Impact and Calibration of Nonlinear Reciprocity Mismatch in Massive MIMO

TL;DR

This paper investigates the effects of nonlinear reciprocity mismatch in massive MIMO systems, deriving rate expressions, analyzing performance loss, and proposing a calibration method to mitigate the impact at the base station.

Contribution

It introduces a novel calibration approach using polynomial approximation and over-the-air training to address nonlinear reciprocity mismatch in massive MIMO.

Findings

Nonlinear mismatch significantly impacts system performance.

Calibration at the base station effectively mitigates the mismatch.

Proposed method improves achievable rate in simulations.

Abstract

Time-division-duplexing massive multiple-input multiple-output (MIMO) systems estimate the channel state information (CSI) by leveraging the uplink-downlink channel reciprocity, which is no longer valid when the mismatch arises from the asymmetric uplink and downlink radio frequency (RF) chains. Existing works treat the reciprocity mismatch as constant for simplicity. However, the practical RF chain consists of nonlinear components, which leads to nonlinear reciprocity mismatch. In this work, we examine the impact and the calibration approach of the nonlinear reciprocity mismatch in massive MIMO systems. To evaluate the impact of the nonlinear mismatch, we first derive the closed-form expression of the ergodic achievable rate. Then, we analyze the performance loss caused by the nonlinear mismatch to show that the impact of the mismatch at the base station (BS) side is much larger than that at the user equipment side. Therefore, we propose a calibration method for the BS. During the calibration, polynomial function is applied to approximate the nonlinear mismatch factor, and over-the-air training is employed to estimate the polynomial coefficients. After that, the calibration coefficients are computed by maximizing the downlink achievable rate. Simulation results are presented to verify the analytical results and to show the performance of the proposed calibration approach.