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337
Output MAI Distributions of Linear MMSE Multiuser Receivers in DSCDMA Systems
 IEEE TRANS. INFORM. THEORY
, 2001
"... Multipleaccess interference (MAI) in a codedivision multipleaccess (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. In this paper, we study the behavior of the output MAI of the minimum meansquare error (MMSE) receiver employed in t ..."
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Cited by 64 (8 self)
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Multipleaccess interference (MAI) in a codedivision multipleaccess (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. In this paper, we study the behavior of the output MAI of the minimum meansquare error (MMSE) receiver employed in the uplink of a directsequence (DS)CDMA system. We focus on imperfect powercontrolled systems with random spreading, and establish that in a synchronous system 1) the output MAI of the MMSE receiver is asymptotically Gaussian, and 2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of singleuser Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signaltointerference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system.
adaptive interference suppression for near–far resistant acquisition and demodulation of DSCDMA signals
 IEEE Trans. Signal Processing
, 1997
"... Abstract — Two key operations required of a receiver in a directsequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of ..."
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Cited by 52 (6 self)
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Abstract — Two key operations required of a receiver in a directsequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of both these operations is limited by multipleaccess interference, especially for conventional matched filterbased methods, whose performance displays an interference floor and is vulnerable to the nearfar problem. Recent work has shown that, provided timing information is available for a given transmission, it can be demodulated reliably using blind or trainingsequencebased adaptive interference suppression techniques. These techniques are nearfar resistant, unlike the matched filter demodulator, and do not require explicit knowledge of the interference parameters, unlike nonadaptive multiuser detectors. In this paper, we present a blind adaptive interference suppression technique for joint acquisition and demodulation, which has the unique feature that the output of the acquisition process is not simply the timing of the desired transmission, but a nearfar resistant demodulator that implicitly accounts for knowledge of the timing and amplitudes of all transmissions to suppress the multipleaccess interference. The only knowledge required by the scheme is that of the desired transmission’s signature sequence, so that it is amenable to a decentralized implementation. On the other hand, it can be efficiently implemented as a centralized scheme in which the bulk of the computations for the adaptation are common to all transmissions that need to be acquired or demodulated. I.
Soft Handoffs in CDMA Mobile Systems
, 1997
"... This article presents an overview of soft handoff, an idea which is becoming quite important because of its use in the IS95 codedivision multiple access (CDMA) cellular phone standard. The benefits and disadvantages of using soft handoff over hard handoff are discussed, with most results drawn fr ..."
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Cited by 49 (1 self)
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This article presents an overview of soft handoff, an idea which is becoming quite important because of its use in the IS95 codedivision multiple access (CDMA) cellular phone standard. The benefits and disadvantages of using soft handoff over hard handoff are discussed, with most results drawn from the available literature. The two most wellknown benefits are fade margin improvement and higher uplink capacity, while disadvantages include increased downlink interference and more complex implementation. Handoff parameter optimization is extremely important, so various studies on the tradeoffs to be considered when selecting these parameters are surveyed, from both the link quality and resource allocation perspectives. Finally, research directions and future trends are discussed.
Adaptive power control and MMSE interference suppression
 Wireless Networks
, 1998
"... Power control algorithms assume that the receiver structure is fixed and iteratively update the transmit powers of the users to provide acceptable quality of service while minimizing the total transmitter power. Multiuser detection, on the other hand, optimizes the receiver structure with the assump ..."
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Cited by 48 (4 self)
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Power control algorithms assume that the receiver structure is fixed and iteratively update the transmit powers of the users to provide acceptable quality of service while minimizing the total transmitter power. Multiuser detection, on the other hand, optimizes the receiver structure with the assumption that the users have fixed transmitter powers. In this study, we combine the two approaches and propose an iterative and distributed power control algorithm which iteratively updates the transmitter powers and receiver filter coefficients of the users. We show that the algorithm converges to a minimum power solution for the powers, and an MMSE multiuser detector for the filter coefficients. 1.
Algebraic Methods for Deterministic Blind Beamforming
, 1998
"... Deterministic blind beamforming algorithms try to separate superpositions of source signals impinging on a phased antenna array by using deterministic properties of the signals or the channels such as their constant modulus or directionsofarrival. Progress in this area has been abundant over the p ..."
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Cited by 39 (6 self)
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Deterministic blind beamforming algorithms try to separate superpositions of source signals impinging on a phased antenna array by using deterministic properties of the signals or the channels such as their constant modulus or directionsofarrival. Progress in this area has been abundant over the past ten years and has resulted in several powerful algorithms. Unlike optimal or adaptive methods, the algebraic methods discussed in this review act on a fixed block of data and give closedform expressions for beamformers by focusing on algebraic structures. This typically leads to subspace estimation and generalized eigenvalue problems. After introducing a simple and widely used multipath channel model, the paper provides an anthology of properties that are available, and generic algorithms that exploit them.
