Showing papers by "Ali H. Sayed published in 2002"

Multi-input multi-output fading channel tracking and equalization using Kalman estimation

Abstract: This paper addresses the problem of channel tracking and equalization for multi-input multi-output (MIMO) time-varying frequency-selective channels. These channels model the effects of inter-symbol interference (ISI), co-channel interference (CCI), and noise. A low-order autoregressive model approximates the MIMO channel variation and facilitates tracking via a Kalman filter. Hard decisions to aid Kalman tracking come from a MIMO finite-length minimum-mean-squared-error decision-feedback equalizer (MMSE-DFE), which performs the equalization task. Since the optimum DFE for a wide range of channels produces decisions with a delay /spl 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. Applications include synchronous multiuser detection of independent transmitters, as well as coordinated transmission through many transmitter/receiver antennas, for increased data rate.

Ability of adaptive filters to track carrier offsets and channel nonstationarities

Abstract: This paper studies the tracking performance of adaptive filters operating in the presence of two sources of nonstationarities: carrier frequency offsets and random channel variations. Both impairments are common in digital communications due to mismatches between transmitter and receiver carrier generators and channel fading. The paper derives expressions for the mean-square error and shows how filter performance is degraded under such nonstationary conditions. Selections of step sizes for optimal tracking performance are derived, different adaptive algorithms are compared, and supporting simulation results are provided.

SNAP&TELL: a multi-modal wearable computer interface for browsing the environment

Abstract: This paper gives an overview of a multi-modal wearable computer system 'SNAP&TELL', which performs real-time gesture tracking combined with audio-based system control commands to recognize objects in the environment including outdoor landmarks. Our system uses a single camera to capture images which are then processed using several algorithms to perform segmentation based on color fingertip shape analysis, robust tracking, and invariant object recognition, in order to quickly identify the objects encircled (SNAPshot) by the user's pointing gesture. In turn, the system returns an audio narration, telling the user information concerning the object's classification, historical facts, usage, etc. This system provides enabling technology for the design of intelligent assistants to support "Web-On-The-World" applications, with potential uses such as travel assistance, business advertisement, the design of smart living and working spaces, and pervasive wireless services and internet vehicles.

Method and apparatus for resolving multipath components for wireless location finding

Abstract: A method and apparatus that provides an accurate estimate of the time and amplitude of arrival of the first arriving overlapping multipath components (rays) in wireless locating finding systems. Overlapping fading multipath components for mobile-positioning are resolved by exploiting the fact that multipath components fade independently. Although fast channel fading is usually considered a challenge to the location finding process, it is used as an additional tool to detect and resolve overlapping multipath rays. A projection technique is also provided that exploits all possible a-priori channel information into a adaptive filtering algorithm, thus providing needed robustness to divergence of the adaptive algorithm that might result from possible severe data matrix ill-conditioning and high noise levels, which are common in wireless location applications.

Adaptive frequency-domain equalization of space-time block-coded transmissions

Abstract: We develop an adaptive equalization scheme for space-time block-coded (STBC) transmissions. The scheme is based on a modified low-complexity version of the fast-converging RLS algorithm. Complexity reduction is achieved by exploiting the rich structure of STBC.

Robust exponential filtering for uncertain systems with stochastic and polytopic uncertainties

Abstract: This paper addresses the problem of robust exponential filtering for discrete uncertain systems with mixed stochastic and deterministic uncertainties, in addition to unmodelled nonlinearities and measurement and process noises with bounded variances

Closed loop power control techniques

Abstract: The present invention provides a method for predicting channel power fading in a wireless communications system. This prediction method estimates the channel power fading via oversampling of the received and transmitted powers. The prediction method is then combined with several proposed structures for closed loop power control. The resulting structures result in improved performance in wireless communications systems.

Optimum error nonlinearities for long adaptive filters

Abstract: In this paper, we consider the class of adaptive filters with error nonlinearities. In particular, we derive an expression for the optimum nonlinearity that minimizes the steady-state error and attains the limit mandated by the Cramer-Rae bound of the underlying estimation process.

SNAP&TELL: A Vision-Based Wearable System To Support 'Web-On-The-World' Applications

Abstract: This paper gives an overview of a vision-based wearable computer system 'NAP&TELL' 1 , which performs realtime gesture tracking for recognizing objects in the scene including outdoor landmarks. Our system uses a single camera to capture images which are processed using several algorithms to perform segmentation based on color, fingertip shape analysis, robust tracking, and invariant object recognition, in order to quickly identify the objects encircled by a user's pointing gesture. In turn, the system returns information concerning the object such as its classification, historical facts, etc. This system provides enabling technology for the design of intelligent assistants to support ”Web-On-TheWorld” applications, with potential uses such as travel assistance, business advertisement, the design of smart living and working spaces, and pervasive wireless services and internet vehicles.

State-space estimation with uncertain models

Abstract: There are always discrepancies between design models and the actual physical systems or phenomena that they model. Regardless of their source, such perturbations can degrade the performance of otherwise optimal designs. This article discusses a design strategy for models with bounded perturbations. In comparison to other robust formulations, the resulting procedure performs data regularization as opposed to de-regularization. Applications in state-space estimation and adaptive filtering are discussed.

Reducing power control errors in wireless communication system

Abstract: The present invention provides a method for predicting channel power fading in a wireless communications system. This prediction method estimates the channel power fading via oversampling of the received and transmitted powers. The prediction method is then combined with several proposed structures for closed loop power control. The resulting structures result in improved performance in wireless communications systems.

Recursive Construction of Multichannel Transmission Lines with a Maximum Entropy Property

Abstract: J. P. Burg gave a now-famous solution of the problem of finding the maximum-entropy extension of a section {c o, c 1,…, c n } of a scalar (stationary) covariance sequence, with a nice interpretation in terms of the reflection coefficients of an associated discrete transmission line: namely, that the maximum-entropy extension is obtained by adding trivial sections with zero reflection coefficients. This work inspired various extensions and generalizations, mostly without any associated physical interpretation. In this paper we shall use the generalized transmission lines associated with the basic triangular matrix factorization algorithms of the Displacement Structure Theory to obtain several interesting maximum entropy extension results. One is the perhaps surprising fact that Burg’s zero-reflection-coefficient extension solution may not always hold when the coefficients {c i } are matrix-valued.

Predictive ratio closed-loop power control for CDMA wireless systems

Abstract: In this work, we discuss the limitations of conventional closed loop power control (CLPC). We derive a closed-form expression for the control error. We then propose two algorithms that reduce this error. Simulations show a reduced power error compared to current CLPC techniques.

Inverse closed-loop power control for CDMA wireless systems

Abstract: In this work, two algorithms for closed loop power control in CDMA wireless systems are proposed. These algorithms aim to minimize the power control error by predicting the inverse of the channel fading: For this purpose, a channel fading prediction scheme is also proposed.