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48
Large System Performance of Linear Multiuser Receivers in Multipath Fading Channels
 IEEE Trans. Inform. Theory
, 2000
"... A linear multiuser receiver for a particular user in a codedivision multipleaccess (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable cha ..."
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Cited by 68 (3 self)
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A linear multiuser receiver for a particular user in a codedivision multipleaccess (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. In this paper, we study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signaltointerference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant. We employ a random model for the spreading sequences and the limiting signaltointerference ratio expressions are independent of the actual signature sequences, depending only on the system loading and the channel statistics: background noise power, energy profile of resolvable multipaths, and channel coherence time. The effect of channel uncertainty on the performance of multiuser receivers is succinctly captured by the notion of effective interference.
A linear receiver for directsequence spreadspectrum multipleaccess systems with antenna arrays and blind adaptation
 IEEE Trans. Inform. Theory
, 1998
"... Abstract — A linear receiver for directsequence spreadspectrum multipleaccess 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 anten ..."
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Cited by 21 (4 self)
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Abstract — A linear receiver for directsequence spreadspectrum multipleaccess 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 multipleaccess 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. Index Terms—Adaptive signal detection, antenna arrays, codedivision multiaccess, interference suppression, pseudonoisecoded communication. I.
Noncoherent Multiuser Detection for CDMA Systems with Nonlinear Modulation: A NonBayesian Approach
 in Proc. IEEE Int. Symp. Information Theory
, 2001
"... This paper considers the problem of multiuser detection for a system in which each user employs nonlinear modulation, with an emphasis on noncoherent detection techniques which do not require knowledge of the users' channel parameters at the receiver. Our goals are to gain fundamental insight into t ..."
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Cited by 18 (6 self)
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This paper considers the problem of multiuser detection for a system in which each user employs nonlinear modulation, with an emphasis on noncoherent detection techniques which do not require knowledge of the users' channel parameters at the receiver. Our goals are to gain fundamental insight into the capabilities of multiuser detection in such a setting, and to provide practical algorithms that perform better than conventional matched filter reception. We begin by providing fundamental performance benchmarks by considering coherent maximum likelihood (ML) detection, which requires knowledge of the users' channel parameters, as well as noncoherent detection, formulated in a nonBayesian generalized likelihood ratio test (GLRT) framework. The asymptotic performance of each detector, as the noise level vanishes, is characterized, yielding simple geometric criteria for nearfar resistance. In general, both the ML and GLRT detectors have complexity which is exponential in the number of users. We therefore propose the more practical sequential decision projection (SDP) detector, which has complexity which is quadratic in the number of users. It is shown that the SDP detector has nonzero asymptotic efficiency if the users' powers are suitably disparate.
MMSE interference suppression for timing acquisition and demodulation in directsequence CDMA systems
 IEEE Trans. Commun
, 1998
"... Abstract — It has been recently shown that minimummeansquarederror (MMSE) demodulators are effective means of interference suppression in codedivision multipleaccess (CDMA) systems. The MMSE demodulator can be implemented adaptively using an initial training sequence, followed by decisiondirect ..."
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Cited by 17 (4 self)
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Abstract — It has been recently shown that minimummeansquarederror (MMSE) demodulators are effective means of interference suppression in codedivision multipleaccess (CDMA) systems. The MMSE demodulator can be implemented adaptively using an initial training sequence, followed by decisiondirected adaptation. This requires that the symbollevel timing of the desired user be known prior to training. In this paper we remove this requirement by providing a method for timing acquisition in which the output of the acquisition process is a near–farresistant demodulator which automatically accounts for the delays and amplitudes of both the desired signal and the interference without explicitly estimating these parameters. The only requirements are a training sequence for the desired user and a finite uncertainty regarding the symbol timing. The latter condition can be realized by using a periodic training sequence even if the absolute timing uncertainty is arbitrarily large. Index Terms—CDMA, direct sequence, equalization, interference suppression, MMSE, near–far problem, timing acquisition, training. I.
