Fuzzy decorrelating detector for non-Gaussian CDMA channel

A fuzzy decorrelating detector for CDMA non-Gaussian channels is proposed. The proposed scheme combines a linear decorrelator with a nonlinear preprocessor based on fuzzy logic and rank ordering that is incorporated to combat impulsive noise. Simulation results show that the proposed technique is very robust in combating impulsive noise.     

A blind adaptive decorrelating detector for CDMA systems

The decorrelating detector is known to eliminate multiaccess interference when the signature sequences of the users are linearly independent, at the cost of enhancing the Gaussian receiver noise. We present a blind adaptive decorrelating detector which is based on the observation of readily available statistics. The algorithm recursively updates the filter coefficients of a desired user by using the output of the current filter. Due to the randomness of the information bits transmitted and the ambient Gaussian channel noise, the filter coefficients evolve stochastically. We prove the convergence of the filter coefficients to a decorrelating detector in the mean squared error (MSE) sense. We develop lower and upper bounds on the MSE of the receiver filter from the convergence point and show that with a fixed step size sequence, the MSE can be made arbitrarily small by choosing a small enough step size. With a time-varying step size sequence, the MSE converges to zero implying an exact convergence. The proposed algorithm is distributed, in the sense that no information about the interfering users such as their signature sequences or power levels is needed. The algorithm requires the knowledge of only two parameters for the construction of the receiver filter of a desired user: the desired user's signature sequence and the variance of the additive white Gaussian (AWG) receiver noise. This detector, for an asynchronous code division multiple access (CDMA) channel, converges to the one-shot decorrelating detector     

A two-stage decorrelating detector for DS/CDMA systems

This paper presents an improved two-stage decorrelating detection scheme for direct-sequence code-division multiple-access (DS-CDMA) systems. Decorrelator outputs corresponding to each user are tested to determine whether or not a final decision can be made with high reliability. For users with high reliability, a hard decision is made at this stage. For the remaining users, a conditional quantization algorithm is employed before a decision is made. Results indicate that significant performance enhancement can be achieved at the expense of a modest increase in computational complexity. The algorithm is also applied to synchronous as well as asynchronous multipath channels with excellent results. We also extend the two-stage decorrelating detection scheme to time-variant communication scenarios and present an adaptive version of the two-stage decorrelator     

On multipath channel estimation for CDMA systems using multiplesensors

This paper focuses on the design of a multiuser receiver structure for the reverse link of a code-division multiple-access communication system, in the presence of multipath effects and using an antenna array at the base station receiver. The algorithm presented solves the complex multidimensional problem of channel estimation in this complex scenario using a maximum-likelihood approach. This channel estimation technique requires the transmission of a training sequence or feedback of detected data. Once a composite channel-impulse response of each user is estimated, it is directly used in the detection process instead of first extracting the individual channel parameters, such as path delays and attenuation factors. The paper presents a framework that facilitates a computationally efficient solution to the combined problem of channel estimation and detection in a scenario involving multiple users, multiple paths, and multiple sensors at the receiver     

Adaptive decorrelating detectors for CDMA systems

Multi-user detection allows for the efficient use of bandwidth in Code-Division Multiple-Access (CDMA) channels through mitigation of near-far effects and multiple-access noise limitations. Due to its inherent noise and multipath immunity, CDMA multi-access is being considered as a platform for personal communication systems (PCS). As CDMA based digital communication networks proliferate, the need to determine the presence of a new user and integrate knowledge of this new user into the detection scheme becomes more important. The decorrelating detector is a linear multi-user detector that is asymptotically optimal in terms of near far resistance; however, in the presence of a new unknown user, performance of the decorrelator is severely degraded. Adaptive decorrelators are constructed which adaptively augment an existing conventional decorrelator to demodulate a new active user in addition to existing users. Several likelihood ratio based schemes are employed. Both synchronous and asynchronous communication are investigated.This research was supported by the U.S. Army Research Office under Grant DAAH04-93-G-0219.     

