Precoding and Pilot Scheduling in Very Large MIMO Multi-Cell TDD Systems

The performance of very large MIMO multi-cell time division duplexing system is bottlenecked by the problem of uplink pilot contamination. The asymptotic achievable sum rate of precoding schemes, including Matched Filter (MF), single-cell Zero-Forcing (ZF), multi-cell Minimum Mean Square Error (MMSE)-based and maximize Signal-Leakage-Noise Ratio (SLNR)-based, with large arrays in downlink was investigated. As the antenna of base station grows without limit, the effective signal-to-interference ratio of each precoder converges to a ratio only associated with large scale fading coefficients, from which an asymptotic result of achievable sum rate of system can be achieved. A pilot scheduling scheme was proposed by maximizing the system achievable sum rate, which making users among cells with weak interference using same pilots while users with strong interference using orthogonal pilots. It could enhance the performance of system in the presence of pilot contamination. The theoretical analysis and numerical results both verified the results.