Backscatter Communication via Harvest-Then-Transmit Relaying

In backscatter communication (BackCom), the direct link between the tag and the reader is susceptible to the deep shadowing due to the obstruction between them. In this paper, we consider cooperative transmission to relay the tag information. The harvest-then-transmit (HTT) protocol is adopted in the relay, where the relay harvests energy in the first time slot and forwards the tag signal after decoding and re-encoding it. Our objective is to maximize the capacity performance subject to the relay residual energy constraint, the tag circuit energy constraint and the total time constraint by optimizing the power splitting factor and time slots. The resulting optimization problem is shown to be convex, however should be numerically evaluated for optimality. In order to gain more insight and avoid high computational complexity, closed-form expressions are derived for sub-optimal solutions. Simulation results confirm the near-optimal performance of sub-optimal solutions, which are shown to perform much better than traditional schemes.