Maximum likelihood blind channel estimation in the presence of Doppler shifts

Transmitter/receiver motion in mobile radio channels may cause frequency sh ifts in each received path due to Doppler effects. Most blind equalization methods, however, assume time-invariant channels and may not be applicable to fading channels with severe Doppler spread. In this paper, we address th e problem of simultaneously estimating the Doppler shift and channel parame ters in a blind setup. Both deterministic and stochastic maximum likelihood methods are developed and iterative solutions proposed. The stochastic max imum likelihood solution is based on the modified version of the Baum-Welch algorithm, which originated in the study of hidden Markov models. The prop osed methods are well suited for short data records appearing in TDMA syste ms. Identifiability and performance analysis issues are discussed, and Cram er-Rao bounds are derived. In addition, some illustrative simulations are p resented.