Impact of Spatial Filtering on Distortion from Low-Noise Amplifiers in Massive MIMO Base Stations

TL;DR

This paper analyzes how the nonlinearity of low-noise amplifiers in massive MIMO base stations affects signal distortion and how spatial filtering can mitigate these effects, especially in the presence of blockers.

Contribution

It models LNA nonlinearity using polynomial functions and examines the impact on spatial correlation and interference mitigation in massive MIMO systems.

Findings

Distortion combines coherently with maximum-ratio combining.

Spatial processing can filter out certain distortion components.

Blocker-induced distortion depends on spatial direction and LNA linearity.

Abstract

In massive MIMO base stations, power consumption and cost of the low-noise amplifiers (LNAs) can be substantial because of the many antennas. We investigate the feasibility of inexpensive, power efficient LNAs, which inherently are less linear. A polynomial model is used to characterize the nonlinear LNAs and to derive the second-order statistics and spatial correlation of the distortion. We show that, with spatial matched filtering (maximum-ratio combining) at the receiver, some distortion terms combine coherently, and that the SINR of the symbol estimates therefore is limited by the linearity of the LNAs. Furthermore, it is studied how the power from a blocker in the adjacent frequency band leaks into the main band and creates distortion. The distortion term that scales cubically with the power received from the blocker has a spatial correlation that can be filtered out by spatial processing and only the coherent term that scales quadratically with the power remains. When the blocker is in free-space line-of-sight and the LNAs are identical, this quadratic term has the same spatial direction as the desired signal, and hence cannot be removed by linear receiver processing.