electronics Article Design of a Compact and Highly Efficient Energy Harvester System Suitable for Battery-Less Low Cost On-Board Unit Applications Giovanni Collodi * , Stefano Maddio and Giuseppe Pelosi Department of Information Engineering, Università degli Studi di Firenze, Via Santa Marta N.3, 50142 Firenze, Italy; stefano.maddio@unifi.it (S.M.); giuseppe.pelosi@unifi.it (G.P.) * Correspondence: giovanni.collodi@unifi.it; Tel.: +39-055-2758544 Abstract: This study addresses the general problem regarding the power supply in specific on-board unit (OBUs) solutions. In detail, this paper refers to a subset of the so-called electronic toll col- lection (ETC) applications such as assets control and vehicle identification, where simplicity, low costs, and maximum compactness represent the most important features. In this context, the next generation of OBUs, developed specifically with reference to such applications, will involve energy harvester-based battery-less techniques. Previous studies have mainly concentrated on performance optimization by achieving maximum energy transmission to the OBUs. This study discusses a tech- nique suitable for both maximizing performance and minimizing the dimensions of transponder energy harvesters suitable for assets control and vehicle identification operating at 5.8 GHz. The tech- nique assumes that an optimal source impedance exists that maximizes the energy transfer to the transponder, thus enabling its power supply in a battery-less configuration. We discuss a solution based on a compact patch antenna designed to exhibit this optimal source impedance to the RF-to-DC rectifier. This approach avoids the use of a lossy matching network. For the sake of comparison, the same function is compared with an equivalent development, which includes the interstage match- ing network between the antenna and the RF-to-DC rectifier. We introduce experimental results demonstrating that the ultracompact energy harvester optimized at −5 dBm of impinging power is Citation: Collodi, G.; Maddio, S.; Pelosi, capable of increasing both the charge current and energy efficiency from 340 to 450 µA and from 37% G. Design of a Compact and Highly to 47%, respectively. Efficient Energy Harvester System Suit- able for Battery-Less Low Cost On- Keywords: energy harvesting; efficiency; power management; rectifier; battery-less; patch antenna Board Unit Applications. Electronics 2021, 10, 3. https://dx.doi.org/10.3390/ electronics10010003 Received: 21 November 2020 1. Introduction Accepted: 18 December 2020 In recent years, many concepts developed in the field of intelligent transportation Published: 23 December 2020 systems (ITS) have started to merge with the Internet of Things (IoT) to become part of this paradigm [1]. It is well known that, until now, limitations in the present communications Publisher’s Note: MDPI stays neu- technologies have reduced the full exploitation of the IoT vision in terms of mobile applica- tral with regard to jurisdictional claims tions. This translated into the poor development of IoT-based approaches oriented toward in published maps and institutional the infomobility ecosystem. As a consequence, it is expected that full development of the affiliations. 5G communications technology will enable wide implementation of the IoT paradigm in the mobile world, including ITS/IoT systems [2–6]. Nevertheless, regardless of the technology, three central classes of applications may be Copyright: © 2020 by the authors. Li- identified in such a system, i.e., road safety, traffic efficiency, and a wide-ranging category censee MDPI, Basel, Switzerland. This of other utility applications. Up to now, one of the most relevant applications in this third article is an open access article distributed class has been related to toll road systems or electronic toll collection (ETC) [7,8]. Typically, under the terms and conditions of the these systems are mainly linked with electronic payment for highways or main roads, Creative Commons Attribution (CC BY) although the same systems or solutions may also be used for assets control and vehicle license (https://creativecommons.org/ identification (access control or parking payment). The common approach followed in such licenses/by/4.0/). a system is an architecture based on roadside units (RSUs) and on-board units (OBUs) [7–9]. Electronics 2021, 10, 3. https://dx.doi.org/10.3390/electronics10010003 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 3 2 of 13 It is a fact that in specific applications, such as assets control and vehicle identification, cost reduction, low complexity, and high reliability represent fundamental requirements for the whole system. These severe requirements make the replacement of the current approach based on RSUs and OBUs with 5G-based applications very challenging. The technological evolution in such identification systems will be focused on OBU cost and complexity reduction. One of the most relevant contributions in this direction will be the development of battery-less solutions that translate into simplified architec- tures, reduced costs, and reduced logistic chains, thereby resulting in increased diffusion of the system. It is worth noting that the battery-less attribute is maximally exploited in conjunction with electronic components capable of operating at very low power sup- ply [10]. As exemplified in Table1, systems oriented to ETC applications involve OBU with performances, power consumption requirements, levels of complexity, and costs that are not compliant with a battery-less approach. Consequently, only applications that match with lower performances, as well as reduced complexity and power requirement, can be implemented using a battery-less approach. Table 1. Compatibility of on-board unit (OBU) applications with respect to battery-less solutions. OBU OBU OBU OBU Power Application Complexity Battery-Less Performance Consumption and Cost Compatibility ETC High High High Low Asset control Medium Medium Low Medium/low Vehicle identification Low Low Low High In this context, the key function for implementing a battery-less system has become the development of advanced radio-based energy harvesting solutions, a technological approach commonly used by a wide range of applications, including the IoT [11], smart cities [12], and mobile healthcare [13], and, in particular, for completely battery-less sys- tems [14–16]. Concerning the specific class of applications, the harvester system has to ensure a highly efficient reduced charge time in order to enable the OBU functionality, as illustrated in Figure1. The OBU must be charged by the RSU transmitted signal during the so-called Electronics 2021, 10, x FOR PEER REVIEW“TX data and power” time interval. Consequently, the implemented harvesting system3 hasof 14 to charge and activate the OBU in a time that does not exceed the order of ten seconds. FigureFigure 1. 1.Principle Principle of of operation operation of of a a battery-less battery-less OBU. OBU. TheThe aim aim of of the the present present study study are to are illustrate to illustrate an efficient an efficient and compact and energycompact harvester energy solution,harvester compatible solution, withcompatible battery-less with OBU batte design,ry-less based OBU ondesign, the approach based on described the approach in [17] but characterized by the use of an antenna showing a specific impedance. The key pa- described in [17] but characterized by the use of an antenna showing a specific impedance. rameters for improving the harvester performance in this approach consist of maximizing The key parameters for improving the harvester performance in this approach consist of the input current of the DC/DC converter. The solution proposed in [17] suggests that, maximizing the input current of the DC/DC converter. The solution proposed in [17] suggests that, by making use of an RF power generator that shows quite a peculiar impedance to the rectifier, it is possible to obtain optimum charge performances. This impedance is almost never purely resistive, and this target is reached by means of a specific matching network that transforms the input impedance of a 50 Ω antenna into such an optimum impedance [17]. In our approach, the results are obtained by means of a proper patch antenna which has been designed to directly show the optimum impedance to the harvester. The use of such an antenna offers the advantage of minimizing the feed line complexity, avoiding any need for additional circuitry, and thus decreasing the system dimensions and complexity. However, the real advantage consists of the power loss reduction because of the matching network removal. The reduction in losses translates into an increased DC/DC input current and, consequently, an enhanced efficiency. A comparative analysis between some state-of-the-art harvesting solutions, specifically designed for low power RF applications, and the proposed application is carried out, showing the advantage of the present approach with respect to the reference applications [18–20]. The use of this novel harvesting approach in the battery-less OBU design enables the development of systems with a very simple architecture, high compactness, reduced costs, and increased performance for assets control and vehicle identification applications. 2. Harvester Architecture and Operation The reference application for this study is the development of a compact harvester system for a low-cost battery-less OBU working at 5.8 GHz [7,8] and optimized for assets control and vehicle identification