Information-theoretic analysis of MIMO channel sounding

TL;DR

This paper analyzes how phase noise and frequency offsets in TDMS-based MIMO channel sounders can cause significant overestimation of channel capacity, especially in low-rank channels, impacting measurement accuracy.

Contribution

It provides a systematic analysis of measurement errors caused by phase noise in TDMS-based MIMO sounders and highlights their impact on capacity estimation.

Findings

Phase noise can cause overestimation of MIMO channel capacity by several hundred percent.

Impact of phase errors is most significant in low-rank channels, such as line-of-sight scenarios.

Measurement results confirm the theoretical analysis and suggest careful interpretation of TDMS-based MIMO measurements.

Abstract

The large majority of commercially available multiple-input multiple-output (MIMO) radio channel measurement devices (sounders) is based on time-division multiplexed switching (TDMS) of a single transmit/receive radio-frequency chain into the elements of a transmit/receive antenna array. While being cost-effective, such a solution can cause significant measurement errors due to phase noise and frequency offset in the local oscillators. In this paper, we systematically analyze the resulting errors and show that, in practice, overestimation of channel capacity by several hundred percent can occur. Overestimation is caused by phase noise (and to a lesser extent frequency offset) leading to an increase of the MIMO channel rank. Our analysis furthermore reveals that the impact of phase errors is, in general, most pronounced if the physical channel has low rank (typical for line-of-sight or poor scattering scenarios). The extreme case of a rank-1 physical channel is analyzed in detail. Finally, we present measurement results obtained from a commercially employed TDMS-based MIMO channel sounder. In the light of the findings of this paper, the results obtained through MIMO channel measurement campaigns using TDMS-based channel sounders should be interpreted with great care.