Pilot Contamination is Not a Fundamental Asymptotic Limitation in Massive MIMO

TL;DR

This paper demonstrates that pilot contamination in Massive MIMO systems is not a fundamental limitation and can be mitigated with advanced signal processing, leading to unbounded spectral efficiency growth as antennas increase.

Contribution

It proves that with multicell MMSE combining, pilot contamination does not prevent spectral efficiency from growing infinitely, challenging previous beliefs based on simplistic models.

Findings

Spectral efficiency grows unbounded with more antennas under certain conditions.

Multicell MMSE combining effectively mitigates pilot contamination.

The common belief about pilot contamination being a fundamental limit is incorrect.

Abstract

Massive MIMO (multiple-input multiple-output) provides great improvements in spectral efficiency over legacy cellular networks, by coherent combining of the signals over a large antenna array and by spatial multiplexing of many users. Since its inception, the coherent interference caused by pilot contamination has been believed to be an impairment that does not vanish, even with an unlimited number of antennas. In this work, we show that this belief is incorrect and an artifact from using simplistic channel models and suboptimal signal processing schemes. We focus on the uplink and prove that with multicell MMSE combining, the spectral efficiency grows without bound as the number of antennas increases, even under pilot contamination, under a condition of linear independence between the channel covariance matrices. This condition is generally satisfied, except in special cases that are hardly found in practice.