Multiuser Detection for Turbo-coded Ds/cdma a Dissertation Multiuser Detection for Turbo-coded Ds/cdma a Dissertation

Multiuser Detection for Turbo-Coded DS/CDMA. (May 2001)Qinghua Li, B.E., South China University of Technology;M.E., Tsinghua University, BeijingCo–Chairs of Advisory Committee: Dr. Costas N. GeorghiadesDr. Xiaodong Wang Multiuser detection and turbo coding are two of the most powerful techniquesfor enhancing the performance of next-generation wideband code-division multiple-access (CDMA) systems. Multiuser detection for three types of uplink CDMA isaddressed in this work.First, we propose a geometric view of multiuser detection and present results onthe problem of efficient computation of the optimal decision regions (Voronoi diagram)for synchronous uncoded DS/CDMA over known channels. Among other results, itis shown that Verdú’s algebraic notion of “indecomposable error sequences” relatesdirectly to the geometric construction of the Voronoi diagram for multiuser detection.Based on the presented geometric results, we introduce two multiuser detectors whoseperformance compares favorably to some popular existing detectors.Next, we consider the problem of joint channel estimation and data detectionin uplink asynchronous CDMA systems employing aperiodic spreading sequences inthe presence of unknown multipath fading channels and additive white Gaussiannoise, in which case the maximum likelihood sequence estimation is too complex toperform. Sequential blind multiuser receivers are proposed, based on the sequentialexpectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel,the receivers sequentially estimate the channel using the sequential EM algorithm.Moreover, the snapshot estimates of each path are tracked by linear minimum-mean-squared-error (MMSE) filters.Finally, we develop an iterative multiuser receiver for turbo-coded CDMA sys-tems with aperiodic spreading sequences operating in known multipath fading, whichexploits the power of both multiuser detection and turbo coding. The key innovationin the proposed receiver is a low-complexity soft multiuser detector which uses thesame decision statistics as the conventional RAKE receiver (i.e., the outputs of themaximum ratio combiners for all the users), and performs soft-interference cancella-tion and instantaneous MMSE filtering. The soft multiuser detector has a complexityofO(K) per bit per iteration, where K is the number of users.In all three types of channel, simulation results demonstrate that the performanceof the proposed techniques significantly outperforms those of some existing populartechniques. To my wife Ruiyu Wang and my parents.