Efficient User Identification and Semi-Blind Channel Estimation for MU-MIMO Systems

In this letter, we consider an uncoordinated access to the uplink of MU-MIMO cellular networks. Based on randomly rotated modulation constellations, we propose an efficient approach to blind identification of active users and estimation of all involved channels, up to a discrete phase ambiguity. To eliminate this ambiguity, we use short orthogonal pilot sequences. To improve the obtained channel estimates, we also rely on a combination of pilot-based and blind channel estimates. Moreover, since no coordination is assumed with the base station the problem of pilot contamination can occur when some active users use the same pilot sequence. As a solution for this problem, we propose an efficient semi-blind recovery of all contaminated channel estimates.