Differences between AMPS and GSM AMPS uses analog technology GSM uses digital technology AMPS has poor performance for data transfer Both systems use control channels to initiate calls. AMPS uses 21 control channels while GSM uses 3. AMPS is less secure than GSM GSM has the SIM smart card which holds the user’s personal information and phone settings. In AMPS only the HLR has that data. Both systems support ROAMING but GSM allows more compatibility.

Differences between AMPS and GSM AMPS requires less power at the MS and at the BS. GSM’s power control attempts to minimize radio transmission power of the MS and BTS, thus, minimizes the amount of co-channel interference. AMPS instead relies on the digital color code (DCC) for that. AMPS has a cell radius 1.5km - 25km GSM is more flexible with cell sizes.

EXAMPLES IS-41 or IS-136 (D-AMPS) uses TDMA scheme; (AT&T Wireless, BellSouth, Southwestern Bell) IS-95 uses CDMA (Bell Atlantic/NYNEX, Verizon, Sprint PCS) GSM uses TDMA (used worldwide; here Cingular; T-Mobile).

INTRODUCTION: USA: 2G Systems 1G -> AMPS -> problems to serve large number of users. 2G Systems with Digital Modulation techniques (called Digital Cellular) achieved large improvements. Late 80ies, USDC (US Digital cellular system) started to support more users in a fixed spectrum allocation. US Digital Cellular System (D-AMPS: Digital Advanced Mobile Phone System) / \ / \ Air Interface Mobility Management IS-54 IS-41 (new version IS-136)

Reference Model for North American Systems The TIA Committees TR-45 and TR-46 develop performance, compatibility, interoperability, and service standards. TR-45.3 TDMA TR-45.5 CDMA New interfaces when compared to GSM Model: DMH: data message handler – collects billing information IWF: interworking function – allows an MSC to connect to other networks AUX: auxiliary equipment – can connect to an MSC Messaging is carried out by protocols very similar to SS-7

IS-54 IS-54 Architecture similar to AMPS, GSM in terms of MSC, BS, Mobile Stations, HLR, VLRs IS-54 standardized in 1990 (Interim Standard-IS) IS-54 shares same frequencies, frequency reuse plan and base stations as AMPS so that base stations and subscriber units could be equipped with both AMPS and IS 54 channels within the same piece of equipment. Both AMPS and IS-54 cellular carriers provide new customers with IS-54 phones and may gradually replace AMPS base stations with IS-54 BSs channel by channel over time  known as D-AMPS AMPS - D-AMPS (IS-54)

IS-54 * Change from Analog to Digital IS-54 uses TDMA supporting 3 full rate users or 6 half rate users on each AMPS channel. 6 times more capacity than AMPS IS-54 uses the same 45 MHz FDD scheme as AMPS REMARK: * Change from Analog to Digital * Temporary increase in Interference and dropped calls in AMPS; since each BS changed over to digital; the number of analog channels in geographic area is decreased. Compatibility with AMPS: * IS 54 Forward/Reverse control channels use exactly the same signaling technique as AMPS. * Voice Channels are 4–ary pi/4 DQPSK modulation with a channel rate 48.6 kbps.

IS-54 Forward and Reverse control channels use the same 10 kbps FSK signaling scheme as in AMPS REMARK: * IS 136 (formerly IS-54-C) includes pi/4 DQPSK modulation for control channels. * IS-54-C  provides 4-ary keying instead of FSK on control channels REASONS: Increase control channel data rate also provide special services like paging etc..

RADIO INTERFACE (IS-54) GOAL: Smooth transition from AMPS  IS-54 IS 54 designed to operate using both AMPS and IS 54 standards. Multiple Access  TDMA/FDD Modulation  pi/4 DQPSK Channel Bandwidth  30 kHz (sam as in AMPS) Reverse Channel Freq. BW  824-894 MHz (same as in AMPS) Forward Channel Freq. BW  869-894 MHz (same as in AMPS) FW and Reverse Channel Data Rates  48.6 kbps Channel Coding  7 bit CRC and ½ rate convolutional coding of constraint length 6. Users per channels  3 (full rate speech coder of 7.95 kbps/user)  6 (half rate speech coder of 3.975 kbps/user).

