Realizing asynchronous massive MIMO with trellis-based channel estimation and superimposed pilots

In this paper, we investigate channel estimation for the massive Multiple-Input Multiple-Output (MIMO) systems, by deploying superimposed pilots. The pilots are the a priori known spreading sequences, repeated cyclically and superimposed to the signal layer. They reserve a marginal fraction of the total transmit energy. Thus, the initial channel estimates can be initially obtained by separating the users with respect to the corresponding code-division orthogonality with robustness against asynchronicity. Further, during a second stage, iterative channel estimation is considered, by integrating a trellis-based channel estimator, which can significantly enhance the channel estimation during the iterations, by reusing the superimposed pilots. Notice that the so-called turbo-effect can be clearly observed, in which both data detection and channel estimation are jointly improved between consecutive iterations.