Outage Performance Analysis for RIS-Enabling Full-Duplex Cellular Symbiotic Radio Network

Aiming at breaking through the transmission distance limitation faced by symbiotic radio, in this correspondence, reconfigurable intelligent surface (RIS) is introduced to enable symbiotic transmission within cellular range. To avoid high-cost channel estimation, the phase shifting of the RIS is designed only based on the channel state information of the cascaded backscatter channel which is easy to obtain. Outage probabilities of the backscatter device (BD) and the ambient transmission, both in parasitic and commensal modes, are derived based on Lyapunov's theorem and Hermite-Gauss quadrature. Simulation results illustrate that the BD outage performance is highly sensitive to the number of RIS reflection elements ($N$) both in parasitic and commensal modes. When $N$ is sufficiently large, its further increment has little impact on the outage probability of the ambient system, although the phase shift is designed merely to enhance the BD transmission. It also offers a guideline in coordinating $N$ with the ambient transmit power to balance between the outage performance of the ambient system and that of the backscatter system.