  The wireless research community has expressed major interest in the
sub-terahertz band for enabling mobile communications in future wireless
networks. The sub-terahertz band offers a large amount of available bandwidth
and, therefore, the promise to realize wireless communications at optical
speeds. At such high frequency bands, the transceivers need to have larger
apertures and need to be deployed more densely than at lower frequency bands.
These factors proportionally increase the far-field limit and the spherical
curvature of the electromagnetic waves cannot be ignored anymore. This offers
the opportunity to realize spatial multiplexing even in line-of-sight channels.
In this paper, we overview and compare existing design options to realize
high-rank transmissions in line-of-sight channels.
