Design of a Compact and Highly Efficient Energy Harvester System Suitable for Battery-Less Low Cost On-Board Unit Applications

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 collection (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 matching 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].

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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 supply [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 systems [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 applications. This means that the energy available for system activation is the energy supplied by the RSU during the wake up and negotiation time account, as illustrated in Figure 1. Consequently, the harvester must possess a maximum conversion efficiency and minimum charge time and size. Additionally, the operating range between RSU and OBU has to be limited to about 1.5 to 2 m. This leads to optimization of the harvester’s performances with respect to the power impinging to the rectifier, related to the required distance. In our reference application, as explained in Section 5, a power level of −5 dBm has been chosen for the optimization procedure. The proposed harvester block is based on the design principle presented in [17]. The top diagram in Figure 2 illustrates the harvester as designed in [17], where the so-called optimum matching network is used to show the desired impedance to the RF-to-DC conversion section. The bottom diagram in Figure 2 describes the modified architecture

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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 applications. This means that the energy available for system activation is the energy supplied by the RSU during the wake up and negotiation time account, as illustrated in Figure1. Consequently, the harvester must possess a maximum conversion efficiency and minimum charge time and size. Additionally, the operating range between RSU and OBU has to be limited to about 1.5 to 2 m. This leads to optimization of the harvester’s performances with respect to the power impinging to the rectifier, related to the required distance. In our reference application, as explained in Section5, a power level of −5 dBm has been chosen for the optimization procedure. Electronics 2021, 10, x FOR PEER REVIEW The proposed harvester block is based on the design principle presented in [17]. The4 of top 14

diagram in Figure2 illustrates the harvester as designed in [ 17], where the so-called optimum matching network is used to show the desired impedance to the RF-to-DC conversion section.with the Thematching bottom network diagram removed. in Figure The2 describes proposed thesystem modified consists architecture of a chain withcomposed the matchingof the optimum network matched removed. antenna, The proposed the corresponding system consists RF-to-DC of a chain rectifier, composed the specific of the optimumDC/DC boost, matched and antenna, a tank thecapacitor, corresponding represen RF-to-DCting the rectifier,energy thestorage specific for DC/DC the following boost, andsections. a tank capacitor, representing the energy storage for the following sections.

FigureFigure 2.2. Architecture of the the proposed proposed harvesting harvesting sy systemstem (below) (below) compared compared with with the the canonical canonical architecture (above). architecture (above). In order to achieve the objective of this study, the key sub-system is the cascade of the RF-to-DC rectifier and the DC/DC converter blocks. The cascade of these two blocks may be addressed as the RF-to-DC rectifier, which acts as a frequency power converter that transfers RF energy to DC. The intrinsically nonlinear behavior of this power converter requires the harvester block to be analyzed by applying nonlinear techniques. In this application, the actual goal is to maximize the harvester performance that translates into increasing the RF-DC output current as much as possible, which feeds the DC/DC boost, thus, reducing the DC/DC “charge” time [17,21]. To this aim, the DC/DC boost, which is implemented by a BQ25570 evaluation board [22], is modeled with an equivalent circuit showing a variable input termination. With reference to Figure 3 and following the reasoning carried out in [17], the relation between current and voltage at the DC/DC input is given by: ℎ > = (1)

0 ℎ

for the hardware involved, the minimum input voltage is the threshold = 0.34 V.

Figure 3. Schematic representation for the source pull optimization.

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with the matching network removed. The proposed system consists of a chain composed of the optimum matched antenna, the corresponding RF-to-DC rectifier, the specific DC/DC boost, and a tank capacitor, representing the energy storage for the following sections.

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Figure 2. Architecture of the proposed harvesting system (below) compared with the canonical architecture (above). In order to achieve the objective of this study, the key sub-system is the cascade of the RF-to-DCIn order rectiﬁerto achieve and the the objective DC/DC of converter this study, blocks. the key The sub-system cascade of these is the two cascade blocks of maythe RF-to-DC be addressed rectifier as the and RF-to-DC the DC/DC rectiﬁer, converte whichr actsblocks. as a The frequency cascade power of these converter two blocks that transfersmay be addressed RF energy as to the DC. RF-to-DC The intrinsically rectifier, nonlinear which acts behavior as a frequency of this power power converter converter requiresthat transfers the harvester RF energy block to to beDC. analyzed The intrin by applyingsically nonlinear nonlinear behavior techniques. of this power In this application, the actual goal is to maximize the harvester performance that converter requires the harvester block to be analyzed by applying nonlinear techniques. translates into increasing the RF-DC output current I as much as possible, which In this application, the actual goal is to maximizeCGH the harvester performance that feeds the DC/DC boost, thus, reducing the DC/DC “charge” time [17,21]. To this aim, translates into increasing the RF-DC output current as much as possible, which the DC/DC boost, which is implemented by a BQ25570 evaluation board [22], is modeled feeds the DC/DC boost, thus, reducing the DC/DC “charge” time [17,21]. To this aim, the with an equivalent circuit showing a variable input termination. DC/DC boost, which is implemented by a BQ25570 evaluation board [22], is modeled with With reference to Figure3 and following the reasoning carried out in [ 17], the relation an equivalent circuit showing a variable input termination. between current and voltage at the DC/DC input is given by: With reference to Figure 3 and following the reasoning carried out in [17], the relation between current and voltage at the VDC/DC−V input is given by: RFDC TH when V > V  RRFDC IN TH   ℎ > ICHG = (1) = (1)  0 otherwise 0 ℎ for the hardware involved, the minimum input voltage is the threshold VTH = 0.34 V. for the hardware involved, the minimum input voltage is the threshold = 0.34 V.

