Optimization for Wireless Powered Communication Networks

With the step growth of Internet of Things (IoT), energy-constrained wireless devices are deployed throughout our lives. Wireless powered communication network (WPCN) is a promising networking paradigm where wireless devices can be remotely powered by radio frequency (RF) enabled wireless power transfer (WPT) technology. In this chapter, we first given an overview of WPCN, which includes the research background and the recent research results. Then, we briefly discuss the main operation modes of WPCN.

In a WPCN, the distances between the users and the power beacon (PB) or the information receiver (IR) are different, which results in unfair transmission rates among different users. NOMA, the basic principle of which is that the users can achieve multiple access by exploiting the power domain multiplexing, is applied in the WPCN to improve user fairness. However, the successive interference cancellation (SIC) constraints, the prerequisite of applying NOMA successfully, are not considered in most existing literatures. In this chapter, the effect of SIC constraints on the throughput of the WPCN with NOMA is investigated, where the users harvest energy from RF signals radiated by the PB, and then use the harvested energy to simultaneously transmit information to the IR. First, the throughput maximization problem is formulated to find the optimal time and energy allocation scheme. Then, to derive the closed-form solution, the optimization problem is further divided into two sub-problems by exploiting the optimal structure of constraints. Finally, simulations on the effect of SIC constraints show the importance of the distinctness among users’ channel power gains for the WPCN with NOMA.

By exploiting the advantages of the harvest-then-transmit (HTT) mode and the BackCom mode, two schemes that integrating the HTT mode and the BackCom mode are proposed for the WPCN in this chapter, i.e., the hybrid user scheme and the hybrid mode scheme. For the hybrid user scheme, the WPCN consists of two types of users, which adopt the HTT mode and the BackCom mode, respectively. While, each user can work in either the HTT mode or the BackCom mode for the hybrid mode scheme. The system throughput maximization problems are formulated for the proposed schemes and the optimal solutions are derived, respectively. The optimal solution for the hybrid user scheme shows that only the user in the BackCom mode with the largest backscatter rate can be scheduled. Moreover, the optimal users’ working mode permutation is studied for the hybrid mode scheme. Simulation results show the superiority of the proposed schemes over the single mode schemes in terms of system throughput.

In this chapter, a hybrid mode scheme is proposed for a CWPCN. The CWPCN consists of a primary communication system and a secondary communication system, where the cognitive user (CU) can adopt the HTT mode, the AB mode or the BB mode following the proposed hybrid mode scheme. The primary transmitter (PT) and the PB serve as the incident signal sources of the AB mode and the BB mode for the CU’s information backscattering, respectively. When the primary channel is idle, the CU use the harvested energy from the PT and PB to transmit information to the IR following the HTT mode. The optimal time allocation among the three modes is investigated for the sake of maximizing the throughput of the secondary communication system. The closed-form solution is derived and the optimal combination of the working modes is obtained. Numerical results demonstrate the advantage of the proposed scheme over the single mode schemes in terms of system throughput.

A relay cooperation scheme is proposed for a backscatter communication system (BCS), where the user backscatters incident signals from a carrier emitter (CE) to an IR and a relay simultaneously, and then the relay forwards the user’s information to the IR for throughput improvement. Two cases are considered that the relay is with an embedded energy source and the relay is without an embedded energy source. If the relay does not have an embedded energy source, it first harvests energy from the CE and then uses its harvested energy for information forwarding. For both cases, the time allocation problems on the user’s information backscattering, the user’s information forwarding, or the relay’s energy harvesting are formulated to maximize the system throughput, and then closed-form solutions are derived. Simulation results demonstrate the advantage of the proposed relay cooperation scheme with the optimal time allocation in terms of system throughput.

In this chapter, we present a summary of main ideas of this book and discuss the future research directions about WPCN.