Hybrid Ambient Backscatter Communication Systems With Harvest-Then-Transmit Protocols

In this paper, we investigate and analyze a hybrid ambient backscatter (AmBack) scheme with harvest-then-transmit protocols. In this scheme, the whole transmission is divided into three phases (time slots): energy harvesting (EH), AmBack, and data transmission (DT). Depending on the type of the reflection coefficient (RC) for AmBack, two hybrid schemes are considered: variable RC (VRC) and fixed RC (FRC). In the VRC scheme, the RC changes with the channel state information, and the harvested energy in the first phase is totally used for DT. On the other hand, in the FRC scheme, the RC is fixed to be one, and the harvested energy in the first phase is in part used to cover the tag circuit operation in the second phase and in part for DT in the third phase. The resulting optimization problem is convex for the VRC scheme, but non-convex for the FRC scheme. However, by utilizing the maximum principle in convex optimization, we are able to derive closed-form expressions for optimal solutions for both schemes. In order to get more insights, we also analyze the upper bound performance of both schemes. Simulation results demonstrate that the AmBack/EH-AmBack scheme can always achieve the optimal performance.