FigureFigure 3. 3.Schematic Schematic representation representation for for the the source source pull pull optimization. optimization.

As stated in [17], the DC/DC block is described by its variable input termination within the simulation. In particular, the boost converter inside the DC/DC block operates draining as much current as possible. Since the input source shows a non-zero output resistance RRFDC, when the current absorption increases, the input voltage reduces. Consequently, this leads to a variation of DC/DC state parameters. Beneﬁtting from this equivalent circuit model, an effective source-pull analysis may be performed. The source-pull algorithm consists of varying the Zsrc of the ideal generator across any feasible value. For each of the sets, the DC/DC charge process is simulated and an estimated ICGH is computed. Comparing all the collected ICGH values, the optimal Zsrc is formally identiﬁed as the one that leads to the maximum charge current. Figure4 depicts the results of the source-pull procedure carried out with respect to the chosen reference power impinging the rectifier of −5 dBm. The resultant optimal impedance Zopt has a very small real part, namely, 12.1 Ω. This is expected since it must maximize the output current. Instead, quite a high imaginary part, namely, −j22.3 Ω, is found as compared with the real part of 12.1 Ω. The corresponding best current is ~145 µA. This value, as well as Zopt, strongly depends on the diode chosen for the voltage tripler. Electronics 2021, 10, x FOR PEER REVIEW 5 of 14

As stated in [17], the DC/DC block is described by its variable input termination within the simulation. In particular, the boost converter inside the DC/DC block operates draining as much current as possible. Since the input source shows a non-zero output resistance RRFDC, when the current absorption increases, the input voltage reduces. Consequently, this leads to a variation of DC/DC state parameters. Benefitting from this equivalent circuit model, an effective source-pull analysis may be performed. The source-pull algorithm consists of varying the of the ideal generator across any feasible value. For each of the sets, the DC/DC charge process is simulated and an estimated is computed. Comparing all the collected values, the optimal is formally identified as the one that leads to the maximum charge current. Figure 4 depicts the results of the source-pull procedure carried out with respect to the chosen reference power impinging the rectiﬁer of −5 dBm. The resultant optimal impedance has a very small real part, namely, 12.1 Ω. This is expected since it must maximize the output current. Instead, quite a high imaginary part, namely, −j22.3 Ω, is found as compared with the real part of 12.1 Ω. The corresponding best current is ~145 Electronics 2021, 10, 3 μA. This value, as well as , strongly depends on the diode chosen for the voltage5 of 13 tripler.

FigureFigure 4. 4. LociLoci for for IICHGCHG currentcurrent as as the the results results of of source-pull source-pull procedure. procedure.

3.3. Antenna Antenna Design Design InIn most most automotive automotive applications, applications, the the OBU OBU is is typically typically placed placed below below the the front front window window of the vehicle pointing toward the RSU when the car is moving [23]. Hence, the most of the vehicle pointing toward the RSU when the car is moving [23]. Hence, the most suitable antenna to be implemented in an OBU is a patch antenna, with a major lobe that is suitable antenna to be implemented in an OBU is a patch antenna, with a major lobe that almost hemispherical. This feature permits the link with the RSU. Many solutions have is almost hemispherical. This feature permits the link with the RSU. Many solutions have been proposed for the patch, depending also on the polarization on the communication. been proposed for the patch, depending also on the polarization on the communication. Here, we refer to the case of linear polarization operating at 5.8 GHz, as prescribed by the Here, we refer to the case of linear polarization operating at 5.8 GHz, as prescribed by the European Telecommunications Standards Institute (ETSI) standard [7]. European Telecommunications Standards Institute (ETSI) standard [7]. The key to the success of the proposed approach consists of the ability to implement The key to the success of the proposed approach consists of the ability to implement the necessary optimal input impedance Zopt with a minimum waste of power. Therefore, the necessary optimal input impedance with a minimum waste of power. Therefore, the antenna becomes a critical component for system implementation. The goal of the the antenna becomes a critical component for system implementation. The goal of the antenna design procedure is to provide a reasonable match with the computed Zopt, while antennapursuing design the best procedure gain. is to provide a reasonable match with the computed , whileAs pursuing previously the best discussed, gain. a relevant improvement in OBU design is given by the architectureAs previously simpliﬁcation, discussed, from a whichrelevant a low-costimprovement and small in OBU size systemdesign canis given be obtained. by the architecture simplification, from which a low-cost and small size system can be obtained. Electronics 2021, 10, x FOR PEER REVIEWOn this basis, the use of a patch antenna, which is a planar technology component, allows 6 of 14 Onus tothis build basis, the the entire use of harvesting a patch antenna, system wh in aich single is a planar step. Therefore, technology high component, integration allows and usmaximum to build the compactness entire harvesting can be achieved.system in a In single addition, step. theTherefore, patch antenna,high integration as an unbal- and maximumanced structure, compactness can be can directly be achieved. connected In addition, with the the network patch withoutantenna, theas an use unbalanced of a balun structure,and a matching can be network, directly connected which would with increase the network the system without size. the use of a balun and a With reference to Figure 5, the proposed antenna consists of a rectangular matchingWith network, reference which to Figure woul5d, theincrease proposed the system antenna size. consists of a L0xW0 rectangular patch, coupled with a pair of identical rectangular patch parasites, one for each patch, coupled with a pair of identical L1xW1 rectangular patch parasites, one for each side. side.The parasitesThe parasites are placed are at placed a small distanceat a smallg, and distance thus are excited, and by thus proximity are excited coupling by proximity couplingfrom the fringingfrom the ﬁeld fringing of the main field patch. of Thisthe main is a common patch. strategy This is to a enhance common the antennastrategy to enhance theperformance, antenna performance, either in termsof either gain or in bandwidth terms of [gain24,25 ].or bandwidth [24,25].

