  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.
