Max-min energy-efficient resource allocation for wireless powered backscatter networks

In this letter, we present the first attempt to solve an energy efficiency (EE) based max-min fairness problem for a wireless powered backscatter network where a power beacon (PB), which is a dedicated radio frequency (RF) power resource, and multiple backscatter devices work in the same frequency band. Each backscatter transmitter harvests energy from the signal transmitted by the PB, modulates its own information on the received signal, and backscatters the modulated signal to its associated receiver. We propose to ensure max-min fairness among the backscatter links by jointly optimizing the PB transmission power and the backscatter reflection coefficients. For analytical tractability, we solve the optimization problem for the case of two co-channel backscatter links by employing Lagrange dual decomposition when it is convex, and analyzing the monotonicity of the constraints when it is non-convex. Based on the obtained closed-form expressions of the optimal PB transmission power and the optimal backscatter reflection coefficients, we propose an iterative algorithm for max-min EE resource allocation. Simulation results show that the proposed iterative algorithm converges very fast and achieves a much fairer EE performance among backscatter links than maximizing the system EE of the network.