Pilot Contamination is Not a Fundamental Asymptotic Limitation in Massive MIMO

Massive MIMO provides great improvements in spectral efficiency, by coherent combining 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 it is basically an artifact from using simplistic channel models and combining schemes. We prove that with multi-cell 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 which can be hardly found in practice.