Wideband Communications Lecture 20-23: MC-CDMA Aliazam Abbasfar
Outline • IS-95/ WCDMA • MC-DS-CDMA • MC-CDMA • OFDMA
IS-95 • DSSS • 1.2288 Mchips/sec ( BW = 1.25 MHz) • FDD • Downlink • Synchronous CDMA • Quadrature spreading • 2 Extended M-seq. (215-chip long) • Offsets identify BTS (synched) • 64 orthogonal Walsh codes • Codes 0 : pilot, 1 : sync, 2: paging • max. 61 data channel (users) • Coherent reception • Uplink • Asynchronous CDMA • No pilot channel (Non-coherent reception) • Long M-seq. (242-1 long) to distinguish users • Walsh coded symbols • OQPSK • Fast power control
WCDMA • DSSS • Pulse shape : SRC with roll off 0.22 • 3.84 Mcps (BW = 3.88*1.22 = 4.68 MHz) • FDD (UL: 1920–1980, DL: 2110–2170 MHz) and TDD • Downlink • Synchronous CDMA • Complex spreading • 2 Gold codes (M-seq of 218-1) • BTS are unsynched • OVSF codes • SF:4-512 • Codes 0 : pilot, 1 : sync • max. 255 data channel (users) • Coherent QPSK • Uplink • Asynchronous CDMA • Complex spreading • 2 Gold codes • Coherent QPSK • Send pilots • Fast power control
Multi-carrier CDMA • Multi-carrier modulation is very good at combating ISI • Low complexity equalizer • High performance • combine Multi-carrier modulations with CDMA • Types: • MC-DS-CDMA ( synch/asynchronous) • Spreading in Time • MC-CDMA ( synchronous) • Spreading in Frequency • Hybrid approaches • 2D spreading • Spreading in time and frequency
MC-DS-CDMA • Multiple DS-CDMA with different carriers • Parallel sub-channels each with DSSS • Sub-channels have narrower band (reduced ISI) • RAKE or equalizer receiver for each • Few sub-channels makes the PAPR small • Very narrowband sub-channels can be implemented efficiently by OFDM/DMT • Spreading is done over OFDM/DMT symbols
MC-CDMA • Signature waveforms are all between 0-T • xk(t) = Xksk(t) • Xk : kth user data symbol (bk bits) • Rk = bk / T • orthonormal bases : fn (t)=rect(t/T) ej2pf0 t • sk(t) = Sck,nfn (t) ; n=0,…,N-1 • Vector representation : sk = [ck,0 ck,1 … ck,N-1]T : codes • Each user uses all the tones • The tone coefficient is determined by its code • Signature waveforms at RX • rk(t) = sk(t)*hk(t) = Sck,nHk,nfn (t) ; n=0,…,N-1 • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk : RX codes • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nfn (t) + n(t) • r = A X + n ; A=[r1 r2 … rK] • High spectral efficiency
MC-CDMA codes • Walsh/Hadamard codes • Orthogonal at TX • NOT orthogonal at RX when ISI • Golay/Zadoff-Chu • Codes with good PAPR • Pseudo noise sequences • M-sequence/Gold/Kassami codes • Simple to generate • Good cross-correlation property • Low rate convolutional codes • Coding gain in addition to processing gain • Bad MAI
Hybrid methods • Two dimensional spreading • Easy to implement in OFDM-CDMA • # of dimensions = N x # data symbols • # of Time slots x # of tones • r(t) = SSXk,irk,i(t) + n(t) • r = S Ai Xi + n • Ai=[r1 r2 … rK] • Ai are orthogonal • Parallel channels • Separate dimensions • ri = Ai Xi + ni • Ai=[r1 r2 … rK]
Single user detection • Only one user channel coefficients and code are available • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nfn (t) + n(t) • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk • r = A X + n ; A=[r1 r2 … rK] • Matched filter (Correlator) receiver • Equalizer + despreader • zk = sk* G r • MRC: G = Hk* • EGC: G = Hk* / |Hk| • ZF: G = (Hk*Hk)-1Hk* • MMSE: G = (Hk*Hk+ s2I)-1Hk*
Multi-user detection • All user channel coefficients and codes are available • r = A X + n ; A=[r1 r2 … rK] • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk • Optimum: min |r – A X|2 • De-correlator : z = (A*A)-1 A* r • MMSE : z = (A*A + s2I)-1 A* r • Interference cancellation • Successive (SIC) • Parallel (PIC)
Pre-equalization • Channel is known at TX (e.g. TDD) • Spreader + pre-equalizer • xk(t) = Xksk(t) • sk(t) = Sck,nGk,nfn (t) ; n=0,…,N-1 • Codes : sk = [Gk,0ck,0 Gk,1ck,1 … Gk,N-1ck,N-1]T • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nGk,nfn (t) + n(t) • rk = [Hk,0Gk,0ck,0 Hk,1Gk,1ck,1 … Hk,N-1Gk,N-1ck,N-1]T = Hk Gksk • r = A X + n ; A=[r1 r2 … rK] • Pre-equalizers: • MRC: G = Hk* • EGC: G = Hk* / |Hk| • ZF: G = (Hk*Hk)-1Hk* • MMSE*: G = (Hk*Hk+ s2I)-1Hk* • Power constraint scale the G coefficients
New Multiple Access schemes • MC-TDMA or OFDM-TDMA • Time slots are assigned to users • OFDM modulation for users (OFDM symbols in time slots) • In many wireless standard, e.g. WiFi ( 802.11a/g) • OFDMA • Assign sub-carrier frequencies to users • Synchronous users • DL : TX sends an OFDM symbol for all users • UL: RX receives an OFDM symbol from all users • Very sensitive to synchronization (ICI) • Time, clock, and Freq • Users are synchronized by BTS • OFDMA + Code division multiplexing (CDM) • Users are assigned L sub-carriers • L Codes are used for each user • Frequency diversity (per user)
Frequency hopping OFDMA • Time varying sub-carrier frequency assignment • Each user is given a hopping pattern • Frequency diversity • Orthogonal patterns • Independent of channels • Latin square • Nc orthogonal patterns = a family • Using all freqs only once • Nc-1 families with maximum one collision per pair • Cross-correlation = 1/Nc • If Nc is prime simple formula
Reading • Fazel & Kaiser 2.1, 3.1, 3.2, and 3.3
