Channel Estimation and Pilot Contamination in Massive MIMO: Challenges, Trends, and Emerging Solutions

Massive multiple input multiple output (MIMO) is an important technology to 5G and beyond wireless communication systems because it is capable of improving the spectral efficiency, energy efficiency, and link reliability. However, precise channel state information (CSI) acquisition is a key requirement for obtaining these benefits. Channel estimation in Massive MIMO is a challenging task especially because of the problem of pilot contamination in which pilot signals from neighboring cells interfere and degrades estimation quality. This paper explores channel estimation techniques and pilot contamination mitigation strategies in Massive MIMO networks from both foundational and emerging perspectives. We describe how different estimation methods are implemented, including least squares (LS), minimum mean square error (MMSE), and compressed sensing (CS) based ones. Moreover, we investigate the effect of the pilot contamination and discuss mitigation approaches, including optimization of pilot reuse, advanced precoding, and deep learning-based approaches. Finally, we highlight open research challenges and future directions.