Abstract
This paper proposes a novel transmission policy for an intelligent reflecting surface (IRS) assisted wireless powered sensor network (WPSN). An IRS is deployed to enhance the performance of wireless energy transfer (WET) and wireless information transfer (WIT) by intelligently adjusting phase shifts of each reflecting elements. To achieve its self-sustainability, the IRS needs to collect energy from the ES to support its control circuit operation. Our proposed policy for the considered system is called IRS assisted harvest-then-transmit time switching (IRS-HTT-TS) which schedules the transmission time slots by switching between energy collection and energy reflection modes. We study the performance of the proposed transmission policy in terms of the achievable sum throughput, and investigate a joint design of the transmission time slots, the power allocation, as well as the discrete phase shifts of the WET and WIT. This formulates the problem as a mixed-integer non-linear program (MINLP), which is NP-hard and non-convex. To deal with this problem, we first relax it to the one with continuous phase shifts. Consequently, we propose a two-step approach and decompose the original problem into two sub-problem, each being solved separately. Specifically, we independently solve the first sub-problem with respect to the phase shifts of the WIT in terms of closed-form expression. Then, we consider two cases to solve the second sub- problem. For the special case without the circuit power of each sensor node, the Lagrange dual method and the Karush-Kuhn- Tucker (KKT) conditions are applied to derive the optimal closed- form transmission time slots, power allocation, and phase shift of the WET. Moreover, we exploit the second sub-problem for the general case with the circuit power of each sensor node, which can be solved via employing a semi-definite programming (SDP) relaxation.