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Direct Sequence (DS) Code Division Multiple Access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity

This paper addresses the problem of channel tracking and equalization for multi-input multi-output (MIMO) time-variant frequencyselective channels. These channels model the corrupting effects of intersymbol interference (ISI), co-channel interference (CCI), and noise. A firstorder autoregressive model describes the MIMO channel variation, and tracking is performed by a Kalman filter. Hard decisions to aid Kalman tracking come from a MIMO finite-length minimum-mean-squared-error decision-feedback equalizer (MMSE-DFE), which performs the equalization task. Since the optimum DFE for a wide range of channels produces decisions with a delay \Delta ? 0, the Kalman filter tracks the channel with a delay. A channel prediction module bridges the time gap between the channel estimates produced by the Kalman filter and those needed for the DFE adaptation. The proposed algorithm offers good tracking behavior for multi-user fading ISI channels at the expense of higher complexity than conventional adaptive algorithms. Appropriate coding options for this system are also discussed. Applications include synchronous multiuser detection of independent transmitters, as well as coordinated transmission through many transmitter/receiver antennas, for increased data rate.

In this paper, Tomlinson-Harashima precoding for multiple-input/multiple-output systems including multiple-antenna and multi-user systems is studied. It is shown that nonlinear preequalization offers significant advantages over linear preequalization which increases average transmit power. Moreover, it outperforms decision-feedback equalization at the receiver side which is applicable if joint processing at the receiver side is possible, and which suffers from error propagation. A number of aspects of practical importance are studied. Loading, i.e., the optimum distribution of transmit power and rate is discussed in detail. It is shown that the capacity of the underlying MIMO channel can be utilized asymptotically by means of non-linear precoding.

Spectral efficiency of CDMA systems with linear interference suppression and channel coding is calculated. For interference suppression by decorrelation (zero-- forcing, ZF) an analytical solution is given. For interference suppression according to the minimum mean squared error (MMSE) criterion a numerical approximation is applied. Hereby, it turns out, that the optimum modulation scheme and the optimum code rate are depending on both signal--to--noise ratio and power efficiency of the applied channel code. I. Introduction Code--division multiple--access (CDMA) is a promising technique for future mobile communication systems. Its main advantages are a wideband transmitter signal, which combats the frequency--selective fading of mobile radio channels, and its ability to work without synchronization among the different users. Furthermore, the same carrier frequency can be used in every cell. Caused by multiuser interference which appears in CDMA systems with randomly chosen signature ...

We view the asynchronous random code-division multiple-access (CDMA) channel as a time-varying convolutional code. We study the case where the users encode their data, and, therefore, the single user transmitters and the CDMA channel appear as the concatenation of two coding systems. At the receiver we employ serial Turbo decoding strategies. Unlike conventional Turbo codes where both the inner and outer code may be selected, in our case, the inner code is due to the CDMA channel which we assume to be random. Nevertheless, the decoding system resembles the decoder of a serial Turbo code and single-user performance is obtained even for numbers of users approaching the spreading code length. Index Terms---Code-division multiple access, iterative methods, random codes, Turbo codes. I. INTRODUCTION T HE OUTPUT of a code-division multiple-access (CDMA) channel is a linear transformation of the input. In the case where the channel output noise is whitened, using the noise whitening-match...

Existing multiuser code division multiple access (CDMA) detectors either have to rely on strict power control or near-perfect parameter estimation for reliable operation. In this paper, a novel adaptive multiuser CDMA detector structure is introduced. Using either an extended Kalman filter (EKF) or a recursive least squares (RLS) formulation, adaptive algorithms which jointly estimate the transmitted bits of each user and individual amplitudes and time delays may be derived. The proposed detectors work in a tracking mode after initial delay acquisition is accomplished using other techniques not discussed here. Through computer simulations, we show that the algorithms perform better than a bank of single-user receivers in terms of near-far resistance. Practical issues such as the selection of adaptation parameters are also discussed in some detail. I. Introduction The area of multiuser CDMA detectors has attracted a lot of attention in the literature in the last few years, ever since t...

In this paper, Tomlinson-Harashima precoding, a nonlinear pre-equalization technique, is proposed for transmission over multiple-input/multiple-output channels. Instead of equalizing intersymbol interference (temporal equalization) here spatial equalization, i.e., equalization of the multi-user interference, or combined spatial/temporal equalization is performed. It is shown that this MIMe precodinglike its sISe counterpart offers significant advantages over linear pre-equalization and over decisionfeedback equalization, as is done in BLAST-like schemes. Using channel coding, MIMe precoding is able to achieve higher power efficiencies at lower coding delays than competing schemes.

This paper considers the problem of multiuser detection for a system in which each user employs nonlinear modulation, with an emphasis on noncoherent detection techniques which do not require knowledge of the users' channel parameters at the receiver. Our goals are to gain fundamental insight into the capabilities of multiuser detection in such a setting, and to provide practical algorithms that perform better than conventional matched filter reception. We begin by providing fundamental performance benchmarks by considering coherent maximum likelihood (ML) detection, which requires knowledge of the users' channel parameters, as well as noncoherent detection, formulated in a non-Bayesian generalized likelihood ratio test (GLRT) framework. The asymptotic performance of each detector, as the noise level vanishes, is characterized, yielding simple geometric criteria for near-far resistance. In general, both the ML and GLRT detectors have complexity which is exponential in the number of users. We therefore propose the more practical sequential decision projection (SDP) detector, which has complexity which is quadratic in the number of users. It is shown that the SDP detector has nonzero asymptotic efficiency if the users' powers are suitably disparate.

In this paper we consider a CDMA system with error control coding. Optimal joint decoding is prohibitively complex. Instead we propose a sequential approach for handling the multiple access interference and the error control decoding. The error control decoding is implemented via single user soft--input decoders utilising metrics generated by linear algebraic multiuser metric generators. The decorrelator, and a new scheme termed the projection receiver are utilised as metric generators. For a synchronous system the coded performance of the projection receiver metric is shown to be superior to the decorrelator even though they are equally complex. Also the theoretical degradation relative to the single user bound is derived. Keywords--- CDMA, random codes, convolutional codes I.