Adaptive Channel Estimation for TDD Massive MIMO Systems with Heterogeneous User Coherence Times

In massive multiple-input-multiple-output (MIMO) systems, users periodically send pilot sequences for channel estimation in a fixed time interval, which results in pilot redundancy when a user undergoes longer coherence time than the time interval defined by LTE specification. In this paper, we propose an adaptive channel estimation scheme for time-divisionduplex massive MIMO systems, which reuses the aged channel estimations by exploiting the temporal channel correlations underlying the channel aging effect. We also derive the net sum spectral efficiency of the proposed scheme and design a novel method to determine the channel estimation interval for each user. Numerical results demonstrate significant performance gains as compared to conventional ones, indicating that our proposed scheme is adaptive to diverse communication scenarios.