CHANNELS (IS-54) Control Channels  AMPS Control Channels 42 Primary AMPS Control Channels  Primary + IS 54 (42 additional control channels) dedicated for IS 54 use only (Secondary Control Channels)

DATA CHANNELS (IS-54) Digital Traffic Channel (DTC) carries user information (speech or user data) Reverse DTC: carries speech from mobile to BS. Forward DTC: carries speech from base to mobile

DATA CHANNELS (IS-54) Coded Digital Verification Color Code Channel (CDVCC) Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH)

DATA CHANNELS (IS-54) Coded Digital Verification Color Code Channel (CDVCC) Is a 12 bit message sent in every time slot. (Function similar to SAT (Supervisory Audio Tone) as in AMPS  allows each BS and its mobiles to confirm that they are properly connected during a call. It is an 8-bit number ranging from 1 and 255 protected with 4 additional channel coding bits (12,8) Hamming code. BS  CDVCC values on Forward Voice Channel  Each subscriber using TDMA channel must receive, decode and retransmit the same CDVCC value to BS on the reverse voice channel. Handshake: If not, then time slot will be relinguished for other users.

DATA CHANNELS (IS-54) Slow Associated Control Channel (SACCH) Sent in every time slot. Carries various control and supervisory message between mobile and BS, e.g., power level changes and handoff requests. Also used by mobile to report results of signal strength measurements of neighboring base stations  to help BS to do MAHO (Mobile Assisted Handoff).

DATA CHANNELS (IS 54) Fast Associated Control Channel (FACCH) (Signaling Channel) Important control or specialized traffic data between BS and mobiles; e.g., call release instructions, MAHO, mobile status requests. The FACCH data when transmitted takes the place of user info data within a frame.

Frame Structure for IS-54 (Mobile to BS) 1 Frame=1944 bits (97 symbols)= 40 ms 1 2 3 4 5 6 25 frames/sec 6.67msec 324 bits (260 bits user data) G R Data SYN DATA S CD DATA G  Guard Time (6 bits) (when no signal is transmitted) R  Ramp Time (6 bits) (to allow transmitter to reach its full output power level) Data  16 bits SYN  28 bits DATA  122 bits SACCH  12 bits CDVCC  12 bits (helps to identify the frequency channel to which mobile is tuned) DATA  122 bits

Mobility Management IS-41 (new version IS-136) IS 41 <-> GSM Procedure for delivering calls to mobile users in GSM very similar to IS 41. Note that names, contents, lengths of messages may be different.

IS-41 Standard for MSC-MSC interface IS-41 is primarily used in the core network to provide services such as automatic roaming, authentication, intersystem handoff, short message service, etc. All wireless network elements such as the MSC, HLR, VLR, EIR, and AUC, use this messaging protocol to communicate among themselves Signaling protocol stack very similar to SS-7 Intersystem handoff: handoff involving two BSSs controlled by different MSCs.

IS-41: Intersystem Handoff The current MSC will request a RSS measurement from the candidate MSC. Once RSS measurements indicate the candidate MSC as suitable for handoff, the two MSCs will complete the intersystem handoff. Three types of handoff: Handoff forward: transfer from one MSC to another MSC of a new system Handoff backward: transfer from the new MSC back to the old MSC Handoff third: transfer from an MSC in a second system to a MSC of a third system During handoff, IS-41 signaling messages will carry terminal information, call information, and air-interface information (serving and destination cells and channels). It also performs authentication procedures between the two systems.

Differences IS-41 - GSM When the new VLR receives the registration affirmation (REGNOT IS-41) from HLR, it assigns a new TMSI to the terminal for the new RA. HLR also provides new VLR with all relevant subscriber profile information required for call handling (e.g., call screening lists, etc.) as part of affirmation message. Thus, in contrast to IS-41, authentication and subscriber profile information are obtained from both HLR and VLR and not just the HLR.

