ABSTRACT
Massive multiple-input multiple-output (MIMO) is leading 5G technology, which aims to equip base stations with large antenna arrays that simultaneously serve many active UEs in same time-frequency resource. Massive MIMO also associated with technologies of large-scale antenna systems, very large MIMO, hyper MIMO, full-dimension MIMO, etc. Massive MIMO is scaled-up version of conventional MIMO with significant improvement in data throughput, spectral, and energy efficiency by using relatively linear signal processing. It offers improved spectral efficiency owing to large multiplexing gain or by exploiting excess degrees of freedom. Extra degrees of freedom help in focusing energy with extreme narrow beam on smaller regions, which results in higher throughput and energy efficiency. Other massive MIMO benefits include low power consumption, reduced air-interface latency, lower interuser interference, and simplification of MAC layer protocols. Besides all advantages, its performance gets limited due to pilot contamination. It occurs due to reuse of orthogonal pilots in multicell scenarios. The number of orthogonal pilots is further upper-bounded by coherence interval and coherence bandwidth of system. This limitation paves the way to either use massive MIMO system in TDD mode or to explore novel precoding/beamforming techniques, which mitigate the directed interference that occurred due to pilot contamination.
