Realizing Asynchronous Massive MIMO with Trellis-Based Channel Estimation and Superimposed Pilot

In this paper, we investigate channel estimation for the massive Multiple-Input Multiple-Output (MIMO) systems, by deploying superimposed pilots. The superimposed pilots are the a priori known spreading sequences, repeated cyclically beyond the signal layer. They reserve a marginal fraction of the total transmit energy. Thus, the initial channel estimates can be firstly obtained by separating the users with respect to the corresponding code division orthogonality, and can combat against the 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.