Analyzing the Impact of Molecular Re-Radiation on the MIMO Capacity in High-Frequency Bands

TL;DR

This paper investigates how molecular re-radiation in high-frequency bands affects MIMO channel capacity, revealing it can significantly enhance capacity and enable open-loop precoding in millimeter wave and terahertz systems.

Contribution

It models the scattering and noise effects of molecular re-radiation, demonstrating its positive impact on MIMO capacity and system design in high-frequency bands.

Findings

Re-radiation can increase MIMO capacity up to 3 times in mmWave and 6 times in terahertz.

Re-radiation provides NLoS paths, improving spatial multiplexing.

Open-loop precoding becomes viable at high SNR due to re-radiation effects.

Abstract

In this paper, we show how the absorption and re-radiation energy from molecules in the air can influence the Multiple Input Multiple Output (MIMO) performance in high-frequency bands, e.g., millimeter wave (mmWave) and terahertz. 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 60 GHz, 180 GHz and 320 GHz. Hence, when hit by electromagnetic waves, molecules will get excited and absorb energy, which leads to an extra path loss and is known as molecular attenuation. Meanwhile, the absorbed energy will be re-radiated towards a random direction with a random phase. These re-radiated waves also interfere with the signal transmission. Although, the molecular re-radiation was mostly considered as noise in literature, recent works show that it is correlated to the main signal and can be viewed as a composition of multiple delayed or scattered signals. Such a phenomenon can provide non-line-of-sight (NLoS) paths in an environment that lacks scatterers, which increases spatial multiplexing and thus greatly enhances the performance of MIMO systems. Therefore in this paper, we explore the scattering model and noise models of molecular re-radiation to characterize the channel transfer function of the NLoS channels created by atmosphere molecules. Our simulation results show that the re-radiation can increase MIMO capacity up to 3 folds in mmWave and 6 folds in terahertz for a set of realistic transmit power, distance, and antenna numbers. We also show that in the high SNR, the re-radiation makes the open-loop precoding viable, which is an alternative to beamforming to avoid beam alignment sensitivity in high mobility applications.