Mitigating pilot contamination in massive MIMO system — 5G: An overview

5G, a future generation cellular networks is expected to meet the increasing demand of continuous growing mobile users and the emergence of services such as internet of things (IoT), machine-to-machine communication (M2M). Massive MIMO has been identified as a promising technology that offers orders-of-magnitude improvements in spectral-efficiency over 4G technologies. However, one of the major limiting factors is pilot contamination when TDD protocol is considered. Different methods have been proposed in literature to mitigate effect of pilot contamination in multi-cellular systems. This paper provides an overview of the proposed approaches in mitigating the effect of pilot contamination in massive MIMO systems. First, we classify the methods proposed under two categories: pilot-based approach and subspace-based approach. We examine the problems of pilot contamination and open issues are discussed.