Research ArticleCompact and Multiband MIMO Dielectric Resonator Antenna forAutomotive LTE Communications

Tzu-Ling Chiu ,1,2 Laure Huitema ,2 Olivier Pajona,1 and Thierry Monediere2

1Ethertronics Inc., an AVX Group Company, 425 Rue de Goa, 06600 Antibes, France2XLIM Laboratory, Faculté des Sciences et Techniques, 123 avenue Albert Thomas, 87060 Limoges Cedex 03, France

Correspondence should be addressed to Tzu-Ling Chiu; lynn.chiu@avx.com

Received 9 May 2018; Revised 16 August 2018; Accepted 18 September 2018; Published 27 December 2018

Academic Editor: Ahmed Toaha Mobashsher

Copyright © 2018 Tzu-Ling Chiu et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A compact and multiband dielectric resonator antenna (DRA) designed for LTE automotive solutions is presented in this paper.The proposed MIMO system is located on the vehicle rooftop within a limited space of 120mm× 70mm× 65mm. To cover allthe LTE standard frequency bands used around the world, the antenna is matched around 790MHz–860MHz, 1700MHz–2200MHz, and 2500MHz–2700MHz frequency bands with a ∣S11∣ lower than −6 dB while presenting a minimum totalefficiency of 50% with a maximum realized gain better than 1 dB on all these frequency bands. The DRA is then mounted andmeasured on a real vehicle rooftop in order to see its performances in real operation conditions. Finally, to improve both thequality and reliability of the wireless link, two DRAs are placed within a small area to reconfigure their radiation patterns oneach frequency band. Measured performances, which are in good agreement with the simulated results, are used to validate ifthe antenna design is suitable for LTE MIMO systems to be integrated on an automotive. The MIMO system is evaluated usingthe envelope correlation coefficient (ECC), and the obtained value for the proposed antenna is lower than 0.25.

1. Introduction

The explosive growth of wireless communications and theincreasing demand for high data rate throughput push thedeployment of Long Term Evolution (LTE) communicationsystems. From the beginning, the LTE standardization effortfocused on enhancing Universal Terrestrial Radio Access(UTRA) and optimizing the Third Generation PartnershipProject’s (3GPP’s) radio access architecture. Its initial releasewas finalized in 2008, and now, the LTE standard works onfour frequency bands all over the world [1].

The Multiple Input–Multiple Output (MIMO) tech-nique is one of the key technologies in the LTE system.Indeed, a MIMO system can help to overcome the damageeffects of multipath. It is now a commonly used method toincrease the downlink and uplink data rates without addi-tional spectrum allocation.

Several printed multiband MIMO antenna systems,based on the LTE standards for wireless devices [2, 3], havebeen published in the literature in the past few years. Amongthese papers, only a few of them are focusing on the lowest

LTE band, which operates around 800MHz. Indeed, at thesefrequencies, the antenna needs to be electrically small to beintegrated within the wireless devices. Nevertheless, it iswell known that, due to fundamental physics limits describedin [4], small antennas are limited, generally in terms ofradiation performances (efficiency) and bandwidth (qualityfactor). In addition, the placement of many antennas (twoin our case) close together increases the mutual couplingbetween them and affects the diversity performances ofMIMO systems [5, 6].

In [7], a MIMO system made of four cylindrical dielectricresonator antenna (DRA) elements was developed for the2.45GHz and 5.8GHz WLAN bands. In [8], authors showedthat with proper excitations for different characteristic modesof planar antennas, which are orthogonally placed, goodmatching and low coupling and correlation can be achievedfor working frequencies lower than 1GHz. In [9], a studyon how to decrease the coupling of two cylindrical DRAs ispresented at 10GHz. Other solutions to decrease the cou-pling of a MIMO system are based on the excitation oforthogonal modes either by using printed antennas [10] or

HindawiInternational Journal of Antennas and PropagationVolume 2018, Article ID 8231081, 15 pageshttps://doi.org/10.1155/2018/8231081

dielectric resonator antennas [11–13]. Another solution is toorthogonally integrate the two MIMO antennas [8].

