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139
Bandwidth Scaling for Fading Multipath Channels
, 1999
"... We show that very large bandwidths on fading multipath channels cannot be effectively utilized by spread spectrum systems that (in a particular sense) spread the available power uniformly over both time and frequency. The approach is to express the input process as an expansion in an orthonormal set ..."
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Cited by 106 (12 self)
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We show that very large bandwidths on fading multipath channels cannot be effectively utilized by spread spectrum systems that (in a particular sense) spread the available power uniformly over both time and frequency. The approach is to express the input process as an expansion in an orthonormal set of functions each localized in time and frequency. The fourth moment of each coefficient in this expansion is then uniformly constrained. We show that such a constraint forces the mutual information to 0 inversely with increasing bandwidth. Simply constraining the second moment of these coefficients does not achieve this effect. The results suggest strongly that conventional direct sequence CDMA systems do not scale well to extremely large bandwidths. To illustrate how the interplay between channel estimation and symbol detection affects capacity, we present results for a specific channel and CDMA signaling scheme.
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 102 (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.
The transport capacity of wireless networks over fading channels
, 2005
"... We consider networks consisting of nodes with radios, and without any wired infrastructure, thus necessitating all communication to take place only over the shared wireless medium. The main focus of this paper is on the effect of fading in such wireless networks. We examine the attenuation regime ..."
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Cited by 89 (4 self)
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We consider networks consisting of nodes with radios, and without any wired infrastructure, thus necessitating all communication to take place only over the shared wireless medium. The main focus of this paper is on the effect of fading in such wireless networks. We examine the attenuation regime where either the medium is absorptive, a situation which generally prevails, or the path loss exponent is greater than 3. We study the transport capacity, defined as the supremum over the set of feasible rate vectors of the distance weighted sum of rates. We consider two assumption sets. Under the first assumption set, which essentially requires only a mild time average type of bound on the fading process, we show that the transport capacity can grow no faster than (), where denotes the number of nodes, even when the channel state information (CSI) is available noncausally at both the transmitters and the receivers. This assumption includes common models of stationary ergodic channels; constant, frequencyselective channels; flat, rapidly varying channels; and flat slowly varying channels. In the second assumption set, which essentially features an independence, time average of expectation, and nonzeroness condition on the fading process, we constructively show how to achieve transport capacity of ( ) even when the CSI is unknown to both the transmitters and the receivers, provided that every node has an appropriately nearby node. This assumption set includes common models of independent and identically distributed (i.i.d.) channels; constant, flat channels; and constant, frequencyselective channels. The transport capacity is achieved by nodes communicating only with neighbors, and using only pointtopoint coding. The thrust of these results is that the multihop strategy, toward which much protocol development activity is currently targeted, is appropriate for fading environments. The low attenuation regime is open.
Optimal Power Control, Scheduling and Routing in UWB Networks
, 2004
"... UltraWide Band (UWB) is an emerging wireless physical layer technology that uses a very large bandwidth. We are interested in finding the design objectives of the medium access (MAC, namely, power control and scheduling) and routing protocols of a multihop, besteffort, UWB network. Our objective ..."
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Cited by 82 (5 self)
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UltraWide Band (UWB) is an emerging wireless physical layer technology that uses a very large bandwidth. We are interested in finding the design objectives of the medium access (MAC, namely, power control and scheduling) and routing protocols of a multihop, besteffort, UWB network. Our objective is to maximize flow rates (more precisely, logutility of flow rates) given node power constraints. The specificity of UWB is expressed by the linear dependence between rate and signaltonoise ratio at the receiver. It is known that, in wireless networks, different routing strategies can imply differences in MAC protocol design. Hence we search for the jointly optimal routing, scheduling and power control.
Ultrawideband radio design: The promise of highspeed, shortrange wireless connectivity
 in Proc. of the IEEE. Special Issue on Gigabit Wireless
, 2004
"... This paper provides a tutorial overview of ultrawideband (UWB) radio technology for highspeed wireless connectivity. Subsequent to establishing a historical and technological context, it describes the new impetus for UWB systems development and standardization resulting from the FCC’s recent decisi ..."
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Cited by 72 (1 self)
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This paper provides a tutorial overview of ultrawideband (UWB) radio technology for highspeed wireless connectivity. Subsequent to establishing a historical and technological context, it describes the new impetus for UWB systems development and standardization resulting from the FCC’s recent decision to permit unlicensed operation in the [3.1,10.6] GHz band subject to modified Part 15 rules and indicates the potential new applications that may result. Thereafter, the paper provides a system architect’s perspectives on the various issues and challenges involved in the design of link layer subsystems. Specifically, we outline current developments in UWB system design concepts that are oriented to highspeed applications and describe some of the design tradeoffs involved. Keywords—Personal area networks, wireless, ultrawide band. I. INTRODUCTION AND BACKGROUND
Broadband fading channels: signal burstiness and capacity
 IEEE Trans. Inform. Theory
, 2002
"... Abstract—Médard and Gallager recently showed that very large bandwidths on certain fading channels cannot be effectively used by direct sequence or related spreadspectrum systems. This paper complements the work of Médard and Gallager. First, it is shown that a key informationtheoretic inequality ..."
