Mitigation Pilot Contamination Based on Matching Technique for Uplink Cell-Free Massive MIMO Systems

A cell-free massive MIMO is a new wireless architecture that integrates user-centric network densification with distributed massive multiple-input multiple-output (MIMO) operation. Cell-free massive MIMO networks can provide a high quality of service (QoS) in terms of low latency, ultra-high reliability and high speed. However, because this system is affected by the pilot contamination phenomenon when a large number of users and a small number of available pilots exists, the QoS will deteriorate due to the low accuracy of the channel estimation. In this paper, we address this problem by presenting two novel schemes of pilot assignment and pilot power control design based on the matching technique for the uplink of cell-free massive MIMO systems to maximize spectral efficiency. We first formulate an assignment optimization problem in order to find the best possible pilot sequence to be used by utilizing genetic algorithm (GA) and then propose a Hungarian matching algorithm to solve this formulated problem. Regarding the power control design, we formulate a minimum-weighted assignment problem to assign pilot power control coefficients to the estimated channel's minimum mean-squared error by considering the access point (AP) selection. Then, we also propose the Hungarian algorithm to solve this problem. Simulation results show that our proposed schemes outperform the state-of-the-art techniques concerning both the pilot assignment and the pilot power control design by achieving a 15% improvement in the spectral efficiency. Finally, the computational complexity analysis is provided for the proposed schemes compared with the state-of-the-art techniques.