A Comprehensive Survey of Pilot Contamination in Massive MIMO—5G System

TLDR

Massive MIMO promises higher data capacity for mobile networks, but requires accurate channel state information, typically obtained via TDD, where non‑orthogonal pilot schemes in multi‑cell networks lead to pilot contamination. This survey examines pilot contamination in massive MIMO, identifying additional sources such as hardware impairments and non‑reciprocal transceivers. The authors review theoretical analyses of pilot contamination’s impact on achievable rates and classify mitigation strategies into pilot‑based and subspace‑based approaches. Open issues include training overhead, deployment scenarios, computational complexity, channel reciprocity usage, and future trends in pilot contamination mitigation.

Abstract

Massive MIMO has been recognized as a promising technology to meet the demand for higher data capacity for mobile networks in 2020 and beyond. Although promising, each base station needs accurate estimation of the channel state information (CSI), either through feedback or channel reciprocity schemes in order to achieve the benefits of massive MIMO in practice. Time division duplex (TDD) has been suggested as a better mode to acquire timely CSI in massive MIMO systems. The use of non-orthogonal pilot schemes, proposed for channel estimation in multi-cell TDD networks, is considered as a major source of pilot contamination in the literature due to the limitations of coherence time. Given the importance of pilot contamination in massive MIMO systems, we provide an extensive survey on pilot contamination, and identify other possible sources of pilot contamination, which include hardware impairment and non-reciprocal transceivers. We review established theories that have analyzed the effect of pilot contamination on the performance of massive MIMO systems, particularly on achievable rates. Next, we categorize the different proposed mitigation techniques for pilot contamination using the following taxonomy: pilot-based approach and subspace-based approach. Finally, we highlight the open issues, such as training overhead, deployment scenario, computational complexity, use of channel reciprocity, and conclude with broader perspective and a look at future trends in pilot contamination in massive MIMO systems.