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EE 6331, Spring, 2009 Advanced Telecommunication

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1 EE 6331, Spring, 2009 Advanced Telecommunication
                                                            Zhu Han Department of Electrical and Computer Engineering Class 22 Apr. 16th, 2009

2 Outline Review CDMA OFDM 2G-3G-4G Exam2 until this class
Convolutional code Encoder Decoder: Viterbi decoding Turbo Code LDPC Code TCM modulation CDMA OFDM 2G-3G-4G Exam2 until this class Project 2 due on the exam ECE6331 2

3 Example Convolutional encoder, k = 1, n = 2, L=2
Convolutional encoder is a finite state machine (FSM) processing information bits in a serial manner Thus the generated code is a function of input and the state of the FSM In this (n,k,L) = (2,1,2) encoder each message bit influences a span of C= n(L+1)=6 successive output bits = constraint length C Thus, for generation of n-bit output, we require n shift registers in k = 1 convolutional encoders ECE6331

4 Generator sequences ECE6331

5 Representing convolutional codes compactly: code trellis and state diagram
Input state ‘1’ indicated by dashed line State diagram Code trellis Shift register states ECE6331

6 Distance for some convolutional codes
Lower the coding rate, larger the L, then larger the distance ECE6331

7 Puncture Code A sequence of coded bits is punctured by deleting some of the bits in the sequence according to some fixed rule. The resulting coding rate is increased. So a lower rate code can be extended to a sequence of higher rate codes. ECE6331

8 The largest metric, verify that you get the same result!
Note also the Hamming distances! ECE6331

9 The Viterbi algorithm Problem of optimum decoding is to find the minimum distance path from the initial state back to initial state (below from S0 to S0). The minimum distance is the sum of all path metrics that is maximized by the correct path Exhaustive maximum likelihood method must search all the paths in phase trellis (2k paths emerging/ entering from 2 L+1 states for an (n,k,L) code) The Viterbi algorithm gets its efficiency via concentrating into survivor paths of the trellis Decoder’s output sequence for the m:th path Received code sequence ECE6331

10 The maximum likelihood path
Smaller accumulated metric selected After register length L+1=3 branch pattern begins to repeat 1 1 (Branch Hamming distances in parenthesis) First depth with two entries to the node The decoded ML code sequence is whose Hamming distance to the received sequence is 4 and the respective decoded sequence is (why?). Note that this is the minimum distance path. (Black circles denote the deleted branches, dashed lines: '1' was applied) ECE6331

11 Parallel Concatenated Codes
Instead of concatenating in serial, codes can also be concatenated in parallel. The original turbo code is a parallel concatenation of two recursive systematic convolutional (RSC) codes. systematic: one of the outputs is the input. Systematic Output Input Encoder #1 MUX Parity Output Interleaver Encoder #2 ECE6331

12 Iterative Decoding There is one decoder for each elementary encoder.
Each decoder estimates the a posteriori probability (APP) of each data bit. The APP’s are used as a priori information by the other decoder. Decoding continues for a set number of iterations. Performance generally improves from iteration to iteration, but follows a law of diminishing returns. Deinterleaver APP APP Interleaver systematic data Decoder #1 Decoder #2 hard bit decisions parity data DeMUX Interleaver ECE6331

13 Performance as a Function of Number of Iterations
K=5, r=1/2, L=65,536 ECE6331

14 LDPC Introduction Low Density Parity Check (LDPC)
History of LDPC codes Proposed by Gallager in his 1960 MIT Ph. D. dissertation Rediscovered by MacKay and Richardson/Urbanke in 1999 Features of LDPC codes Performance approaching Shannon limit Good block error correcting performance Suitable for parallel implementation Advantages over turbo codes LDPC do not require a long interleaver LDPC’s error floor occurs at a lower BER LDPC decoding is not trellis based ECE6331

15 Pro and Con ADVANTAGES Near Capacity Performance .. Shannon’s Limit
Some LDPC Codes perform better than Turbo Codes Trellis diagrams for Long Turbo Codes become very complex and computationally elaborate … and make my head hurt ! Low Floor Error Decoding in the Log Domain is quite fast. DISADVANTAGES Long time to Converge to Good Solution Very Long Code Word Lengths for good Decoding Efficiency Iterative Convergence is SLOW Takes ~ 1000 iterations to converge under standard conditions. Due to the above reason transmission time increases i.e. encoding, transmission and decoding Hence Large Initial Latency (4086,4608) LPDC codeword has a latency of almost 2 hours ECE6331

