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337
Linear Multiuser Receivers: Effective Interference, Effective Bandwidth and User Capacity
 IEEE Trans. Inform. Theory
, 1999
"... Multiuser receivers improve the performance of spreadspectrum and antennaarray systems by exploiting the structure of the multiaccess interference when demodulating the signal of a user. Much of the previous work on the performance analysis of multiuser receivers has focused on their ability to re ..."
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Cited by 358 (13 self)
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Multiuser receivers improve the performance of spreadspectrum and antennaarray systems by exploiting the structure of the multiaccess interference when demodulating the signal of a user. Much of the previous work on the performance analysis of multiuser receivers has focused on their ability to reject worst case interference. Their performance in a powercontrolled network and the resulting user capacity are less wellunderstood. In this paper, we show that in a large system with each user using random spreading sequences, the limiting interference effects under several linear multiuser receivers can be decoupled, such that each interferer can be ascribed a level of effective interference that it provides to the user to be demodulated. Applying these results to the uplink of a single powercontrolled cell, we derive an effective bandwidth characterization of the user capacity: the signaltointerference requirements of all the users can be met if and only if the sum of the effective bandwidths of the users is less than the total number of degrees of freedom in the system. The effective bandwidth of a user depends only on its own SIR requirement, and simple expressions are derived for three linear receivers: the conventional matched filter, the decorrelator, and the MMSE receiver. The effective bandwidths under the three receivers serve as a basis for performance comparison.
A ratesplitting approach to the Gaussian multipleaccess channel
 IEEE TRANS. INFORM. THEORY
, 1996
"... It is shown that any point in the capacity region of a Gaussian multipleaccess channel is achievable by singleuser coding without requiring synchronization among users, provided that each user “splits” data and signal into two parts. Based on this result, a new multipleaccess technique called rat ..."
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Cited by 130 (2 self)
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It is shown that any point in the capacity region of a Gaussian multipleaccess channel is achievable by singleuser coding without requiring synchronization among users, provided that each user “splits” data and signal into two parts. Based on this result, a new multipleaccess technique called ratesplitting multiple accessing (RSMA) is proposed. RSMA is a codedivision multipleaccess scheme for the Muser Gaussian multipleaccess channel for which the effort of finding the codes for the M users, of encoding, and of decoding is that of at most 2M  1 independent pointtopoint Gaussian channels. The effects of bursty sources, multipath fading, and intercell interference are discussed and directions for further research are indicated.
Blind PARAFAC receivers for DSCDMA systems
 IEEE TRANS. SIGNAL PROCESSING
, 2000
"... This paper links the directsequence codedivision multiple access (DSCDMA) multiuser separationequalizationdetection problem to the parallel factor (PARAFAC) model, which is an analysis tool rooted in psychometrics and chemometrics. Exploiting this link, it derives a deterministic blind PARAFAC ..."
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Cited by 129 (20 self)
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This paper links the directsequence codedivision multiple access (DSCDMA) multiuser separationequalizationdetection problem to the parallel factor (PARAFAC) model, which is an analysis tool rooted in psychometrics and chemometrics. Exploiting this link, it derives a deterministic blind PARAFAC DSCDMA receiver with performance close to nonblind minimum meansquared error (MMSE). The proposed PARAFAC receiver capitalizes on code, spatial, and temporal diversitycombining, thereby supporting small sample sizes, more users than sensors, and/or less spreading than users. Interestingly, PARAFAC does not require knowledge of spreading codes, the specifics of multipath (interchip interference), DOAcalibration information, finite alphabet/constant modulus, or statistical independence/whiteness to recover the informationbearing signals. Instead, PARAFAC relies on a fundamental result regarding the uniqueness of lowrank threeway array decomposition due to Kruskal (and generalized herein to the complexvalued case) that guarantees identifiability of all relevant signals and propagation parameters. These and other issues are also demonstrated in pertinent simulation experiments.
MultiInput MultiOutput Fading Channel Tracking and Equalization Using Kalman Estimation
, 2002
"... This paper addresses the problem of channel tracking and equalization for multiinput multioutput (MIMO) timevariant frequencyselective channels. These channels model the corrupting effects of intersymbol interference (ISI), cochannel interference (CCI), and noise. A firstorder autoregressive mod ..."
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Cited by 98 (0 self)
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This paper addresses the problem of channel tracking and equalization for multiinput multioutput (MIMO) timevariant frequencyselective channels. These channels model the corrupting effects of intersymbol interference (ISI), cochannel interference (CCI), and noise. A firstorder autoregressive model describes the MIMO channel variation, and tracking is performed by a Kalman filter. Hard decisions to aid Kalman tracking come from a MIMO finitelength minimummeansquarederror decisionfeedback equalizer (MMSEDFE), which performs the equalization task. Since the optimum DFE for a wide range of channels produces decisions with a delay \Delta ? 0, the Kalman filter tracks the channel with a delay. A channel prediction module bridges the time gap between the channel estimates produced by the Kalman filter and those needed for the DFE adaptation. The proposed algorithm offers good tracking behavior for multiuser fading ISI channels at the expense of higher complexity than conventional adaptive algorithms. Appropriate coding options for this system are also discussed. Applications include synchronous multiuser detection of independent transmitters, as well as coordinated transmission through many transmitter/receiver antennas, for increased data rate.
