Abstract — The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive softoutput decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in Turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low-complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached. Index Terms — Coded CDMA, instantaneous MMSE filtering, multiuser detection, soft interference cancellation, Turbo processing.

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 use of decorrelating detectors for a dual rate DS/CDMA system that serves both low bit rate and high bit rate users. All users have the same BPSK modulation technique and the same chip rate. The differences in bit transmission rates result in different processing gains for each class of user. We assume that in an interval of duration T 0 , a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T 0 yields an M bit processing delay for high rate users and a computational complexity that grows with M . In this paper, we propose a decorrelator that generates bit decisions for each high rate user in every subinterval of duration T 0 =M . To decode a low rate user, a soft decoding rule applies maximal ratio combining on M separate decorrelated outputs of each low rate user. The soft decoding dual rate decorrelator eliminates the bit processing delay for high rate users and also reduces t...

The optimum receiver to detect the bits of multiple CDMA users has exponential complexity in the number of active users in the system. Consequently, many suboptimum receivers have been developed to achieve good performance with less complexity. In this paper, we take the approach of approximating the solution of the optimum multiuser detection problem (OMUD) using nonlinear programming relaxations. First, we observe that some popular suboptimum receivers indeed correspond to relaxations of the optimal detection problem. In particular, one proposed approximation method yields to iterative solutions which correspond to previously proposed heuristic nonlinear detectors. Using nonlinear programming approach, we identify the convergence properties of these iterative detectors. Secondly, we propose a relaxation that yields a receiver which we call the generalized MMSE detector. We give a simple iterative implementation of the detector. Its performance is evaluated and comparisons to other s...

This paper introduces the problem of noncoherent detection for nonorthogonal multipulse modulation in the context of the synchronous multiuser Gaussian channel. Each user sends an M-ary information symbol by transmitting one of possibly nonorthogonal waveforms. Furthermore, the signals of one user are allowed to be correlated with the signals of all other users. A key idea proposed here is that of a noncoherent decorrelative receiver front end. Like its counterpart in single-pulse modulation, this front end eliminates multiuser interference. It therefore reduces the multiuser detection problem into decoupled single-user problems over equivalent noise-enhanced single-user channels. Each equivalent single-user channel is rather general and can be described as one where the waveforms employed are not only correlated, but are also of unequal energies. Several new results pertaining to the design and analysis of optimum and suboptimum noncoherent detectors for this single-user channel are obtained. In the multiuser channel, these detectors constitute the post-decorrelative processing units for each user.

. Multiuser detection by iterated soft--decision interference cancellation is known to achieve almost single user performance in low and moderate interference scenarios. In this paper, it is shown how this detection scheme can be further improved by adaptive calculation of the soft--decision rule. This enables a significantly higher number of active users signaling within the same bandwidth to be detected, implying that spectral efficiency is increased. 1 Introduction Code--division multiple--access (CDMA) is a promising multiple--access technique for future mobile communication systems. Its main advantages are firstly a wideband transmitter signal reducing the negative effects of frequency --selective fading, and secondly the fact, that in cellular environments the same carrier frequency can be used in every cell. An important drawback of CDMA systems is the distortion caused by multiple--access interference (MAI). Since the pioneering work of [1], who has shown that single user perf...

We consider the convergence in norm of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that asymptotically, linear parallel interference cancellation diverges for systems loads of greater than about 17%. Using known results from the theory of iterative solutions for linear systems we derive optimal or near optimal relaxation parameters for parallel (first and second order stationary, Chebyshev) and serial cancellation (successive relaxation) methods. An analytic comparison of the asymptotic convergence factor for the various methods is given. Simulations are used to verify results for finite size systems. Keywords--- Interference cancellation, iterative methods, multi-stage receivers, multi-user receivers, CDMA, random sequences I.

Abstract — An adaptive multistage parallel interference cancellation technique based on a partial interference cancellation (IC) approach [1] was proposed in [2] for multipath fading channels. In this paper, the proposed technique is applied to develop a receiver structure in an AWGN environment. Unlike the scheme in [1], the weighting factors in this proposed scheme are derived by minimizing the meansquare error between the received signal and its estimate through an LMS algorithm. Neither training sequence nor pilot signal is needed. The complexity of the proposed adaptive multistage PIC structure is much lower than that of linear multiuser detectors. Simulation results show superior performance of the proposed receiver structure over an AWGN channel and in various conditions. I.

We consider the convergence properties of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that parallel interference cancellation diverges for systems loads of greater than 17%. Using known results from the theory of iterative solutions for linear systems, we derive optimal or near optimal relaxation parameters for parallel (first and second order stationary, Chebyshev) and serial cancellation (successive relaxation) methods. An analytic comparison of the asymptotic convergence factor for the various methods is given. Simulations are used to verify results for finite size systems.

This paper presents analysis of a multistage parallel interference canceller receiver for a synchronous multiuser CDMA system. At every stage, new estimates of the multiple access interference are formed from infinitely soft decisions for the users' data. These new estimates are then used to cancel multiple access interference and form new decision statistics for the users' data. We show that in limit, when the number of stages increases infinitely, this receiver has the performance of a decorrelating detector. For an arbitrary finite number of stages, closed form expressions for the probability of error and the asymptotic multiuser efficiency of this receiver are presented and evaluated for performance comparisons with the conventional and decorrelating detectors. 1 Introduction The design of CDMA receivers has received increasing attention in the recent years. A major limiting factor for the performance of the conventional CDMA receiver is the near-far effect. Formally proposed CDM...