Pilot contamination mitigation strategies in massive MIMO systems

Compared with the traditional multi-user MIMO (multiple-input and multiple-output), massive MIMO aims to serve tens of users with hundreds of antennas on each base station. All users can use the same time-frequency resources through space division multiple access, leading to vast improvement on spectral efficiency. However, to achieve the benefits, channel state information is usually required, and the acquisition is difficult in massive MIMO systems. Theoretically, each user should be assigned with orthogonal pilot sequences to avoid interference; however, due to the huge number of users (much more than available orthogonal pilot sequences) in service, pilot reuse in adjacent cells is inevitable, causing inter-cell interference. This phenomenon is often referred to as pilot contamination (PC) and is believed to be the fundamental limit on system capacity of massive MIMO systems. To solve this problem, many methods have been proposed since 2010, when the concept of massive MIMO was first proposed. In this study, the authors reviewed these methods, categorised them into four groups and compared their advantages and limitations. Although a survey on PC has been conducted by Elijah et al., where they tried to cover various aspects of the PC issue, their work focuses on the analysis of rationale and limitations of different contamination mitigation methods. Besides, performance evaluations are conducted and presented.