Achieving full mutualism with massive passive devices for multiuser MIMO symbiotic radio

Symbiotic radio (SR) is one of the attractive communication technologies to facilitate large-scale Internet of Things (IoT) connections with enhanced energy and spectrum efficiency, where passive backscatter devices (BDs) modulate their information over the radio frequency (RF) signals emitted by the active primary transmitters (PTs). Meanwhile, the primary transmission can be strengthened with the additional multipaths created by BDs. To capitalize to a greater extent on the mutualism relationship between the two types of communication, this paper studies multiple-input multiple-output (MIMO) SR communication systems including multiple PTs and massive BDs. We derive the achievable rate expressions of each PT and BD, as well as the sum rate expressions of primary and secondary communication, respectively. Then asymptotic analysis is given to derive the active and passive communication rates with a large number of BDs. Furthermore, for the general case with a finite number of BDs, we study the precoding optimization problem to maximize the sum rate of primary communication, while ensuring that the sum rate of secondary communication and the individual rate for each PT and BD in the prescribed active and passive user sets satisfy the specified thresholds. Simulation results are presented to verify the analytical studies.