OVERVIEW IS-54 GSM IS-95 Access Technology TDMA/FDD TDMA/FDD CDMA (DS)/FDD Freq Band: BS 869-894 935-960 (1805-1880) 869-894 Mobile 824-849 890-915 (1710-1785) 824-849 Channel Spacing 30kHz 200kHz 1250kHz Modulation pi/4 DQPSK GMSK BPSK/QPSK Power Max/ Average Milliwatts 600/200 1000/125 600 Speech Rate (kbps 7.95 13 8 (variable rate) Frame Duration (ms) 40 4.615 20 Coding ½ rate ½ rate ½ rate conv for Forward convolutional conv. 1/3 rate conv. for Reverse; CRC DQPSK: Differential Quadratic Phase Shift Keying; QPSK: Quadrature Phase Shift Keying; GFSK: Gaussian Freq. Shift Keying; BPSK: Binary Phase Shift Keying; GMSK: Gaussian Minimum Shift Keying

IS-95 Digital Cellular: Two different directions for the air-interface: IS-136 standard based on TDMA IS-95 standard based on CDMA Interoperability was only possible via dual mode telephones IS-41 standard has now evolved to support both IS-136 and IS-95

IS-95 IS-95 is the North American digital cellular standard that employs CDMA as the Access Method as well as the Air-Interface. It was developed by Qualcomm around 1990. CDMA/AMPS dual mode phones were produced by Qualcomm in 1994. Also called cdmaOne. In Dec. 1993, the TIA published Qualcomm’s air-interface specifications as the interim standard IS-95. Formally adopted in July 1993 and revised in May 1995.

IS-95 CDMA Digital AMPS increased capacity of AMPS by factor 3 First Code Division Multiple Access (CDMA) cellular system was developed in 1990, claimed to increase capacity of AMPS by factor 20.

IS-95 CDMA GOAL: Low Cost Light-weight hand-held portable units Battery Life Spectrally efficient Low link budgets Minimum number of base stations Excellent grade of service Excellent scalability Reduction of dropped calls Reduction of fading and poor voice quality

CDMA CDMA is both an Access Method and an Air-Interface. Similarities: Core fixed network infrastructure  GSM core network structure Radio resource management, mobility management, and security  same as in TDMA (D-AMPS) systems. There are differences in terms of * Handling the power control * Employing soft handoff

Access Method CDMA CDMA (Code Division Multiple Access) All terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel Each sender has a unique random number, the sender XORs the signal with this random number The receiver can “tune” into this signal if it knows the pseudo random number; tuning is done via a correlation function 28

Access Method CDMA CDMA (Code Division Multiple Access) User data stream encoded with ½ convolutional code rate, interleaved and spread by one of 64 orthogonal spreading sequences (Walsh functions). Each MS in a given cell is assigned a different spreading sequence, providing perfect separation among signals from different users. 28

Access Method CDMA CDMA (Code Division Multiple Access) To reduce interference between MSs which use same spreading sequence in different cells and to provide the desired spectral characteristic, all signals in a cell are SCRAMBLED using a pseudo-random sequence of length 2^15 chips. 28

CDMA (Code Division Multiple Access ) Frequency User 1 . User 2 . . User n Time Code

CDMA (Advantages and Disadvantages) All terminals can use the same frequency, no planning needed Huge code space compared to frequency space Interference (e.g. white noise) is not coded Forward error correction and encryption can be easily integrated Disadvantages: Higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) All signals should have the same strength at a receiver 28

CDMA: Further Advantages and Disadvantages CDMA provides an increase in system capacity when compared with the analog and TDMA systems. CDMA improves quality of voice by using a better voice coder. CDMA has resistance to multipath and fading. CDMA implements soft handoffs. CDMA has less power consumption (about 10% of analog or TDMA phones) because of implementation of power control. CDMA does not require frequency planning because all cells employ the same frequency at the same time. Disadvantage: Necessity for power control and complexity.

Comparison TDMA/FDMA/CDMA Approach TDMA FDMA CDMA Idea time into disjoint time-slots, demand driven or fixed patterns segment the frequency band into disjoint sub-bands spread the spectrum Terminals all terminals are active for short periods of time on the same frequency every terminal has its own frequency, uninterrupted all terminals can be active at the same place at the same moment, separation synchronization in the time domain frequency domain code plus special receivers established, fully simple, established, robust flexible, less frequency handover Dis- guard space needed (multipath propagation), synchronization difficult inflexible, frequencies are a scarce resource complex receivers, needs more complicated power control for senders Comment standard in fixed networks, together with FDMA/SDMA used in many mobile networks typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA advantages Advantages Signal using orthogonal codes segment sending digital, flexible planning needed, soft filtering in the

Air-Interface CDMA The air-interface in CDMA is not symmetrical on the forward and reverse channels (separated by 45MHz) One Forward Channel (1.25 MHz in 824-849 MHz bands): transmissions originate at a single transmitter (BS) and transmissions for all users are synchronized. One Reverse Channel (1.25 MHz in 869-894 MHz bands): mobile terminals transmit whenever they have to.

