Special Issue on Wireless Powered Communications Networks
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Power supply is a long-standing problem for mobile devices and sensor nodes. When entering the era of Internet-of-Things (IoT) and 5G, with the significant growing number of IoT devices and the increasing dependence on mobile devices, it is expected that the power supply issue will be more demanding in the future. Wireless power transfer is a technology that allows replenishing the battery without physical connections. Compared to the cordless power supply technologies that rely on pure energy harvesting, wireless power transfer provides an active and controlled way for power delivery to mobile devices or sensor nodes. Since the power is targeted for wireless communication, jointly considering wireless power transfer and wireless communication is essential, which leads to the research of wireless powered communication networks. However, there are several challenges that need to be addressed for practical design of wireless powered communication networks, such as the concerns of power transfer efficiency and constraint, delay constraint, resource allocation, relaying, MAC protocols, and prototyping under different network architectures and various advanced systems.
This special issue includes four papers that provide new designs and overview for wireless powered communications networks. In the paper “Optimal-stopping spectrum sensing in energy harvesting cognitive radio systems”, a cognitive radio system in which the secondary user (SU) is powered by energy harvested exclusively from the environment is considered. Optimizing the SU’s expected achievable throughput leads to a parametrized optimal stopping problem. An efficient algorithm is proposed to derive the optimal save-ratio and spectrum sensing rule, which is significantly faster than the conventional simulated annealing algorithm. A suboptimal solution is also proposed to further reduce the computational complexity, in which the problem is formulated as a mixed-integer non-linear programming problem.
In the paper “Simultaneous wireless information and power transfer in multi-antenna systems”, a comprehensive literature survey on simultaneous wireless information and power transfer (SWIPT) in multi-antenna systems is provided. Firstly, recent contributions on the theoretical analysis of SWIPT from the information theoretic standpoint is reviewed. Secondly, the implementation aspects of multi-antenna SWIPT are discussed. Then, dynamic SWIPT optimization is reviewed, followed by the introduction of recent works on secure SWIPT. Finally, researches on combining SWIPT with state-of-the- art techniques are overviewed.
In the paper “Robust downlink beamforming for multiuser SWIPT systems with imperfect CSI”, a multiuser multiple-input-single-output simultaneous wireless information and power transfer downlink system with imperfect channel is considered. The worst-case harvested energy of all users is maximized with the signal-to-interference and noise ratio requirement satisfied, assuming unknown channel estimate error. The beamforming vector optimization is formulated as a semi-definite programming problem transformed by the S-procedure method. With semidefinite relaxation, optimal beamforming vector is obtained.
Finally in the paper “Throughput and energy efficiency of wireless powered multi-tier MIMO HetNets“, wireless powered multi-tier multi-input multi-output (MIMO) heterogeneous networks (HetNets), where the multi-antenna base stations perform downlink transmission, are considered. Two cases are considered: energy harvesting with and without energy beamforming. By using tools of stochastic geometry, throughput and energy efficiency are analyzed. Closed-form and tractable results are obtained to conclude that the proposed wireless-powered MIMO HetNets can achieve higher energy efficiency compared to the conventional HetNets without energy harvesting, and the energy efficiency performance can be further improved by introducing well-designed energy beamforming across tiers.