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82
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 99 (6 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 27 (6 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 i ..."
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Cited by 21 (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.
Performance of adaptive linear interference suppression in the presence of dynamic fading
 Communications, IEEE transactionon
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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 18 (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.
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.
Subspace multiuser detection for multicarrier CDMA
 IEEE Trans. Commun
, 2000
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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 10 (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.
A Blind Frequency Domain Method For DsCdma Synchronization Using Antenna Arrays
, 1998
"... A blind frequency domain method for estimating the propagation delays of DSCDMA signals is presented. The algorithm is formulated using an antenna array, and uses the fact that the spatially and temporally correlated additive noise, consisting of both interfering users and thermal noise, will be as ..."
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Cited by 10 (2 self)
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A blind frequency domain method for estimating the propagation delays of DSCDMA signals is presented. The algorithm is formulated using an antenna array, and uses the fact that the spatially and temporally correlated additive noise, consisting of both interfering users and thermal noise, will be asymptotically uncorrelated on the FFT frequency grid. The algorithm is computationally simple, and can be efficiently implemented using the FFT algorithm. Simulations illustrate that nearfar resistance can be achieved using spatial diversity. 1. INTRODUCTION Directsequence code division multiple access (DSCDMA) is widely considered to be a promising technology for future wireless communication networks. Due to the nearfar problem for CDMA cellular systems in a multiuser environment, there has lately been an increased interest in robust timedelay estimation and blind adaptive interference suppression (see [1] and the many references therein). Nearfar resistant estimators are of great im...