We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions." Such random environment may arise in a DS-CDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Assuming that such random directions can be tracked by the receiver, the resulting SIR performance is a function of the directions and therefore also random. We study the asymptotic distribution of this random performance in the regime where both the number of users and the number of degrees of freedom in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the SIR distribution decreases like 1 , and the SIR distribution is asymptotically Gaussian. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems.

In this paper , we devise theoretical grounds for constructing channel models for Multi-input Multioutput (MIMO) systems based on information theoretic tools. The paper provides a general method to derive a channel model which is consistent with one's state of knowledge. The framework we give here has already been fruitfully explored with success in the context of Bayesian spectrum analysis and parameter estimation. For each channel model, we conduct an asymptotic analysis (in the number of antennas) of the achievable transmission rate using tools from random matrix theory. A central limit theorem is provided on the asymptotic behavior of the mutual information and validated in the finite case by simulations. The results are both useful in terms of designing a system based on criteria such as quality of service and in optimizing transmissions in multiuser networks .

Linear multiuser detectors for vector channels with crosstalk are approximated by weighted matrix polynomials. The weight optimization problem is overcome using convergence results from random matrix theory. The results are also extended to receivers with subsequent successive decoding.

Synchronous code-division multiple-access communication systems with randomly chosen spreading sequences and capacity-achieving forward error correction coding are analyzed in terms of spectral efficiency. Emphasis is on the penalties paid by applying single user coding in conjuction with suboptimal multiuser receivers as opposed to optimal joint decoding which involves complexity that is exponential in the number of users times the codeword length. The conventional, the decorrelating and the (re-encoded) decorrelating decision-feedback detectors are analyzed in the nonasymptotic case for spherical random sequences. The re-encoded minimum mean squared error (MMSE) decision-feedback receiver achieving the same performance as joint multiuser decoding for equal power users is shown to be suboptimal in the case of equal rates.

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...

We present a large-system performance analysis of blind and group-blind multiuser detection methods. In these methods, the receivers are estimated based on the received signal samples. In particular, we assume binary random spreading, and let the spreading gain N , the number of users K, and the number of received signal samples M , all go to infinity, while keeping the ratios K/N and M/N fixed. Under such a scenario, we characterize the asymptotic performance of the direct-matrix inversion (DMI) blind linear MMSE receiver, the subspace blind linear MMSE receiver, and the group-blind linear hybrid receiver. We first derive the asymptotic average output signal-to-interference-plus-noise ratio (SINR), for each of these receivers. Our results reveal an interesting "saturation" phenomenon: The output SINR of each of these receivers converges to a finite limit as the signal-to-noise ratio (SNR) of the desired user increases, which is in stark contrast to the fact that the output SINR achieved by the exact linear MMSE receiver can get arbitrarily large. This indicates that the capacity of a wireless system with blind or group-blind multiuser receivers is not only interference-limited, but also estimation-error-limited. We then show that for both the blind and group-blind receivers, the output residual interference has an asymptotic Gaussian distribution, independent of the realizations of the spreading sequences. The Gaussianity indicates that in a large system, the bit error rate (BER) is related to the SINR simply through the Q function.

Abstract- A family of multiuser detectors is ana-lyzed which require neither matrix inversions nor oth-er operations with significant complexity. The time complexity per bit of most of them is independent of the number of users. Nevertheless, their spectral ef-ficiency for random spreading sequences is shown to be not far behind that of linear MMSE detection. I.

We investigate a sub-optimal reducedcomplexity iterative technique for joint detection and estimation for sets of constrained sequences and derive analytical results concerning convergence regions and xed points. We apply this theory to the problem of multiple-user decoding for multi-path fading channels.

| Based on correspondence between codedivision multiple-access (CDMA) multiuser detection problem and statistical mechanics, I present a novel analytical result on the average-case performance of CDMA multiuser detectors, in the large-system limit and under some simplifying assumptions, using tools borrowed from statistical mechanics. I.

The purpose of this paper is to review some important models and methods in receiver design for multiuser communication, and to outline some open problems of potential interest to researchers with a signal processing and information/communication theory background. We also present a new interference cancellation technique that permits operation in an oversaturated regime, in which the number of users exceeds the number of signaling dimensions.