A decorrelating detector for use with antenna arrays

This paper addresses the problem of synchronous CDMA communications when, due to bandwidth limitations, signature waveforms have relatively large correlations. A receiver which uses the multiple received signals available from an antenna array is proposed and its performance analyzed. This receiver may be interpreted as a multichannel extension of the decorrelating detector and therefore shares its immunity to near-far effects. The receiver uses the discrimination afforded by the antenna array to effectively reduce the correlation between users' transmitted signals thereby avoiding the reduction in performance which occurs in the single channel decorrelator when signature correlations become significant.This work was supported by the National Science Foundation under Grant MIP-9202081. This paper was presented in part at the International Conference on Communications, ICC'94, New Orleans, Louisiana.     

A decorrelating RAKE receiver for CDMA communications overfrequency-selective fading channels

Previous studies showed that multiuser detection in code-division multiple-access (CDMA) communications can be performed without explicit knowledge of users' channel characteristics in a frequency-selective fading environment. However, the computations of these blind approaches are an order of magnitude higher than existing adaptive minimum output energy (MOE) receivers which require at least knowledge of the desired user's channel response. Although the high-complexity problem can be alleviated by constrained adaptive filtering, the tradeoff is a significant drop in receiver performance, especially when the multipath pattern is time varying. In this paper, we present an adaptive receiver for CDMA communications over frequency-selective, and possibly time-varying, wireless channels. A salient feature of the new receiver is that it has complexity and performance comparable to that of the well-known MOE receivers, and yet requires no knowledge of the desired user's channel characteristics     

A receiver of simple structure for antenna array CDMA systems

In code division multiple access systems, the two-dimensional (2-D) RAKE structure had been proposed using multiple antennas to increase the capacity. It consists of multiple fingers of a beamformer and a correlator. In this paper, we consider a simpler receiver structure. We only use one finger, which consists of one pair of spatial and temporal filters to combine signals. It is shown that the performance of the receiver system is comparable with that of the 2-D RAKE receiver. Furthermore, it is observed that the proposed receiver and the 2-D RAKE receiver have limited near-far resistance by the generation of the space domain due to antenna arrays     

A semiblind method for transmit antenna arrays in CDMA systems

Using a transmit antenna array (TAA), the downlink beamforming is able to improve the performance of wireless communication systems. We propose a semiblind downlink beamforming method based on channel reciprocity of the wireless channels for code-division multiple-access (CDMA) systems equipped with TAA. In the proposed semiblind method, it is shown that the feedback delay can result in performance degradation under a time-varying fading channel environment. Hence, the performance of the semiblind method is satisfactory only when the maximum Doppler frequency is low or the feedback delay is not significant. To overcome this difficulty, the channel prediction is considered. Using the channel predictor, the semiblind method can provide a reasonable performance for a moderate maximum Doppler frequency (about a few hundred hertz). For theoretical performance analysis, upper, and lower bounds of the bit error probability under a fading channel environment are derived     

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     

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.     

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     

Improved base station circular antenna array receiver for wireless CDMA systems

A Spatial interpolation technique for the upsampling of uniform circular arrays (UCAs), beyond spatial aliasing, is considered. The UCA interpolation algorithm is used as a preprocessing procedure in the reverse link to improve the performance of a cellular code division multiple access (CDMA) system. The motivation is to enhance the system's capacity and array resolution and reduce the fading and coupling effects with minimal receiver hardware and cost. At the base station, we propose to place the antenna elements uniformly on an imaginary circumference farther apart than/2, where is the free-space wavelength. The resulting UCA possesses superior resolution, less coupling effects than a corresponding UCA with the same number of elements and circumferential spacing less than or equal to/2, and possibly higher diversity gain. In order to increase the system's capacity and eliminate any spatial aliasing, we propose to interpolate the UCA to within the spatial Nyquist rate. This is achieved by placing a virtual antenna element halfway on the circumference between every two adjacent antenna elements. Simulations results are provided to support our claims.     

A linear receiver for direct-sequence spread-spectrummultiple-access systems with antenna arrays and blind adaptation

A linear receiver for direct-sequence spread-spectrum multiple-access communication systems under the aperiodic random sequence model is considered. The receiver consists of the conventional matched filter followed by a tapped delay line with the provision of incorporating the use of antenna arrays. It has the ability of suppressing multiple-access interference (MAI) and narrowband interference in some weighted proportions, as well as combining multipath components without explicit estimation of any channel conditions. Under some specific simplified channel models, the receiver reduces to the minimum variance distortionless response beamformer, the RAKE receiver, a notch filter, or an MAI suppressor. The interference rejection capability is made possible through a suitable choice of weights in the tapped delay line. The optimal weights can be obtained by straightforward but computationally complex eigenanalysis. In order to reduce the computational complexity, a simple blind adaptive algorithm is also developed     

Adaptive antenna arrays for the reverse link of CDMA cellularcommunication systems

The use of an antenna array in the reverse link of cellular communication systems is proposed. A recursive least squares (RLS) algorithm is used to adjust the weight coefficients of the array. The improvement in performance is quantified by comparison with the omnidirectional antenna system using realistic simulations     

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 multi‐user CDMA receiver utilizing decorrelating detector with additional dummy pilot response

A pilot assisted CDMA system which uses extended spreading sequences with guard sequences under a quasi‐synchronous condition is capable of separating the interference components included in the input of a de‐correlating receiver by solving a system of linear equations. The performance of such a system, however, depends on the property of the de‐correlating matrix consisting of the received pilots, which correspond to the respective user spreading sequences and the multi‐path channel conditions. That is, the regularity of the matrix often tends to degrade, and the rank reduces occasionally primarily due to the multi‐paths, resulting in solutions that are vulnerable to AWGN. The present paper proposes an effective technique to solve this problem by introducing a virtual user into a group of real users that are to be served. The simulation results indicate a remarkable improvement in the bit‐error‐rate (BER) performance. In addition, based on the BER performance, the system has a RAKE‐like function that has power‐sum characteristics. Copyright © 2003 John Wiley & Sons, Ltd.     