Air-Interface CDMA Forward Channels (BS MOBILE): Pilot (1 Channel): Provides a reference signal to all MSs within a cell for demodulation. It is also used for signal strength comparison. Determines when to handoff. Synchronization (1 Channel): Used to acquire initial time synchronization. The sync message includes the system and network identification, coding information, and the paging channel data rate. Operates at 1200kbps.

Air-Interface CDMA Forward Channels: Paging (7 channels) As in GSM, used to page the MS when there is an incoming call, and to carry the control messages for call setup. Operates at 9600,4800, 2400 bps) Forward Traffic (63 channels): Carries the actual user information. Two possible rate sets, RS1 and RS2. RS1 supports date rates of 9.6, 4.8, 2.4, and 1.2 kbps. RS2 supports 14.4, 7.2, 3.6, and 1.8 kbps. Multiplexed with power control information.

Air-Interface CDMA Reverse Channels (MOBILE  BS): Access (max. 32 channels): Used by the MS to transmit information to the BS such as call origination, response to a page and so on. Fixed Data Rate 4800 kbps. Random access channel user uniquely identified by their long codes. Reverse Traffic: Supports voice data at two rate sets: RS1 and RS2. It is used to send information related to the signal strength of the pilot and frame error rate statistics to one BS or multiple BSs. It is also used to transmit control information to the BS such as a handoff completion message. Operates on variable data rate.

IS-95 CDMA Channels Overview Types of Channels Forward Channels Pilot Synchronization Paging Traffic Reverse channels Access Application System mon. Sync. Signaling Voice/data

IS-95 CDMA Interesting Features Multiple users can share same frequency Soft capacity limit: more users raises noise floor linearly, no absolute limit on number of users - performance degrades gradually for all users Multipath fading is reduced by signal spreading

IS-95 CDMA Interesting Features (cont) Spatial diversity provides soft handoff: MSC monitors signal of a user from multiple base stations and chooses best version of signal at any time Self-Jamming is a problem: Because spreading sequences of different users are not exactly orthogonal When despreading, other users can contribute significantly to receiver decision statistic

IS-95 CDMA Interesting Features (cont) Near-Far Problem: If power of multiple users are unequal, the strongest received mobile signal will capture demodulator at the base station Base stations must implement power control to ensure that each mobile within coverage area provides same signal level to base station receiver

IS-95 CDMA Interesting Features (cont) The system can move a call from digital to analog when the call enters the coverage area of a cell that does not have CDMA capability. The opposite does not work.

Mobility and Radio Resource Management in IS-95 Soft Handoff Note: Unlike channelized wireless systems that assign different radio channels during a handoff (called a hard handoff), spread spectrum mobiles share the same channel in every cell, thus the term handoff does not mean a physical change in the assigned channel, but rather that a different base station handles the radio communication task. Refers to the process by which an MS is in communication with multiple candidate BSs before finally deciding to communicate its traffic through one of them.

Mobility and Radio Resource Management in IS-95 Soft Handoff Reason to implement it is based on the near-far problem and the associated power control mechanism: If an MS moves far away from a BS and continues to increase power to compensate for the near-far problem, it might cause a lot of interference to MSs in neighboring cells. To avoid the above situation and to ensure that an MS is connected to the BS with the largest RSS (Received Signal Strength), a soft handoff strategy is implemented.

Soft Handoff IS-95 defines three types of soft handoff: (a) Softer Handoff: between two sectors of the same cell (b) Soft Handoff: between two sectors of different cells (c) Soft-Softer Handoff: includes two sectors from the same cell and a third sector from a different cell.

Soft Handoff A controlling BS coordinates the addition or deletion of other BSs The primary BS uses a handoff direction message (HDM) to indicate the pilot channels to be used or removed. At some point, the primary BS is also changed after handoff. The signals from multiple BSs are combined in the BSC or MSC and processed as a single call. This is achieved by using a frame selector join message. A frame selector remove message is used to remove the old BS.