On the Average Near–Far Resistance for MMSE Detection of Direct Sequence CDMA Signals with Random Spreading
, 1999
"... The performance of a near–farresistant, finitecomplexity, minimum mean squared error (MMSE) linear detector for demodulating direct sequence (DS) codedivision multiple access (CDMA) signals is studied, assuming that the users are assigned random signature sequences. We obtain tight upper and lowe ..."
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Cited by 14 (1 self)
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The performance of a near–farresistant, finitecomplexity, minimum mean squared error (MMSE) linear detector for demodulating direct sequence (DS) codedivision multiple access (CDMA) signals is studied, assuming that the users are assigned random signature sequences. We obtain tight upper and lower bounds on the expected near–far resistance of the MMSE detector, averaged over signature sequences and delays, as a function of the processing gain and the number of users. Since the MMSE detector is optimally near–farresistant, these bounds apply to any multiuser detector that uses the same observation interval and sampling rate. The lower bound on near–far resistance implies that, even without power control, linear multiuser detection provides near–farresistant performance for a number of users that grows linearly with the processing gain.
Performance of adaptive linear interference suppression for DSCDMA in the presence of flat Rayleigh fading
 Proc. IEEE VTC'97
, 1997
"... Abstract—Adaptive linear interference suppression for directsequence (DS) codedivision multiple access (CDMA) is studied in the presence of time and frequencyselective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decision ..."
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Cited by 14 (2 self)
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Abstract—Adaptive linear interference suppression for directsequence (DS) codedivision multiple access (CDMA) is studied in the presence of time and frequencyselective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decisiondirected and blind adaptive algorithms, which do not require a training sequence, are considered. Modifications to least squares adaptive algorithms are presented which are compatible with differential coding and detection. For frequencyselective fading, adaptive algorithms are presented based upon different assumptions concerning knowledge of the desired user’s channel. Specifically, the cases considered are as follows: 1) perfect knowledge of the desired channel; 2) knowledge of only the relative path delays; and 3) knowledge of only one delay corresponding to the strongest path. Computer simulation results are presented which compare the performance of these algorithms with the analogous RAKE receivers. These results show that for case 3), even slow fading can cause a significant degradation in performance. Effective use of channel parameters in the adaptive algorithm reduces the sensitivity to fade rate, although moderate to fast fading can significantly compromise the associated performance gain relative to the RAKE receiver. Index Terms—CDMA, differential detection, fading, interference suppression, multipath. I.
Performance analysis of MMSE receivers for DSCDMA in frequencyselective fading channels
 IEEE Trans. Commun
, 2000
"... Abstract—The performance of the minimum meansquared error (MMSE) receiver for detection of direct sequence code division multiple access is considered in various fading channel models. Several modifications to the basic MMSE receiver structure which have been recently proposed for use on nonselecti ..."
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Cited by 14 (2 self)
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Abstract—The performance of the minimum meansquared error (MMSE) receiver for detection of direct sequence code division multiple access is considered in various fading channel models. Several modifications to the basic MMSE receiver structure which have been recently proposed for use on nonselective fading channels are reviewed and shown to represent different approximations to a single common form. The performance of this general structure is analyzed as well as various extensions suitable for frequencyselective fading channels. Particular attention is given to the performance advantage gained through knowledge of the fading parameters of the various transmission paths of each user’s signal. It is shown that having this knowledge is not particularly useful on a flat fading channel unless the loading is very heavy and even then the difference in performance is only minimal. On the other hand, having this knowledge is crucial in a multipath fading channel and the inability to learn the fading channel parameters will lead to substantial degradation in capacity. A heuristic explanation to support this result based on a dimensionality argument is also presented. Index Terms—Codedivision multiple access, fading channels, multiuser channels, signal detection, spreadspectrum communication. I.
