Enhancing Spectral Efficiency Using Aged CSI in Massive MIMO Systems

Abstract

In time-division duplex (TDD) massive multiple-input multiple-output (MIMO) systems, the spatial multiplexing gain cannot be fully achieved since available pilot resources in each channel coherence time interval are limited. In this paper, we propose OpSAC, an Opportunistic user Scheduling algorithm that uses Aged Channel state information (CSI) to increase the spatial multiplexing gain without incurring additional pilot overhead. Assuming the base station (BS) employs precoders of maximum ratio transmission (MRT) and zero-forcing (ZF), we first derive closed- form lower bounds on the achievable sum-rate under channel aging. According to the analysis results, we develop a heuristic solution which opportunistically schedules additional users by exploiting aged CSI to enhance spectral efficiency. Through simulation, we confirm that our analysis is accurate, and show the impact of channel aging on the performance of massive MIMO systems. In addition, it is shown that OpSAC achieves near-optimal performance and considerably outperforms the conventional user scheduling algorithm that only uses current CSI.