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Unformatted text preview: IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 46, NO. 2, FEBRUARY 1998 211 Maximum-Likelihood Synchronization, Equalization, and Sequence Estimation for Unknown Time-Varying Frequency-Selective Rician Channels Brian D. Hart, Member, IEEE , and Desmond P. Taylor, Fellow, IEEE Abstract— This paper develops a receiver structure to perform jointly maximum-likelihood (ML) synchronization, equalization, and detection of a linearly modulated signal transmitted over a time-varying frequency-selective Rician-faded channel, corrupted by additive white Gaussian noise (AWGN). The receiver is partic- ularly suited to a fast-fading channel, where other receivers that rely on estimating the channel cannot track it quickly enough. The signal mean and autocovariance are needed, and a scheme is proposed for estimating these quantities adaptively. The receiver processes the specular and diffuse components (corresponding to the signal mean and autocovariance) separately. Processing the known specular component is the classical detection problem. The unknown diffuse component is processed by predictors . We show that the predictors can achieve synchronization in a novel manner, if synchronization is required. A union bound on the receiver’s bit-error rate (BER) is derived, and it tightly bounds simulated BER’s in fast fading at high signal-to-noise ratios (SNR’s). Index Terms— Dispersive channels, error analysis, linear pre- diction, maximum-likelihood detection, parameter estimation, Ri- cian channels, synchronization, time-varying channels. I. INTRODUCTION W HEN communicating with fast moving mobile termi- nals in a multipath channel, the receiver observes a delay and Doppler-spread signal. In the time domain, this Doppler spread is experienced as a time-varying channel. If the Doppler spread is significant compared to the symbol rate, then the channel becomes difficult to track, and most existing receiver structures exhibit an error floor, where an increase in signal-to-noise ratio (SNR) does not improve the bit-error rate (BER) , . Several approaches have been considered in the literature to surmount the problem, particularly for frequency-flat channels –. It is instructive to consider receiver structures that are actually optimal for the time-varying frequency-selective Rician-fading channel model. Different maximum-likelihood Paper approved by S. Ariyavisitakul, the Editor for Wireless Techniques and Fading of the IEEE Communications Society. Manuscript received June 18, 1996; revised January 27, 1997 and September 2, 1997. This paper was presented in part at the IEEE Communications Theory Workshop, Tucson, AZ, April 1997. B. D. Hart is with Telecommunications Engineering, Research School of Information Sciences and Engineering, Australian National University, Canberra 0200, ACT, Australia (e-mail: Brian.Hart@anu.edu.au)....
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