Showing papers by "Ali H. Sayed published in 2005"
TL;DR: An overview of wireless location challenges and techniques with a special focus on network-based technologies and applications is provided.
Abstract: Wireless location refers to the geographic coordinates of a mobile subscriber in cellular or wireless local area network (WLAN) environments. Wireless location finding has emerged as an essential public safety feature of cellular systems in response to an order issued by the Federal Communications Commission (FCC) in 1996. The FCC mandate aims to solve a serious public safety problem caused by the fact that, at present, a large proportion of all 911 calls originate from mobile phones, the location of which cannot be determined with the existing technology. However, many difficulties intrinsic to the wireless environment make meeting the FCC objective challenging. These challenges include channel fading, low signal-to-noise ratios (SNRs), multiuser interference, and multipath conditions. In addition to emergency services, there are many other applications for wireless location technology, including monitoring and tracking for security reasons, location sensitive billing, fraud protection, asset tracking, fleet management, intelligent transportation systems, mobile yellow pages, and even cellular system design and management. This article provides an overview of wireless location challenges and techniques with a special focus on network-based technologies and applications.
1,308 citations
TL;DR: The effect of IQ imbalances on OFDM receivers is studied, and system-level algorithms to compensate for the distortions are proposed, and bounds on the achievable performance of the compensation algorithms are derived and evaluated as a function of the physical distortion parameters.
Abstract: The implementation of orthogonal frequency division multiplexing (OFDM)-based physical layers suffers from the effect of In-phase and Quadrature-phase (IQ) imbalances in the front-end analog processing. The IQ imbalances can severely limit the achievable operating signal-to-noise ratio (SNR) at the receiver and, consequently, the supported constellation sizes and data rates. In this paper, the effect of IQ imbalances on OFDM receivers is studied, and system-level algorithms to compensate for the distortions are proposed. The algorithms include post-fast Fourier transform (FFT) least-squares and least mean squares (LMS) equalization, as well as pre-FFT correction using adaptive channel/distortion estimation and special pilot tones to enable accurate and fast training. Bounds on the achievable performance of the compensation algorithms are derived and evaluated as a function of the physical distortion parameters. A motivation is included for the physical causes of IQ imbalances and for the implications of the approach presented in this paper on designing and implementing wireless transceivers.
390 citations
18 Mar 2005
TL;DR: This paper considers an alternative approach based on maximizing the signal-to-leakage ratio (SLR) for designing transmit beamforming vectors in a multi-user system and finds that it outperforms the conventional beamforming scheme.
Abstract: Multi-user multiple-input multiple-output (MU-MIMO) wireless systems can provide a substantial gain in network downlink throughput by allowing multiple users to communicate in the same frequency and time slots. The challenge is to design transmit beamforming vectors for every user while limiting the co-channel interference (CCI) from other users. One approach is to perfectly cancel the CCI at every user, which requires a relatively large number of transmit antennas. In this paper, we consider an alternative approach based on maximizing the signal-to-leakage ratio (SLR) for designing transmit beamforming vectors in a multi-user system. One advantage of the proposed scheme is that it does not impose a restriction on the number of available transmit antennas; it also outperforms the conventional beamforming scheme.
195 citations
TL;DR: Simulation results show significant improvement in the achievable BER of the proposed MIMO receivers for space-time block-coded OFDM systems in the presence of IQ imbalances.
Abstract: Orthogonal frequency division multiplexing (OFDM) is a widely recognized modulation scheme for high data rate communications. However, the implementation of OFDM-based systems suffers from in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. Such imbalances are caused by the analog processing of the received radio frequency (RF) signal, and they cannot be efficiently or entirely eliminated in the analog domain. The resulting IQ distortion limits the achievable operating SNR at the receiver and, consequently, the achievable data rates. The issue of IQ imbalances is even more severe at higher SNR and higher carrier frequencies. In this paper, the effect of IQ imbalances on multi-input multioutput (MIMO) OFDM systems is studied, and a framework for combating such distortions through digital signal processing is developed. An input-output relation governing MIMO OFDM systems is derived. The framework is used to design receiver algorithms with compensation for IQ imbalances. It is shown that the complexity of the system at the receiver grows from dimension (n/sub R//spl times/n/sub T/) for ideal IQ branches to (2n/sub R//spl times/2n/sub T/) in the presence of IQ imbalances. However, by exploiting the structure of space-time block codes along with the distortion models, one can obtain efficient receivers that are robust to IQ imbalances. Simulation results show significant improvement in the achievable BER of the proposed MIMO receivers for space-time block-coded OFDM systems in the presence of IQ imbalances.
