A two-stage adaptive multiuser detector in an additive white Gaussian noise code-division multiple-access channel is proposed and analyzed. Its first stage is an asynchronous one-shot decorrelator which in terms of computational complexity only requires inversion of K symmetric K/spl times/K matrices for all K users. In addition, the K inversions can be done in parallel, and the computed results for one user can be reused by all other users as well, resulting in a latency of only one bit, same as its synchronous counterpart. The decorrelated tentative decisions are utilized to estimate and subtract multiple-access interference in the second stage. Another novel feature of the detector is the adaptive manner in which the multiple-access interference estimates are formed, which renders prior estimation of the received signal amplitudes and the use of training sequences unnecessary. Adaptation algorithms considered include steepest descent (as well as its stochastic version), and a recursive least squares-type algorithm that offers a faster transient response and better error performance. Sufficient conditions for the receiver to achieve convergence are derived. The detector is near-far resistant, and is shown to provide substantial steady-state error performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals.

This paper describes a differentially coherent decorrelating detector for a K-user reverse link code-division multiple-access environment that exhibits time-varying Rayleigh fading. The channel is modeled as providing only a single fading path for each user and with no additional means to achieve diversity. The design of the detector is based on using fractionally sampled matched filter outputs to simultaneously achieve two goals: 1) the novel realization of a one shot decorrelator with lower computational complexity and 2) the forming of the maximum-likelihood decision rule on the decorrelated outputs, which results in an effective increase of the correlation in the fading process. Analytical evaluation and simulation of the error probability of the detector demonstrates significant lowering of the error floor in comparison to the decorrelating detector that employs conventional differentially coherent detection.

A coherent multiuser decorrelating detector for an asynchronous CDMA, time-varying Rayleigh fading channel is proposed and analyzed. The detector employs fractionally sampled correlators' outputs at time instants corresponding to users' relative delays to simultaneously achieve two goals: the novel realization of a one-shot decorrelator with lower computational complexity; and to exploit a form of the time diversity for improved error performance compared to symbol spaced sampling. The decision statistics are formed from the decorrelator output according to the maximal ratio combining rule. The proposed detector performance is compared to that of a conventional symbol-spaced receiver in a single-user environment, and the impact of the channel estimation error is illustrated.

A thorough investigation on the convergence and stability of an adaptive synchronous CDMA receiver is presented. The receiver consists of a decorrelator at the first stage and an adaptive interference canceler at the second stage. By using a steepest descent algorithm that minimizes the output signal energy to adaptively control the weights, neither the knowledge of the users' received amplitudes nor the use of training sequences is required. The system is near-far resistant, and its error performance approaches the single-user bound when the interferers' SNRs are high. Sufficient conditions for the receiver to achieve convergence are derived, and their properties are analyzed.

For personal, indoor, and mobile wireless radio communications (PIMRC) uplink multiuser CDMA channels, received signals are time asynchronous, and suffer from amplitude fading. The relative signals' bit delays, energy variations, and hence the cross-correlation matrix of the received signal, are unknown. Using a novel adaptive algorithm, we propose in this paper a decorrelating detector suitable for particular CDMA uplink channels. The proposed asynchronous CDMA adaptive decorrelator consists of a bank of matched filters (matched to the users' signature sequences), followed by a sampler at the bit-rate of 1/T and controllable weights. It is near-far resistant, and can be used by itself if signal-to-noise ratios are high, or as a first stage in the two-stage adaptive canceller previously proposed for the synchronous CDMA case.

A multiuser detector in an additive white Gaussian noise, code division multiple access channel is proposed and analyzed. The detector utilizes the decorrelated tentative decisions obtained in the first stage to estimate and subtract multiple access interference in the second stage. The novel feature of the detector is the adaptive manner in which the multiple access interference estimates are formed, which renders prior estimation of the received signal amplitudes unnecessary. The detector is near-far resistant, and is shown to provide substantial performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals.