Pilot sequences allocation in TDD massive MIMO systems

Massive multiple-input multiple-output (MIMO) has been proposed as a key technology for the future fifth generation (5G) cellular networks. In time division duplex (TDD) massive MIMO systems, pilot contamination caused by channel estimation error is crucial to the system performance. In this paper, we propose a pilot sequences allocation strategy to mitigate the pilot contamination. In this strategy, the pilot sequences sets are identical for center users, but mutually orthogonal for edge users in different cells. With mitigated pilot contamination, we analytically determine the approximate system capacity which is accurate when the number of antennas at the base station tends to infinite. The simulation results show that the proposed pilot sequences allocation strategy achieves higher system capacity than the traditional pilot sequences allocation strategy whose sequences reuse rate is one or three. There also exists an optimal number of pilot sequences in different SNR to maximize the system capacity.