16 Trellis Coded Modulation
Combine both encoding and modulation. (using Euclidean distance only) Allow parallel transition in the trellis. Has significant coding gain (3~4dB) without bandwidth compromise. Has the same complexity (same amount of computation, same decoding time and same amount of memory needed). Has great potential for fading channel. Widely used in Modem ECE6331

17 Set Partitioning Branches diverging from the same state must have the largest distance. Branches merging into the same state must have the largest distance. Codes should be designed to maximize the length of the shortest error event path for fading channel (equivalent to maximizing diversity). By satisfying the above two criterion, coding gain can be increased. ECE6331

18 Spread-spectrum transmission
Three advantages over fixed spectrum Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background. Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently. ECE6331

19 PN Sequence Generator Pseudorandom sequence
Randomness and noise properties Walsh, M-sequence, Gold, Kasami, Z4 Provide signal privacy ECE6331 19

20 Direct Sequence (DS)-CDMA
It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal. ECE6331

21 Direct Sequence Spread Spectrum
Unique code to differentiate all users Sequence used for spreading have low cross-correlations Allow many users to occupy all the frequency/bandwidth allocations at that same time Processing gain is the system capacity How many users the system can support ECE6331 21

22 Spreading & Despreading
Source signal is multiplied by a PN signal: 6.134, 6.135 Processing Gain: Despreading Spread signal is multiplied by the spreading code Polar {±1} signal representation ECE6331

23 Direct Sequence Spreading
ECE6331 23

24 Spreading & Despreading

25 CDMA – Multiple Users One user’s information is the other’s interferences If the interference structure can be explored, multiuser detection Match filter Decorrelator MMSE decodor Successive cancellation Decision feedback ECE6331 25

26 CDMA Example R Receiver (a base station) Data=1011… Data=0010… A B
Transmitter (a mobile) Transmitter Codeword=101010 Codeword=010011 Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading. ECE6331 26

27 CDMA Example – transmission from two sources
A Data A Codeword A Signal B Data B Codeword B Signal Transmitted A+B Signal ECE6331 27

28 CDMA Example – recovering signal A at the receiver
A+B Signal received A Codeword at receiver Integrator Output Comparator Output Take the inverse of this to obtain A ECE6331 28

29 CDMA Example – recovering signal B at the receiver
A+B Signal received B Codeword at receiver Integrator Output Comparator Output Take the inverse of this to obtain B ECE6331 29

30 CDMA Example – using wrong codeword at the receiver
A+B Signal received Wrong Codeword Used at receiver Integrator Output Comparator Output X Noise Wrong codeword will not be able to decode the original data! ECE6331 30

31 Near Far Problem and Power Control
At a receiver, the signals may come from various (multiple sources. The strongest signal usually captures the modulator. The other signals are considered as noise Each source may have different distances to the base station In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. Therefore the receiver power at the base station for all mobile users should be close to eacother. This requires power control at the mobiles. Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power. B pr(M) M M M M ECE6331 31

32 Frequency Hopping Spread Spectrum
Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. Military, bluetooth ECE6331

33 Hybrid Spread Spectrum Techniques
FDMA/CDMA Available wideband spectrum is frequency divided into number narrowband radio channels. CDMA is employed inside each channel. DS/FHMA The signals are spread using spreading codes (direct sequence signals are obtained), but these signal are not transmitted over a constant carrier frequency; they are transmitted over a frequency hopping carrier frequency. ECE6331 33

34 Hybrid Spread Spectrum Techniques
Time Division CDMA (TCDMA) Each cell is using a different spreading code (CDMA employed between cells) that is conveyed to the mobiles in its range. Inside each cell (inside a CDMA channel), TDMA is employed to multiplex multiple users. Time Division Frequency Hopping At each time slot, the user is hopped to a new frequency according to a pseudo-random hopping sequence. Employed in severe co-interference and multi-path environments. Bluetooth and GSM are using this technique. ECE6331 34

35 Orthogonal frequency-division multiplexing
Special form of Multi-Carrier Transmission. Multi-Carrier Modulation. Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel (using Sub-Carriers) ECE6331

36 OFDM bit loading Map the rate with the sub-channel condition
Water-filling ECE6331

37 OFDM Time and Frequency Grid
Put different users data to different time-frequency slots ECE6331

38 Guard Time and Cyclic Extension...
A Guard time is introduced at the end of each OFDM symbol for protection against multipath. The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval. Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI. ECE6331

39 OFDM Transmitter and Receiver

40 Pro and Con Advantages Can easily be adopted to severe channel conditions without complex equalization Robust to narrow-band co-channel interference Robust to inter-symbol interference and fading caused by multipath propagation High spectral efficiency Efficient implementation by FFTs Low sensitivity to time synchronization errors Tuned sub-channel receiver filters are not required (unlike in conventional FDM) Facilitates Single Frequency Networks, i.e. transmitter macro-diversity. Disadvantages Sensitive to Doppler shift. Sensitive to frequency synchronization problems Inefficient transmitter power consumption, since linear power amplifier is required. ECE6331

