  A relentless trend in wireless communications is the hunger for bandwidth,
and fresh bandwidth is only to be found at ever-higher frequencies. While 5G
systems are seizing the mmWave band, the attention of researchers is shifting
already to the terahertz range. In that distant land of tiny wavelengths,
antenna arrays can serve for more than power-enhancing beamforming. Defying
lower-frequency wisdom, spatial multiplexing becomes feasible even in
line-of-sight conditions. This paper reviews the underpinnings of this
phenomenon, and it surveys recent results on the ensuing information-theoretic
capacity. Reconfigurable array architectures are put forth that can closely
approach such capacity, practical challenges are discussed, and supporting
experimental evidence is presented.
