A systematic approach to decision feedback multiuser detection is introduced for the joint detection of symbols of K simultaneously transmitting users of a synchronous correlated waveform multiple-access (CWMA) channel with Gaussian noise. A new performance criterion called symmetric energy is defined which is a low-noise indicator of the joint error rate that at least one user is detected erroneously. Even the best linear detectors can perform poorly in terms of symmetric energy compared to the maximum-likelihood detector. A general class of decision feedback detectors is defined with O(K) implementational complexity per user. The symmetric energy of arbitrary DFD's and bounds on their asymptotic effective energy (AEE) performance are obtained along with an exact bit-error rate and AEE analysis for the decorrelating DFD. The optimum DFD that maximizes symmetric energy is obtained. Each one of the optimum, decorrelating, and conventional DFD's, that correspond to the orders in which the users can be detected, are shown to outperform the linear optimum, decorrelating, and conventional detectors, respectively, in terms of symmetric energy. Moreover, algorithms are obtained for determining the choice of order of detection for the three DFD's which guarantee that they uniformly (user-wise) outperform their linear counterparts. In addition to optimality in symmetric energy, it is also shown that under certain conditions, the optimum DFD achieves the AEE performance of the exponentially complex maximum-likelihood detector for all users simultaneously. None of the results of this paper make the perfect feedback assumption. The implications of our work on power control for multiuser detection are also discussed.

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.

This paper analyzes the performance of the linear parallel interference cancellation (LPIC) multiuser detector in a synchronous multiuser communication scenario with binary signaling, nonorthogonal multiple access interference, and an additive white Gaussian noise channel. The LPIC detector has been considered in the literature lately due to its low computational complexity, potential for good performance under certain operating conditions, and close connections to the decorrelating detector. In this paper, we compare the performance of the two-stage LPIC detector to the original multistage detector proposed by Varanasi and Aazhang for CDMA systems. The general M-stage LPIC detector is compared to the conventional matched filter detector to describe operating conditions where the matched filter detector outperforms the LPIC detector in terms of error probability at any stage M. Analytical results are presented that show that the LPIC detector may exhibit divergent error probability perf...

Limitations on the interference suppression capabilities of linear multiuser detection due to dimensional constraints are studied in this paper. Generalized linear multiuser detection is defined for CDMA systems with arbitrary modulation and coding schemes and channel characteristics. Conditions are derived for the existence of linear decorrelators in this general framework and a solution for the generalized linear decorrelator is presented when it exists. This theory is applied to determine the ecacy of linear multiuser detection in important scenarios that include the presence of user asynchronism, multipath propagation, error control coding and block modulation (such as M-ary orthogonal modulation in IS-95 uplink). Ways to overcome the dimensional limitations of linear multiuser detection are to use larger symbol constellations that span a smaller number of dimensions, or to employ nonlinear techniques such as nonlinear multistage interference cancellation.

Abstract—An approach to successive interference cancellation is presented that exploits the structure of the combined signal constellation in a multiuser system. The asymptotic conditional efficiency of a successive detector is defined, based on the conditional probability of error at high signal-to-noise ratio (SNR), as a quantitative measure for evaluating detector performance at each stage of successive detection. The joint successive interference canceller (JSIC) that jointly detects consecutive users in an ordered set is proposed as an improvement over the conventional successive interference canceller (SIC). The maximal asymptotic conditional efficiency successive interference canceller (MACE-SIC) and its JSIC equivalent (MACE-JSIC) are also derived as the multiuser detectors that achieve the highest asymptotic conditional multiuser efficiency at each stage of successive detection among all possible SIC and JSIC detectors, respectively, given any particular ordering of user signals. The ordering of users achieving the highest asymptotic conditional efficiency at each stage of successive detection is derived. Performance bounds based on the signal constellation structure are derived to quantify the gain of the MACE-JSIC detector compared to the MACE-SIC detector. Index Terms—Asymptotic efficiency, convex optimization, detection, interference cancellation, multiuser detection. I.

Abstract — A structural approach to multi-stage detection is proposed where the joint information between interfering users is utilized to give better soft decisions on the user bits in each stage or iteration. The key idea is to use the structure of the multi-user signal constellation to achieve higher performance in terms of the bit-error-rate, particularly at high SNR, and reduce the possibility of error propagation or limit cycles. A maximal asymptotic efficiency detector is employed as the first stage to generate better initial estimates of the user bits, followed by a local maximum likelihood kernel for subsequent stages. The proposed detector is particularly attractive for heavily correlated or over-loaded multi-user systems. The proposed multi-stage detector also includes the possibility of known memoryless non-linearity in the system, e.g., the non-linearity introduced due to saturation by the downlink RF amplifier in satellite communication systems. I.

Abstract—A generalized concept of interference suppression is introduced for multiuser systems with a known memoryless transmit non-linearity, such as in the downlink amplifier of a satellite communication system. By considering the operation of commonly used multiuser detection techniques from a viewpoint of constellation structure, we extend these notions to nonlinear multiuser systems. We also analyze the performance of such multiuser detectors in terms of their asymptotic multiuser error exponents, which reduce to the asymptotic multiuser effective energies for the respective detectors in the absence of nonlinearity. The asymptotic conditional error exponent is introduced as a quantitative measure for evaluating nonlinear multiuser detectors based on the conditional probability of error at high signal-to-noise ratio (SNR). The optimal affine detector that maximizes the asymptotic error exponent for a given user is derived