## 1 Introduction  
In wireless sensor networks (WSNs), energy is one of the most critical resources because sensors are often low-cost, energy-constrained, resource-constrained nodes [1; 2]. The energy harvesting (EH) technique [3; 4] has been considered as a viable solution to prolong battery lifetime, improve network performance, and provide green communication for WSNs. Therefore, it has received significant interest from the wireless communication community. Besides conventional EH techniques powered by external energy sources such as solar, wind energy, piezoelectric shoe inserts, thermoelectricity, acoustic noise, etc. [5; 6; 7], radio frequency (RF) energy harvesting (EH) has recently become a promising technique for WSNs since it allows information and energy to be transmitted simultaneously [8; 9; 10; 11; 12; 13]. In [8], the authors first dealt with the fundamental trade-off between transmitting energy and information at the same time over single input single output (SISO) additive white Gaussian noise (AWGN) channels. Based on these pioneering works, [PERSON]. [9; 10] proposed more practical designs, by assuming that the receivers are capable of performing EH and information decoding separately. [PERSON] and [PERSON] [9] studied multiple input multiple output (MIMO) transmission with practical designs that separate the operation of information decoding and EH receivers. Based on the time switching (TS) and power switching (PS) receiver architectures, [PERSON]. [10; 11] proposed two relaying protocols, namely, time switching-based relaying (TSR) and power switching-based relaying (PSR), to enable EH and information processing at the relay. Following that, [PERSON]. [12; 13] showed interest in the application of simultaneous wireless information and power transfer (SWIPT) for wireless communication systems. The authors in [12] studied the joint beamforming and power splitting design for a multi-user multiple-input single-output (MISO) broadcast system, where a multi-antenna base station (BS) simultaneously transmits information and power to a set of single-antenna mobile stations (MSs). Different from [12], [PERSON]. [13] considered a large-scale network with multiple transmitter-receiver pairs where receivers conducted a PS technique to harvest energy from RF signals. Most of the above works focused on EH using radio frequency (RF) transmitted from the source node. However, in practical communication networks, the RF signal is severely degraded due to the huge path loss between the source node and the receiver. Therefore, these systems are only suitable for short distance communications. To overcome this issue, [PERSON]. [14] proposed a novel hybrid network with randomly deployed power beacons (PB) to provide a practically infinite battery lifetime for mobiles. PB-assisted wireless energy transfer has recently attracted a lot of attention from many researchers [15; 16; 17]. The authors of [15] analyzed the throughput of a distributed PB assisted wireless powered communication network via time division multiple access (TDMA) and under i.n.i.d. Nakagami-\(m\) fading distribution. The PB-assisted technique has been also studied in the device to device (D2D) communication system [16; 17], due to the benefits of D2D systems, i.e., low latency, high spectral efficiency, and low transmit power [18]. In [19; 20], multi-hop PB-assisted relaying schemes were studied and investigated. More specifically, the authors in [21; 22] proposed novel multi-hop multi-path PB-assisted cooperating networks with path selection methods to enhance the system performance. Besides energy, another consequence of the explosive growth of wireless services is the spectrum scarcity problem. To solve this problem, the concept of cognitive radio (CR) was first introduced by [PERSON] in [23], where licensed users (primary users (PUs)) can share their bands to unlicensed users (secondary users (SUs)) provided that quality of service (QoS) of the primary network is still guaranteed.