Code Division Multiple Access

Transcription

1 Code Division Multiple Access Ashutosh Deepak Gore EESA Lecture Series 11 th August 2005

2 Challenges in wireless mobile communications Higher loss rate: R < C = B log 2 (1 + N S ) [Shannon] Restrictive allocation of frequency bands - costly spectrum Lower transmission rates Higher delay and delay jitter Lower security and simpler active attacking Locating a user and routing the call

3 Time-varying channel Interference Path loss Fading due to multipath Shadowing Characteristics of the wireless channel P r = P t f 2 r α where 2 < α 4

4 Spatial channel reuse Handoff Pilot channel Logical coverage area is a circle Cellular concept Hexagons completely cover the area, which leads to minimum number of base stations Sectoring reduces inter-cell interference Downlink / Forward link: BS to MS Uplink / Reverse link / Multiple access: MS to BS

5 Multiple Access Multiple non-cooperative users sharing a common channel have to send information to a common receiver. Fixed/assigned multiple access techniques like: Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) GSM (hybrid TDMA/FDMA) are suitable for Constant Bit Rate (CBR) traffic, but not for bursty traffic. Bursty traffic: Peak arrival rate Average arrival rate 1

6 CDMA: Underlying principles Direct Sequence Spread Spectrum System Capacity of multiple access channel can be achieved only if users are interfering with each other and their signals are noise-like [multiuser information theory]. Spread Spectrum is a technique in which transmission bandwidth W message bandwidth R Near-orthogonal codes are used to demodulate the data of different users, i.e., if c i (t) and c j (t) are the signature waveforms of different users, then T 0 c i (t)c j (t)dt 0

7 DS-CDMA transmitter Baseband data spectrum RF spectrum A 0 B 0 B f A 1 0 W f 0 f T s t t Data waveform Spreading waveform Carrier Modulation T c t

8 DS-CDMA receiver Despreading and Demodulation Filtering recovered baseband data waveform Received spectrum Interference and Jamming Demodulated spectrum I 1 A 1 0 W f 0 f I 0 A 0 B 0 B f

9 Reverse link power control The base station issues power control signals to all users in the cell to ensure that the power received at the base station from every user is equal. Let M = number of users in the cell I 0 = interference spectral density W = spread bandwidth E b = energy per bit R = bit rate Then M W/R E b /I 0 M can be increased by relaxing BER or decreasing rate. Any interference reduction technique can improve QoS (Quality of Service) and/or number of users. Soft capacity

10 Voice Activity Detection (VAD) Multi-cell interference Sectoring Features/Issues M 5 W/R E b /I 0 In IS-95, with W/R = 128 and E b /I 0 = 7dB, we obtain M = 128. Universal Frequency Reuse Resource allocation of each user s channel is energy. Unified approach for interference control and channel allocation.

11 Spreading Codes A noise-like and random signal has to be generated at the transmitter. The same signal must be generated at the receiver in synchronization. We limit the complexity by specifying only one bit per sample, i.e., a binary sequence. Desirable properties of the binary random sequences (Golomb s postulates): Relative frequencies of 0 and 1 should be 1 2 (Balance property). A fraction 2 1 of all run lengths should be of length n (Run property). n If the random sequence is cyclically shifted by any non-zero integer, the resulting sequence should have an equal number of agreements and disagreements with the original sequence (Autocorrelation property).

12 PN sequences PseudoNoise (PN) sequences are deterministically generated sequences that nearly satisfy Golomb s postulates. Periodic binary sequences can be generated by a linear feedback shift register (LFSR). Let r = number of stages in the LFSR P = period of any sequence generated by the LFSR Then P 2 r 1 If the feedback connections are derived from an irreducible polynomial over GF (2 r ), then P = 2 r 1. Such an LFSR is known as a Maximal Length Shift Register (MLSR) and its output is a PN sequence.

13 LFSR implementation of PN sequences c 1 c 2 c i 1 c i c n 1 x x 2 x i x n R 1 R 2 R i R n output sequence clock LFSR configuration of f(x) = 1 + c 1 x + c 2 x c i x i c n 1 x n 1 + x n

14 Autocorrelation function of PN sequence 1 R c (t) PT c Tc 1 P T 0 c t PT c

15 PN sequences specified in IS-95 A long PN sequence (r = 42) is used to scramble user-specific data on the uplink as well as downlink. Long PN sequences of different users are separated by multiples of 512 bits. A short PN sequence (r = 15) is specific to a base station and its period is (2 15 1)T c = 27ms. Two short PN sequences are used to separate the quadrature components of the uplink waveform.

16 RAKE receiver Mobile station receives multiple attenuated and delayed replicas of the original signal (multipath diversity channels). Two multipath signals are resolvable only if their relative delay exceeds the chip period T c. Amplitudes and phases of multipath components are found by correlating the received waveform with multiple delayed versions of the signal (delay = nt c ). Searcher performs the above task for up to 3 different multipath signals. 3 parallel demodulators (RAKE fingers) isolate the multipath components and the RAKE receiver combines them.

17 Soft Handoff In GSM, hard handoff occurs at a cell boundary. In CDMA, a mobile commences communication with a new BS without interrupting communication with the power-controlling BS (base station diversity). Diversity combining at mobile switching center (MSC) improves resistance to fading. Increases capacity in heavily-loaded systems. Increases coverage in lightly-loaded systems.

18 CDMA: More features Orthogonal Walsh codes are used on the downlink to separate data of different users. Base station transmits a known sequence of symbols on the pilot channel (W 0 ), which is used by a mobile for channel estimation. Diversity techniques: Frequency diversity Time diversity Space diversity

19 Comparison of IS-95 and CDMA-2000 Feature IS-95 CDMA 2000 RF channel BW 1.25 MHz 1.25/5/10/15/20 MHz Chip rate Mcps /3.6864/ Mcps Modulation BPSK with QPSK with quadrature spreading quadrature spreading Pilot-based FL Yes FL Yes demodulation RL No RL Yes Turbo codes No Yes FL transmit diversity No Yes User data rate kbps 9.6kbps - 2 Mbps

20 References J. Lee and L.E. Miller, CDMA Systems Engineering Handbook. London, Artech House, S. Glisic and B. Vucetic, Spread Spectrum CDMA Systems for Wireless Communications. London, Artech House, A.J. Viterbi, CDMA: Principles of Spread Spectrum Communications. Addison Wesley Wireless Communications Series, Vijay K. Garg, IS-95 and CDMA2000: Cellular/PCS Systems Implementation, Pearson Education, Questions? Thank you!

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