Multi-Functional Reflection Modulation Design for Multi-RIS Empowered Symbiotic Radios

This paper investigates a multi-user multiple-input single-output (MU-MISO) symbiotic radio (SR) system empowered by multiple reconfigurable intelligent surfaces (RISs). In this system, each RIS adopts a multi-functional reflection modulation scheme to simultaneously transmit data from Internet of Things (IoT) sensors and enhance the primary transmission. However, allocating all reflected power exclusively to IoT data transmission could compromise the capability of RISs to enhance the primary transmission. To address this issue, we propose a flexible power allocation scheme that partitions the power reflected from the RISs into two components: one for enhancing the primary transmission and the other for supporting IoT communication, fully exploiting the multi-functional potential of RISs. Subsequently, we formulate a joint optimization problem aimed at minimizing the transmit power subject to the rate requirements for both primary and RIS transmissions, involving the joint optimization of the beamforming at the base station (BS) and the reflection modulation design at the RISs. Given the non-convex constraints and the coupled variables within the problem, we develop an alternating optimization algorithm combined with difference-of-convex programming to efficiently solve the problem and determine the power allocation scheme. Furthermore, to reduce computational complexity, we consider uniform power allocation across all reflecting elements and introduce weighted parameters to flexibly balance between assisting primary transmission and supporting IoT communication. Simulation results demonstrate that our proposed power allocation scheme effectively balances the demands of the primary and RIS transmission, significantly reducing the required transmit power.