Radio frequency energy harvesting for wireless sensor networks

The need for the battery-free devices is increasing dramatically in many applications including medical and environmental monitoring, electric car charging, and wireless sensor networks.^1–4 For wireless sensor networks, energy harvesting (EH) techniques based on various types of energy resources such as solar energy, thermal energy, radio frequency (RF) energy, and piezoelectric energy have been developed. Of these techniques, we particularly focus on the ambient RF EH. The systems harvesting ambient RF energy exploit energy sources already present in the surrounding environments, namely, TV, radio, cellular, satellite, and Wi-Fi systems.^5,6

Ambient wireless EH technologies are generally used for low-power standalone electronics. EH from the surrounding RF environments can be an option to recharge wireless sensor networks. The amount of energy harvested from ambience primarily depends on the type of the harvesting device and the type of energy source. The harvested energy from ambience and the density of incident power according to the type of energy source are summarized in Georgiou et al. ⁷ In the case of wireless LAN, for instance, the peak power density ranges from 30 pW/cm² to 13.36 mW/cm² with typical transmit power, channel status, and the distance between transmitter and receiver.

If ambient energy sources are not available, wireless power transfer (WPT) can be a powerful technique to recharge devices for their sustainable operation. ⁸ The WPT can be classified into near-field (NF) WPT, and far-field (FF) WPT, depending on the distance between the transmitter and the receiver.^9–11 Most of NF-WPT techniques are induction-based: magnetic induction (or inductive power transfer (IPT)) and electrostatic induction (or capacitive power transfer (CPT)). ¹² The IPT is able to recharge the sensor devices with charging power ranging from low level (<1 W) to high level (>1 kW). ¹³ Particularly, the high-power IPT has recently emerged as an attractive means of electric car charging. The CPT is suitable for low-power biomedical devices and mobile devices. ¹⁴ The FF-WPT is based on magnetic resonance with RF. In RF-based WPT, a dedicated power beacon can be used for recharging while data are transmitted simultaneously. Wireless systems in support of simultaneous power and data transfer at different frequency bands are known as out-band system, whereas those working in the same frequency band are known as in-band systems. ¹⁰

In the paper “Wireless-powered cooperative energy aware anycast routing in wireless sensor networks,” authors present a wireless-powered cooperative energy aware anycast routing protocol. Selection of relays with optional capability of EH and routing to destination are investigated with wireless sensor networks. They found from their study that maximization of lifetime and reduction in energy consumption of cooperative wireless sensor networks can be obtained by relays capable of EH. In another paper “Performance analysis of the clustering-based multi-hop wireless EH sensor networks over symmetric and asymmetric fading channels,” authors investigated clustering-based multi-hop relaying sensor network. They also considered relays capable of EH and proposed a relay selection scheme. It is found from their work that gain obtained from path loss reduction and relay diversity can mitigate potential performance degradation incurred by poor RF-to-DC conversion efficiency. They also showed comparative performance of their scheme with conventional multi-hop relaying scheme without clustering.