Showing papers by "David Tse published in 2000"
TL;DR: The main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant.
Abstract: A linear multiuser receiver for a particular user in a code-division multiple-access (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. 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 signal-to-interference 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 signal-to-interference 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.
347 citations
TL;DR: This work studies the signal-to-interference performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions", and shows that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the Sir distribution decreases like 1/N, and the SIR distribution is asymptotically Gaussian.
Abstract: We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions." Such a random environment may arise in a DS-CDMA 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 K and the number of degrees of freedom N in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the SIR distribution decreases like 1/N, and the SIR distribution is asymptotically Gaussian. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems.
177 citations
TL;DR: It is shown that while the effective bandwidth of the decorrelator and the MMSE receiver is higher in an asynchronous system than that in a synchronous system, it progressively decreases with the increase in the length of the observation window and is asymptotic to that of the synchronous systems.
Abstract: The performance of linear multiuser receivers in terms of the signal-to-interference ratio (SIR) achieved by the users has been analyzed in a synchronous CDMA system under random spreading sequences. In this paper, we extend these results to a symbol-asynchronous but chip-synchronous system and characterize the SIR for linear receivers-the matched-filter receiver the minimum mean-square error (MMSE) receiver and the decorrelator. For each of the receivers, we characterize the limiting SIR achieved when the processing gain is large and also derive lower bounds on the SIR using the notion of effective interference. Applying the results to a power controlled system, we derive effective bandwidths of the users for these linear receivers and characterize the user capacity region: a set of users is supportable by a system if the sum of the effective bandwidths is less than the processing gain of the system. We show that while the effective bandwidth of the decorrelator and the MMSE receiver is higher in an asynchronous system than that in a synchronous system, it progressively decreases with the increase in the length of the observation window and is asymptotic to that of the synchronous system, when the observation window extends infinitely on both sides of the symbol of interest. Moreover, the performance gap between the MMSE receiver and the decorrelator is significantly wider in the asynchronous setting as compared to the synchronous case.
114 citations
TL;DR: This correspondence analyzes the high signal-to-noise ratio (SNR) performance of optimum multiuser detectors for synchronous direct-sequence spread spectrum with random spreading in an additive white Gaussian noise channel and shows that the optimum asymptotic efficiency of a K-user system with spreading gain N converges to 1 almost surely as K/spl rarr//spl infin.
Abstract: This correspondence analyzes the high signal-to-noise ratio (SNR) performance of optimum multiuser detectors for synchronous direct-sequence spread spectrum with random spreading in an additive white Gaussian noise channel. Under very general conditions on the received powers, we show that the optimum asymptotic efficiency of a K-user system with spreading gain N converges to 1 almost surely as K/spl rarr//spl infin/, and K/N is kept equal to an arbitrary nonzero constant. Therefore, the asymptotic behavior of the minimum bit error rate is equivalent to that of a single-user system.
65 citations
23 Sep 2000
TL;DR: In this paper, the authors explore the capacity of dual-antenna-array systems via theoretical analysis and simulation experiments and present expressions for the asymptotic growth rate of capacity with n for both independent and correlated fading cases.
Abstract: Wireless systems using multi-element antenna arrays simultaneously at the both transmitter and receiver promise a much higher capacity than conventional systems. Previous studies have shown that single-user systems employing n-element transmit and receive arrays can achieve a capacity proportional to n, assuming independent Rayleigh fading between pairs of antenna elements. We explore the capacity of dual-antenna-array systems via theoretical analysis and simulation experiments. We present expressions for the asymptotic growth rate of capacity with n for both independent and correlated fading cases; the latter is derived under some assumptions about the fading correlation structure. We show that the capacity growth is linear in n in both the independent and correlated cases, but the growth rate is smaller in the latter case. We compare the predictions of our asymptotic theory to the capacities of channels simulated using ray tracing, and find good agreement even for moderate n, i.e., 1/spl les/n/spl les/16. Our results address both the cases when the transmitter does and does not know the channel realization.
34 citations
25 Jun 2000
TL;DR: It is established that the Gaussianity of the output multiple-access interference of linear MMSE receiver in a large DS-CDMA network is the same as that of the input data.
