  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.
