Multistage Symbol-by-Symbol Bayesian Interference Cancellation for UMTS-CDMA Links Affected by Severe Multipath

Transcription

1 1 Multistage Symbol-by-Symbol Bayesian Interference Cancellation for UMTS-CDMA Links Affected by Severe Multipath Roberto Cusani 1, Jari Mattila, Marco Di Felice 1 1 INFO-COM Dpt., University of Rome La Sapienza, Italy Helsinki University of Technology, Commun. Lab., Finland

2 Outline Introduction CDMA system model Multistage parallel interference cancellation (MPIC) partial cancellation Bayesian cancellation Simulation results application to TDD-CDMA for UMTS channel estimation Conclusions

3 CDMA system model 3 y () 1 ( n) ( 1 d ) ( n) Code #1 Channel #1 AWGN CC 1 Code #1 $ ( 1) d ( n) Σ y ( K ) ( n ) M U D d ( K ) ( n ) Code #K Channel #K CC K Code #K $ ( K ) d ( n) Transmitted symbols Spreading Coherent Despreading Received combining sequences Data out received sequences of K users after coherent combining and despreading: K L g Í Í ( k) ( i, k) ( i) ( k ) y ( n) = g ( m) d ( n- m) + w ( n) i= 1 m=-l g {g (i,k) (n)} are S-CIRs {w (k) (n)} are noise sequences w (n) d (n) g (1,1) (n) g (1,K) (n) Σ y (n) w (K) (n) d (K) (n) g (K,K) (n) g (K,1) (n) Σ y (K) (n)

4 Partial interference cancellation 4 partial cancellation: partial cancellation 1) in multipath channel: y (n) y () (n) received sequences Decision Device d 1 g (1,) (n) ~ I ~ y 0 1-p 1 y 1 (n) tentative symbol estimate from the previous stage d Decision Device () d 1 g (,1) (n) ~ I () p 1 p 1 y 1 () (n) 1-p 1 d () out scale the cleaned signal sample by a fraction p 1 (0 < p 1 < 1) and the symbol estimate from the previous stage by 1 - p 1 ~ y () 0 ( y k ) ( n p y k ) ~ ( n I k ) ( n 1 p y k ) ( ) = ( ) - ( ) + ( - ) ~ ( n ) value of fraction increases for successive stages 1) Divsalar, Simon, Raphaeli, Improved Parallel Interference Cancellation for CDMA, 1998

5 MPIC detector with partial interference cancellation 5 soft tentative decisions for QPSK constellation symbols {+1,-1,+j,- j} are obtained from the hyperbolic tangent non-linearity ~ d = F[ y] = % & ' tanh( a yr), if abs( yr) abs( yi) j tanh( a y ), if abs( y ) < abs( y ) I R I where α is a scale factor and y = y R + j y I = y (k) (n) multistage successive interference cancellation (MSIC) subtracts the interference with the most up-to-date symbol decisions

6 Multistage Bayesian (MB) interference cancellation 6 MPIC/MSIC receiver with symbol-by-symbol adaptive soft decisions for multipath channel and any complex modulation interference power estimate determines the scale factor of the non-linearity y (n) y () (n) received sequences d 1 g (1,) (n) ~ I 1 - y 1 (n) + d d () 1 g (,1) (n) ~ () I 1 y () 1 (n) + - d () out symbol-by-symbol adaptive soft decision device

7 Bayesian non-linearity 7 Bayesian non-linearity: c m + j s m, m=0,,m -1, are the constellation symbols the scale factor α is: s I s W { } d% = E d y == M -1 { } Í ( cm + jsm) exp a ( yrcm + yism) m= 0 M -1 Í m= 0 exp { a ( yc R m+ ys I m) } 7 a = 1 s = 1 s + s WI W I is the variance of the sum of ISI and MAI is the variance of the observation noise w (k) (n) magnitude: phase:

8 Estimate of the total interference 8 for a practical measure of s = s + s, we employ symbol-by-symbol: s WI m WI W I 1 6 m m J L K min y- g c + js, m= 0,, M -1 g is the main tap of S-CIR (typically real) using an averaged interference estimate gives worse performance hard-limiting (α = ) is used for the last stage

9 Application to TDD-CDMA for UMTS 9 ETSI-SMG proposals for UMTS at the unified chip rate of Mcps: (now changed to 3.84 Mcps) Wideband W-CDMA (FDD) with spreading factors SF = 56,18,,4 Time-Division TD-CDMA (TDD) with: small spreading factor SF=16 small number of simultaneously active users (max K=8) multiplexing of users over 16 timeslots (now 15) on the same channel User #1 User # User # time

10 Vehicular B channel, TDD downlink 10 Veh. B channel, QPSK, downlink, K=8, N=40, Q=16, Orth. Codes, Mchip/s 5 1.E-01 BER 1.E-0 Relative power (db) E CIR taps 1.E-04 1.E-05 ZF, channel known MPIC, Ns=4, channel known MSIC, Ns=3, channel known MB, Ns=4, channel estimated via correl. MB, Ns=4, channel known Ideal IC, channel known 5 10 Eb/No (db) 15 modulation SF=16, Orthogonal Walsh codes 8 users, block length = 40

11 Channel Estimation for TDD downlink 11 from signal theory: x(t) channel estimation for downlink: h(t) y(t) R xy (τ)=h(τ) R xx (τ) if R xx (τ)=δ(τ), then R xy (τ)=h(τ) Spreaded Midamble Spreaded Channel Received Received Received spreaded spreaded midamble sum of K users sum of K users Midamble the channel estimate is obtained by computing the cross-correlation between the (known) transmitted midamble and the received one cross-correlation channel taps

12 Vehicular B channel, TDD uplink 1 Veh. B channel, QPSK, uplink, K=8, N=40, Q=16, Orth. Codes, Mchip/s 5 1.E-01 BER 1.E-0 Relative power (db) E CIR taps 1.E-04 1.E-05 ZF, channel known MB, Ns=4, channel estimated via correl. MPIC, Ns=4, channel known MSIC, Ns=4, channel known MB, Ns=4, channel known Ideal IC, channel known 5 10 Eb/No (db) 15 modulation SF=16, Orthogonal Walsh codes 8 users, block length = 40

13 Channel Estimation for TDD uplink 13 User #1 User #K Spreaded Midamble Spreaded Midamble Spreaded Spreaded Channel Channel AWGN MAI between midambles is present Solution based on cross-correlation and multistage IC: for user #i, the cross-correlation method gives a tentative channel estimate other users regenerate the received midamble pertaining to user #i and subtract it from the received midamble before estimating (via crosscorrelation) its own channel the procedure is repeated until that a satisfactory channel estimate is obtained for all users

14 Conclusions 14 We propose for multipath channel: MPIC detector with symbol-by-symbol MB interference cancellation MPIC detector with partial interference cancellation realistic channel estimation procedure for TDD-CDMA both detectors have approximately the same complexity MB-canceller has a performance close to the ideal cancellation with known channel and has a minor loss with realistic channel estimates MB appears to be a good candidate for TDD and FDD receivers