1:00, SPCOM-P8.1
COMPUTATIONALLY EFFICIENT MULTIUSER DETECTION FOR APERIODICALLY SPREADING CDMA SYSTEMS
Z. XU
Multiuser CDMA systems with aperiodic spreading codes have received considerable attention. One major difficulty in multiuser detection is the user's time-varying signature. In a multipath communication environment, this signature is not a priori known, making direct design of the detector more intractable. In this paper, multiuser detection is performed by a two-step approach: estimate the unknown channel coefficients and construct the detector based on the MMSE criterion. We model the interference as colored Gaussian noise with unknown correlations and estimate the channel parameters based on the correlation matching idea. The spreading codes are assumed random with known up to the fourth order statistics, resulting in computationally efficient method. In the case of unknown statistics however, the method is still applicable by estimating those statistics from given codes. To guarantee identifiability, both the channel's output and the output of the matched filter for the desired user are considered in one cost function. Comparisons with previously proposed method and other existing methods are made by simulations.

1:00, SPCOM-P8.2
ADAPTIVE MINIMUM-BER LINEAR MULTIUSER DETECTION
S. CHEN, A. SAMINGAN, B. MULGREW, L. HANZO
An adaptive minimum bit error rate (MBER) linear multiuser detector (MUD) is proposed for DS-CDMA systems. Based on the approach of kernel density estimation for approximating the bit error rate (BER) from training data, a least mean squares (LMS) style adaptive algorithm is developed for training linear MUDs. Computer simulation results show that this adaptive MBER linear MUD outperforms an existing LMS-style adaptive MBER algorithm previously reported in Globecom'98 by Yeh, Lopes and Barry.

1:00, SPCOM-P8.3
IMPROVED CDMA MULTIUSER RECEIVERS ROBUST TO TIMING ERRORS
W. ZHA, S. BLOSTEIN
We investigate an improved multiuser detector for asynchronous CDMA uplink channels when there exist timing delay estimation errors. We first formulate a robust decorrelator with a capacity limit of 50%. To increase the system capacity, we further propose a robust successive interference cancellation (SIC) implementation. The proposed robust SIC detector adds only a residual error estimation procedure onto the standard SIC detector. Computer simulation results show that its performance and capacity are very close to those of the ideal decorrelating detector which assumes perfect knowledge of time delay.

1:00, SPCOM-P8.4
FSLE AND FSDF JOINT DETECTORS FOR LONG CODE DS-CDMA
F. HORLIN, T. SARTENAER, L. VANDENDORPE, L. VANDENDORPE
We investigate detection structures for a DS-CDMA multiuser uplink using codes with a period of several symbols, named long codes. We first propose a multirate description of the uplink. Then based on this formalism we revisit a number of popular receiver structures : rake receivers, minimum mean square error linear and decision feedback fractionally spaced detectors.

1:00, SPCOM-P8.5
A CLASS OF ASYMPTOTICALLY OPTIMUM ITERATED-DECISION MULTIUSER DETECTORS
A. CHAN, G. WORNELL
A promising class of nonlinear multiuser detectors is introduced for CDMA systems. These ``iterated-decision'' multiuser detectors use optimized multipass algorithms to successively cancel multiple-access interference (MAI) from received data and generate symbol decisions whose reliability increases monotonically with each iteration. They significantly outperform decorrelating detectors and linear minimum mean-square error (MMSE) multiuser detectors, but have the same order of computational complexity. When the ratio of the number of users to the spreading factor is below a certain threshold, iterated-decision multiuser detectors asymptotically achieve the performance of the ``optimum'' multiuser detector, i.e., maximum-likelihood (ML) decoding.

1:00, SPCOM-P8.6
MULTIUSER DETECTION IN FLAT RAYLEIGH FADING CHANNEL USING AN APPROXIMATE MMSE ALGORITHM
J. HU, R. BLUM
We study multiuser detection for synchronous DS-CDMA system in a flat Rayleigh fading channel. An approximate MMSE detector which is independent of the magnitude of the received signal is proposed. This detector is not only easier to implement but also easier to analyze than the exact MMSE detector. This is especially true for the case employing ML sequences. In this case we have developed a closed form expression for its performance. Numerical simulations for CDMA systems employing ML and random sequences are provided. These results demonstrate that the performance difference between the exact and approximate MMSE algorithms is very small which makes the later a very good approximation.

1:00, SPCOM-P8.7
MULTIUSER DETECTION OF SYNCHRONOUS CDMA SIGNALS BY THE GIBBS COUPLER
Y. HUANG, P. DJURIC
Code-division multiple-access (CDMA) is a multiplexing technique which has become a driving force behind the rapidly advancing communications industry. In order to recover transmitted signals at the receiver when CDMA is used, multiuser detection techniques are engaged. In the past, various techniques have been developed to approach the performance of the optimum multiuser detector, and in this paper we have the same objective. We propose a new multiuser detector which is developed under the Bayesian framework and implemented by a novel efficient perfect sampling algorithm called the Gibbs coupler. The simulation results demonstrate excellent performance of the proposed detector.

1:00, SPCOM-P8.8
SUBSPACE ADAPTIVE DETECTION FOR ASYNCHRONOUS MULTIUSER CDMA WITH FREQUENCY DOMAIN CHANNEL RESPONSE AND DELAY ESTIMATION
X. TANG, Y. BAR-NESS
Channel estimation, particularly delay estimation in asynchronous CDMA channel, is essential for most receivers. The correlated time domain data sequence will become asymptotically uncorrelated in frequency domain. By taking Discrete Fourier Transform (DFT) of the received signals, we can take advantage of this asymptotical property to simplify the estimation algorithm. The proposed scheme can jointly estimate the path delay and the complex channel response with lower computational complexity. Furthermore, having such estimates, it is possible to use adaptive subspace MOE detector, previously implemented in synchronous channels, for asynchronous multiuser CDMA. Using simulation, it was shown that this scheme can provide comparable performance to that obtained with Wiener filter.

1:00, SPCOM-P8.9
EFFECT OF IMPERFECT CHANNEL ESTIMATION ON SYNCHRONOUS MULTI-RATE DS/CDMA SYSTEMS WITH HIGH SPREADING FACTORS
J. FONOLLOSA, X. MESTRE
This paper analyzes the influence of channel estimation errors on the performance of linear multi-user receivers. Assuming randomized codes and asymptotically high spreading factors and noise power, we show that the performance of the decorrelating and the minimum mean squared error (MMSE) receivers tend to the same limit in terms of signal to noise ratio and bit error rate (but not in terms of mean squared error). Using these results and assuming Gaussian-distributed channel estimators, we derive two simple approximations to the bit error rate and compare them with the actual values via simulation.