FigureFigure 5. 5. Layout of of the the proposed proposed harvesting harvesting antenna. antenna.

It is well known that this kind of antenna can be described by the cavity model, which represents the field under the patch as a superimposition of resonant modes. These modes depend on the working frequency, and thus the patch sizes and the position of the feeding network, which allows us to only excite some modes. The proposed patch antenna is designed to excite the first resonant mode, from which a radiated far field with a maximum gain in the normal direction is obtained. This first resonant mode is related to a certain input patch impedance trend and is equal to zero at the center of the patch and the maximum on the edge. Therefore, different values of can be obtained by changing and . As discussed, for the sake of miniaturization and efficiency, a complex matching network must be avoided to prevent energy waste. This requirement spurs the idea of synthesizing at the edge of the antenna an impedance , which is compatible with the required , after the simplest impedance transformations. The rectangular patch is designed to use a low-cost commercial substrate ( =3.75, = 0.02 and ℎ = 1.575 mm) and, as previously mentioned, to show a maximum gain of ~6.5 dB in the broadside direction. With reference to Figure 6, the input impedance of the synthesized patch, evaluated at 5.8 GHz, is compared to the desired . This evaluation is carried out graphically by means of various Smith charts. The idea is to pursue the nominal matching to exploiting only a section of transmission line of length . Accordingly, the effective normalized values of and change as well, and so does the length of the rotation needed to transform one into the other. In particular, Figure 6a shows the case of a transmission line of impedance = 50 Ω. The impedance seen by the patch after the rotation corresponding to , which is the best possible in this case, does not accurately synthesize the real part of . In the case of = 100 Ω, described by Figure 6b, the optimal impedance is almost exactly hit after a rotation corresponding to the length . Finally, the case depicted in Figure 6c shows the rotation corresponding to around the reference impedance = 200 Ω. In this case, the real part of the input impedance is slightly larger than the real part of .

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It is well known that this kind of antenna can be described by the cavity model, which represents the field under the patch as a superimposition of resonant modes. These modes depend on the working frequency, and thus the patch sizes and the position of the feeding network, which allows us to only excite some modes. The proposed patch antenna is designed to excite the first resonant mode, from which a radiated far field with a maximum gain in the normal direction is obtained. This first resonant mode is related to a certain input patch impedance trend and is equal to zero at the center of the patch and the maximum on the edge. Therefore, different values of Zpatch can be obtained by changing Wslit and Lslit. As discussed, for the sake of miniaturization and efficiency, a complex matching network must be avoided to prevent energy waste. This requirement spurs the idea of synthesizing at the edge of the antenna an impedance Zpatch, which is compatible with the required Zopt, after the simplest impedance transformations. The rectangular patch is designed to use a low-cost commercial substrate (εr = 3.75, tanδ = 0.02 and h = 1.575 mm) and, as previously mentioned, to show a maximum gain of ~6.5 dB in the broadside direction. With reference to Figure6, the input impedance of the synthesized patch, evaluated at 5.8 GHz, is compared to the desired Zopt. This evaluation is carried out graphically by means of various Smith charts. The idea is to pursue the nominal matching to Zopt exploiting only a section of transmission line of length l. Accordingly, the effective normalized values of Zpatch and Zopt change as well, and so does the length of the rotation needed to transform one into the other. In particular, Figure6a shows the case of a transmission line of impedance Z0A = 50 Ω. The impedance seen by the patch after the rotation corresponding to lA, which is the best possible in this case, does not accurately synthesize the real part of Zopt. In the case of Z0B = 100 Ω, described by Figure6b, the optimal impedance Zopt is almost exactly hit after a rotation corresponding to the length lB. Finally, the case depicted in Figure6c Electronics 2021, 10, x FOR PEER REVIEWshows the rotation corresponding to lC around the reference impedance Z0C = 200 Ω. In7 of this 14

case, the real part of the input impedance Zin is slightly larger than the real part of Zopt.