Adaptive decorrelating detectors for CDMA systems

Multi-user detection allows for the efficient use of bandwidth in Code-Division Multiple-Access (CDMA) channels through mitigation of near-far effects and multiple-access noise limitations. Due to its inherent noise and multipath immunity, CDMA multi-access is being considered as a platform for personal communication systems (PCS). As CDMA based digital communication networks proliferate, the need to determine the presence of a new user and integrate knowledge of this new user into the detection scheme becomes more important. The decorrelating detector is a linear multi-user detector that is asymptotically optimal in terms of near far resistance; however, in the presence of a new unknown user, performance of the decorrelator is severely degraded. Adaptive decorrelators are constructed which adaptively augment an existing conventional decorrelator to demodulate a new active user in addition to existing users. Several likelihood ratio based schemes are employed. Both synchronous and asynchronous communication are investigated.This research was supported by the U.S. Army Research Office under Grant DAAH04-93-G-0219.     

Adaptive path selective fuzzy decorrelating detector under impulsive noise for multipath fading CDMA systems

This letter proposes a novel adaptive path selective fuzzy decorrelating (APSFD) detector for direct-sequence code- division multiple-access (DS-CDMA), systems with time-varying multipath fading channels under impulsive noise. The proposed detector combines adaptive path selective decorrelating (APSD) detector with a fuzzy median filter which is based on fuzzy rank ordering of samples to eliminate the effect of impulsive noise. Simulation results clearly show that the proposed detector eliminates the effect of impulsive noise and improves the performance of APSD detector under high impulsive noise.     

Joint decorrelating multiuser detection and channel estimation inasynchronous CDMA mobile communications channels

The asymptotic efficiencies of two decorrelators, path-by-path and channel-matched decorrelators, are analyzed in fading multipath propagation environments, and based upon the analytical results, a new joint multiuser detection and channel estimation scheme is proposed for asynchronous code division multiple access (CDMA) mobile communications channels. In the path-by-path decorrelator, each of the received signals corresponding to one of the multiple propagation paths is regarded as an independent interference source. On the contrary, in the channel-matched decorrelator, each composite signal transmitted from an identical user is regarded as a response of the multipath channel to the corresponding user's spreading sequence. The asymptotic efficiency of the path-by-path decorrelator is shown to drop rapidly as the number of simultaneous users increases. It is shown that the asymptotic efficiency can be made independent of the number of the propagation paths by the channel-matched decorrelator at the expense of requiring knowledge about the fading complex envelopes of all the propagation paths. The proposed joint multiuser detection and channel estimation scheme uses both path-by-path and channel-matched decorrelators. The path-by-path decorrelator is used for providing the channel estimator with the (noisy) channel information path-by-path, and decisions are made on the output of the channel-matched decorrelator. The decision results are fed back to the channel estimator, and used as the reference signals. The received complex envelope of each of the propagation paths is estimated in the channel estimator. Results of a series of exhaustive computer simulations are presented in order to demonstrate the overall performance of the proposed scheme, both in non-fading and fading multipath propagation environments     

Differentially coherent decorrelating detector for CDMA single-pathtime-varying Rayleigh fading channels

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     

Blind adaptive decorrelating detector for DS-CDMA systems:auxiliary-vector detector

A blind adaptive decorrelating detector is proposed which consists of two vectors: one is the pre-assigned spreading code of the desired user and the other is orthogonal to it. The orthogonal vector is adjusted adaptively to minimise the multiple access interference (MAI). This blind adaptive detector requires no more knowledge than does the conventional single-user receiver. Simulation results show that the obtained performance is close to that of the decorrelator     

RBF network assisted adaptive path selective decorrelating detector under impulsive noise for multipath fading CDMA systems

This paper proposes a novel radial basis function assisted adaptive path selective decorrelating detector for direct sequence code division multiple access systems with time-varying multipath fading channels under impulsive noise. The proposed detector combines adaptive path selective decorrelating detector (APSDD) with a radial basis function (RBF) network to eliminate the effect of impulsive noise. Simulation results clearly show that the proposed detector eliminates the effect of impulsive noise and improves the performance of APSDD under high impulsive noise.     

