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191
Iterative (turbo) soft interference cancellation and decoding for coded CDMA
 IEEE Trans. Commun
, 1999
"... Abstract — The presence of both multipleaccess interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath codedivision multipleaccess (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuse ..."
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Cited by 446 (18 self)
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Abstract — The presence of both multipleaccess interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath codedivision multipleaccess (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DSCDMA system. The receiver performs two successive softoutput decisions, achieved by a softinput softoutput (SISO) multiuser detector and a bank of singleuser SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in Turbo decoding. Given the multipath CDMA channel model, a direct implementation of a slidingwindow SISO multiuser detector has a prohibitive computational complexity. A lowcomplexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum meansquare error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed lowcomplexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signaltonoise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and singleuser performance can be approached. Index Terms — Coded CDMA, instantaneous MMSE filtering, multiuser detection, soft interference cancellation, Turbo processing.
Spectral Efficiency of CDMA with Random Spreading
 IEEE TRANS. INFORM. THEORY
, 1999
"... The CDMA channel with randomly and independently chosen spreading sequences accurately models the situation where pseudonoise sequences span many symbol periods. Furthermore, its analysis provides a comparison baseline for CDMA channels with deterministic signature waveforms spanning one symbol per ..."
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Cited by 312 (23 self)
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The CDMA channel with randomly and independently chosen spreading sequences accurately models the situation where pseudonoise sequences span many symbol periods. Furthermore, its analysis provides a comparison baseline for CDMA channels with deterministic signature waveforms spanning one symbol period. We analyze the spectral efficiency (total capacity per chip) as a function of the number of users, spreading gain, and signaltonoise ratio, and we quantify the loss in efficiency relative to an optimally chosen set of signature sequences and relative to multiaccess with no spreading. White Gaussian background noise and equalpower synchronous users are assumed. The following receivers are analyzed: a) optimal joint processing, b) singleuser matched filtering, c) decorrelation, and d) MMSE linear processing.
Sparse solution of underdetermined linear equations by stagewise orthogonal matching pursuit
, 2006
"... Finding the sparsest solution to underdetermined systems of linear equations y = Φx is NPhard in general. We show here that for systems with ‘typical’/‘random ’ Φ, a good approximation to the sparsest solution is obtained by applying a fixed number of standard operations from linear algebra. Our pr ..."
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Cited by 278 (23 self)
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Finding the sparsest solution to underdetermined systems of linear equations y = Φx is NPhard in general. We show here that for systems with ‘typical’/‘random ’ Φ, a good approximation to the sparsest solution is obtained by applying a fixed number of standard operations from linear algebra. Our proposal, Stagewise Orthogonal Matching Pursuit (StOMP), successively transforms the signal into a negligible residual. Starting with initial residual r0 = y, at the sth stage it forms the ‘matched filter ’ Φ T rs−1, identifies all coordinates with amplitudes exceeding a speciallychosen threshold, solves a leastsquares problem using the selected coordinates, and subtracts the leastsquares fit, producing a new residual. After a fixed number of stages (e.g. 10), it stops. In contrast to Orthogonal Matching Pursuit (OMP), many coefficients can enter the model at each stage in StOMP while only one enters per stage in OMP; and StOMP takes a fixed number of stages (e.g. 10), while OMP can take many (e.g. n). StOMP runs much faster than competing proposals for sparse solutions, such as ℓ1 minimization and OMP, and so is attractive for solving largescale problems. We use phase diagrams to compare algorithm performance. The problem of recovering a ksparse vector x0 from (y, Φ) where Φ is random n × N and y = Φx0 is represented by a point (n/N, k/n)
Blind Adaptive Interference Suppression For DirectSequence CDMA
 IEEE TRANS. COMMUN
, 1994
"... Direct Sequence (DS) Code Division Multiple Access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity ..."
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Cited by 82 (7 self)
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Direct Sequence (DS) Code Division Multiple Access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity
An energyefficient approach to power control and receiver design in wireless networks
 IEEE Trans. Commun
, 2005
"... ..."
Decision Feedback Multiuser Detection: A Systematic Approach
, 1999
"... A systematic approach to decision feedback multiuser detection is introduced for the joint detection of symbols of K simultaneously transmitting users of a synchronous correlated waveform multipleaccess (CWMA) channel with Gaussian noise. A new performance criterion called symmetric energy is defi ..."