Networked MIMO with Clustered Linear Precoding
, 2008
"... A clustered base transceiver station (BTS) coordination strategy is proposed for a large cellular MIMO network, which includes full intracluster coordination–to enhance the sum rate–and limited intercluster coordination–to reduce interference for the cluster edge users. Multicell block diagonaliz ..."
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Cited by 90 (19 self)
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A clustered base transceiver station (BTS) coordination strategy is proposed for a large cellular MIMO network, which includes full intracluster coordination–to enhance the sum rate–and limited intercluster coordination–to reduce interference for the cluster edge users. Multicell block diagonalization is used to coordinate the transmissions across multiple BTSs in the same cluster. To satisfy perBTS power constraints, three combined precoder and power allocation algorithms are proposed with different performance and complexity tradeoffs. For intercluster coordination, the coordination area is chosen to balance fairness for edge users and the achievable sum rate. It is shown that a small cluster size (about 7 cells) is sufficient to obtain most of the sum rate benefits from clustered coordination while greatly relieving channel feedback requirement. Simulations show that the proposed coordination strategy efficiently reduces interference and provides a considerable sum rate gain for cellular MIMO networks.
Iterative construction of optimum signature sequence sets in synchronous CDMA systems
 IEEE Trans. Inform. Theory
, 1989
"... Abstract—Recently, optimum signature sequence sets that maximize the capacity of singlecell synchronous code division multiple access (CDMA) systems have been identified. Optimum signature sequences minimize the total squared correlation (TSC); they form a set of orthogonal sequences, if the number ..."
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Cited by 89 (9 self)
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Abstract—Recently, optimum signature sequence sets that maximize the capacity of singlecell synchronous code division multiple access (CDMA) systems have been identified. Optimum signature sequences minimize the total squared correlation (TSC); they form a set of orthogonal sequences, if the number of users is less than or equal to the processing gain, and a set of Welch bound equality (WBE) sequences, otherwise. We present an algorithm where users update their transmitter signature sequences sequentially, in a distributed fashion, by using available receiver measurements. We show that each update decreases the TSC of the set, and produces better signature sequence sets progressively. We prove that the algorithm converges to a set of orthogonal signature sequences when the number of users is less than or equal to the processing gain. We observe and conjecture that the algorithm converges to a WBE set when the number of users is greater than the processing gain. At each step, the algorithm replaces one signature sequence from the set with the normalized minimum mean squared error (MMSE) receiver corresponding to that signature sequence. Since the MMSE filter can be obtained by a distributed algorithm for each user, the proposed algorithm is amenable to distributed implementation. Index Terms—Code division multiple access (CDMA), distributed interference avoidance, minimum mean squared error (MMSE), optimum signature sequence sets, Welch bound equality (WBE) sequences. I.
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
Power Control and Capacity of Spread Spectrum Wireless Networks
 Automatica
, 1999
"... Transmit power control is a central technique for resource allocation and interference management in spreadspectrum wireless networks. With the increasing popularity of spreadspectrum as a multiple access technique, there has been significant research in the area in recent years. While power contr ..."
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Cited by 77 (5 self)
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Transmit power control is a central technique for resource allocation and interference management in spreadspectrum wireless networks. With the increasing popularity of spreadspectrum as a multiple access technique, there has been significant research in the area in recent years. While power control has been considered traditionally as a means to counteract the harmful effect of channel fading, the more general emerging view is that it is a flexible mechanism to provide QualityofService to individual users. In this paper, we will review the main threads of ideas and results in the recent development of this area, with a bias towards issues that have been the focus of our own research. For different receivers of varying complexity, we study both questions about optimal power control as well as the problem of characterizing the resulting network capacity. Although spreadspectrum communications has been traditionally viewed as a physicallayer subject, we argue that by suitable abstr...
Wireless systems and interference avoidance
 IEEE Trans. Wireless Commun
, 2002
"... Abstract—Motivated by the emergence of programmable radios, we seek to understand a new class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference r ..."
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Cited by 74 (12 self)
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Abstract—Motivated by the emergence of programmable radios, we seek to understand a new class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference ratio as a metric and a general signal space approach, we present a class of iterative distributed algorithms for synchronous systems which results in an ensemble of optimal waveforms for multiple users connected to a common receiver (or colocated independent receivers). That is, the waveform ensemble meets the Welch Bound with equality and, therefore, achieves minimum average interference over the ensemble of signature waveforms. We derive fixed points for a number of scenarios, provide examples, look briefly at ensemble stability under user addition and deletion as well as provide a simplistic comparison to synchronous codedivision multipleaccess. We close with suggestions for future work. Index Terms—Adaptive modulation, codedivision multipleaccess systems, codeword optimization, interference avoidance, multiuser
Linear Multiuser Receivers in Random Environments
 IEEE Trans. Inform. Theory
, 2000
"... We study the signaltointerference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions." Such random environment may arise in a DSCDMA system with random signature sequences, or in a system with antenna diversi ..."
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Cited by 67 (2 self)
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We study the signaltointerference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions." Such random environment may arise in a DSCDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Assuming that such random directions can be tracked by the receiver, the resulting SIR performance is a function of the directions and therefore also random. We study the asymptotic distribution of this random performance in the regime where both the number of users and the number of degrees of freedom in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum meansquare error (MMSE) receiver, the variance of the SIR distribution decreases like 1 , and the SIR distribution is asymptotically Gaussian. We compute closedform expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finitesized systems.