There are several feed mechanisms for the DRA [14] suchas probe near the DRA [15], microstrip line [16], aperture-coupled [17], and dielectric image guide coupling [18]. More-over, DRAs can have different shapes such as cylindrical,rectangular, pyramidal, spherical, and hemispherical. Theshape of the DRA, its feeding mechanism, and its wallboundary condition play an important role in its operatingfrequencies and impedance bandwidths.

In [19, 20], we presented the simulation of a beam steer-ing system for an automotive application. In this frameworkand to steer the beam, a phase shifter has been designedand integrated. Now, in this paper, a MIMO LTE systemhaving a pattern diversity is developed. The antenna develop-ment is based on modal analyses in order to cover all theLTE bands while being miniature. Resonance frequenciesof a rectangular-shaped DRA are studied. Some faces canbe completely or partially coated with a metal in order toobtain the wanted working frequencies. The shape must alsobe easy to manufacture for commercial purposes. The DRAis designed to cover the LTE spectrum operated on four dis-tinct frequency bands: LTE 800DD (band 20), LTE 1800+(band 3), LTE 2100 (band 1), and LTE 2600 (band 7). Inorder to have a MIMO system for the LTE standard, theantenna also needs to have reconfigurable radiation patternsin these four frequency bands.

Based on commercial LTE wireless module specifica-tions, a ∣S11∣ lower than –6 dB with a minimum total effi-ciency of 50% on the four frequency bands is required. TheMIMO antenna system performances are evaluated withthe envelope correlation coefficient (ECC). A value under0.5 is required.

The proposed MIMO system is designed to be inte-grated on the rooftop of a vehicle within a limited space of120mm× 70mm× 65mm corresponding to λ0/3 × λ0/5 ×λ0/6 at 800MHz as presented in Figure 1.

The roof of the car is considered as a large ground plane.For the ease of measurement and manufacture, we initiallyused a 200mm× 200mm-FR4 substrate entirely coated bycopper. Finally, the prototype was mounted and measuredinside an anechoic chamber on a car rooftop. The corre-sponding performances are presented.

2. Antenna Design and Configuration

2.1. DRA Development. To integrate two antennas withinthe allocated space, each radiating element needs to be min-iaturized. In this framework, we set the dimensions of onedielectric resonator (DR) at 22mm× 13mm× 50mm to bevertically placed on a finite FR4 substrate (Figure 2). Anten-na’s characteristics (dimensions, quality factor, bandwidth,gain, and efficiency) depend on both the relative permittivityand the loss tangent of the dielectric material. The selecteddielectric resonator relative permittivity is chosen close to10 in order to obtain the widest impedance bandwidth,while having the smallest possible dimensions. Therefore,the chosen DR material is the Rogers TMM10 with a rela-tive permittivity of 9.2 and a loss tangent of 0.0022.

For the modal analysis, the FR4 substrate is considered tohave the same lateral dimensions as the dielectric resonator(see Figure 2) with a copper metallic ground on the bottomside. Its relative permittivity is 4.1 with a loss tangent of0.012 at 1GHz. Within these dimensions and considering adielectric resonator placed on a FR4 substrate coated by cop-per, the first resonant mode is the TE101 at 2GHz (Figure 3).

It should be noticed that this resonance frequency hasbeen determined considering evanescent modes at the inter-face between the dielectric resonator and the air, i.e., withoutconsidering a perfect magnetic condition (PMC). The metal-lic conditions are obtained using copper layers. To proceed,a vacuum cavity surrounded by PMC conditions is inte-grated around the DRA during the eigenmode analyses(Figure 3(b)). The thickness of the cavity is chosen such thatwe obtain a convergence of the resonance frequency of themode with eigenmode analyses. These eigenmode analysesare then validated by the electromagnetic simulation by inte-grating an excitation port and checking both the resonancefrequencies and the 3D shapes of each mode.

