Analytical Performance Evaluation of STAR-RIS Assisted Terahertz Wireless Communications

This paper considers simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted terahertz communications, in which a multi-antenna full-duplex base station serves two half-duplex users (U1 and U2) in the downlink and uplink, respectively. It is assumed that U1 and U2 are located in the reflection and transmission spaces. The STAR-RIS is considered under energy splitting (ES) and mode switching (MS) protocols to provide simultaneous full-space coverage to both users, but at the expense of inter-user interference (IUI). The paper aims to evaluate the impacts of various practical factors on the system performance, including IUI, quantization errors due to discrete phase shifters of the STAR-RIS, beam misalignment due to highly-directive antennas, and random fog. Thus, it derives accurate expressions for the ergodic capacity, outage probability, and symbol error rate. Numerical and simulation results reveal that the system performance is more severely impacted by beam misalignment and random fog than by other factors. It is also observed that the ES protocol outperforms the MS protocol in the downlink, but the situation is reversed in the uplink due to coupled phase shifts. Moreover, in the high-signal-to-noise ratio regime, the system performance is restricted due to IUI signals.