One of the main challenges in implementing sensor devices for internet of things (IoTs), is the means for the operating power supply. RF energy harvesting (RFEH) presents a promising solution as RF power is a suitable choice particularly for cases where solar harvesting is not feasible. However, in spite of RF communication system design being a well-established, there are several challenges poised for the implementation of the RFEH systems especially for harvesting the ambient RF signals. The challenges can be widely categorized as the overall conversion efficiency, bandwidth, and form factor. In this article, an exhaustive survey on the different RFEH system that is reported is carried out and discussed. Important design issues are identified with insights drawn. First, we have presented the challenges in designing antennas for RFEH systems. This is followed by rectifier circuits and matching networks, and eventually a general frame work for designing of ambient RFEH systems is deduced.IoT sensors, RF energy harvesting, rectennas, rectifier, matching circuit
| INTRODUCTIONRecent advances in technology has led to the development of Internet of Things (IoT), Wearable Electronics, 5G Wireless Systems, etc, which requires widespread deployment of sensors often positioned at remote places 1 with the capacity of communicating wirelessly with each other. Power is often the limiting factor, as we have to depend on battery sources. This leads to the tedious task of disposing and replacing enormous number of batteries causing environmental pollution.Energy harvesting (EH) provides a green as well as sustainable solution to this challenge. This involves absorbing ambient energy and converting it into electricity to power up the battery/sensors. 2 Various sources of ambient energy are solar energy, wind energy, tidal energy, electromagnetic energy, thermal energy, mechanical energy, etc. The block diagram of a typical EH system is shown in Figure 1. An energy harvester converts the incident ambient energy into electrical energy. Power management unit (PMU) conditions the electrical energy into a form suitable for a particular application. Solar energy, even though the most abundant of the sources, suffers from intermittent supply. 3 Due to its continuous availability, radio frequency (RF) source is an alternative. However, it suffers from low incident power levels. This can be improved by using a dedicated source and an efficient RF to DC conversion circuit. Low power consumption in the sensors and a widespread availability of RF power makes it as a tangible option for energy harvesting.The RFEH process involves harvesting electrical energy from RF signals (which could be from a dedicated, ambient, or unknown RF source). The power density from the various RF sources ranges from 0.1 (ambient RF) to 1000 (dedicated RF) μW/cm 2 . The various RF energy sources are, namely, WiFi, WLAN, Digital TV, AM, FM, Bluetooth, etc. Due to its ubiquitous nature, it can be used to power wireless sensor nodes, 4 wireless body area networks, ...
