  The prospect of ultra-massive multiple-input multiple-output (UM-MIMO)
technology to combat the distance problem at the Terahertz (THz)-band is
considered. It is well-known that the very large available bandwidths at THz
frequencies come at the cost of severe propagation losses and power
limitations, which result in very short communication distances. Recently,
graphene-based plasmonic nano-antenna arrays that can accommodate hundreds of
antenna elements in a few millimeters have been proposed. While such arrays
enable efficient beamforming that can increase the communication range, they
fail to provide sufficient spatial degrees of freedom for spatial multiplexing.
In this paper, we examine spatial modulation (SM) techniques that can leverage
the properties of densely packed configurable arrays of subarrays of
nano-antennas, to increase capacity and spectral efficiency, while maintaining
acceptable beamforming performance. Depending on the communication distance and
the frequency of operation, a specific SM configuration that ensures good
channel conditions is recommended. We analyze the performance of the proposed
schemes theoretically and numerically in terms of symbol and bit error rates,
where significant gains are observed compared to conventional SM. We
demonstrate that SM at very high frequencies is a feasible paradigm, and we
motivate several extensions that can make THz-band SM a future research trend.
