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Showing papers by "Ali H. Sayed published in 2007"


Journal ArticleDOI
TL;DR: This paper proposes designing precoders by maximizing the so-called signal-to-leakage-and-noise ratio (SLNR) for all users simultaneously, and it also avoids noise enhancement.
Abstract: In multiuser MIMO downlink communications, it is necessary to design precoding schemes that are able to suppress co-channel interference. This paper proposes designing precoders by maximizing the so-called signal-to-leakage-and-noise ratio (SLNR) for all users simultaneously. The presentation considers communications with both single- and multi-stream cases, as well as MIMO systems that employ Alamouti coding. The effect of channel estimation errors on system performance is also studied. Compared with zero-forcing solutions, the proposed method does not impose a condition on the relation between the number of transmit and receive antennas, and it also avoids noise enhancement. Simulations illustrate the performance of the scheme

871 citations


Journal ArticleDOI
TL;DR: An adaptive distributed strategy is developed based on incremental techniques that addresses the problem of linear estimation in a cooperative fashion, in which nodes equipped with local computing abilities derive local estimates and share them with their predefined neighbors.
Abstract: An adaptive distributed strategy is developed based on incremental techniques. The proposed scheme addresses the problem of linear estimation in a cooperative fashion, in which nodes equipped with local computing abilities derive local estimates and share them with their predefined neighbors. The resulting algorithm is distributed, cooperative, and able to respond in real time to changes in the environment. Each node is allowed to communicate with its immediate neighbor in order to exploit the spatial dimension while limiting the communications burden at the same time. A spatial-temporal energy conservation argument is used to evaluate the steady-state performance of the individual nodes across the entire network. Computer simulations illustrate the results.

706 citations


Journal ArticleDOI
TL;DR: The paper develops a dynamic antenna scheduling strategy for downlink MIMO communications, where a subset of the receive antennas at certain users is selectively disabled, and shows that increasing the number of antennas at some users can degrade the SLNR performance at other users.
Abstract: The paper develops a dynamic antenna scheduling strategy for downlink MIMO communications, where a subset of the receive antennas at certain users is selectively disabled. The proposed method improves the signal-to-leakage-plus-noise (SLNR) ratio performance of the system and it relaxes the condition on the number of transmit-receive antennas in comparison to traditional zero-forcing and time-scheduling strategies. The largest value that the SLNR can achieve is shown to be equal to the maximum eigenvalue of a certain random matrix combination, and the probability distribution of this eigenvalue is characterized in terms of a Whittaker function. The result shows that increasing the number of antennas at some users can degrade the SLNR performance at other users. This fact is used to propose an antenna scheduling scheme that leads to improvement in terms of SINR outage probabilities

242 citations


Journal ArticleDOI
TL;DR: This paper proposes and study two-hop multisensor relay strategies that achieve minimum mean-square-error (MSE) performance subject to either local or global power constraints.
Abstract: A fundamental task in a wireless sensor network is to broadcast some measured data from an origin sensor to a destination sensor. Since the sensors are typically small, power limited, and low cost, they are only able to broadcast low-power signals. As a result, the propagation loss from the origin to the destination nodes can attenuate the signals beyond detection. One way to deal with this problem is to pass the transmitted signal through relay nodes. In this paper we propose and study two-hop multisensor relay strategies that achieve minimum mean-square-error (MSE) performance subject to either local or global power constraints. The capacity of the resulting relay link and its diversity order are studied. The effect of channel uncertainties on system performance is examined and a modified relay scheme is proposed.

195 citations


Journal ArticleDOI
TL;DR: The proposed compensation scheme can effectively mitigate the ICI caused by phase noise and improve the BER of OFDM systems and helps simplify the oscillator and circuitry design in terms of implementation cost and power consumption.
Abstract: Phase noise causes significant degradation in the performance of orthogonal frequency division multiplexing (OFDM)-based wireless communication systems. The presence of phase noise can reduce the effective signal-to-noise ratio (SNR) at the receiver, and consequently, limit the bit error rate (BER) and data rate. In this paper, the effect of phase noise on OFDM wireless systems is studied, and a compensation scheme is proposed to mitigate the common phase error and intercarrier interference (ICI) caused by phase noise. In the proposed scheme, the communication between the transmitter and receiver blocks consists of two stages. In the first stage, block-type pilot symbols are transmitted and the channel coefficients are jointly estimated with the phase noise in the time domain. In the second stage, comb-type OFDM symbols are transmitted such that the receiver can jointly estimate the data symbols and the phase noise. It is shown both by theory and computer simulations that the proposed scheme can effectively mitigate the ICI caused by phase noise and improve the BER of OFDM systems. Another benefit of the proposed scheme is that the sensitivity of OFDM receivers to phase noise can be significantly lowered, which helps simplify the oscillator and circuitry design in terms of implementation cost and power consumption.