(a) = = (b) = = (c) = = Figure 6. The same antenna input impedance normalized with respect to three different characteristic impedances , Figure 6. The same antenna input impedance normalized with respect to three different characteristic impedances Z0A,Z0B, , . In each case, the impedance is also highlighted. Z0C. In each case, the Zopt impedance is also highlighted.

Apparently, the the best best solution solution is theis the case case where whereZ0 = Z0=B. However,. However, with referencewith reference to the source-pullto the source-pull result inresult Figure in 4Figure, it can 4, be it seencan be how seen the how area the around area around the best the impedance best impedanceZopt is a plateau. is a plateau. Hence, theHence, third the solution third issolution only marginally is only marginally worse than worseZ0B in than terms of impedance, in terms of butimpedance, it is reasonably but it betteris reasonably for the loss better and for compactness, the loss and considering compactness, how muchconsidering smaller howlC is thanmuchlB smaller. Furthermore, is than higher transmission. Furthermore, line higher impedance transmis impliession a line narrower impedance width, implies resulting a innarrower a positive width, effect resulting on spurious in a radiationpositive reduction,effect on spurious and hence radiatio antennan reduction, efficiency. and hence antennaIn view efficiency. of this consideration, a ﬁnal optimization was carried out starting from this heuristicallyIn view of good this condition. consideration, The besta final gain opti andmization match was are pursuedcarried out by starting exploiting from all this the availableheuristically degrees good of condition. freedom andThe thebest ones gain given and bymatch the parasiticare pursued patches. by exploiting The best designall the available degrees of freedom and the ones given by the parasitic patches. The best design exhibits the gain in Figure 7, with a maximum of 6.75 dB at 5.8 GHz. The corresponding is (15–j25) Ω, in agreement with the nominal value of (12.1–j 22.3) Ω.

Figure 7. Simulated pattern of the proposed antenna.

Figure 8 shows the simulated reflection coefficient of the proposed antenna. The optimal condition at the nominal frequency of 5.8 GHz is clearly highlighted. It is worthwhile to remark that the proposed harvester operates in a narrowband around the center frequency. Nevertheless, the 10 dB return loss bandwidth extends from 5.7 GHz to 5.9 GHz, confirming the effectiveness of the proposed solution implemented without a matching network.

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(a) = = (b) = = (c) = =

Figure 6. The same antenna input impedance normalized with respect to three different characteristic impedances , , . In each case, the impedance is also highlighted.

Apparently, the best solution is the case where =. However, with reference to the source-pull result in Figure 4, it can be seen how the area around the best impedance is a plateau. Hence, the third solution is only marginally worse than in terms of impedance, but it is reasonably better for the loss and compactness, considering how much smaller is than . Furthermore, higher transmission line impedance implies a narrower width, resulting in a positive effect on spurious radiation reduction, and hence antenna efficiency. In view of this consideration, a final optimization was carried out starting from this

Electronics 2021, 10, 3 heuristically good condition. The best gain and match are pursued by7 of exploiting 13 all the available degrees of freedom and the ones given by the parasitic patches. The best design exhibits the gain in Figure 7, with a maximum of 6.75 dB at 5.8 GHz. The corresponding exhibits is (15–j25) the gain inΩ Figure, in agreement7, with a maximum with the of nomi 6.75 dBnal at value 5.8 GHz. of The(12.1–j corresponding 22.3) Ω. Zin is (15–j25) Ω, in agreement with the nominal value of (12.1–j 22.3) Ω.

Figure 7. Simulated pattern of the proposed antenna. Figure 7. Simulated pattern of the proposed antenna. Figure8 shows the simulated reflection coefficient of the proposed antenna. The optimal conditionFigure at8 theshows nominal the frequency simulated of 5.8 reflection GHz is clearly coefficient highlighted. of It isthe worthwhile proposed to antenna. The Electronics 2021, 10, x FOR PEER REVIEW 8 of 14 remarkoptimal that condition the proposed at harvester the nominal operates frequency in a narrowband of 5.8 around GHz the centeris clearly frequency. highlighted. It is Nevertheless, the 10 dB return loss bandwidth extends from 5.7 GHz to 5.9 GHz, confirming theworthwhile effectiveness to of remark the proposed that solutionthe proposed implemented harvester without operates a matching in network.a narrowband around the center frequency. Nevertheless, the 10 dB return loss bandwidth extends from 5.7 GHz to 5.9 GHz, confirming the effectiveness of the proposed solution implemented without a matching network.

FigureFigure 8. Simulated8. Simulated reﬂection reflection coefﬁcient coeffi evaluatedcient evaluated with respect with to the respect optimal to impedance the optimalZopt. impedance .