This mode is presenting a symmetry in the x = 6 5 mmplane. Therefore, the size of the DR can be halved by placinga metal plate in this plane while keeping the same resonantmode. It also means that if the global dimensions of theDRA are kept while integrating a copper coating at x =

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Figure 1: Dedicated space of automotive antenna.

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Figure 2: Maximum dimensions of the antenna to be integratedwithin the dedicated space presented in Figure 1.

2 International Journal of Antennas and Propagation

13mm, the resonance frequency of the mode will decreaseat 1GHz. This second solution has been chosen in orderto reach the lowest LTE band. The modal analysis is pre-sented in Figure 4(a). From this modal analysis, when consid-ering the metallic plate, the E-field is mostly concentratedon its upper side. Therefore, this metallic plate can be cutwithout excessively disturbing the mode. Figure 4(b) showsthat the field inside the DR with the metallic plate halvedremains mostly the same.

An excitation port is then integrated in order to computethe antenna input impedance and make the analogy withthe previous eigenmode resolution, which did not integratean excitation. In this framework, input impedances for thesethree previous cases have been plotted in Figure 5. Asexpected, the fundamental mode of the DRA without metal-lic condition (Figure 5) has its resonance frequency twicehigher than the DRA with the metallic condition. This reso-nance frequency is little when the lateral face is partially

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3International Journal of Antennas and Propagation

coated by metal since it is resonating at 1.1GHz for the cutmetal plate condition (Figures 4(b) and 5). Starting fromthe metal plate condition, we can change the metallic partshapes in order to create hybrid modes between the two firstmodes at 1.1GHz and 2.85GHz, respectively (Figure 5).

Some parametric studies on the metallic plate showedthat we can create two hybrid modes without disturbing thefirst resonating mode. On Figure 6, the proposed antennastructure is detailed and shows the surface where metallicareas are integrated. Table 1 details the DRA dimensions.

The input impedance of the optimized design with thecorresponding modal analysis is presented in Figure 7.

Since this antenna is developed to be integrated onan automobile rooftop, it is studied on a 200mm× 200mm-FR4 substrate entirely coated by copper. Figure 7 is theDRA response on the proposed 200mm× 200mm groundplane. For higher frequencies, both eigenmode analyses ofthe DR and the input impedance presented in Figure 7show that the parasitic strip on the side wall creates hybridmodes. From modal analyses presented in [21–23] andregarding the 3D fields from the eigenmode study, theexcited hybrid modes (corresponding to the second andthird modes) at 1.8GHz and 2.42GHz are TE 3/2 01 andTE 3/2 11, respectively.

The resonance frequency of the fundamental mode isonly slightly shifted compared with the metal plate cutcondition since it is equal to 1.18GHz. The integrationof a large ground plane means that the global dimensionsof the radiating element are increasing, which involves thedecrease of the Q factor. Indeed, from the input impedance,we can determine the antenna quality factor by using equa-tion (1) [24] where ω0 is resonant angular frequency, R0 isthe resistance, and Z0 is the input impedance:

Q ω0 =2R0 ω0

ω0 Z0 ω01

The Q factor is reaching a maximum value of 30 at1.04GHz for the antenna on the 200mm× 200mm groundplane while it equals 100 for the DRA alone (without alarge ground plane). It is lower than 5 for the other modesof the antenna mounted on the ground plane. A high Qfactor implies that it is difficult to match the antenna ona wide bandwidth.

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Figure 6: Geometry of the proposed DRA unit (a) in 3D view, (b)side view for SMA connector location, and (c) top view formatching circuitry connection of DRA.

Table 1: Design parameters of the proposed antenna.

