Millimeter Wave vs. THz Energy Harvesting for Autonomous Reconfigurable Intelligent Surfaces

Abstract

The aim of this work is to examine the efficacy of wireless energy harvesting for autonomous reconfigurable intelligent surfaces that is performed either at millimeter wave or the lower THz bands. Towards this, we first consider an architecture in which a subset of unit cells is dedicated to energy harvesting and the rest to information transmission to a receiver through reflection. Subsequently, we compute the RIS energy consumption per frame according to the considered channel-estimation protocol. Based on it, we formulate an optimization problem that has as aim the maximization of the average rate under the constraint of meeting the RIS long-term energy consumption demands and provide its closed-form solution. Finally, numerical results are provided that target the performance comparison between the RIS-assisted links operating at the 28 GHz and 140 GHz bands. They reveal that the energy harvesting becomes more effective at the 140 GHz band, owing to the larger amount of unit cells that can be accommodated onto the same physical space.