Uplink sum rate analysis with flexible pilot reuse

Pilot contamination caused by non-orthogonal pilot sequences reuse in adjacent cells has become one of the main impairments for massive multiple-input multiple-output (MIMO) systems. A scheme named flexible pilot reuse is proposed in this paper to reduce the effect. The main idea is that users suffered from mild pilot contamination (PC) are allocated the same set of orthogonal pilots, while the users suffered from severe PC in adjacent cells all use orthogonal pilots. That is users in a cell have different reuse factors according to the severity of PC. We derive closed-form approximate expressions of uplink achievable rate for both mild and severe PC groups, then analyze the optimal reuse factor and obtain the suitable scope of cell users for the design of flexible pilot reuse. Simulation results show that the uplink sum rate is improved significantly with the proposed scheme compared to existing integer reuse.