149 citations


Journal ArticleDOI
TL;DR: Algorithms are developed to compensate for in-phase and quadrature-phase IQ imbalances in an OFDM system and include post-FFT least-squares and adaptive equalization, as well as a pre-distortion scheme at the transmitter and a pre -FFT correction at the receiver.
Abstract: The implementation of OFDM-based systems suffers from impairments such as in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. Such imbalances are caused by the analog processing of the radio frequency (RF) signal and can be present at both the transmitter and receiver. The resulting IQ distortion limits the achievable operating SNR at the receiver and the achievable data rates. In this paper, the effect of both the transmitter and receiver IQ imbalances in an OFDM system is studied and algorithms are developed to compensate for such distortions in the digital domain. The algorithms include post-FFT least-squares and adaptive equalization, as well as a pre-distortion scheme at the transmitter and a pre-FFT correction at the receiver

143 citations


Journal ArticleDOI
TL;DR: The article describes recent adaptive estimation algorithms over distributed networks that rely on local collaborations and exploit the space-time structure of the data.
Abstract: The article describes recent adaptive estimation algorithms over distributed networks. The algorithms rely on local collaborations and exploit the space-time structure of the data. Each node is allowed to communicate with its neighbors in order to exploit the spatial dimension, while it also evolves locally to account for the time dimension. Algorithms of the least-mean-squares and least-squares types are described. Both incremental and diffusion strategies are considered.

119 citations


Proceedings ArticleDOI
15 Apr 2007
TL;DR: The resulting adaptive networks are robust to node and link failures and present a substantial improvement over the non-cooperative case asserting that cooperation improves estimation performance.
Abstract: Distributed adaptive algorithms are proposed to address the problem of estimation in distributed networks. We extend recent work by relying on static and adaptive diffusion strategies. The resulting adaptive networks are robust to node and link failures and present a substantial improvement over the non-cooperative case asserting that cooperation improves estimation performance. The distributed algorithms are peer-to-peer implementations suitable for networks with general topologies.

113 citations


Journal ArticleDOI
TL;DR: In this article, MIMO processing is shown to increase the information capacity of communication links linearly as the minimum number of transmitters/receivers increases.
Abstract: In this article we discuss the application of MIMO processing to multimode fiber links. MIMO processing is shown to increase the information capacity of communication links linearly as the minimum number of transmitters/receivers increases. The fundamentals of optical MIMO fiber links are presented, and the promises and challenges of such systems are elaborated

102 citations


Proceedings ArticleDOI
26 Dec 2007
TL;DR: This paper proposes optimal cooperation strategies for spectrum sensing to combat the effects of destructive channels and malfunctioning devices, and proposes two optimization schemes to control the combining weights, and compare their performance.
Abstract: Spectrum sensing is a key enabling functionality in cognitive radio (CR) networks, where the CRs act as secondary users that opportunistically access free frequency bands. Due to the effects of channel fading, individual CRs may not be able to reliably detect the existence of a primary radio, who is a licensed user for the particular band. In this paper, we present optimal cooperation strategies for spectrum sensing to combat the effects of destructive channels and malfunctioning devices. Our approach conducts spectrum sensing based on the linear combination of local test statistics from individual secondary users. We propose two optimization schemes to control the combining weights, and compare their performance. Our first approach is to optimize the probability distribution function of the global test statistics at the fusion center. For the second scheme, we maximize the global detection sensitivity under constraints on the false alarm probability. Simulation results illustrate the significant cooperative gain achieved by the proposed strategies.

87 citations


Proceedings ArticleDOI
01 Feb 2007
TL;DR: In this article, the authors provide an overview of recent work on distributed adaptive algorithms focusing mainly on incremental and diffusion strategies and comments on the mean-square-error performance of the incremental solution.
Abstract: The article provides an overview of recent work on distributed adaptive algorithms. The work focuses mainly on incremental and diffusion strategies and comments on the mean-square-error performance of the incremental solution.

