Performance of THz Communications Over Cascaded RISs: A Practical Solution to the Over-Determined Formulation

Abstract

As a last resort for radio frequency communication, Terahertz bands (0.1-10 THz) are targeted thanks to their ability to provide ultra-wide bandwidth satisfying varieties of emerging applications. However, the waves propagating at THz frequencies suffer from high losses due to absorption in air molecules which forces their use into short-range links. We propose a multi-hop system to extend the range of THz links with the aid of multiple passive reconfigurable intelligent surfaces (RIS). In contrast with existing literature, we address the challenge of the over-determined system of equations arising from optimizing the phases of cascaded RISs taking into account the practical case of highly directional antennas and spatially correlated channels in THz bands. We propose an efficient method to jointly optimize the phases induced by the RISs in order to maximize the power at the receiver side.