Adaptive Detection for DS-CDMA GRAEME WOODWARD, STUDENT MEMBER, IEEE, AND BRANKA S. VUCETIC, MEMBER, IEEE A review of adaptive detection techniques for direct-sequence code division multiple access (CDMA) signals is given. The goal is to improve CDMA system performance and capacity by re- ducing interference between users. The techniques considered are implementations of multiuser receivers, for which background material is given. Adaptive algorithms improve the feasibility of such receivers. Three main forms of receivers are considered. The minimum mean square error (MMSE) receiver is described and its performance illustrated. Numerous adaptive algorithms can be used to implement the MMSE receiver, including blind techniques, which eliminate the need for training sequences. The adaptive decorrelator can be used to eliminate interference from known in- terferers, though it is prone to noise enhancement. Multistage and successive interference cancellation techniques reduce interference by cancellation of one detected signal from another. Practical problems and some open research topics are mentioned. These typically relate to the convergence rate and tracking performance of the adaptive algorithm. Keywords—Adaptive equalizers, adaptive signal detection, code division multiaccess, decorrelation, interference suppression, lat- tice filters, least mean square methods, least squares methods, multiaccess communication, Wiener filtering. I. INTRODUCTION Direct-sequence code division multiple access (DS- CDMA) is an important component of numerous recent communications systems, proposed and implemented. Applications include terrestrial cellular, wireless multi- media systems, and personal satellite mobile systems. To achieve increased data throughput and user capacities, it is important to optimize channel utilization. This paper will describe how adaptive detection algorithms can be applied to achieve this goal. A review of major techniques will be presented. Increasing CDMA channel utilization is rooted in multiuser communications research, so a brief background of this field will be given. For a detailed discussion of multiuser receivers, the reader is referred to [1]. All adaptive detectors to be described in this paper are approximate realizations of multiuser receiver structures. Conventional asynchronous DS-CDMA systems allow each user to transmit and receive independently. Each Manuscript received December 1, 1997. The work of G. Woodward was supported by Telstra Research Laboratories, Melbourne, Australia. The work of B. S. Vucetic was supported by an Australian Research Council Grant. The authors are with the Department of Electrical Engineering, Univer- sity of Sydney, Sydney NSW 2006 Australia. Publisher Item Identifier S 0018-9219(98)04241-8. receiver performs a simple correlation between the received baseband signal and the corresponding user’s spreading sequence. In an additive white Gaussian noise (AWGN) channel with orthogonal spreading sequences, this approach would be optimal. Due to the asynchronicity of users and the need to support numerous users, such orthogonality is impossible, even on a hypothetical AWGN channel. Thus, the residual cross correlations are treated like AWGN in the classical analysis of these systems. This is acceptable as a first-order approximation due to the wide-band quasiwhite spectra of the spreading sequences typically used. However, this approach ignores the highly structured, cyclostation- ary statistics of the interference. System performance is rendered multiple-access interference (MAI) limited, and channel utilization is correspondingly low. The recently adopted CDMA terrestrial cellular standard IS-95 [2] is based on the conventional CDMA receiver but uses a RAKE 1 front-end and powerful, yet complex, error-control coding in an effort to alleviate this problem. Due to the nonorthogonality of practical spreading se- quences, the conventional correlator receiver suffers from the near–far problem. This implies that the cross correlation between the spreading sequence of the user of interest and the signal from a strong interferer can be larger than the correlation with the signal from the desired user. Detection is rendered unreliable. The classical way to deal with this is power control, whereby all users’ transmit powers are controlled so that the powers received from all users are equal. This adds complexity to the system, and inaccuracies in power control have a detrimental impact on performance. The optimal multiuser receiver of Verd´ u [3] demonstrated that DS-CDMA is not fundamentally MAI limited and can be near–far resistant. The optimal receiver comprises a bank of matched filters (MF’s), one for each user, followed by a Viterbi algorithm for maximum likelihood sequence estimation (MLSE). Significantly, it was demonstrated that the output from the bank of matched filters provided a sufficient statistic for optimum detection. Use of such a receiver allows a tradeoff between complexity of signal selection and receiver complexity in a multiuser envi- ronment. The requirement to select spreading sequences 1 A tapped delay line receiver structure designed to collect energy from multipath signal components. Its form and function resemble a garden rake. 0018–9219/98$10.00  1998 IEEE PROCEEDINGS OF THE IEEE, VOL. 86, NO. 7, JULY 1998 1413