Proceedings ArticleDOI
17 Jun 2007
TL;DR: A diffusion recursive least-squares algorithm where nodes need to communicate only with their closest neighbors, which has no topology constraints, and requires no transmission or inversion of matrices, therefore saving in communications and complexity.
Abstract: We consider the problem of distributed estimation in adaptive networks where a collection of nodes are required to estimate in a collaborative manner some parameter of interest from their measurements. The centralized solution to the problem uses a fusion center, thus requiring a large amount of energy for communication. We propose a diffusion recursive least-squares algorithm where nodes need to communicate only with their closest neighbors. The algorithm has no topology constraints, and requires no transmission or inversion of matrices, therefore saving in communications and complexity. We show that the algorithm is stable and analyze its performance comparing it to the centralized global solution.

Journal ArticleDOI
TL;DR: It is shown both by theory and computer simulations that the two methods can effectively enlarge the transmission range of UWB devices.
Abstract: Ultrawideband (UWB) radio is a promising solution for high-rate wireless communications over short ranges. In this paper, the performance of multiband orthogonal frequency division multiplexing (MB-OFDM) UWB systems is analyzed using the Saleh-Valenzuela channel model in terms of the packet error rate and the transmission range for indoor environments. The performance improvements resulting from the use of linear precoding and multiple antenna techniques are also analyzed and compared. It is shown both by theory and computer simulations that the two methods can effectively enlarge the transmission range of UWB devices.

01 May 2007
TL;DR: The application of MIMO processing to multimode fiber links is shown to increase the information capacity of communication links linearly as the minimum number of transmitters/receivers increases.
Abstract: In this article we discuss the application of MIMO processing to multimode fiber links. MIMO processing is shown to increase the information capacity of communication links linearly as the minimum number of transmitters/receivers increases. The fundamentals of optical MIMO fiber links are presented, and the promises and challenges of such systems are elaborated.

Journal ArticleDOI
TL;DR: A robust energy-aware clustering architecture (REACA) for large-scale wireless sensor networks is proposed and it is shown that by exploiting traffic locality, clustering can achieve performance improvement both in capacity and in power consumption over general-purpose ad hoc networks.
Abstract: The emergence of wireless sensor networks has imposed many challenges on network design such as severe energy constraints, limited bandwidth and computing capabilities. This kind of networks necessitates network protocol architectures that are robust, energy-efficient, scalable, and easy for deployment. This paper proposes a robust energy-aware clustering architecture (REACA) for large-scale wireless sensor networks. We analyze the performance of the REACA network in terms of quality-of-service, asymptotic throughput capacity, and power consumption. In particular, we study how the throughput capacity scales with the number of nodes and the number of clusters. We show that by exploiting traffic locality, clustering can achieve performance improvement both in capacity and in power consumption over general-purpose ad hoc networks. We also explore the fundamental trade-off between throughput capacity and power consumption for single-hop and multi-hop routing schemes in cluster-based networks. The protocol architecture and performance analysis developed in this paper provide useful insights for practical design and deployment of large-scale wireless sensor network.

Proceedings ArticleDOI
26 Aug 2007
TL;DR: The mean-square performance of a diffusion least mean-squares protocol proposed in recent work to address the problem of distributed estimation is studied and closed form expressions for themean-square deviation and the excess mean- square error of the adaptive network are derived.
Abstract: We study the mean-square performance of a diffusion least mean-squares protocol proposed in recent work to address the problem of distributed estimation [1, 2]. By relying on energy conservation arguments [8] we derive closed form expressions for the mean-square deviation (MSD) and the excess mean-square error (EMSE) of the adaptive network. Examples show a good agreement between simulations and theory.

Proceedings ArticleDOI
15 Apr 2007
TL;DR: The framework enables the system to achieve a desired estimation fidelity level and to improve the network lifetime andSimulations illustrate the effectiveness of the proposed sampling schemes.
Abstract: We consider an estimation network of many distributed sensors, where each senor takes a noisy measurement of some unknown parameter. Due to energy limitation, the network selects only a subset of sensors for data fusion as long as the distortion is tolerable. In this paper, we present a sampling framework based on linear minimum variance unbiased estimation. The framework enables the system to achieve a desired estimation fidelity level and to improve the network lifetime. Simulations illustrate the effectiveness of the proposed sampling schemes.