Soft Handoff The pilot channels of each cell are involved in the handoff mechanism. Only channel not subject to power control and providing a measure of the RSS. The MS maintains a list of pilot channels that it can hear and classifies them into: Active Set: pilots continuously monitored or used by the MS (MS has three RAKE fingers that allows it to monitor or use up to three pilots). Candidate Set: can have at most six pilots, not in the active set but with sufficient RSS to be used. Neighbor Set: contains pilots that belong to neighboring cells and are intimated to the MS by a message on the paging channel. Remaining Set: all other possible pilots in the system.

Soft Handoffs and Thresholds Whenever the strength of a pilot falls below a threshold, the MS starts a dwell timer. Unless the pilot strength goes back above the threshold before the timer expires, the MS will drop it from a given set. There is a trade-off in setting high and low values for these thresholds and timers.

Soft Handoffs and Thresholds If the strength of a candidate pilot channel is above the pilot detection threshold (T_ADD), this pilot must be added to the active set and the MS enters a soft handoff region. If T_ADD is too small, there may be false alarms caused by noise or interfering signals. If T_ADD is too large, useful pilot are not detected, and the call may be dropped.

Handoff Thresholds in IS-95

Handoff Thresholds in IS-95 As soon as the strength of the pilot exceeds T_ADD, it is transferred to the candidate set, and the MS sends the new pilot strength measurement to the BS that is transmitting the current pilot. The BS sends a handoff direction message to the MS. The pilot is transferred to the active set. The MS acquires a traffic channel and sends a handoff completion message. After the pilot strength falls below T_DROP, the handoff timer is started.

Handoff Thresholds in IS-95 If it is still below T_DROP after the timer expires, the MS sends another pilot strength measurement to the BS associated with the pilot. The BS responds with HDM without the pilot in it. The MS moves the pilot to the neighbor set. The MS then sends a handoff completion message. At some point, the BS may send it a neighbor update list message that no longer contains the pilot and it is moved into the remaining set.

Power Control in CDMA Co-channel and adjacent channel interference are not the major problems in CDMA. Why? The interference is from other users transmitting in the frequency band at the same time. To avoid the near-far problem, it is important to implement good power control. Signal strength may be good in CDMA, but frames may still be received in error because of interference. Thus, Frame Error Rate (FER) is used for power control decisions rather than the signal strength used in other PCS systems.

Open Loop Reverse Link Power Control in IS-95 RULE: Use a transmit power that is inversely proportional to the received signal strength of pilots from all BSs. On the access channel, the MS sends a request using a weak signal if the pilot is strong. An ACK may not be received because the transmit power was low or because of collisions. If no ACK is received, a stronger signal is transmitted. This is repeated until a maximum power level is reached. The process is repeated after a back-of delay.

Open Loop Reverse Link Power Control in IS-95 Up to 15 attempts can be made to obtain a traffic channel. Disadvantages: Assumption that forward and reverse channels are identical; slow response time; and using the total power received from all BSs in calculating the required transmit power.

Closed Loop Reverse Link Power Control in IS-95 On Downlink channel -> A power control bit is transmitted every 1.25 ms (800 times per second). A “0” indicates the MS should increase the power and a “1” indicates it should decrease it. Every 1.25 ms, in the BS, the receiver determines the received signal to interference ratio by sampling it 16 times. If it is above a preset target, the MS is instructed to reduce its power by 1 dB.

Closed Loop Reverse Link Power Control in IS-95 Inner-Loop Power Control: If below the target, the MS is instructed to increase its power by 1 dB. The target value controls the long-term frame error rate. Outer-Loop Power Control: The target is varied over time to reflect accurate values related to velocity, fading, environment, and so on. It is reduced (increased) by x dB every 20 ms if the FER is small (large) enough.

Closed Loop Reverse Link Power Control in IS-95

Forward Link Power Control Employed to reduce inter-cell interference. Wthin a cell, multiple users employ orthogonal sequences and the primary source of interference is from users of other cells or from multi-path. A mobile assisted power control is employed: The MS periodically reports the FER on the forward link to the BS, which will then adjust its transmit power accordingly. A maximum transmit power is defined to avoid excessive interference. A minimum transmit power is defined to avoid allowing voice quality to drop.

Overview of 2G Systems (S/I)min N AMPS ~18 dB 7 GSM ~11 dB 4 IS-54 ~16 dB 7 IS-95 ~15 dB 1