Turbo synchronization: an EM algorithm interpretation

Abstract
This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchro- nization parameters can be implemented by means of the itera- tive expectation-maximization (EM) algorithm (1). Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algo- rithm proposed in (2) for carrier phase recovery turns out to be a particular instance of this implementation. The proposed math- ematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with itera- tive demodulation and decoding (3).

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