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

Abstract

In this paper, we show how the absorption and reradiation energy from molecules in the air can influence the Multiple Input Multiple Output (MIMO) performance in highfrequency 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.

Intelligent Surface Optimization in Terahertz under Two Manifestations of Molecular Re-radiation

“…This re-radiation happens almost isotropically and the energy is distributed in all the directions for the THz band (unlike the forward scattering we see in visible light due to small particles) [16]. In the literature, however, it is commonly assumed that the whole absorbed power is available at the Rx node through re-radiation [5], [8]. Even though this is never explicitly mentioned, this follows from the assumptions that a single absorption/re-radiation event happens over the whole propagation path, and that all of the re-radiated power is scattered in the direction of the Rx node.…”

“…In addition, there is a second school of thought, which models this phenomenon as scattering, where the existence of multiple scattered copies of the signal due to re-radiation has some basis in the literature [6], [7]. Following that, [8] recently modeled the THz channel as a Rician channel, where the Rician factor is calculated from the molecular absorption coefficient. Please note that this scattering could also lead to delay dispersion [4], but that is beyond the scope of this paper.…”

In Situ MIMO-WPT Recharging of UAVs Using Intelligent Flying Energy Sources

“…Since there is almost a pure LoS MIMO channel between a pair of tUAV and rUAV during the recharging process as the tUAV adjusts its position to achieve this, H n is a rank one matrix and an optimal beamforming can be achieved by an SVD-based beam-former [39,40] when only one strong beam is formed by transmitter antennas as optimal energy beamforming with a gain of n t [37,41],…”

“…For example, the wavelength of frequencies below 1 GHz is very large for MIMO. Additionally, 1-7 GHz is highly saturated for current wireless communications [40]. On the other hand, high path loss in higher frequency is helpful to minimize the interference of WPT to ground stations.…”

In-situ MIMO-WPT Recharging of UAVs Using Intelligent Flying Energy Sources

“…Since there is almost a pure LoS MIMO channel between a pair of tUAV and rUAV during the recharging process as the tUAV adjusts its position to achieve this, H n is a rank one matrix and an optimal beamforming can be achieved by an SVD-based beamformer [36,37] when only one strong beam is formed by transmitter antennas as optimal energy beamforming with a gain of n t [34,38],…”