  A method for channel estimation in wideband massive Multiple-Input
Multiple-Output (MIMO) systems using covariance identification is developed.
The method is useful for Frequency-Division Duplex (FDD) at either sub-6GHz or
millimeter wave (mmWave) frequency bands and takes into account the beam squint
effect caused by the large bandwidth of the signals. The method relies on the
slow time variation of the channel covariance matrix and allows for the
utilization of very short training sequences thanks to the exploitation of the
channel structure, both in the covariance matrix identification and the channel
estimation stages. As a consequence of significantly reducing the training
overhead, the proposed channel estimator has a computational complexity lower
than other existing approaches.