175 citations
TL;DR: In this article, a multiple-input multiple-output (MIMO) optical link based on coherent optics and its ability to exploit the inherent information capacity of multimode fiber is presented.
Abstract: We present a multiple-input multiple-output (MIMO) optical link based on coherent optics and its ability to exploit the inherent information capacity of multimode fiber. A coherent implementation differs from previous work in optical MIMO by allowing the system to tolerate smaller modal delay spreads, because of a much larger carrier frequency, and yet maintain the necessary diversity needed for MIMO operation. Furthermore, we demonstrate the use of MIMO adaptive equalization to mitigate intersymbol interference when exceeding the bandwidth-length product of the link. The impact of phase noise is studied with numerical simulation.
162 citations
TL;DR: This paper proposes dynamic rate and power control algorithms for distributed wireless networks that also account for the congestion levels in a network.
Abstract: There is a fundamental tradeoff between power consumption, data transmission rates, and congestion levels in a wireless network. These three elements influence the performance of rate and power control strategies, and they need to be coordinated judiciously. This paper proposes dynamic rate and power control algorithms for distributed wireless networks that also account for the congestion levels in a network. The design is pursued by formulating state-space models with and without uncertain dynamics and by determining control signals that help meet certain performance criteria (such as robustness and desired levels of signal-to-interference ratio). Simulation results illustrate the performance of the proposed control schemes.
96 citations
TL;DR: This letter provides a unified mean-square performance analysis of the class of data reusing adaptive algorithms, and the derivation relies on energy conservation arguments, and it does not restrict the regression data to being Gaussian.
Abstract: This letter provides a unified mean-square performance analysis of the class of data reusing adaptive algorithms. The derivation relies on energy conservation arguments, and it does not restrict the regression data to being Gaussian. Simulation results show that there is a relatively good match between theory and practice.
49 citations
18 Mar 2005
TL;DR: This paper studies the steady-state performance of an adaptive convex combination of transversal filters and shows its universality in the sense that the combination performs, in steady- state, at least as well as its best component.
Abstract: Combination approaches can improve the performance of adaptive schemes. In this paper, we study the steady-state performance of an adaptive convex combination of transversal filters and show its universality in the sense that the combination performs, in steady-state, at least as well as its best component. We specialize the results to a convex combination of LMS filters using energy conservation arguments.
32 citations
TL;DR: This letter shows that the matrix structure with 2/spl times/2 Alamouti sub-blocks remains invariant under several nontrivial matrix operations, including matrix inversion, Schur complementation, Riccati recursion, triangular factorization, and QR factorization.
Abstract: This letter shows that the matrix structure with 2/spl times/2 Alamouti sub-blocks remains invariant under several nontrivial matrix operations, including matrix inversion, Schur complementation, Riccati recursion, triangular factorization, and QR factorization.
31 citations
TL;DR: A robust power and rate control algorithm is proposed for distributed wireless networks where the network dynamics is modelled as an uncertain discrete-time state-delayed system.
30 citations
05 Jun 2005
TL;DR: 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.
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.
05 Jun 2005
TL;DR: A multi-sensor relay strategy that achieves path-loss saving and improved power efficiency and an mean-square error design is pursued and the performance is shown to improve as the number of relay sensors (N) increases.
Abstract: In this paper we propose a multi-sensor relay strategy that achieves path-loss saving and improved power efficiency. In the proposed distributed scheme, the relay sensors do not need to share information about the received signals. An mean-square error design is pursued and the performance is shown to improve as the number of relay sensors (N) increases. Specifically, it is shown that the average power usage per sensor and the total average power drop as O(1/N/sup 2/) and O(1/N), respectively.
18 Mar 2005
TL;DR: This paper proposes a globally distributed synchronization algorithm for wireless sensor networks that controls the network synchronization and the other sensors periodically broadcast synchronization pulses in an effort to overcome the effect of channel fading.