FigureFigure7 shows 7 shows the simulated the simulated radiation radiation pattern. The pattern. maximum The gain maximum of 6.6 dB isgain of 6.6 dB is achieved in the boresight direction. This good performance proves that the design condi- tionsachieved imposed in forthe the boresight optimal impedance direction. do This not g affectood theperformance radiative performance proves that of the the design condi- antenna.tions imposed The absence for ofthe an optimal externalmatching impedance network do not is a strongaffect aidthe in radiative keeping a highperformance of the levelantenna. of efﬁciency. The absence of an external matching network is a strong aid in keeping a high levelThe of proposed efficiency. antenna was designed to show the optimum impedance to the harvester. However, in order to minimize the OBU dimensions, the designed optimum impedance antennaThe must proposed also be used antenna for OBU was communications. designed to The show complete the blockoptimum diagram impedance of the to the har- systemvester. is However, illustrated in in Figure order9. Note,to minimize the introduction the OBU of adimensions, control unit that the drives designed the optimum im- modulator.pedance Theantenna modulator must block also is be directly used connected for OBU to communications. the antenna. The complete block diagram of the system is illustrated in Figure 9. Note, the introduction of a control unit that drives the modulator. The modulator block is directly connected to the antenna.

Figure 9. OBU full block diagram.

The description of the full system exceeds the aims of the present study but the reference system configuration with the optimum-matching network makes use of a modulator block based on a cold FET architecture, directly connected to the 50 Ω interconnection between the antenna and the optimum matching network. In a system based on the proposed harvester configuration, the modulator block must work on a different impedance. It has been demonstrated that such a block may be connected directly to the antenna and harvester even if they show an impedance different from 50 Ω. As illustrated in [20] it is possible to evaluate the condition under which the present harvester design is compatible with a backscattering ASK modulation scheme based on a two-symbol modulation (Si with i = 0, 1). In particular, with reference to Figure 10 and according to the results showed in [20], is possible to make explicit the impedance condition for modulator implementation as:

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Figure 8. Simulated reflection coefficient evaluated with respect to the optimal impedance .

Figure 7 shows the simulated radiation pattern. The maximum gain of 6.6 dB is achieved in the boresight direction. This good performance proves that the design conditions imposed for the optimal impedance do not affect the radiative performance of the antenna. The absence of an external matching network is a strong aid in keeping a high level of efficiency. The proposed antenna was designed to show the optimum impedance to the harvester. However, in order to minimize the OBU dimensions, the designed optimum im- Electronics 2021, 10, 3 pedance antenna must also be used for OBU communications. The complete block8 of dia- 13 gram of the system is illustrated in Figure 9. Note, the introduction of a control unit that drives the modulator. The modulator block is directly connected to the antenna.

FigureFigure 9. 9.OBU OBU full full block block diagram. diagram.

TheThe description description of of the the full full system system exceeds exceeds the the aims aims of of the the present present study study but but the the refer- ref- enceerence system system conﬁguration configuration with with the the optimum-matching optimum-matching network network makes makes use ofuse a modulatorof a modu- blocklator block based based on a coldon a FETcold architecture,FET architecture, directly directly connected connected to the to 50theΩ 50interconnection Ω interconnec- betweention between the antenna the antenna and the and optimum the optimum matching matching network. network. InIn aa system based based on on the the proposed proposed harvester harvester configuration, conﬁguration, the themodulator modulator block block must mustwork work on a ondifferent a different impedance. impedance. It has It been has been demonstrated demonstrated that thatsuch such a block a block may may be con- be connectednected directly directly to tothe the antenna antenna and and harvester harvester even even if if they show anan impedanceimpedance differentdifferent fromfrom 50 50Ω Ω. . AsAs illustrated illustrated in in [ 20[20]] it it is is possible possible to to evaluate evaluate the the condition condition under under which which the the present present Electronics 2021, 10, x FOR PEER REVIEWharvesterharvester designdesign isis compatiblecompatible withwith aa backscatteringbackscattering ASK modulation scheme scheme based based9 ofon on 14 a

atwo-symbol two-symbol modulation modulation ( (SSi iwithwith i i== 0, 0, 1). 1). In In particular, particular, with reference toto FigureFigure 10 10 and and accordingaccording to to the the results results showed showed in in [20 [20],], is possibleis possible to maketo make explicit explicit the the impedance impedance condition condi- fortion modulator for modulator implementation implementation as: as: |)| ≫||, ||) ( |Z (s )| max(|Z |, |Z |) MOD 0 HARV RF ZMOD(s1) ) (2) mASK = 1 − |ΓBO| 2 Z + 1 =1−|Γ| 2 RF +1

FigureFigure 10. 10.Principle Principle scheme scheme for for calculating calculating the the impedance impedance conditions conditions for for modulator. modulator.

UnderUnder this this condition, condition, the the modulator modulator does does not signiﬁcantlynot significantly affect affect the matching the matching between be- thetween antenna the antenna and the and harvester the harvester during theduring “TX the Data “TX and Data Power” and Power” time interval. time interval.

