EM algorithm-based estimation of amplitude, carrier phase and noise variance in multiuser turbo receivers

The current paper addresses the issue of estimating the users received carrier phase offsets and received amplitudes as well as the noise variance in an asynchronous DS-CDMA environment with frequency nonselective propagation channels. The user propagation delays are assumed to be known or estimated beforehand. The proposed synchronizer is based on the expectation-maximization (EM) algorithm and takes benefit from the soft information delivered by the receiver which is of turbo type. Besides, this paper explains which training symbols should be preferably chosen to initialize this synchronizer. It also compares two methods for combining the training-based estimate and the EM estimate. Performance of the resulting synchronizer is illustrated by simulation results.