RIS Reflection and Placement Optimisation for Underlay D2D Communications in Cognitive Cellular Networks

Reconfigurable intelligent surface (RIS) is considered a promising technology in sixth-generation (6G) networks due to its ability to configure the phase of signals using low-cost reflecting elements. An RIS-aided device-to-device (D2D) communication system operating in underlay mode is considered in this work. Assuming the presence of a direct link between the D2D pair, we present a closed-form expression of signal-to-interference and noise ratio (SINR) at the D2D user. We aim to solve the SINR maximisation problem by jointly optimising the power allocated to the D2D source and the RIS placement. A closed-form global optimal solution to the proposed optimisation problem has been obtained. Numerical results are presented to validate the analytical framework. Our proposed solution can yield a significant gain of around 4.65 dB over the benchmark.