Smart Solutions in Smart Spaces: Getting the Most from Far-Field Wireless Power Transfer

Abstract

MotivationNowadays there is an almost unlimited number of monitoring applications, such as structural health, logistic, security, healthcare and agriculture, which are planning to be based on a large deployment of co-operative wireless microsystems, with sensing capabilities, moving closer to the effective realization of the paradigm of the Internet of Things. The main open challenge is the reliability of maintenance-free devices, with life-time duration, especially from the energy sustainability point of view. Such systems are required to power themselves, by harvesting energy from the ambient, thus eliminating battery needs. To minimize energy requirements, wake-up radios able to be activated by signals as low as -50 dBm are already available [1]. RF/microwave energy sources are foreseen as one of the best candidates to comply with energy autonomy, either because they are widely distributed in humanized environments or because they can be efficiently provided on demand. These two different ways of providing RF energy can be referred to as RF energy harvesting (EH) and wireless power transmission (WPT), respectively. In both cases a delicate design of the RF power transfer link, consisting of the nonlinear sub-systems and the radiating elements, is required, providing that their characteristics are carefully optimized depending on the particular contest and scenario. Intensive industrial and academic activities has been devoted to this field: several and concurrent techniques and circuit solutions have been proposed and tested to ensure nonintermittent, sufficient wireless power transfer, with the lowest possible density. In this way, it is possible to comply with maximum transfer efficiency and minimum EM interference and pollution, at the same time. Typical frequencies adopted for these purposes are in the UHF and SHF bands around 400-800 MHz, for terrestrial TV signals, and around 900, 1800, 2400 and 5800 MHz, for different wireless standards: since the geometrical area to be covered are usually of the order of few meters, the devices to be powered are in the far-field region of their known or unknown RF/microwave sources. This article reviews some of the recent and promising circuit and antenna solutions and discusses reliable sub-systems adopted for receiving and transmitting subsystems, their associated radiating elements, with a focus on minimizing the power budget for enabling device operations.

Large-Scale Multiantenna Multisine Wireless Power Transfer

“…Then, v out,q can be expressed as a function of the transmit waveform, by combining (1), (2) and (6). In the term f LPF y 2 q (t) = f LPF Re{ỹ q (t)ỹ q (t) +ỹ q (t)ỹ * q (t)} , where Re{ỹ q (t)ỹ q (t)} can be removed as it only contains the non-DC components.…”

“…Low-power applications such as the Internet of Things and radio frequency identification networks are expected to benefit from far-field Wireless Power Transfer (WPT) systems [2]. WPT utilizes a dedicated source to radiate electromagnetic energy through a wireless channel and a rectenna at the receiver to convert this energy into DC power.…”

“…This work has been partially supported by the EPSRC of UK, under grant EP/P003885/ 1. revealed that this efficiency is also a function of the input waveforms and can be boosted by multi-sine WPT i.e. transmitting a superposition of sinewaves over frequencies with uniform frequency spacing [2]- [5]. In the context of microwave theory, methods for improving the energy transfer efficiency have been studied in smart power beaming [2] (not leveraging multi-sine signals) and spatially combined multi-sine signals [3].…”

“…transmitting a superposition of sinewaves over frequencies with uniform frequency spacing [2]- [5]. In the context of microwave theory, methods for improving the energy transfer efficiency have been studied in smart power beaming [2] (not leveraging multi-sine signals) and spatially combined multi-sine signals [3]. The main limitation of those methods is not only the lack of a systematic approach to design waveforms, but also the fact that they operate without Channel State Information (CSI) at the Transmitter (CSIT) 1 , which in communications is usually used to turn multipath fading into a benefit for users.…”

“…In the scenario where multiple frequency components can be utilized for WPT, the linear model favours a waveform strategy where the transmit power is allocated to a single frequency component [8], [13]. Unfortunately, this contradicts experimental results that show the benefits of a power allocation over multiple frequency components to boost the RF-to-DC conversion efficiency [2]- [5]. The reason is that the 4th-order truncation is necessary for characterizing the basic diode rectification process [5].…”