Pilot contamination in multi-cell massive-MIMO systems in 5G wireless communications

Massive-MIMO systems are the emerging systems for the new generation 5G. Massive-MIMO systems have the particularity of increasing the performance of the system while providing a very large number of antennas to serve a very large number of users. However, the orthogonal pilot sequences which can be served are limited, then non-orthogonal pilot sequences are used to satisfy the requirements. These nonorthogonal pilot sequences in the adjacent cells overlap with each other; Consequently, they cause the problem of pilot contamination. This paper presents the problem of pilot contamination in the uplinks and the downlinks. The pilot contamination reduce the performance of the system where the identical pilot sequences interferes with each other and consequently an estimate is obtained contaminating channels of different user.