Blind channel estimation in multi-rate CDMA systems

Multi-rate CDMA is a potentially attractive multiple access method for future broad-band multimedia wireless networks that must support integrated voice/data traffic. The primary impairment for such multi-rate systems is the multipath nature of radio channels that results in intra-user inter-chip interference (ICI) and multi-user interference (MUI) between different users' symbols. Explicit knowledge of the channel is typically needed for high performance detectors (such as coherent demodulation). We propose a subspace method for channel estimation in multi-rate CDMA systems. A unified signal model that applies to three multi-rate CDMA schemes proposed in the literature is developed. The computational complexity for multi-rate scenarios is large and variable-accordingly, a modified approach is devised that offers performance/complexity trade-offs. Performance analysis is conducted based on a close-form expression for the mean square error of the estimator, supported by simulation results that investigate the effectiveness of our method     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

Blind channel estimation for long code multiuser CDMA systems

In CDMA systems with long codes, the users' signatures change in each bit period, impeding the estimation of the time-invariant multipath parameters. In this paper, correlation-matching methods are employed to blindly estimate those multipath parameters. For given code sequences, the output correlation matrix (parameterized by the unknown channel coefficients) is compared with its instantaneous approximation. By minimizing the Frobenius norm of the resulting error matrix, the channel parameters can be estimated up to a complex scalar ambiguity. Both batch and adaptive algorithms are derived. Under the assumption of i.i.d. code sequences, identifiability up to a complex scalar ambiguity for each channel is guaranteed, and the asymptotic convergence of the proposed algorithm is established. Furthermore, step-size selection for the adaptive version is investigated. When only the code sequence of the user of interest is available, a single user receiver is also derived. Simulation results verify those claims and provide comparisons with other methods     

DOA and channel parameter estimation for wideband CDMA systems

We consider the uplink of a direct-sequence code-division multiple-access system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters (path gains and delays) and the directions of arrival of the signal echoes from a user entering the network. We propose an estimator that operates in an iterative fashion and exploits knowledge of the transmitted symbols (training sequence). Compared to other existing schemes, it is simpler to implement as it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional searches. Computer simulations indicate that the proposed scheme is useful even in applications over rapidly varying channels where the training sequence must be short compared with the channel decorrelation time.     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

A spatial-temporal decorrelating receiver for CDMA systems withbase-station antenna arrays

We investigate multiuser signal detection with a base-station antenna array for a synchronous DS-CDMA uplink using nonorthogonal codes in Rayleigh fading channels. We have developed a new formulation for a spatial-temporal decorrelating detector using the maximum-likelihood criteria. The detector is shown to be near-far resistant. We propose to implement the spatial-temporal decorrelating receiver iteratively by applying the space-alternating generalized expectation-maximization (SAGE) algorithm. Simulation results show that the SAGE-based decorrelating receiver significantly outperforms the conventional single-user receiver and with performance close to that of a spatial-temporal decorrelating receiver with known channel parameters. We have observed that adding base-station antennas can actually improve convergence of the proposed iterative receiver     

Adaptive power techniques for blind channel estimation in CDMA systems

The problem of blind adaptive channel estimation in code-division multiple access (CDMA) systems is considered. Motivated by the iterative power method, which is used in numerical analysis for estimating singular values and singular vectors, we develop recursive least squares (RLS) and least mean squares (LMS) subspace-based adaptive algorithms in order to identify the impulse response of the multipath channel. The schemes proposed in this paper use only the spreading code of the user of interest and the received data and are therefore blind. Both versions (RLS and LMS) exhibit rapid convergence combined with low computational complexity. With the help of simulations, we demonstrate the improved performance of our methods as compared with the already-existing techniques in the literature.     

Blind multiuser channel estimation in asynchronous CDMA systems

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor     

Multi-user detector for multi-carrier CDMA systems

Blind multi-user detection is of special interest for multiple access interference (MAI) mitigation in code-division multiple-access (CDMA) systems since it is impractical to assume perfect knowledge of parameters such as spreading codes, time delays and amplitudes of all the users in a rapidly changing mobile environment. In [1], the blind decorrelating detector and the minimum mean square error (MMSE) detector were developed based on subspace estimation and derived in closed forms of the signal subspace components. To combat frequency-selective fading, the subspace approach to blind multi-user detection was extended to multi-carrier CDMA (MCCDMA) systems, e.g. in [2]. In this Letter, we develop further the algorithm presented in [1, 2], and show how the condition of the signal subspace can be improved by using not only the autocorrelation, but also the pseudo-autocorrelation of the received observations, leading to improved performance in blind multi-user detection.