Research On The Pilot Scheduling Schemes For Multi-cell Multi-user Massive MIMO Systems

Massive multi-input multi-output(MIMO) system at the base station side equipped with large-scale antenna array uses the same time-frequency resources to provide service for users, which increases the user connection number density as well as improves energy efficiency and system capacity. However in the mobile communication applications, the pilot sequences length and pilot scale are limited by complex radio propagation environment and low channel coherence time, pilot contamination caused by inter-cell pilot reuse may lead to channel estimation error which affects the performance of received signal detection and transmit precoding. This work focuses on multiple cell multiple user massive MIMO system pilot scheduling schemes, which aims to design the effective pilot scheduling schemes. The proposed schemes reduce the pilot contamination and improve the comprehensive performance of the system. The main contents are as follows:(1) The cause and impact of pilot contamination, as well as typical mitigating pilot contamination measures in the multiple cell multiple user massive MIMO system are analyzed. In allusion to the multiple cell multiple user massive MIMO system model based on TDD protocol, the impact of pilot contamination on achievable rate of both uplink and downlink is analyzed respectively. The typical mitigating pilot contamination measures are briefly summarized sequentially.(2) Based on the random pilot scheduling scheme, this thesis proposes a pilot scheduling scheme named pilot scheduling based on link gain to perform user grouping(PS-UG). The design concept of PS-UG is to choose a tradeoff between mitigating pilot contamination(achieved by full reuse pilot scheduling scheme) and pilot overhead(achieved by full orthogonal pilot scheduling scheme). The proposed PS-UG scheme divides the users into the center user group and the edge user group according to different levels of pilot contamination, in which the center users group consists of the users in all cells suffering from slight pilot contamination, the edge users group consists of the users in the adjacent cells suffering from severe pilot contamination. The proposed PS-UG scheme assigns orthogonal pilot sequences to the edge users in the adjacent cells, while the center users of all cells reuse pilot sequences. The simulations choose channel estimation normalized MSE, uplink SINR and achievable rate as metrics of pilot scheduling scheme effectiveness and reliability, analyze and evaluate the performance of the proposed PS-UG scheme in two cases, where one case is with fixed edge users and variable base station antenna number, the other is with fixed base station antennas and variable edge user number.(3) One main drawback of the SPR scheduling scheme is that the user grouping threshold setting merely considers the average value of large scale fading factor, ignoring the interference of the edge users who are far away to target cell in adjacent cells. This thesis proposes an improved soft pilot reuse(ISPR) scheduling scheme, which completes the first user grouping based on the SPR scheduling scheme, then performs a second screening according to the selection criteria for edge users in adjacent cells causing a small interference to the target cell. The simulations choose channel estimation normalized MSE, uplink SINR and achievable rate as metrics of pilot scheduling scheme effectiveness and reliability, analyze and evaluate the performance of the proposed ISPR scheduling scheme in two cases, where one case is with fixed grouping parameter and variable base station antenna number, the other is with fixed base station antennas and variable grouping parameter.