  In this paper, we compare the performance of two main MIMO techniques,
beamforming and multiplexing, in the Terahertz (THz) band. The main problem
with the THz band is its huge propagation loss, which is caused by the
tremendous signal attenuation due to molecule absorption of the
electro-magnetic wave. To overcome the path loss issue, massive MIMO has been
suggested to be employed in the network and is expected to provide Tbps for a
distance within a few meters. In this context, beamforming is studied recently
as the main technique to take advantage of MIMO in THz and overcome the very
high path loss with the assumption that the THz communication channel is
Line-of-Sight (LoS) and there are not significant multipath rays. On the other
hand, recent studies also showed that the well-known absorbed energy by
molecules can be re-radiated immediately in the same frequency. Such
re-radiated signal is correlated with the main signal and can provide rich
scattering paths for the communication channel. This means that a significant
MIMO multiplexing gain can be achieved even in a LoS scenario for the THz band.
Our simulation results reveal a surprising observation that the MIMO
multiplexing could be a better choice than the MIMO beamforming under certain
conditions in THz communications.