Parameter a b c d1 d2Value (mm) 13 22 50 109 20

Parameter w1 w2 w3 w4 w5 w6Value (mm) 4.5 6.3 13.5 3.4 7 4

Parameter h1 h2 h3 h4 h5 h6 h7Value (mm) 9.7 2 24.3 14 20.5 8 14.5

4 International Journal of Antennas and Propagation

The interest of exciting hybrid modes in this design isthat the real part of the input impedance remains around50Ω while having an imaginary part around 0Ω between1.4GHz and 2.6GHz. A wider impedance bandwidth is

therefore obtained compared to the other cases, as presentedby the ∣S11∣ parameters on Figure 8.

It should be reminded that the antenna needs to workon the lowest LTE band between 790MHz and 860MHz.

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5International Journal of Antennas and Propagation

Therefore, a T-type matching circuitry is placed on the FR4substrate just before the DRA to match the antenna onthe serial resonance of the first mode, i.e., at 800MHz. Thismatching circuit is presented in Figure 9(a). Measured touch-stone files with a homemade TRL kit for inductor and capac-itor from Murata LQG and GRM series are used in theelectromagnetic simulation.

2.2. DRA Performance. To validate the design, a prototype ofthe DRA was built as shown in Figure 9(b). Its performanceswere measured in an anechoic chamber.

Measurements were performed from 0.7GHz to 2.7GHz,where the four LTE bands are operating. Simulated and mea-sured results are in a good agreement, and they exhibit abroadband and multiband DRA since it covers 790MHz–860MHz, 1575MHz–2200MHz, and 2500MHz–2700MHzfrequency bands with a ∣S11∣ lower than −6 dB as shown inFigure 10.

3D radiation patterns have been measured inside ananechoic chamber; they are compared with the simulatedresults in Figure 11 on three different planes, i.e., the x0yplane (θ = 90o), x0z plane (φ = 0o), and y0z plane (φ = 90o),at the center of the four LTE frequency bands. The cross-polarized field is high, and the antenna is not exhibiting a lin-ear polarization due to the DRA location, which is close tothe edge of the ground plane. However, a pure polarization

in this design is not needed since LTE specifications depend

on the total radiated field Etot = E2θ + E2

φ, including both

the co- and the cross-polarizations.For LTE antennas in automobile applications, it is an

advantage if the radiation pattern for all azimuth anglesshows an omnidirectional shape. Figure 11 demonstrates thatthe proposed antenna can have the almost omnidirectionalradiation pattern in the x0y plane for four LTE bands.

From the 3D radiation patterns, we can deduce both themaximum realized gain and the total efficiency according tothe frequency as presented in Figure 12. Measurement resultsare in good agreement with the simulated ones and exhibitthat the antenna efficiency is greater than 50% for the firstLTE band around 800MHz and better than 75% for the otherthree bands.

A radome, with the geometry described in Figure 1, isfinally integrated and enclosed the antenna. The used mate-rial is a painting plastic. Figure 13 compares the measured∣S11∣ parameter and efficiency according to frequency withand without this radome. As expected, the antenna per-formances are more disturbed on the lowest band sincethe radome is closer in terms of λ0 at lower frequencies.Indeed, on the first LTE band, the working frequency is alittle bit shifted to the lower frequencies, i.e., there is a20MHz frequency shift around 800MHz. The antenna

0.79 0.86 1 1.5 1.575 2 2.2 2.5 2.7Frequency (GHz)

MeasuredSimulated

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Port 1

C = 0.5 pF L = 9.1 nH

C = 3 pF

DRA

(a) (b)

Figure 9: (a) Matching circuitry of DRA. (b) Photograph of the antenna.

6 International Journal of Antennas and Propagation

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Figure 11: Simulated and measured 2D radiation patterns for the proposed DRA.

7International Journal of Antennas and Propagation

efficiency is a little bit lower than for the ideal case butremains higher than 50%.

