No abstract

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.

Effective channel estimation for massive multiple-input-multiple-output (MIMO) systems based on frequency division duplex (FDD) protocol is a crucial problem. To reduce pilot overhead, we consider the joint sparsity of the multi-path channel in delay-angle domain. Based on the joint sparsity, we propose a decoupling pilot design scheme. In the proposed scheme, the pilot design is decoupled into two parts. In the first part, a global optimal selection greedy iterative algorithm (GOS-GIA) is proposed to obtain the near-optimal pilot subcarrier pattern. In the second part, a random Rademacher distribution pilot matrix is used as the angle-domain pilot matrix. Accordingly, a two-stage channel estimation strategy is also provided and analyzed. The first stage of the strategy is concerned with retrieving the positions of non-zero dominant taps in the delay domain. The second stage focuses on estimating the channel coefficients at these taps. Algorithm complexity analysis shows that the GOS-GIA can reduce the computational complexity at least one order of magnitude, and the proposed channel estimation strategy reduces the computational complexity by more than two orders of magnitude. Simulation results verify that the proposed pilot design scheme achieves better performance with lower pilot overhead. INDEX TERMS Angle domain, channel estimation, compressive sensing, delay domain, massive multiple-input-multiple-output, pilot design, sparsity.