Wireless-Powered Communication with Energy Accumulation

Abstract

In this thesis, I focus on the system design and performance analysis of wireless communication networks with radio frequency (RF) energy harvesting. Battery re-placement/recharging has always been a challenging issue in wireless communication networks, especially in large-scale networks like wireless sensor networks (WSNs). Recently, RF energy harvesting (EH) technology has been developed as a new viable solution to extend the lifetime of wireless networks via enabling wireless devices to harvest energy from RF signals. Inspired by this technique, wireless-powered communication networks (WPCNs) have attracted an upsurge of research interest. According to the protocols proposed in current literature, an EH user will exhaust all the energy it harvests straightway during this transmission block. This may be a sub-optimal solution, since this little amount of energy may not be able to contribute to an effective transmission. Therefore, in this thesis, I consider energy accumulation at each EH node by providing an energy storage (e.g. a rechargeable battery), so that it can an accumulate sufficient amount of harvested energy before transmission and wait to transmit in an appropriate time slot.

To begin with, I first investigate wireless energy harvesting (WEH) technique in cooperative communication networks where the source and relay can both communicate with the destination. I refer to this kind of network as wireless-powered cooperative communication networks (WPCCNs). This WEH technique offers a new cooperation manner for wireless devices since the relay node is now able to harvest energy from the source’s information. In this thesis, I consider the relay as a wireless-powered node that has no external power supply; but it is equipped with an EH unit and a rechargeable battery so that it can harvest and accumulate energy from RF signals broadcast by the source. By fully incorporating the EH feature of the relay, an opportunistic relaying protocol was developed, termed accumulate-then-forward (ATF), for the considered WPCCN with a direct link. The discrete Markov chain is adopted to model the dynamic charging and discharging behaviors of the relay battery. Based on this, I derive a closed-form expression for the exact system outage probability of the proposed ATF protocol. Numerical results show that the ATF scheme can outperform the direct transmission one, especially when the amount of energy consumed by the relay for information forwarding is optimized.

In order to further take advantage of the direct link between the source and destination, an incremental accumulate-then-forward (IATF) scheme was proposed to the considered WPCCNs. In the IATF protocol, the source sends its information to the destination via the direct link and requests the relay to cooperate only when it is necessary, such that the relay has more chances to accumulate energy. By modelling the charging/discharging behaviors of the relay battery as a finite-state Markov chain, I derive a closed-form expression for the system outage probability of the proposed IATF. Numerical results validate my theoretical analysis and show that the IATF scheme can significantly outperform the direct transmission scheme without cooperation and the ATF scheme. A comparison of these two schemes is also given to show examples of their advantages and disadvantages.

One of the key features of WEH technique is one-to-many, where multiple EH nodes can harvest energy simultaneously from one single RF signal. The energy from broadcasted RF signals can thus be transferred to several EH nodes rather than only one EH node, which can increase the energy transfer efficiency of the whole communication network. Contrast to the single EH node previously, in the last chapter of this thesis, I investigate a WPCN with multiple EH nodes and focus on the multi-user scheduling problem. I assume a power beacon (PB) is deployed in WPCN dedicating to charge the EH nodes by broadcasting RF energy signals. With this PB-assisted WEH technique, the considered WPCN consists of a PB, an access point (AP) and multiple EH users attempting to transmit information toward the AP. In this thesis, I consider that only one EH user can be selected to forward its information, and the rest of the nodes will continue to harvest energy. In order to harvest energy effectively and transmit information efficiently, I proposed several types of multi-user scheduling schemes based on the availability of the channel state information (CSI), i.e. multi-user scheduling scheme without CSI, multi-user scheduling scheme using channel state information at the receiver (CSIR), and multi-user scheduling scheme using channel state information at the transmitter (CSIT). Two more Proportional Fairness scheduling schemes are proposed in order to achieve the fairness among all the EH users. Simulation results show that the availability of CSI can significantly improve the access outage probability, where the average probability of the user successfully connect to the AP is shown. Moreover, the fairness among different users can be dramatically improved by two Proportional Fairness schemes.