Exploring Hybrid Active and Passive Multiple Access via Slotted ALOHA-Driven Backscatter Communications

In conventional backscatter communication (BackCom) systems, various centralized access schemes, i.e., time/frequency division multiple access, are commonly adopted to prevent transmission collisions among multiple backscatter devices (BDs). However, as the number of BDs proliferates, there will be high overhead, and more seriously, the proactive inter-user coordination may be unaffordable for the passive BDs, which are of scarce concern in existing works and remain a challenge to solve. To this end, in this paper, we propose a slotted ALOHA-based random access for multi-BD BackCom systems, where each BD is selected randomly and coexists with an active device for hybrid transmission. To excavate and evaluate the performance, a resource allocation problem targeting max-min average throughput is formulated, where transmit antenna selection, receive beamforming design, reflection coefficient (RC) adjustment, power control, and access probability determination are jointly optimized. To deal with this intractable problem, we first transform the objective function with the max-min form into an equivalent linear one, and then decompose the resulting problem into three sub-problems. Next, a block coordinate descent-based greedy algorithm, incorporating maximum ratio combining, semi-definite programming, and variable substitution, to achieve a sub-optimal solution. Besides, we derive closed-form solutions for both RC and transmit power to facilitate analysis. Simulation results demonstrate that the proposed algorithm outperforms benchmark algorithms in terms of throughput and fairness.