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Cited by 69 (11 self)
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A systematic approach to decision feedback multiuser detection is introduced for the joint detection of symbols of K simultaneously transmitting users of a synchronous correlated waveform multipleaccess (CWMA) channel with Gaussian noise. A new performance criterion called symmetric energy is defined which is a lownoise indicator of the joint error rate that at least one user is detected erroneously. Even the best linear detectors can perform poorly in terms of symmetric energy compared to the maximumlikelihood detector. A general class of decision feedback detectors is defined with O(K) implementational complexity per user. The symmetric energy of arbitrary DFD's and bounds on their asymptotic effective energy (AEE) performance are obtained along with an exact biterror rate and AEE analysis for the decorrelating DFD. The optimum DFD that maximizes symmetric energy is obtained. Each one of the optimum, decorrelating, and conventional DFD's, that correspond to the orders in which the users can be detected, are shown to outperform the linear optimum, decorrelating, and conventional detectors, respectively, in terms of symmetric energy. Moreover, algorithms are obtained for determining the choice of order of detection for the three DFD's which guarantee that they uniformly (userwise) outperform their linear counterparts. In addition to optimality in symmetric energy, it is also shown that under certain conditions, the optimum DFD achieves the AEE performance of the exponentially complex maximumlikelihood detector for all users simultaneously. None of the results of this paper make the perfect feedback assumption. The implications of our work on power control for multiuser detection are also discussed.
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.
Downlink capacity of interferencelimited MIMO systems with joint detection
 IEEE Trans. Wireless Commun
, 2004
"... The capacity of downlink cellular multipleinput multipleoutput (MIMO) cellular systems, where cochannel interference is the dominant channel impairment, is investigated in this paper, mainly from a signalprocessing perspective. Turbo spacetime multiuser detection (ST MUD) is employed for intrac ..."
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Cited by 62 (7 self)
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The capacity of downlink cellular multipleinput multipleoutput (MIMO) cellular systems, where cochannel interference is the dominant channel impairment, is investigated in this paper, mainly from a signalprocessing perspective. Turbo spacetime multiuser detection (ST MUD) is employed for intracell communications, and is shown to closely approach the ultimate capacity limits in Gaussian ambient noise for an isolated cell. Then it is combined with various multiuser detection methods for combating intercell interference. Among various multiuser detection techniques examined, linear minimummeansquareerror (MMSE) MUD and successive interference cancellation are shown to be feasible and effective. Based on these two multiuser detection schemes, one of which may outperform the other for different settings, an adaptive detection scheme is developed, which together with a Turbo ST MUD structure offers substantial performance gain over the well known VBLAST techniques with coding in this interferencelimited cellular environment. The obtained multiuser capacity is excellent in high to medium signaltointerference ratio scenario. Nonetheless, numerical results also indicate that a further increase in system complexity, using basestation cooperation, could lead to further significant increases of the system capacity. The asymptotic multicell MIMO capacity with linear MMSE MUD preprocessing is also derived, and this analysis agrees well with the simulation results.
Asymptotic normality of linear multiuser receiver outputs
 IEEE TRANS. INFORM. THEORY
, 2002
"... This paper proves largesystem asymptotic normality of the output of a family of linear multiuser receivers that can be arbitrarily well approximated by polynomial receivers. This family of receivers encompasses the singleuser matched filter, the decorrelator, the minimum mean square error (MMSE) ..."
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Cited by 47 (6 self)
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This paper proves largesystem asymptotic normality of the output of a family of linear multiuser receivers that can be arbitrarily well approximated by polynomial receivers. This family of receivers encompasses the singleuser matched filter, the decorrelator, the minimum mean square error (MMSE) receiver, the parallel interference cancelers, and many other linear receivers of interest. Both with and without the assumption of perfect power control, we show that the output decision statistic for each user converges to a Gaussian random variable in distribution as the number of users and the spreading factor both tend to infinity with their ratio fixed. Analysis reveals that the distribution conditioned on almost all spreading sequences converges to the same distribution, which is also the unconditional distribution. This normality principle allows the system performance, e.g., the multiuser efficiency, to be completely determined by the output signaltointerference ratio (SIR) for large linear systems.
Support Vector Machine Multiuser Receiver for DSCDMA Signals in Multipath Channels
 IEEE Trans. Neural Networks
, 2000
"... The problem of constructing an adaptive multiuser detector (MUD) is considered for directsequence codedivision multipleaccess (DSCDMA) signals transmitted through multipath channels. The emerging learning technique, called support vector machines (SVMs), is proposed as a method of obtaining a no ..."
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Cited by 40 (14 self)
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The problem of constructing an adaptive multiuser detector (MUD) is considered for directsequence codedivision multipleaccess (DSCDMA) signals transmitted through multipath channels. The emerging learning technique, called support vector machines (SVMs), is proposed as a method of obtaining a nonlinear MUD from a relatively small training data block. Computer simulation is used to study this SVM MUD, and the results show that it can closely match the performance of the optimal Bayesian oneshot detector. Comparisons with an adaptive radial basis function (RBF) MUD trained by an unsupervised clustering algorithm are discussed.