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Cited by 62 (5 self)
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Abstract—Médard and Gallager recently showed that very large bandwidths on certain fading channels cannot be effectively used by direct sequence or related spreadspectrum systems. This paper complements the work of Médard and Gallager. First, it is shown that a key informationtheoretic inequality of Médard and Gallager can be directly derived using the theory of capacity per unit cost, for a certain fourthorder cost function, called fourthegy. This provides insight into the tightness of the bound. Secondly, the bound is explored for a widesensestationary uncorrelated scattering (WSSUS) fading channel, which entails mathematically defining such a channel. In this context, the fourthegy can be expressed using the ambiguity function of the input signal. Finally, numerical data and conclusions are presented for directsequence type input signals. Index Terms—Channel capacity, fading channels, spread spectrum, widesensestationary uncorrelated scattering (WSSUS) fading channels. I.
Channel coherence in the low SNR regime
, 2005
"... While capacity in the limit of vanishing SNR per degree of freedom is known to be linear in SNR for fading and nonfading channels, regardless of channel side information at the receiver, such asymptotic results hide the cost of channel variability in terms of performance. In this paper, rather than ..."
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Cited by 36 (4 self)
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While capacity in the limit of vanishing SNR per degree of freedom is known to be linear in SNR for fading and nonfading channels, regardless of channel side information at the receiver, such asymptotic results hide the cost of channel variability in terms of performance. In this paper, rather than maintain a fixed channel model with a given coherence and consider a vanishing SNR, we present a model in which coherence, peak energy and SNR are considered jointly. In particular, we show that channel variability, characterized by coherence, when considered in terms of SNR, determines a sublinear term in SNR which reduces the coherent capacity, itself linear in SNR. We explicitly characterize this sublinear term and show how coherence, when considered jointly with SNR, affects the peakiness required in transmissions. We examine how, by using suboptimal training schemes, we may achieve rates that trail coherent capacity by a sublinear term in SNR.
The noncoherent Rician fading channel  Part I : Structure of the capacityachieving input
 IEEE TRANS. WIRELESS COMMUN
, 2005
"... Transmission of information over a discretetime memoryless Rician fading channel is considered, where neither the receiver nor the transmitter knows the fading coefficients. First, the structure of the capacityachieving input signals is investigated when the input is constrained to have limited p ..."
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Cited by 36 (5 self)
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Transmission of information over a discretetime memoryless Rician fading channel is considered, where neither the receiver nor the transmitter knows the fading coefficients. First, the structure of the capacityachieving input signals is investigated when the input is constrained to have limited peakedness by imposing either a fourth moment or a peak constraint. When the input is subject to second and fourth moment limitations, it is shown that the capacityachieving input amplitude distribution is discrete with a finite number of mass points in the lowpower regime. A similar discrete structure for the optimal amplitude is proven over the entire signaltonoise ratio (SNR) range when there is only a peakpower constraint. The Rician fading with the phasenoise channel model, where there is phase uncertainty in the specular component, is analyzed. For this model, it is shown that, with only an average power constraint, the capacityachieving input amplitude is discrete with a finite number of levels. For the classical averagepowerlimited Rician fading channel, it is proven that the optimal input amplitude distribution has bounded support.
A joint PHY/MAC architecture for lowradiated power THUWB wireless adhoc networks
 Wireless Communications and Mobile Computing Journal, Special Issue on Ultrawideband (UWB) Communications
, 2005
"... Due to environmental concerns and strict constraints on interference imposed on other networks, the radiated power of emerging pervasive wireless networks needs to be strictly limited, yet without sacrificing acceptable data rates. Pulsed TimeHopping UltraWideband (THUWB) is a radio technology th ..."
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Cited by 33 (12 self)
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Due to environmental concerns and strict constraints on interference imposed on other networks, the radiated power of emerging pervasive wireless networks needs to be strictly limited, yet without sacrificing acceptable data rates. Pulsed TimeHopping UltraWideband (THUWB) is a radio technology that has the potential to satisfy this requirement. Although THUWB is a multiuser radio technology, nonzero crosscorrelation between timehopping sequences, timeasynchronicity between sources and a multipath channel environment make it sensitive to strong interferers and nearfar scenarios. While most protocols manage interference and multipleaccess through power control or mutual exclusion, we base our design on rate control, a relatively unexplored dimension for multipleaccess and interference management. We further take advantage of the nature of pulsed THUWB to propose an interference mitigation scheme that alleviates the need for an exclusion scheme. A source is always allowed to send and continuously adapts its channel code (hence its rate) to the interference experienced at the destination. In contrast to power control or exclusion, our MAC layer is local to sender and receiver and does not need coordination among neighbors not involved in the transmission. We show by simulation that we achieve a significant increase in network throughput compared to alternative designs.