Abstract: We establish the Gaussianity of the output multiple-access interference (MAI) of linear MMSE receiver in a large DS-CDMA network.
31 citations
25 Jun 2000
TL;DR: The high SNR channel capacity is computed, and a geometric interpretation of sphere packing in the Grassmann manifold is given, which shows the capacity of multi-antenna channel where the fading coefficients are unknown to either transmitter or receiver.
Abstract: In this paper, we study the capacity of multi-antenna channel where the fading coefficients are unknown to either transmitter or receiver. The high SNR channel capacity is computed, and a geometric interpretation of sphere packing in the Grassmann manifold is given.
22 citations
29 Oct 2000
TL;DR: It is shown that, as the bandwidth increases to infinity, the capacity goes to zero when signaling is constrained in its second moment and peak signal amplitude, consistent with similar results which do not consider doubly block-fading channels.
Abstract: We consider wideband fading channels which are block-fading in time and in frequency (doubly block-fading). We show that, as the bandwidth increases to infinity, the capacity goes to zero when signaling is constrained in its second moment and peak signal amplitude. This result is consistent with similar results which do not consider doubly block-fading channels. None of these results, however, offer a range for the optimal spreading bandwidth, because they consider only upper bounds to capacity. While we know that spreading over very large bandwidths is detrimental in terms of capacity, we wish to determine over what range of bandwidths spreading is beneficial. We are able to give a range for the optimal spreading bandwidth by combining our upper bound with a suitable lower bound.
21 citations
01 Jan 2000
TL;DR: With no restriction on the input signal other than an av- erage power constraint, as the bandwidth gets large, one can achieve communication rates over a multipath fading channel equal to the capacity of an infinite bandwidth additive white Gaussian channel of the same signal-to- noise ratio (SNR) without fading.
Abstract: In this paper we will investigate the capacity and mu- tual information of a broadband fading channel consisting of a finite number of time-varying paths. We will show that the ca- pacity of the channel in the wideband limit is the same as that of a wideband Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be "peaky" in time or frequency. In particular, we show that if white-like signals are used instead (as is common in spread-spec- trum systems), the mutual information is inversely proportional to the number of resolvable paths with energy spread out, and in fact approaches as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter is defined in terms of system parameters to delineate the threshold on over which such over- spreading phenomenon occurs. To answer these questions, we study in this paper the capacity and mutual information of multipath fading channels without the a priori assumption of knowledge of the channel at the re- ceiver. We consider a channel having a finite number of paths and a large transmission bandwidth . To state the results we introduce the notion of the number of resolvable paths : This is the number of paths one would see if one could only differ- entiate paths whose delays differ by more than . The fol- lowing three results are presented in this paper: 1) With no restriction on the input signal other than an av- erage power constraint, as the bandwidth gets large, one can achieve communication rates over a multipath fading channel equal to the capacity of an infinite bandwidth additive white Gaussian channel of the same signal-to- noise ratio (SNR) without fading. Moreover, this can be achieved by frequency shift keying and noncoherent de- tection.
10 citations
01 Dec 2000
TL;DR: This paper considers "vector multiple access channels" (VMAC) where each user has multiple "degrees of freedom" and studies the effect of power allocation as a function of the channel state on the "sum capacity" defined as the maximum sum of rates of users per unit degree of freedom at which the users can jointly reliably transmit.
Abstract: This paper considers "vector multiple access channels" (VMAC) where each user has multiple "degrees of freedom" and studies the effect of power allocation as a function of the channel state on the "sum capacity" defined as the maximum sum of rates of users per unit degree of freedom at which the users can jointly reliably transmit, in an information theoretic sense. A concrete example of a VMAC is a MAC with multiple antennas at the receiver where the antennas provide spatial degrees of freedom. Our main result is the identification of a simple dynamic power allocation scheme that is optimal in a large system, i.e., in the regime of a large number of users and a correspondingly large number of antennas. A key feature of this policy is that, for any user, it depends only on the instantaneous amplitude of the slow fading component of the vector channel of that user alone and the structure of the policy is "waterfilling".
