Distributed Hybrid Active-Passive RIS-Assisted THz Wireless Systems: Performance Analysis and Optimization

In this paper, we propose distributed hybrid active-passive reconfigurable intelligent surfaces (H-RIS) for Terahertz (THz) wireless communication systems in the presence of hardware impairments and alpha - mu small-scale fading. At first, the end-to-end channel is characterized by a gamma distribution approximation by means of the Lyapunov-central limit theorem. Based on this, analytic expressions of the outage probability and the ergodic capacity of the system are derived. Also, the scaling-law for the received signal-to-noise ratio (SNR) is analyzed in association with the H-RIS scheme, in comparison with fully active RIS and fully passive RIS counterparts. Next, the transmit power is optimized to achieve maximum energy-efficiency in practical scenarios where only partial channel state information (CSI) is available. Furthermore, impacts of hardware impairments, RIS phase-shift errors, as well as numbers of active and passive RIS elements on the system performance are evaluated. Finally, simulations are provided to validate the accuracy of the theoretical analysis.