4.4. AssemblyAssembly WithWith reference reference to to Figure Figure 11 11a,a, the the design design showing showing the the best best performance performance in in the the simula- simu- tionlation was was assembled assembled with with the the harvester harvester directly direct connectedly connected to the to patch, the patch, and thenand fabricatedthen fabri- oncated a commercial on a commercial dielectric dielectric substrate substrate ISOLA ISOLA FR408. FR408. The calculated final values for the parameters are reported in Table 2.

Table 2. List of the geometric parameters of the antenna design. All dimensions are in mm.

g 15.93 12.5 9.53 2.33 1.58 0.266 0.125 1.23 2.21 45 0.7 2.8 3.1 0.67 30 1nH 100pF BAT1503

It is noteworthy that the occupied area is only 30 × 45 mm2 as compared the solution based on a 50 Ω antenna, shown in Figure 11b, which is significantly much larger with an area of 35 × 62 mm2. The latter is employed as a reference in the following section to comparatively validate the result of the proposed solution based on an antenna directly showing the desired impedance.

(a) (b)

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|)| ≫||, ||) ) (2) =1−|Γ| 2 +1

Figure 10. Principle scheme for calculating the impedance conditions for modulator.

Under this condition, the modulator does not significantly affect the matching between the antenna and the harvester during the “TX Data and Power” time interval.

4. Assembly With reference to Figure 11a, the design showing the best performance in the simulation was assembled with the harvester directly connected to the patch, and then fabri- cated on a commercial dielectric substrate ISOLA FR408. The calculated final values for the parameters are reported in Table 2.

Table 2. List of the geometric parameters of the antenna design. All dimensions are in mm.

g 15.93 12.5 9.53 2.33 1.58 0.266 0.125 1.23 2.21 45 0.7 2.8 3.1 0.67 30 1nH 100pF BAT1503

It is noteworthy that the occupied area is only 30 × 45 mm2 as compared the solution based on a 50 Ω antenna, shown in Figure 11b, which is significantly much larger with an area of 35 × 62 mm2. The latter is employed as a reference in the following section to com- Electronics 2021, 10, 3 9 of 13 paratively validate the result of the proposed solution based on an antenna directly showing the desired impedance.

(a) (b)

Figure 11. The proposed harvesting solution (a) as compared with a reference harvesting solution (b).

The calculated ﬁnal values for the parameters are reported in Table2.

Table 2. List of the geometric parameters of the antenna design. All dimensions are in mm.

L0 W0 L1 W1 g Wslit Wslit Win l 15.93 12.5 9.53 2.33 1.58 0.266 0.125 1.23 2.21

L2 W2 L3 W3 Wsub Lsub LCD 45 0.7 2.8 3.1 0.67 30 1 nH 100 pF BAT1503

It is noteworthy that the occupied area is only 30 × 45 mm2 as compared the solution based on a 50 Ω antenna, shown in Figure 11b, which is signiﬁcantly much larger with an area of 35 × 62 mm2. The latter is employed as a reference in the following section to comparatively validate the result of the proposed solution based on an antenna directly showing the desired impedance. It can be observed that, without the need of a matching network, the dimensions of the device are very compact. Indeed, Lm = 13.5 mm is the length of the input section of the reference antenna, while Ln = 18.5 mm is the length of the matching network needed for the transformation of 50 Ω in Zopt. It is noteworthy that the gain of the reference antenna, measured at the 50 Ω section, exceeds 7 dB.

5. Experimental Validation The performance of the proposed harvester solution is compared with the performance of a system based on [17] (the reference system). The prototype of the proposed solution (Figure 12a) and the reference system were evaluated, experimentally, in operative conditions as illustrated in Figure 12b. In order to reproduce the effective operative condition, a specific experimental setup was prepared. The RSU, compliant with ETC applications, was implemented making use of a signal generator supplying the CW signal at 5.8 GHz. The transmitting antenna, visible in Figure 12b, together with the receiving antenna, is fully compliant with the antenna used in RSU for ETC. The impinging power to the antenna section was measured making use of a calibrated power meter connected to a certified antenna. Finally, the ICGH current supplied by the rectifier to the BQ25570 circuit was evaluated using a precision multimeter. Electronics 2021, 10, x FOR PEER REVIEW 10 of 14

Figure 11. The proposed harvesting solution (a) as compared with a reference harvesting solution (b).

It can be observed that, without the need of a matching network, the dimensions of the device are very compact. Indeed, =13.5 mm is the length of the input section of the reference antenna, while =18.5 mm is the length of the matching network needed for the transformation of 50 Ω in . It is noteworthy that the gain of the reference antenna, measured at the 50 Ω section, exceeds 7 dB.

5. Experimental Validation The performance of the proposed harvester solution is compared with the perfor- Electronics 2021, 10, 3 mance of a system based on [17] (the reference system). The prototype10 of 13of the proposed solution (Figure 12a) and the reference system were evaluated, experimentally, in operative conditions as illustrated in Figure 12b.