Improved robust multiuser detection in nonGaussian channels
 IEEE Signal Processing Lett
, 2001
"... Abstract — In many wireless systems where multiuser detection techniques may be applied, the ambient channel noise is known through experimental measurements to be decidedly nonGaussian, due largely to impulsive phenomena. The performance of many multiuser detectors can degrade substantially in the ..."
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Cited by 35 (3 self)
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Abstract — In many wireless systems where multiuser detection techniques may be applied, the ambient channel noise is known through experimental measurements to be decidedly nonGaussian, due largely to impulsive phenomena. The performance of many multiuser detectors can degrade substantially in the presence of such impulsive ambient noise. In this paper, we develop robust multiuser detection techniques for combating multipleaccess interference and impulsive noise in CDMA communication systems. These techniques are based on the Mestimation method for robust regression. Analytical and simulation results show that the proposed robust techniques offer significant performance gain over linear multiuser detectors in impulsive noise, with little attendant increase in computational complexity. We also develop a subspacebased technique for blind adaptive implementation of the robust multiuser detectors, which requires only the signature waveform and the timing of the desired user in order to demodulate that user’s signal. The robust multiuser detection technique and its blind adaptive version can be applied to both synchronous and asynchronous CDMA channels. Index Terms—Impulsive noise, Mestimation, multiuser detection, signal subspace tracking.
Onoff random access channels: A compressed sensing framework
, 2009
"... This paper considers a simple on–off random multiple access channel, where n users communicate simultaneously to a single receiver over m degrees of freedom. Each user transmits with probability λ, where typically λn < m ≪ n, and the receiver must detect which users transmitted. We show that whe ..."
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Cited by 30 (7 self)
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This paper considers a simple on–off random multiple access channel, where n users communicate simultaneously to a single receiver over m degrees of freedom. Each user transmits with probability λ, where typically λn < m ≪ n, and the receiver must detect which users transmitted. We show that when the codebook has i.i.d. Gaussian entries, detecting which users transmitted is mathematically equivalent to a certain sparsity detection problem considered in compressed sensing. Using recent sparsity results, we derive upper and lower bounds on the capacities of these channels. We show that common sparsity detection algorithms, such as lasso and orthogonal matching pursuit (OMP), can be used as tractable multiuser detection schemes and have significantly better performance than singleuser detection. These methods do achieve some near–far resistance but—at high signaltonoise ratios (SNRs)—may achieve capacities far below optimal maximum likelihood detection. We then present a new algorithm, called sequential OMP, that illustrates that iterative detection combined with power ordering or power shaping can significantly improve the high SNR performance. Sequential OMP is analogous to successive interference cancellation in the classic multiple access channel. Our results thereby provide insight into the roles of power control and multiuser detection on randomaccess signalling.
Ensuring Convergence of the MMSE Iteration for Interference Avoidance to the Global Optimum
, 2003
"... Viswanath and Anantharam [1] characterize the sum capacity of multiaccess vector channels. For a given number of users, received powers, spreading gain, and noise covariance matrix in a codedivision multipleaccess (CDMA) system, the authors of [1] present a combinatorial algorithm to generate a se ..."
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Cited by 29 (1 self)
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Viswanath and Anantharam [1] characterize the sum capacity of multiaccess vector channels. For a given number of users, received powers, spreading gain, and noise covariance matrix in a codedivision multipleaccess (CDMA) system, the authors of [1] present a combinatorial algorithm to generate a set of signature sequences that achieves the maximum sum capacity. These sets also minimize a performance measure called generalized total square correlation (TSC ).
RealTime Algorithms And Architectures For Multiuser Channel Estimation And Detection In Wireless BaseStation Receivers
 in Wireless Basestation Receivers,” Submitted to IEEE Journal in Selected Areas in Communication (JSAC
, 2002
"... This paper presents efficient algorithms and architecture designs that can meet realtime requirements of multiuser channel estimation and detection in future wireless basestation receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their realti ..."
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Cited by 29 (14 self)
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This paper presents efficient algorithms and architecture designs that can meet realtime requirements of multiuser channel estimation and detection in future wireless basestation receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their realtime implementation difficult on current Digital Signal Processor (DSP)based receivers. A maximumlikelihood based multiuser channel estimation scheme requiring matrix inversions is redesigned from an implementation perspective for a reduced complexity, iterative scheme with a simple fixedpoint VLSI architecture. A reducedcomplexity, bitstreaming multiuser detection algorithm that avoids the need for multishot detection is also developed for a simple, pipelined VLSI architecture. Thus, we show that realtime solutions, with 34 orders of magnitude performance improvements over DSPs, can be achieved for next generation wireless systems by (1) designing the algorithms from an implementation ...
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.