41 OFDM Applications ADSL and VDSL broadband access via telephone network copper wires. IEEE a and g Wireless LANs. The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB. The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T. The IEEE or WiMax Wireless MAN standard. The IEEE or Mobile Broadband Wireless Access (MBWA) standard. The Flash-OFDM cellular system. Some Ultra wideband (UWB) systems. Power line communication (PLC). Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications. ECE6331

42 The IEEE a/g Standard Belongs to the IEEE system of specifications for wireless LANs. covers both MAC and PHY layers. Five different PHY layers. 802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps PHY Layer very similar to ETSI’s HIPERLAN Type 2 ECE6331

43 4G Road Map cdma2000 IS 95 B GSM TDMA EDGE UWC-136 GPRS W-CDMA
1XRTT/3XRTT cdma2000 CDMA (IS 95 A) IS 95 B GSM TDMA EDGE UWC-136 GPRS W-CDMA 4G 1999 2000 2001 2002 cdmaOne IS-95A 3X No 3X IS-95B 1X 2G 2.5G 3G Phase 1 3G Phase 2 ECE6331

44 2G: IS-95A (1995) Known as CDMAOne Chip rate at 1.25Mbps
Convolutional codes, Viterbi Decoding Downlink (Base station to mobile): Walsh code 64-bit for channel separation M-sequence 215 for cell separation Uplink (Mobile to base station): M-sequence 241 for channel and user separation Standard IS-95, ANSI J-STD-008 Multiple Access CDMA Uplink Frequency MHz Downlink Frequency MHz Channel Separation 1.25 MHz Modulation Scheme BPSK/QPSK Number of Channel 64 Channel Bit Rate 1.25 Mbps (chip rate) Speech Rate 8~13 kbps Data Rate Up to 14.4 kbps Maximum Tx Power 600 mW ECE6331

45 2.5G: IS-95B (1998) Increased data rate for internet applications
Up to 115 kbps (8 times that of 2G) Support web browser format language Wireless Application Protocol (WAP) ECE6331

46 3G Technology Ability to receive live music, interactive web sessions, voice and data with multimedia features Global Standard IMT-2000 CDMA 2000, proposed by TIA W-CDMA, proposed by ARIB/ETSI Issued by ITU (International Telecommunication Union) Excellent voice quality Data rate 144 kbps in high mobility 384 kbps in limited mobility 2 Mbps in door Frequency Band MHz Convolutional Codes Turbo Codes for high data rates ECE6331

47 3G: CDMA2000 (2000) CDMA 1xEV-DO CDMA 1xEV-DV Channel Bandwidth:
peak data rate 2.4 Mbps supports mp3 transfer and video conferencing CDMA 1xEV-DV Integrated voice and high-speed data multimedia service up to 3.1 Mbps Channel Bandwidth: 1.25, 5, 10, 15 or 20 MHz Chip rate at Mbps Modulation Scheme QPSK in downlink BPSK in uplink ECE6331

48 3G: CDMA2000 Spreading Codes
Downlink Variable length orthogonal Walsh sequences for channel separation M-sequences 3x215 for cell separation (different phase shifts) Uplink M-sequences 241 for user separation (different phase shifts) ECE6331

49 3G: W-CDMA (2000) Stands for “wideband” CDMA Channel Bandwidth:
5, 10 or 20 MHz Chip rate at Mbps Modulation Scheme QPSK in downlink BPSK in uplink Downlink Variable length orthogonal sequences for channel separation Gold sequences 218 for cell separation Uplink Gold sequences 241 for user separation ECE6331

50 4G OFDM 4G is being developed to accommodate the quality of service (QoS) and rate requirements set by forthcoming applications like wireless broadband access, Multimedia Messaging Service (MMS), video chat, mobile TV, HDTV content, Digital Video Broadcasting (DVB), minimal service like voice and data, and other streaming services for "anytime-anywhere". Baseband techniques[9] OFDM: To exploit the frequency selective channel property MIMO: To attain ultra high spectral efficiency Turbo principle: To minimize the required SNR at the reception side Adaptive radio interface Modulation, spatial processing including multi-antenna and multi-user MIMO Relaying, including fixed relay networks (FRNs), and the cooperative relaying concept, known as multi-mode protocol 3GPP is currently standardizing LTE Advanced as future 4G standard ECE6331

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