A Framework for Over-the-air Reciprocity Calibration for TDD Massive MIMO Systems

TL;DR

This paper introduces a unified framework for over-the-air reciprocity calibration in TDD massive MIMO systems, proposing novel antenna grouping methods that enhance calibration accuracy and efficiency while minimizing service disruption.

Contribution

It presents a new family of calibration algorithms based on antenna grouping, compares estimators against the Cramér-Rao bound, and distinguishes between coherent and non-coherent measurement accumulation.

Findings

Antenna grouping improves calibration speed and accuracy.

Non-coherent accumulation reduces impact on data service.

Maximum likelihood estimation approaches the Cramér-Rao bound.

Abstract

One of the biggest challenges in operating massive multiple-input multiple-output systems is the acquisition of accurate channel state information at the transmitter. To take up this challenge, time division duplex is more favorable thanks to its channel reciprocity between downlink and uplink. However, while the propagation channel over the air is reciprocal, the radio-frequency front-ends in the transceivers are not. Therefore, calibration is required to compensate the RF hardware asymmetry. Although various over-the-air calibration methods exist to address the above problem, this paper offers a unified representation of these algorithms, providing a higher level view on the calibration problem, and introduces innovations on calibration methods. We present a novel family of calibration methods, based on antenna grouping, which improve accuracy and speed up the calibration process compared to existing methods. We then provide the Cram\'er-Rao bound as the performance evaluation benchmark and compare maximum likelihood and least squares estimators. We also differentiate between coherent and non-coherent accumulation of calibration measurements, and point out that enabling non-coherent accumulation allows the training to be spread in time, minimizing impact to the data service. Overall, these results have special value in allowing to design reciprocity calibration techniques that are both accurate and resource-effective.