  Interference during the uplink training phase significantly deteriorates the
performance of a massive MIMO system. The impact of the interference can be
reduced by exploiting second order statistics of the channel vectors, e.g., to
obtain minimum mean squared error estimates of the channel. In practice, the
channel covariance matrices have to be estimated. The estimation of the
covariance matrices is also impeded by the interference during the training
phase. However, the coherence interval of the covariance matrices is larger
than that of the channel vectors. This allows us to derive methods for accurate
covariance matrix estimation by appropriate assignment of pilot sequences to
users in consecutive channel coherence intervals.
