Energy Efficiency Maximization in Bistatic Backscatter Communications with QoS Constraint

Bistatic backscatter radio is expected to be applied in Internet of Things since it can achieve information transfer while consuming extreme low energy. However, no effort have been made to design an energy-efficient resource allocation algorithm with Quality of Service (QoS) requirement, and this observation motivates our work. In this paper, we consider a classical bistatic backscatter system, where the backscatter node first harvests energy from the dedicated power source in the sleep phase, then backscatters its information to the receiver and harvests energy simultaneously through a reflection coefficient in the active phase. We formulate an optimization problem to maximize the achievable energy efficiency, subject to the energy causality and QoS requirement, by optimizing the transmit power at the dedicated power source, the reflection coefficient, the time for sleep phase and active phase. As the formulated problem is a non-convex fractional programming problem and hard to solve, by means of the fractional programming, we firstly transform it in the fractional form as an equivalent problem in the subtractive form. By introducing several auxiliary variables and employing the monotonicity of the objective function, we show that the transformed problem can be reduced to one of the following two convex problems corresponding to two scenarios, i.e., the transmitter always operates in the active state and the dedicated power source adopts the maximum allowed power. Simulation results are provided to verify our findings and show the advantage of the proposed scheme in terms of energy efficiency.