Journal ArticleDOI
TL;DR: A robust energy-aware clustering architecture (REACA) for large-scale wireless sensor networks is proposed and it is shown that by exploiting traffic locality, clustering can achieve performance improvement both in capacity and in power consumption over general-purpose ad hoc networks.
Abstract: The emergence of wireless sensor networks has imposed many challenges on network design such as severe energy constraints, limited bandwidth and computing capabilities. This kind of networks necessitates network protocol architectures that are robust, energy-efficient, scalable, and easy for deployment. This paper proposes a robust energy-aware clustering architecture (REACA) for large-scale wireless sensor networks. We analyze the performance of the REACA network in terms of quality-of-service, asymptotic throughput capacity, and power consumption. In particular, we study how the throughput capacity scales with the number of nodes and the number of clusters. We show that by exploiting traffic locality, clustering can achieve performance improvement both in capacity and in power consumption over general-purpose ad hoc networks. We also explore the fundamental trade-off between throughput capacity and power consumption for single-hop and multi-hop routing schemes in cluster-based networks. The protocol architecture and performance analysis developed in this paper provide useful insights for practical design and deployment of large-scale wireless sensor network.

Proceedings ArticleDOI
26 Aug 2007
TL;DR: In this article, a robust and low complexity scheme to estimate and track carrier frequency from the received signals at the Earth end is proposed, which employs a hierarchical arrangement of convex linear prediction cells that is dynamically adapted to respond to channel conditions.
Abstract: In the Mars rover missions the signals transmitted back to Earth travel under low SNR conditions in highly non-stationary channels [1, 2]. During the entry, descent and landing phase (EDL), the spacecraft high dynamics yields severe Doppler effects. We propose a robust and low complexity scheme to estimate and track carrier frequency from the received signals at the Earth end. The method employs a hierarchical arrangement of convex linear prediction cells that is dynamically adapted to respond to the channel conditions. The adaptive combination is able to outperform the best individual estimator in the set, leading to a universal scheme for frequency estimation and tracking. In order to compensate the lag error effect, we explore an efficient forward and backward aggregation scheme that improves considerably the frequency RMS error as compared to the original method [3].

Proceedings ArticleDOI
25 Apr 2007
TL;DR: This paper presents a distributed sampling and estimation framework based on innovations diffusion, within which the sensor selection and estimation are accomplished through local computation and communications between sensor nodes.
Abstract: This paper considers an estimation network of many distributed sensors with a certain correlation structure. Due to limited communication resources, the network selects only a subset of sensor measurements for estimation as long as the resulting fidelity is tolerable. We present a distributed sampling and estimation framework based on innovations diffusion, within which the sensor selection and estimation are accomplished through local computation and communications between sensor nodes. In order to achieve energy efficiency, the proposed algorithm uses a greedy heuristics to select a nearly minimum number of active sensors in order to ensure the desired fidelity for each estimation period. Extensive simulations illustrate the effectiveness of the proposed sampling scheme.

Journal ArticleDOI
TL;DR: The proposed filter can be used to mitigate the dispersion penalty in other fiber optic applications as well and increases the overall achievable signal-to-noise-and-distortion ratio to 52 dB in the presence of linear distortions.
Abstract: Time-stretched analog-to-digital converters (ADCs) have offered revolutionary enhancements in the performance of electronic converters by reducing the signal bandwidth prior to digitization. An inherent limitation of the time-stretched ADC is the frequency-selective response of the optical system that reduces the effective number of bits for ultrawideband signals. This paper proposes a solution based on spatio-temporal digital processing. The digital algorithm exploits the optical phase diversity to create a flat RF frequency response, even when the system's transfer function included deep nulls within the signal spectrum. For a 10times time-stretch factor with a 10-GHz input signal, simulations show that the proposed solution increases the overall achievable signal-to-noise-and-distortion ratio to 52 dB in the presence of linear distortions. The proposed filter can be used to mitigate the dispersion penalty in other fiber optic applications as well.

Proceedings ArticleDOI
15 Apr 2007
TL;DR: An iterative algorithm is proposed to jointly estimate the channel coefficients and the impairment parameters of OFDM systems and it is shown by computer simulations that the algorithm performs close to its associated Cramer-Rao lower bound.
Abstract: OFDM systems are susceptible to receiver impairments like frequency offset, IQ imbalance and phase noise. In this paper, OFDM channel estimation in the presence of these impairments is studied, and an iterative algorithm is proposed to jointly estimate the channel coefficients and the impairment parameters. It is shown by computer simulations that the algorithm performs close to its associated Cramer-Rao lower bound.