9 citations
23 Sep 2000
TL;DR: The capacity of multiple antenna fading channels is studied on the scenario where the fading coefficients vary quickly; thus an accurate estimation of the coefficients is generally not available to either the transmitter or the receiver.
Abstract: In this paper, we study the capacity of multiple antenna fading channels. We focus on the scenario where the fading coefficients vary quickly; thus an accurate estimation of the coefficients is generally not available to either the transmitter or the receiver. We use a block fading model proposed by Marzetta and Hochwald (see IEEE Trans. on Info. Theory, vol.45, no.1, p.139-57, 1999). The model does not assume any prior knowledge of the fading coefficients, but only assumes that the coefficients remain constant for a coherent interval T as an approximation of the continuously varying channel. We compute the asymptotic capacity of this channel at high SNR in terms of T, the number of transmit antennas M and the number of receive antennas N. While the capacity gain of the coherent multi-antenna channel is min{M,N} bps/Hz for every 3 dB increase in SNR, the corresponding gain for the non-coherent channel turns out to be M/sup */(1-M/sup *//T) bps/Hz, where M/sup */=min{M,N,[T/2]}. The capacity expression has a geometric interpretation of sphere packing in the Grassmann manifold.
Patent•
27 Jun 2000TL;DR: In this article, an access metric was proposed to compare the desirability of granting each channel (i.e., instantaneous channel, instantenous channel, etc.) access with respect to the average data throughput over that channel.
Abstract: A transmitter directed, distributed receiver using path diversity provided by the distribution of the receiver. Advantage is taken of the uncorrelated variations over time in the condition of channels (106a, 106b, 106c, 106d) between a common transmitter and several users (104a, 104b, 104c). The greater the variation in the channel (106a, 106b, 106c) condition of a particular channel (106a, 106b, 106c, 106d) over time, the greater the increase in total system throughput provided. An access metric represents the instantaneous channel (106a, 106b, 106c, 106d) condition of the communication system between each user (104a, 104b, 104c) and the transmitter with respect to the average channel (106a, 106b, 106c, 106d) condition of each channel. Alternatively, the access metric represent the instantenous channel (106a, 106b, 106c, 106d) condition with respect to the average data throughput over that channel (106a, 106b, 106c, 106d). The common transmitting station uses the access metric to directly compare the desirability of granting each channel (106a, 106b, 106c, 106d) access with the desirability of granting each other channel (106a, 106b, 106c, 106d) access. The user (104a, 104b, 104c) that has the greatest access metric is provided access to the channel (106a, 106b, 106c, 106d).
29 Oct 2000
TL;DR: This work explores the capacity of dual-antenna-array systems via theoretical analysis and simulation experiments and presents expressions for the asymptotic growth rate of capacity with n for both independent and correlated fading cases; the latter is derived under some assumptions about the fading correlation structure.
Abstract: Wireless systems using multi-element antenna arrays simultaneously at both transmitter and receiver promise a much higher capacity than conventional systems. Previous studies have shown that single-user systems employing n-element transmit and receive arrays can achieve a capacity proportional to n, assuming independent Rayleigh fading between pairs of antenna elements. We explore the capacity of dual-antenna-array systems via theoretical analysis and simulation experiments. We present expressions for the asymptotic growth rate of capacity with n for both independent and correlated fading cases; the latter is derived under some assumptions about the fading correlation structure. We show that the capacity growth is linear in n in both the independent and correlated cases, but the growth rate is smaller in the latter case. We compare the predictions of our asymptotic theory to the capacities of channels simulated using ray tracing and find good agreement even for moderate n, i.e., 1/spl les/n/spl les/16. Our results address both the cases when the transmitter does and does not know the channel realization.
25 Jun 2000
TL;DR: This work considers a spread spectrum, multi-user channel, with an antenna array at the receiver, and independent flat fading to each antenna from each user, and shows that a curious phenomenon of "resource pooling" arises.
Abstract: We consider a spread spectrum, multi-user channel, with an antenna array at the receiver, and independent flat fading to each antenna from each user. We focus on the case of microdiversity, and show that a curious phenomenon of "resource pooling" arises.