Near-far resistant channel estimation for CDMA systems using thelinear decorrelating detector

The well-known linear decorrelating detector (LDD) for direct-sequence code-division multiple-access (CDMA) systems provides near-far resistant performance when the timing of each user is accurately known. Traditional CDMA acquisition techniques suffer from high differences in power levels. The estimation accuracy for a user overwhelmed by stronger ones is likely to be unsatisfactory; at the same time, the interference from a user undergoing acquisition or tracking is not removed by the standard LDD. In this paper, a fully near-far resistant technique for acquisition and tracking for asynchronous CDMA systems applying the LDD is proposed, considering realistic band-limited signals. This technique is based on the adoption of a pair of special sequences equivalent to a dedicated access channel and is shown to provide a relatively fast and robust means to perform channel estimation both in case of single- and multipath channels     

Adaptive MMSE receiver for multirate CDMA systems

We consider the adaptive receiver for multirate code division multiple access (CDMA) systems under a fading channel environment. The main difficulty that arises in the use of the adaptive receiver for multirate CDMA systems is that the adaptation should resume after the rate change. Hence, the adaptive receiver may not provide a reasonable performance during the transient after the rate change. In order to overcome this difficulty, we investigate an approach that allows updating the weight vectors for all rates simultaneously. For example, in a dual-rate system, the weight vector for the lower rate (the higher rate) can be updated during the period of the higher rate (resp., the lower rate) to avoid the transient after the rate change. The resulting adaptive receiver has multiple parallel adaptive filters. The adaptive filters for each rate can carry out the adaptation simultaneously, regardless of what the current rate is. As a result, the performance is not degraded by the rate change.     

Adaptive receiver structures for asynchronous CDMA systems

Adaptive linear and decision feedback receiver structures for coherent demodulation in asynchronous code division multiple access (CDMA) systems are considered. It is assumed that the adaptive receiver has no knowledge of the signature waveforms and timing of other users. The receiver is trained by a known training sequence prior to data transmission and continuously adjusted by an adaptive algorithm during data transmission. The proposed linear receiver is as simple as a standard single-user detector receiver consisting of a matched filter with constant coefficients, but achieves essential advantages with respect to timing recovery, multiple access interference elimination, near/far effect, narrowband and frequency-selective fading interference suppression, and user privacy. An adaptive centralized decision feedback receiver has the same advantages of the linear receiver but, in addition, achieves a further improvement in multiple access interference cancellation at the expense of higher complexity. The proposed receiver structures are tested by simulation over a channel with multipath propagation, multiple access interference, narrowband interference, and additive white Gaussian noise     

An adaptive multiuser receiver for CDMA systems

A new real-time, digital adaptive multiuser receiver structure is proposed for the uplink in a mobile communications system employing code division multiple access (CDMA). The receiver efficiently implements the decorrelating detector of Lupas and Verdu (1989) and can be adapted to incorporate decision feedback to further improve the detector performance. While the basic receiver design is presented for synchronous CDMA over AWGN channels, experimental evaluation of the receiver for the asynchronous case verifies its robustness for cases when the relative user delays are small compared to the symbol duration as in microcellular scenarios. An efficient decorrelating RAKE combiner for frequency-selective multipath channels is also proposed and analyzed. Performance evaluation of the detector via computer simulation scenarios is conducted in support of analytical results to substantiate its potential for real-time operation     

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 sliding window decorrelating receiver for multiuser DS-CDMAmobile radio networks

We detail the development of a near-far (NF) resistant sliding window decorrelating algorithm (SLWA) that overcomes the near-far problem (NFP) pertaining to the conventional linear correlation receiver (CLCR) and alleviates some of the limitations of the standard decorrelator. The SLWA architecture is extended to incorporate differentially coherent multipath (RAKE) diversity combining techniques and hence achieves simultaneous rejection of multiuser and multipath interference. This paper also presents a novel algorithm for updating the decorrelator coefficients using a fully parallel architecture. We present a mathematical model for the performance of a sliding window scheme, the main contribution of which is the analysis of finite sequence length multipath decorrelation under the practical limitation of incomplete RAKE combining. In addition to numerical results, we present results pertaining to capacity improvements achieved over the CLCR, derived from a simulation of a multiuser direct-sequence code-division multiple-access (DS-CDMA) mobile radio system     

Achieving high-capacity wireless by merging multicarrier CDMA systems and oscillating-beam smart antenna arrays

We establish the network capacity (measured in terms of number of users) of a wireless system merging multicarrier code-division multiple-access (MC-CDMA) and smart antennas with oscillating-beam patterns. The MC-CDMA component supports high performance (in a probability-of-error sense) via frequency diversity and high network capacity via code division. The smart antenna with oscillating-beam pattern further enhances performance via transmit diversity (in the form of an induced time diversity) and further enhances network capacity via spatial division. The proposed merger has been shown to achieve a very high performance by exploiting a two-dimensional time-frequency diversity. We demonstrate the impressive network capacity gains achieved by this merger.