The Impact of Physical Channel on Performance of Subspace-Based Channel Estimation in Massive MIMO Systems

TL;DR

This paper investigates how physical channel characteristics, such as finite angles of arrival, affect the performance of subspace-based channel estimation in massive MIMO systems, revealing limitations and an antenna saturation effect.

Contribution

It extends previous models by incorporating a finite scattering environment, analyzing its impact on spectral properties and estimation performance in massive MIMO.

Findings

Physical channels increase the power ratio needed for effective estimation.

An antenna saturation effect limits performance as the number of antennas grows.

Performance under physical channels approaches that of ideal i.i.d. channels with enough antennas.

Abstract

A subspace method for channel estimation has been recently proposed [1] for tackling the pilot contamination effect, which is regarded by some researchers as a bottleneck in massive MIMO systems. It was shown in [1] that if the power ratio between the desired signal and interference is kept above a certain value, the received signal spectrum splits into signal and interference eigenvalues, namely, the "pilot contamination" effect can be completely eliminated. However, [1] assumes an independently distributed (i.d.) channel, which is actually not much the case in practice. Considering this, a more sensible finite-dimensional physical channel model (i.e., a finite scattering environment, where signals impinge on the base station (BS) from a finite number of angles of arrival (AoA)) is employed in this paper. Via asymptotic spectral analysis, it is demonstrated that, compared with the i.d. channel, the physical channel imposes a penalty in the form of an increased power ratio between the useful signal and the interference. Furthermore, we demonstrate an interesting "antenna saturation" effect, i.e., when the number of the BS antennas approaches infinity, the performance under the physical channel with P AoAs is limited by and nearly the same as the performance under the i.d. channel with P receive antennas.