 of 64 Walsh Codes Long Code Modulator I Short Code Interleaver Walsh Code 63 1.2288 Mbps Convolutional Encoder Symbol Repetition Walsh Code 62 FIR I Walsh Code 61 307.2 kbps 1/3 rate Short Code Scrambler 2x t/2 FIR Q 14.4 kbps 28.8 kbps Walsh Code 2 1/2 Chip Delay 28.8 kbps 4.8 kbps When a CDMA mobile makes an access attempt, it uses the special channel described previously. This slides shows the coding for an Access Channel. The Access Channel message data runs at 4,800 bps. This data is passed through a one-third rate covolutional encoder that triples the data rate to 14,400 bps. The data is then repeated to produce a data stream at 28,800 bps. The rest of the coding is very similar to a standard reverse Traffic channel with the exception that the Access Channel uses a special long code mask as described previously. The net result is that the processing gain for an Access Channel is 3 dB higher than a Traffic Channel (10 x log {1,228,800/4,800} = 24 dB verses 21 dB for full rate traffic). Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Walsh Code 1 Q Short Code Walsh Code 0 Access Channel Message Data Mbps Mbps Access Channel Long Code Copyright © Hewlett-Packard Company 1999

68 Advanced Concepts of CDMA
CDMA Call Completion Base Answers Access Probe using the Channel Assignment Message Mobile Goes to A Traffic Channel Based on the Channel Assignment Message Information Base Station Begins to Transmit and Receive Traffic Channel After each access attempt, the mobile listens to the Paging Channel for a response from the base station. If the base station detects the access probe from the mobile, it responds with a channel assignment message. This message contains all of the information required to get the mobile onto a traffic channel. this messages includes such information as the Walsh code channel to be used for the forward traffic channel, the frequency being used, and the frame offset to indicate the delay between the forward and reverse links.. Once the mobile has acknowledge the channel assignment message, the base station initiates the land link and the mobile moves from the access channel to the traffic channel. At this point, a conversation can take place. Any further signaling required once the traffic channel is established takes place on the traffic channel. To accommodate signaling, EIA/TIA-95-B supports two methods of temporarily grabbing the traffic channel: blank and burst signaling, and dim and burst signaling. Both are similar with the difference that the blank and burst steals a contiguous block of frames to transmit signaling messages, while dim and burst reduces the vocoder rate and then uses the remaining traffic channel time to more slowly send signaling messages. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

69 CDMA Soft Handoff Initiation
Advanced Concepts of CDMA CDMA Soft Handoff Initiation Mobile Finds Second Pilot of Sufficient Power (exceeds T_add Threshold) Mobile Sends Pilot Strength Message to First Base Station Base Station Notifies MTSO MTSO Requests New Walsh Assignment from Second Base Station If Available, New Walsh Channel Info is Relayed to First Base Station Once a call is established, the mobile is constantly searching for other possible cell sites that might be good candidate for soft handoffs. Most of the search time is limited to looking for those PN offsets specified in the neighbor list found on the paging channel. The rake receiver's searcher is the device that scans for other possible cell sites. If the mobile identifies a pilot that exceed the T_add threshold defined by the cell site, it alerts the base station by sending a pilot strength message. The pilot strength message is sent on the traffic channel using either dim and burst or blank and burst signaling. This is the action that initiates a soft handoff. When the pilot strength message is received, the base station passes this request to the MTSO (Mobile Telephone Switching Office). If the MTSO has an available channel card, it passes the request to the second base station to see if a traffic channel is available for the soft handoff request. A second channel card is required at the switch for a soft handoff to allow the MTSO to select the better of the two signals being returned from the two cell sites involved in the soft handoff. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

70 CDMA Soft Handoff Completion
Advanced Concepts of CDMA CDMA Soft Handoff Completion First Base Station Orders Soft Handoff with new Walsh Assignment MTSO Sends Land Link to Second Base Station Mobile Receives Power from Two Base Stations MTSO Chooses Better Quality Frame Every 20 Milliseconds If it is available, the second cell site returns the Walsh Code that will be assigned for the soft handoff. At this point, the original base station orders the soft handoff by using a handoff direction message on the traffic channel using one of the two types of signaling: dim and burst or blank and burst. Once the handoff direction message is acknowledged, the MTSO sends the land link to the second base station who then begins to send the information on the assigned Walsh code traffic channel. The mobile then receives both signals from the two cell sites, each operating of different PN offsets and Walsh coded traffic channels and combines the signals from both base stations by using the two Pilot signals as coherent phase references. In a two way soft handoff, two of the mobile's rakes are used: one for each of the received base station's signals. This provides greatly improved fading performance. At the same time, each of the cell sites independently receives the signal transmitted by the mobile station. Each cell site demodulates the signal and returns the decoded data to the MTSO. The MTSO then compares the two signals on a frame-by-frame basis and selects the better of the two to send to the codec to pass along to the public telephone network. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

71 Ending CDMA Soft Handoff
Advanced Concepts of CDMA Ending CDMA Soft Handoff First BS Pilot Power Goes Low at Mobile Station (drops below T_drop) Mobile Sends Pilot Strength Message First Base Station Stops Transmitting and Frees up Channel Traffic Channel Continues on Base Station Two As the signal from the first base station degrades and then drops below the T_drop threshold , the mobile sends another pilot strength message indicating to the base station that the soft handoff needs to be terminated. At this point the mobile is being power controlled by the second base station (since the first cell probably has a very poor link). The request is passed from the second cell through the MTSO, and the first cell stops transmission and reception of the signal. The mobile is now only on the second cell. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

72 Advanced Concepts of CDMA
CDMA End of Call Mobile or Land Initiated Mobile and Base Stop Transmission Land Connection Broken Finally, the call ends. This can be initiated either from the mobile or the land side. In either case, transmissions are stopped and the land line connection is broken. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

73 Advanced Concepts of CDMA
CDMA Conclusions New Access Method Code Based Designed for Use in Interfering Environment Uses Multipath to Improve Reception in Fading Conditions Has High Capacity 6 Times Analog for 14.4 kbps Voice 10 Times Analog for 9.6 kbps Voice CDMA CDMA provides an advanced technology for cellular applications. It is based on a new access method that uses codes to identify users and base stations. Unlike most existing systems, CDMA is designed for use in a high interference environment. Using rake receiver technology, CDMA uses multipath to improve reception under multipath fading conditions. It provides high quality service to a large number of users. Actual systems today using the 14.4 kbps vocoder are showing about 6 times the capacity of the existing AMPS analog cellular service. Networks using the original or new EVRC 8 kbps vocoder are demonstrating about 10 times the capacity of the existing AMPS analog cellular service. The Korean system has experienced explosive growth and has over 5 million subscribers. PCS CDMA systems are now in operation across the United States with more than 1 million subscribers. Cellular systems are in operation in Seattle and Los Angeles with many other locations being added. CDMA is a technology that is here to stay. All of the currently proposed third generation wireless systems in North America, Japan, and Europe are all based on direct sequence, spread spectrum technology. Notes: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Copyright © Hewlett-Packard Company 1999

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