Interference-Free MU-MISO Symbiotic Radios via RIS Partitioning Design

This paper explores the potential of reconfigurable intelligent surface (RIS) to null interference in a multi-user multiple-input single-output (MU-MISO) symbiotic radio (SR) system. Due to the periodic variations of the RIS phase shifts in the SR systems, it is challenging to use it to achieve interference-free reception in SR systems. To address this issue, we propose a RIS partitioning design for MU-MISO SR. By splitting RIS into two parts, one serves as a relay and removes interference among users, and the other one acts as a secondary transmitter for the Internet of Things (IoT) system, transmitting IoT information by periodically changing its phase shifts. Through this design, the MU-MISO SR system can be decomposed into multiple parallel MISO SR subsystems, each of which only contains the desired user signal and the RIS signal without interference from other users. To do so, we first explore the relationship between the number of reflecting elements and their interference-free capability. Then we formulate a power minimization problem to jointly optimize the transmit beamforming and the phase shifts of partitioned RIS. Finally, simulation results show that the proposed RIS partitioning design could help reduce power consumption, while at the same time achieving interference-free reception in SR.