A New Look at MIMO Capacity in the Millimeter Wave

TL;DR

This paper reveals that atmospheric molecules can enhance MIMO capacity at specific millimeter wave frequencies by creating non-line-of-sight paths, fundamentally changing the understanding of MIMO performance in mmWave communications.

Contribution

It introduces a novel theoretical insight that atmospheric molecular re-radiation can significantly boost MIMO capacity at certain mmWave frequencies, impacting future communication system designs.

Findings

MIMO capacity is highly frequency selective due to molecular re-radiation.

Certain mmWave bands can serve as spectrum windows for high-efficiency MIMO.

Molecular effects can improve spatial multiplexing and diversity in LoS environments.

Abstract

In this paper, we present a new theoretical discovery that the multiple-input and multiple-output (MIMO) capacity can be influenced by atmosphere molecules. In more detail, some common atmosphere molecules, such as Oxygen and water, can absorb and re-radiate energy in their natural resonance frequencies, such as 60GHz, 120GHz, and 180GHz, which belong to the millimeter wave (mmWave) spectrum. Such phenomenon can provide equivalent non-line-of-sight (NLoS) paths in an environment that lacks scatterers, and thus greatly improve the spatial multiplexing and diversity of a MIMO system. This kind of performance improvement is particularly useful for most mmWave communications that heavily rely on line-of-sight (LoS) transmissions. To sum up, our study concludes that since the molecular re-radiation happens at certain mmWave frequency bands, the MIMO capacity becomes highly frequency selective and enjoys a considerable boosting at those mmWave frequency bands. The impact of our new discovery is significant, which fundamentally changes our understanding on the relationship between the MIMO capacity and the frequency spectrum. In particular, our results predict that several mmWave bands can serve as valuable spectrum windows for high-efficiency MIMO communications, which in turn may shift the paradigm of research, standardization, and implementation in the field of mmWave communications.