Characterizing the Rate Region of Active and Passive Communications With RIS-Based Cell-Free Symbiotic Radio.

Thanks to the great potential to alleviate the inter-cell interference issue, cell-free wireless network is regarded as one of the most promising networking architectures in the future. In the meantime, the dramatic increase in the number of wireless devices and their diversified communication rate requirements pose new challenges for cell-free networks. In this paper, we integrate the new symbiotic radio (SR) transmission technique into cell-free networks. In particular, spectral-and energy-efficient passive backscatter communication in SR is achieved by passive reflective beamforming over multiple reconfigurable intelligent surfaces (RISs). On the one hand, distributed access points (APs) in cell-free network collaboratively perform active communication through direct links, and on the other hand, RISs reuse the spectrum and energy of active communication to passively backscatter its own information-bearing signal. Considering the coexistence of active and passive communication demands, we define the rate region of the RIS-based cell-free SR system as the union of all rate pairs achieved by the active and passive communication devices. To characterize the achievable rate region, we formulate an optimization problem to maximize the passive communication rate given a minimum active rate constraint, by jointly optimizing the active transmit beamforming and passive reflective beamforming. An efficient alternating algorithm is proposed to solve the formulated problem. Finally, simulation results are presented to show the rate region of RIS-based cell-free SR systems and demonstrate the effectiveness of the proposed joint beamforming algorithm.