Abstract: This paper proposes a globally distributed synchronization algorithm for wireless sensor networks. In the proposed scheme, there is a master node that controls the network synchronization and the other sensors periodically broadcast synchronization pulses; they also monitor different frequency channels in an effort to overcome the effect of channel fading.
TL;DR: In this article, a Kalman filter solution to active control and its fast-array implementation are provided, where the adaptive control problem is formulated as a state-estimation problem and no interchanging of the adaptive filter and the secondary-path is imposed.
Abstract: A Kalman filter solution to active control and its fast-array implementation are provided. The adaptive control problem is formulated as a state-estimation problem and no interchanging of the adaptive filter and the secondary-path is imposed. Moreover, no estimate of the disturbance signal is needed, and we exploit the structure in the state–space matrices to derive a fast-array implementation. A minimum variance estimate of the controller coefficients and the secondary path state is obtained. When there is no uncertainty in the secondary path, state equivalence with the modified filtered-RLS algorithm is proven. Using exponential forgetting, the analysis shows that in the generation of the filtered reference signal in the modified filtered-RLS, exponential forgetting should be incorporated too. Simulations show the superiority in convergence of the fast-array Kalman algorithm over the fast-array modified filtered-RLS algorithm. Copyright © 2004 John Wiley & Sons, Ltd.
28 Jan 2005
TL;DR: This chapter provides an overview of interesting phenomena pertaining to the learning capabilities of stochastic-gradient adaptive filters, and in particular those of the least-mean-squares (LMS) algorithm.
Abstract: This chapter provides an overview of interesting phenomena pertaining to the learning capabilities of stochastic-gradient adaptive filters, and in particular those of the least-mean-squares (LMS) algorithm. The phenomena indicate that the learning behavior of adaptive filters is more sophisticated, and also more favorable, than was previously thought, especially for larger step-sizes. The discussion relies on energy conservation arguments and elaborates on both the mean-square convergence and the almost-sure convergence behavior of an adaptive filter.
17 Jul 2005
TL;DR: In this article, the tracking performance of an adaptive convex combination of two transversal filters is studied, where individual filters are independently adapted using their own error signals, while the combination is adapted to minimize the error of the overall structure.
Abstract: Combination approaches provide a useful way to improve adaptive filter performance. In this paper, we study the tracking performance of an adaptive convex combination of two transversal filters. The individual filters are independently adapted using their own error signals, while the combination is adapted to minimize the error of the overall structure. We show the universality of the approach with respect to the component filters, i.e., that the overall filter is able to track changes at least as well as the best component filter. Using energy conservation arguments, we then specialize the results to a combination of two LMS filters
17 Jul 2005
TL;DR: A relay strategy for Alamouti space-time coded transmissions is proposed that is optimized to maximize the received SNR and the receiver is able to exploit the orthogonal structure of the code to ensure temporal and spatial diversity.
Abstract: A relay strategy for Alamouti space-time coded transmissions is proposed. The relay structure is optimized to maximize the received SNR and the receiver is able to exploit the orthogonal structure of the code to ensure temporal and spatial diversity
TL;DR: A receiver structure for direct-sequence code-division multiple-access (DS-CDMA) systems is presented by using robust Kalman estimation and post-correlation processing for channel estimation and enhancement in performance is achieved at the same order of complexity as a standard Kalman-based receiver.
Abstract: Kalman filtering has been proposed in the literature for wireless channel estimation, however, it is not sufficiently robust to uncertainties in the channel auto-correlation model as well as to multiple access interference (MAI). This paper presents a receiver structure for direct-sequence code-division multiple-access (DS-CDMA) systems by using robust Kalman estimation and post-correlation (i.e., symbol rate) processing for channel estimation. The proposed structure is also generalized to incorporate multiple-antenna combining and interference cancellation techniques. The resulting receiver outperforms earlier structures in the presence of channel modeling uncertainties, MAI, and low-received signal-to-noise ratio. The enhancement in performance is achieved at the same order of complexity as a standard Kalman-based receiver.
08 Sep 2005
TL;DR: A modified relay scheme is proposed to compensate for the influence of imperfect channel information on system performance and a multi-relay strategy for wireless networks is described.
Abstract: This paper describes a multi-relay strategy for wireless networks and examines the influence of imperfect channel information on system performance. A modified relay scheme is proposed to compensate for such imperfections.
Journal Article•