Achievable Rates and Spectral Efficiency Analysis in Massive MIMO Systems with MMSE Channel Estimation

摘要

Pilot contamination due to the re-use of limited pilot resources from one cell to another in Time Division Duplex multi-cell massive MIMO systems is a serious problem. This is because it severely affects the quality of the Channel State Information. In this paper, we analyze the performance of two pilot allocation schemes and derive closed-form lower bounds on the uplink achievable rates for the cases of maximum-ratio combining (MRC) and zero-forcing (ZF) detectors with minimum mean squared error channel estimation. We consider the case where the users are uniformly distributed in all cells and show the maximum and optimal number of users per cell that maximizes the spectral efficiency. We further analyze the system performance with respect to the signal-to-interference-plus-noise ratio (SINR) and coherence block length and show the superiority of MRC to ZF at low SINR values and vice versa and that orthogonal pilot sequences with ZF detector gives a better performance at high coherence block length. Our simulation results show the accuracy of the derived bounds.