(a) (b)

Figure 12. The proposedFigure 12. harvesterThe proposed prototype harvester (a) and prototype the experimental (a) and the setup experimental (b) used setup for the (b )valida usedtion. for The harvester prototype connectedthe validation. to the DC/DC The boost harvester is also prototype highlighted. connected to the DC/DC boost is also highlighted.

Figure 13 showsIn the order power to reproduce impinging the on effective the device operativ undere test condition, (DUT) in a thespecific operative experimental setup condition, meaningwas prepared. a nominal The EIRP RSU, of compliant 39 dBm. The with DUT ETC is applications, progressively was moved implemented away making use Electronics 2021, 10, x FOR PEER REVIEW 11 of 14 from the RSU keepingof a signal the generator boresight direction.supplying The the power CW signal at the at rectiﬁer 5.8 GHz. section The transmitting is estimated antenna, visi- from the measurementble in Figure at the 12b, antenna together section, with the considering receiving aantenna, gain of 6.6is fully dB, ascompliant previously with the antenna estimated in Sectionused in3. RSU for ETC. The impinging power to the antenna section was measured making use of a calibrated power meter connected to a certified antenna. Finally, the current supplied by the rectifier to the BQ25570 circuit was evaluated using a precision multimeter. Figure13 shows the power impinging on the device under test (DUT) in the operative condition, meaning a nominal EIRP of 39 dBm. The DUT is progressively moved away from the RSU keeping the boresight direction. The power at the rectifier section is estimated from the measurement at the antenna section, considering a gain of 6.6 dB, as previously estimated in Section 3.

FigureFigure 13. 13. ImpingingImpinging powerpower from from the the roadside roadside unit unit (RSU) (RSU) at the at antennathe antenna section section (measured) (measured) and at and atthe the rectiﬁer rectifier section section (evaluated). (evaluated).

As anticipated in Section3, the value of Z was estimated as the result of the source- As anticipated in Section 3, the valueopt of was estimated as the result of the − source-pullpull procedure procedure considering considering the impinging the impinging power at the power5 dBm at at the the − rectiﬁer5 dBm section.at the rectifier With sec- reference to Figure 13, this power level is chosen because it corresponds to the distance tion. With reference to Figure 13, this power level is chosen because it corresponds to the of 1.75 m, the center of the range of interest for application proposed in this paper, as distanceanticipated of 1.75 in Section m, the2 center. of the range of interest for application proposed in this paper, as anticipatedFor the sake in Section of repeatability, 2. subsequent experiments were performed by varying the transmissionFor the sake power of repeatability, rather than the subsequent distance. Figure experiments 14 shows thewere measured performed current by varyingICGH the transmissionfor the two prototypes, power rather as well than as the the distan differencece. Figure of the resulting14 shows currents the measured corresponding current forto the the two two prototypes, devices. The as current well as is the plotted differen againstce of the the impinging resulting powercurrents at thecorresponding rectiﬁer to theinput. two Itdevices. can be highlightedThe current that is plotted the proposed agains solutiont the impinging starts to perform power at better the rectifier than the input. It referencecan be highlighted solution starting that the from proposed the impinging solution power starts of −to11 perform dBm, which better correspond than the reference to 2.8 m, according to Figure 14. solution starting from the impinging power of −11 dBm, which correspond to 2.8 m, according to Figure 14.

Figure 14. Measured charge current versus the impinging power.

At the target optimizing impinging power, the value of the proposed solution is more than 110 μA greater than in the reference solution. The best improvement is observed at −4 dBm, corresponding to 560 μA for the proposed solution and 430 μA for the reference solution. Moreover, Figure 15 describes the comparison of conversion efficiency shown by both the harvester prototype and the reference prototype with respect to the impinging

Electronics 2021, 10, x FOR PEER REVIEW 11 of 13 Electronics 2021, 10, x FOR PEER REVIEW 11 of 13

for the two prototypes, as well as the difference of the resulting currents corresponding to forthe the two two devices. prototypes, The currentas well asis plottedthe difference against of the the impinging resulting currentspower at corresponding the rectifier input. to theIt twocan bedevices. highlighted The current that the is proposedplotted against solution the starts impinging to perfo powerrm better at the than rectifier the reference input. It solutioncan be highlighted starting from that the the impinging proposed solutionpower of starts −11 dBm,to perfo whichrm better correspond than the to reference 2.8 m, ac- Electronics 2021, 10, 3 solutioncording starting to Figure from 14. the impinging power of −11 dBm, which correspond to 2.8 m,11 ac- of 13 cording to Figure 14.

