  TeraHertz (THz) communications can satisfy the high data rate demand with
massive bandwidth. However, severe path attenuation and hardware imperfection
greatly alleviate its performance. Therefore, we utilize the reconfigurable
intelligent surface (RIS) technology and investigate the RIS-aided THz
communications. We first prove that the small-scale amplitude fading of THz
signals can be accurately modeled by the fluctuating two-ray distribution based
on two THz signal measurement experiments conducted in a variety of different
scenarios. To optimize the phase-shifts at the RIS elements, we propose a novel
swarm intelligence-based method that does not require full channel estimation.
We then derive exact statistical characterizations of end-to-end
signal-to-noise plus distortion ratio (SNDR) and signal-to-noise ratio (SNR).
Moreover, we present asymptotic analysis to obtain more insights when the SNDR
or the number of RIS's elements is high. Finally, we derive analytical
expressions for the outage probability and ergodic capacity. The tight upper
bounds of ergodic capacity for both ideal and nonideal radio frequency chains
are obtained. It is interesting to find that increasing the number of RIS's
elements can significantly improve the THz communications system performance.
For example, the ergodic capacity can increase up to 25% when the number of
elements increases from 40 to 80, which incurs only insignificant costs to the
system.