Robust Constrained Linear Receivers for CDMA Wireless Systems
 IEEE Trans. Sig. Proc. Available at http://www.galaxy.gmu.edu/∼kbell/ssaplab/tiansp2.pdf
, 2001
"... For codedivision multiple access (CDMA) communication systems, many constrained linear receivers have been developed to suppress multiple access interference. The linearly constrained formulations are generally sensitive to multipath fading and other types of signal mismatch. In this paper, we deve ..."
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Cited by 7 (1 self)
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For codedivision multiple access (CDMA) communication systems, many constrained linear receivers have been developed to suppress multiple access interference. The linearly constrained formulations are generally sensitive to multipath fading and other types of signal mismatch. In this paper, we develop robust linear receivers by exploring appropriate constraints. Multiple linear constraints are exploited to preserve the output energy that is scattered in multipath channels. In addition, a quadratic inequality constraint on the weight vector norm is used to improve robustness with respect to imprecise signal modeling. These constraints can be applied to the minimum output energy (MOE) detector to mitigate the signal mismatch problem and to the decision directed minimum mean square error (MMSE) detector to prevent error propagation and eliminate the need for training sequences at startup. Adaptive implementations for the proposed detectors are presented using recursive least square (RLS) updating in both the direct form and the partitioned linear interface canceller (PLIC) structure. Simulations are performed in a multipath propagation environment to illustrate the performance of the proposed detectors.
MinimumEntropy Blind Acquisition/Equalization for Uplink DSCDMA
 Allerton Conference
, 1998
"... : In this paper, we consider blind estimation of linear chipspaced receivers for the demodulation of a particular shortcode DSCDMA mobile user under multipath propagation and in the absence of timing information. We propose a family of schemes for blind acquisition and equalization based on Donoh ..."
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Cited by 6 (2 self)
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: In this paper, we consider blind estimation of linear chipspaced receivers for the demodulation of a particular shortcode DSCDMA mobile user under multipath propagation and in the absence of timing information. We propose a family of schemes for blind acquisition and equalization based on Donoho's Minimum Entropy principle and propose a specific algorithm that uses the second and fourthorder moments of a prewhitened chiprate received signal. The proposed algorithm can be considered a nearfar resistant initialization procedure for, and application of, the Constant Modulus Algorithm (CMA) to DSCDMA. 1 Introduction Direct sequence code division multiple access (DSCDMA) systems have received considerable attention as a flexible means of communication between multiple mobile users and centralized base stations. Since CDMA users share the same time and frequency resources, demodulation of a particular user is principally concerned with suppression of interference from other user...
Successive Interference Cancellation for DSCDMA Systems,” Submitted for
 IEEE Transactions on Communications
, 2000
"... Abstract—In this paper, we propose a blind successive interference cancellation receiver for asynchronous directsequence codedivision multipleaccess (DSCDMA) systems using a maximum mean energy (MME) optimization criterion. The covariance matrix of the received vector is used in conjunction with ..."
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Cited by 5 (2 self)
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Abstract—In this paper, we propose a blind successive interference cancellation receiver for asynchronous directsequence codedivision multipleaccess (DSCDMA) systems using a maximum mean energy (MME) optimization criterion. The covariance matrix of the received vector is used in conjunction with the MME criterion to realize a blind successive interference canceler that is referred to as the BICMME receiver. The receiver executes interference cancellation in a successive manner, starting with the most dominant interference component and successively cancelling the weaker ones. The receiver is compared against various centralized and decentralized receivers, and it is shown to perform well in the presence of estimation errors of the covariance matrix, making it suitable for application in timevarying channels. We also analyze properties of the covariance matrix estimates which are relevant to the performance of the BICMME receiver. Further, the BICMME receiver is particularly efficient in the presence of a few strong interferers as may be the case in the downlink of DSCDMA systems where intracell user transmissions are orthogonal. An iterative implementation that results in reduced complexity is also studied. Index Terms—Blind interference cancellation, CDMA downlink, successive interference cancellation. I.