Downlink Coverage and Exposure Analysis in RIS-Assisted THz Network with Nakagami Fading

We develop a stochastic geometry framework to analyze the downlink coverage probability and electromagnetic field exposure (EMFE) of a typical user in a reconfigurable intelligent surface (RIS)-enabled THz cellular network. The framework captures Nakagami-m fading to model the line-of-sight (LOS) channel, molecular absorption, and blockages, and it can be customized to incorporate a variety of non-LOS (NLOS) fading channels and sub-6GHz frequencies. The typical user is supported either by direct or RIS-assisted transmission. For RIS-assisted transmissions, we first model the cascaded channel as a double Nakagami-m random variable and then depict that the received signal power follows a non-central chisquare distribution. Then, we derive closed-form expressions for the conditional Laplace Transform (LT) of the received signal power and aggregated interference from multiple RISs and THz base stations (TBSs). The Gil-Paláez inversion is then used to compute the downlink EMFE and the coverage probability of a typical user. Numerical results validate the accuracy of the derived analytical expressions. We note that while RISs are beneficial from a coverage perspective, they can induce additional EMFE, thus the RIS deployment planning is critical.