Figure 14. Measured charge current 퐼퐶퐺퐻 versus the impinging power. Figure 14. Measured charge current 퐼 versus the impinging power. Figure 14. Measured charge current퐶퐺퐻ICGH versus the impinging power. At the target optimizing impinging power, the 퐼퐶퐺퐻 value of the proposed solution 퐼 is moreAtAt the than the target target 110 optimi optimizingμA greaterzing impinging impingingthan in the power, power, reference the the solution.I퐶퐺퐻CGH valuevalue The of of bestthe the proposed proposedimprovement solution is ob- is 110 μA is servedmore than at −4 110 dBm,µA greatercorresponding greater than than in theinto the560 reference referenceμA for solution. the solution. proposed The The best solution best improvement improvement and 430 isμA observed isfor ob- the − µ µ servedreferenceat 4 at dBm, −4 solution. dBm, corresponding corresponding to 560 to A560 for μA the for proposed the proposed solution solution and 430 andA 430 for theμA referencefor the referencesolution.Moreover, solution. Figure 15 describes the comparison of conversion efficiency shown by bothMoreover, Moreover,the harvester Figure Figure prototype 15 15 describes describes and the the the reference comparison comparison prototype of of conversion conversion with respect efficiency efficiency to the shown impinging shown by by both the harvester prototype and the reference prototype with respect to the impinging bothpower. the harvesterThe measured prototype results and confirm the reference a very similar prototype behavior with forrespect lower to impinging the impinging power power. The measured results confirm a very similar behavior for lower impinging power power.levels. The At themeasured optimi zresultsed impinging confirm power a very reference similar behavior level (−5 for dBm), lower the impinging proposed powersolution levels. At the optimized impinging power reference level (−5 dBm), the proposed solution leveexhibitsls. At thea significant optimized improvement impinging power of ~12.0% reference in conversion level (−5 dBm), efficiency. the proposed solution exhibitsexhibits a significant a significant improvement improvement of of~12.0% ~12.0% in inconversion conversion efficiency. efficiency.

FigureFigure 15. 15. MeasuredMeasured conversion conversion efficiency efficiency versus versus the the impinging impinging power. Figure 15. Measured conversion efficiency versus the impinging power. TheThe efficiency efficiency was evaluated byby comparingcomparing the the power power in in DC, DC, calculated calculated on on the the basis basis of The efficiency was evaluated by comparing the power in DC, calculated on the basis ofthe the DC/DC DC/DC charge charge current currentI CGH퐼퐶퐺퐻and and the the fixed fixed input input voltagevoltage withwith thethe samesame DC/DC,DC/DC, i.e., i.e., of 0.34V,the DC/DC with respect charge tocurrent the impinging 퐼퐶퐺퐻 and power. the fixed A peak input exceeding voltage with 50% isthe observed same DC/DC, at −7.3 i.e., dBm. At the same time, a research concerning state-of-the-art equivalent systems was carried out. The aim was to assess the performances obtained by the proposed solution with respect to the state-of-the-art approaches illustrated in the literature. Table3 summarizes the comparison with the state-of-the-art at 5.8 GHz with respect to charging current and conversion energy efficiency. The data show that for an input power of −5 dBm, the charge current increases from 135 µA to 450 µA, while in terms of efficiency, there is an increase from 16% to 45%. In two of the illustrated solutions, the results were Electronics 2021, 10, 3 12 of 13

obtained making use of ﬁxed load conditions that represented a far simpler constraint with respect to the measurement conditions implemented in the present work.

Table 3. Harvesting performances in comparison with the state-of-the-art approaches.

Design Freq. Input Power η ICHG Load and Condition [18] 5.8 GHz −5 dBm 16% N.A. 1500 Ω, ﬁxed load [19] 5.8 GHz −5 dBm 23% 145 µA 3000 Ω, ﬁxed load [20] 5.8 GHz −5 dBm 16% 135 µA DC/DC, current sink Present work 5.8 GHz −5 dBm 45% 450 µA DC/DC, current sink

6. Conclusions We demonstrated a novel energy harvesting solution based on an optimum antenna design suitable for implementation in an on-board unit for vehicle communications. Exper- imental results, carried out on the bases of a comparison between the present development and the harvesting prototype based on the integration of a 50 Ω antenna showing similar gain combined with the matching network, confirm the better performance of the proposed approach. As compared with the reference harvester, the proposed compact prototype optimized at −5 dBm of impinging power provides an improvement in both charging current (ICGH) and efficiency of about 100 µA and 15%, respectively. The two prototypes show almost equivalent performances up to −11 dBm of impinging power, while for higher value the compact prototype shows a significantly better behavior. Furthermore, on the basis of results illustrated in Table3, it is possible to state that the harvesting approach illustrated in the paper shows the best performance as compared with those of all the other approaches. In particular, Table3 highlights an efficiency that is double with respect to the efficiency obtained in [19] and more than three times the charge current measured in [19,20].

Author Contributions: Conceptualization, G.C. and S.M.; methodology, S.M.; software, S.M.; validation, G.C. and S.M.; formal analysis, S.M.; investigation, G.C.; resources, G.C. data curation, S.M.; writing—original draft preparation, G.C.; writing—review and editing, G.C. and S.M.; visualiza- tion, G.P.; supervision, G.C.; project administration, G.P.; funding acquisition, G.P. Please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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