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Unformatted text preview: IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 46, NO. 2, FEBRUARY 1998 211 MaximumLikelihood Synchronization, Equalization, and Sequence Estimation for Unknown TimeVarying FrequencySelective Rician Channels Brian D. Hart, Member, IEEE , and Desmond P. Taylor, Fellow, IEEE Abstract— This paper develops a receiver structure to perform jointly maximumlikelihood (ML) synchronization, equalization, and detection of a linearly modulated signal transmitted over a timevarying frequencyselective Ricianfaded channel, corrupted by additive white Gaussian noise (AWGN). The receiver is partic ularly suited to a fastfading 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 [11]. We show that the predictors can achieve synchronization in a novel manner, if synchronization is required. A union bound on the receiver’s biterror rate (BER) is derived, and it tightly bounds simulated BER’s in fast fading at high signaltonoise ratios (SNR’s). Index Terms— Dispersive channels, error analysis, linear pre diction, maximumlikelihood detection, parameter estimation, Ri cian channels, synchronization, timevarying channels. I. INTRODUCTION W HEN communicating with fast moving mobile termi nals in a multipath channel, the receiver observes a delay and Dopplerspread signal. In the time domain, this Doppler spread is experienced as a timevarying 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 signaltonoise ratio (SNR) does not improve the biterror rate (BER) [1], [2]. Several approaches have been considered in the literature to surmount the problem, particularly for frequencyflat channels [3]–[5]. It is instructive to consider receiver structures that are actually optimal for the timevarying frequencyselective Ricianfading channel model. Different maximumlikelihood 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 (email: Brian.Hart@anu.edu.au)....
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