Joint Beamforming and Backscatter Communication Design for Symbiotic Radio Networks

Symbiotic radio (SR) is a promising energy-, spectrum-, and cost-efficient communication technology for Internet of Things. This article considers an SR network (SRN) in which backscatter devices (BDs) communicate with a primary receiver (PR) by riding on ambient radio frequency carriers from a primary transmitter (PT). First, both the achievable primary rate and backscatter-link sum rate are derived for BDs adopting spatial-division-multiple-access (SDMA) scheme and dynamic time-division-multiple-access (TDMA) scheme, respectively. Then, two optimization problems are formulated to maximize the BDs' sum rate, by jointly optimizing the PTs' beamforming matrix, and the BDs' power reflection coefficients as well as the backscatter time allocation. For the single-antenna PR case, the optimal beamforming is obtained in semi-closed forms. For the multiantenna PR case, to solve the nonconvex problems with coupled variables, efficient iterative algorithms are proposed based on block coordinate descent and sequential convex approximation techniques. Finally, numerical results show that the proposed design enhances the SRNs' throughput significantly, and give useful insights on the BDs' multiple-access schemes.