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Turbo channel estimation for TDMA systems on highly time and frequency selective channels

In this paper, we consider a Time Division Multiple Access (TDMA) transmission of coded symbols over a time and frequency selective channel. We consider at the receiver a turbo-detector consisting of a Maximum A Posteriori (MAP) equalizer and a MAP decoder. We assume that the channel is unknown at the receiver. Thus, we develop a MAP iterative burst by burst channel estimation algorithm using the Expectation-Maximization (EM) algorithm. It needs a convenient representation of the channel based on the Karhunen-Love expansion theorem. It also needs a posteriori probabilities (APPs) on the coded symbols. These probabilities are provided by the MAP decoder at each iteration of the receiver. Simulations show that the performance of our iterative receiver approaches the performance obtained when the channel is perfectly known.