Wireless Power Transfer is a promising solution to increase the lifetime of wireless nodes and hence alleviate the energy bottleneck of energy constrained wireless networks. In this Chapter, we discuss two power transfer policies; dual-source and single fixed-source, two bidirectional relaying protocols; multiple access and time division broadcast, and two relay receiver structures; time switching and power splitting, are considered to derive closed-form expressions for the outage and throughput of the network in the context of delay-limited transmission. This framework assists the reader not only to quantify the degradation of outage probability and throughput of the networks due to the impairments of realistic transceiver but also to provide realistic insight into the effect of power transfer policies, relaying protocols and receiver structures on outage and throughput of the networks.
TopIntroduction
All living and machine objects rely on both information and power for their existence. Although these two entities are in harmony in nature, in traditional engineering design, information and power are handled by separate systems with limited interaction. While wireless power transfer (WPT) through radio waves has already been employed in various applications (e.g., the radio-frequency identification (RFID) technology, healthcare monitoring, etc.), radio wave-based information transfer and power transfer have largely been designed separately. Many forthcoming applications could benefit from a joint consideration of information and power transfer. For example, wireless implants can be charged and calibrated concurrently with the same signal, wireless sensor nodes can be charged with the control signals received from the access points, and mobile phones can download emails while being wirelessly charged. Initial efforts on WPT have focused on long-distance and high-power applications. However, both the low efficiency of the transmission process and the health concerns for such high-power applications prevented their further development. With sensors and wireless transceivers getting ever smaller and more power- efficient, we envision that radio waves will not only become a major source of power for operating these devices, but also their information and power transmission aspects will be unified. A design that jointly takes into account both information and power can provide significant engineering breakthroughs and lead to potential new applications and services for next-generation sustainable societies.
Two common methods to design co-located receiver architecture for EH are time- switching based (TSB) and power-splitting based (PSB). In TS, the receiver switches in time between energy harvesting and information decoding, while in PSB the receiver splits the received signal into two streams of different power for energy harvesting and information decoding. EH has to be realized by properly allocating the available resources and sharing them among both information transfer and energy transfer. Designing TSB/PSB EH receivers in a point-to- point wireless environment to achieve various trade-offs between wireless information transfer and energy harvesting is considered in (Ng, Lo, & Schober, pp. 635–6370, 2013; Shi, Liu, Xu & Zhang, pp. 3269–3280, 2014).
As a sustainable remedy to strengthening the lifetime of energy constrained wireless devices, EH technique has received considerable attention since it meets the requirements of green communications. Besides to the traditional renewable energy sources such as solar and wind, radio frequency (RF) signals radiated by ambient transmitters can be identified as a viable new intuition for EH. In (Paradiso & Starner, pp. 18–27, 2005; Le & Fiez, pp. 1287-1302, 2008), wireless nodes gather energy of RF signals in the surrounding environment to self-power the transmission. Recently, some critical advances of WPT have largely increased the feasibility of EH in practical wireless applications (Suh & Chang, pp. 1784-1789, 2002; Huang & Lau, pp. 902-912, 2014). With concurrent development in the antenna technology and EH circuit designs, wireless energy transfer is recognized as a valuable candidate for future networks.
Cognitive radio is emerging as a means to improve the wireless spectrum utilization (Hunter & Sanayei, pp. 375--391, 2006). In cognitive radio, secondary users (SUs) are allowed to transmit wireless signals in the same frequency bands that are officially allocated to primary users (PUs). In order to maintain quality-of-service of primary transmission links, the transmit power of SUs should be limited to the maximum interference allowance of PUs. Consequently, this power constraint limits the performance of SUs. In order to tackle the transmit power limitation in cognitive networks, the concept oftwo-way cognitive relay (TWCR) networks has been proposed in (Yang, Alouini & Qaraqe, pp. 1240-1243, 2012; Kim, Duong, Elkashlan, Yeoh & Nallanathan, pp. 992-997, 2013) among others. TWCR networks exploit the advantages of two-way relaying protocols and cognitive radios. Also, they are able to overcome transmit power limitations and boost the system performance.