  Terahertz (THz) systems have the benefit of high bandwidth and hence are
capable of supporting ultra-high data rates, albeit at the cost of high
pathloss. Hence they tend to harness high-gain beamforming. Therefore a novel
hybrid 3D beamformer relying on sophisticated sensor-based beam training and
channel estimation is proposed for Reconfigurable Intelligent Surface (RIS)
aided THz MIMO systems. A so-called array-of-subarray based THz BS architecture
is adopted and the corresponding sub-RIS structure is proposed. The BS, RIS and
receiver antenna arrays of the users are all uniform planar arrays (UPAs). The
Ultra-wideband (UWB) sensors are integrated into the RIS and the user location
information obtained by the UWB sensors is exploited for channel estimation and
beamforming. Furthermore, the novel concept of a Precise Beamforming Algorithm
(PBA) is proposed, which further improves the beam-forming accuracy by
circumventing the performance limitations imposed by positioning errors.
Moreover, the conditions of maintaining the orthogonality of the RIS-aided THz
channel are derived in support of spatial multiplexing. The closed-form
expressions of the near-field and far-field path-loss are also derived. Our
simulation results show that the proposed scheme accurately estimates the
RIS-aided THz channel and the spectral efficiency is much improved, despite its
low complexity. This makes our solution eminently suitable for delay-sensitive
applications.
