How Often Do We Need to Estimate Wireless Channels in Massive MIMO with Channel Aging?

In massive multiple-input-multiple-output (MIMO) systems, wireless channels are estimated at a fixed and short time interval for all users, which causes redundancy since most of users in practice might have a considerably larger coherence time than the prescribed time interval of estimation, and consequently wastes considerable signaling resources on channel acquisition. In this paper, we propose a novel channel estimation scheme for time-division-duplex massive MIMO systems, which can fully exploit the redundancy by exploring the temporal channel correlation underlying the channel aging effect. We also derive a rigorous lower bound on the achievable spectral efficiency, and maximize the lower bound to determine the optimal time interval of channel estimation. Numerical results show that the proposed estimation scheme can offer great spectral efficiency gains over the conventional one, and provide insights on how to put into practice the proposed scheme.