Latency Analysis of an RF Energy Harvesting System with a Backscatter Symbiotic Radio

This paper considers a radio frequency (RF) energy-harvesting (EH) node with a finite battery capacity and an RF backscatter symbiotic radio (SR) in the vicinity that modulates the EH node's RF signal with its own message. With both messages intended for the same receiver, we consider a successive interference cancellation strategy with an automatic repeat request protocol under Rayleigh fading. We derive closed-form expressions for the probability of outage as well as the latency per successfully transmitted bit experienced by the system as a function of (a) the battery capacity of the EH node and (b) the reflection coefficient of the SR. We prove that although the latency is not a convex function of the battery capacity, it exhibits a unique minima which can easily be found through an iterative search. We also analyze the latency as a function of the reflection coefficient and derive an upper bound for its optima. Using an in-depth examination of the trends of the objective function, we propose an iterative algorithm for determining the latency's global minima with respect to the two variables of interest.