Internet of Things (IoT) is an emerging computing concept that describes a structure in which everyday physical objects, each provided with unique identifiers, are connected to the Internet without requiring human interaction.Long-term and self-sustainable operation are key components for realization of such a complex network, and entail energy-aware devices that are potentially capable of harvesting their required energy from ambient sources. Among different energy harvesting methods such as vibration, light and thermal energy extraction, wireless energy harvesting (WEH) has proven to be one of the most promising solutions by virtue of its simplicity, ease of implementation and availability. In this article, we present an overview of enabling technologies for efficient WEH, analyze the life-time of WEH-enabled IoT devices, and briefly study the future trends in the design of efficient WEH systems and research challenges that lie ahead.

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A high-sensitivity fully passive 868-MHz wake-up radio (WUR) front-end for wireless sensor network nodes is presented. The front-end does not have an external power source and extracts the entire energy from the radio-frequency (RF) signal received at the antenna. A high-efficiency differential RFto-DC converter rectifies the incident RF signal and drives the circuit blocks including a low-power comparator and reference generators; and at the same time detects the envelope of the onoff keying (OOK) wake-up signal. The front-end is designed and simulated 0.13μm CMOS and achieves a sensitivity of −33 dBm for a 100 kbps wake-up signal.

A CMOS rectifier with a wide input signal range for radio-frequency identification (RFID) applications is presented. Using quasi-floating gate technique, a gate-biasing scheme is proposed to provide a relatively flat power conversion efficiency (PCE) curve for a wide input voltage (power) range. The proposed technique also enables an efficient operation for input voltage levels well below the standard threshold voltage of the MOS switching transistors. Appropriate bias voltages for different stages of the rectifier are generated through a chain of low-power bandgap reference generators which impose minimal power and area overhead. The proposed rectifier architecture is designed and laid out in a standard 0.13-p,m CMOS technology. For a 2.4 GHz RF input frequency and 30 kO output load, post layout simulation results of the circuit show that a maximum PCE of 66.7% is achieved for an input signal with an amplitude (power) of 0.45 V ( . While a high PCE of 60% is achieved for input voltage (power) levels as low as 0.25 V (-15 dBm), PCE maintains above 60% for a wide input voltage (power) range from 0.25 V to 0.7 V (-15 dBm to -3 dBm).

An adaptive control mechanism to improve the efficiency of magnetically coupled resonators (MCRs) used in wireless power transmission is presented. To minimize the degradation in power transfer efficiency, the proposed system dynamically adjusts the capacitance of MCRs as the distance between the transmitter (TX) and receiver (RX) coils changes. The control unit operates in a self-sufficient manner through rectifying a portion of the AC signal present on TX and RX coils. A proof-of-concept circuit operating at 13.56 MHz is designed in a 0.13 μm CMOS technology and simulation results confirm the validity of the proposed scheme.Index Terms-Wireless power transfer, magnetically coupled resonator, adaptive tunning.

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