On composite channel estimation in wireless massive MIMO systems

A multiuser MIMO (MU-MIMO) TDD system in which the base station (BS) uses a large scale antenna array to communicate with single-antenna mobile users is considered. We present novel schemes to estimate both large-scale and small-scale fading coefficients by uplink pilot sequences. The large-scale fading coefficients (LSFCs) are usually ignored or assumed perfectly known and almost all MIMO channel state information (CSI) estimation studies deal solely with small-scale fading coefficients (SSFCs). We propose low complexity algorithms that give reliable estimates for both FCs. The LSFC estimator takes advantage of the excess spatial samples received and the channel hardening effect while the SSFC estimator is based on a rank-reduction approach. Our estimators require a low training overhead and both analysis and numerical experiments prove that they offer superior mean-squared error performance.