The DRA is then mounted on a real automobile(Figure 14) and measured to determine the antenna effi-ciency and its radiation patterns in an anechoic chamber.

Figure 15 shows the 2D simulated gain patterns comparedto the measurement on the vehicle rooftop. The main beamdirection for both low and high bands occurs at the eleva-tion angle of θ = 60° counting from the zenith. Figure 15 istherefore presenting the comparison of simulated andmeasured gain patterns at θ = 60°.

The maximum realized gain for the required bands isaround 4dB (Figure 16). Compared to the upper hemi-sphere maximum realized gain for the DRA on the200mm× 200mm ground plane, the realized gain is a littlegreater since the ground plane can be considered as infi-nite in this case.

Therefore, the proposed DRA structure has been vali-dated in measurement with performances fitting with theLTE wireless module specifications.

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Figure 12: Measured and simulated maximum realized gains and total efficiencies for the proposed DRA.

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Figure 14: Photo of the measurement for DRA implement on thevehicle rooftop.

8 International Journal of Antennas and Propagation

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Figure 15: 2D radiation patterns of the DRA on the vehicle rooftop.

9International Journal of Antennas and Propagation

2.3. Performances versus the State-of-the-Art. In order tocompare the performances of our fabricated dielectric reso-nator antenna, Table 2 provides a comparison of our resultswith different papers [22–25-32] presenting antennas dedi-cated to LTE bands (at least for the lowest band starting at790MHz). Their dimensions and operating bands with theircorresponding maximum total efficiencies and maximumrealized gains are compared. Some of them are dedicatedfor an integration within a vehicle [26–32] and also for aMIMO application [22, 25, 26, 31, 32].

As observed, our reported antenna is a good trade-offbetween its dimensions and radiated performances. More-over, it covers all the four distinct frequency bands: LTE800DD (band 20), LTE 1800+ (band 3), LTE 2100 (band 1),and LTE 2600 (band 7). As the main objective of this studyis to enhance the quality of the wireless link by employingthe radiation pattern diversity technique, its low sizes andgood efficiency imply that we can integrate two of the previ-ously presented antennas within the dedicated volumedefined in Figure 1.

3. MIMO System Structure and Performances

3.1. Structure of the MIMO Antenna System. The proposedMIMO system integrates two DRAs placed face-to-face insuch a way that they can be embedded in the automotive ded-icated space. Therefore, the distance between the DRAs is18mm along the x-axis and 40mm for the y-axis. EachDRA is independently fed with a 50Ω coaxial cable, andthe system shape and element locations are symmetric as pre-sented in Figure 17.

3.2. Performance of the MIMO Antenna System. S-param-eters are presented in Figures 18(a) and 18(b). Since DRAsare similar and the structure is symmetric, the ∣S11∣ and∣S22∣ parameters are almost similar, and they are wellmatched on the four LTE frequency bands. The systemreciprocity implies that the ∣S21∣ parameter is equal tothe ∣S12∣ parameter. Since the distance between the tworadiating elements equals λ0/8 5 at 800MHz, the coupling

is reaching its maximum of −6 dB on the first workingband. This coupling will therefore result in a lower realizedgain and thus a lower efficiency compared to the antennastudied alone.

Figure 19 plots the total efficiency and maximum realizedgain according to the frequency. As expected, it demonstratesthat the antenna efficiency is lower for frequency bandswhere the coupling is higher than 10 dB. However, this effi-ciency remains more than acceptable since it is higher than50% around 800MHz, higher than 70% between 1.7GHzand 2.2GHz, and better than 60% for the highest LTE fre-quency band.

Figure 20 shows the 3D measured radiation patterns forthe main and the MIMO antennas at 820MHz, 1575MHz,1800MHz, 2150MHz, and 2600MHz. It can be observedthat this device offers the possibility to perform a patterndiversity since the radiation pattern is depending on theexcited DRA. Figure 21 shows the 2D radiation patterns inthe x0y plane (θ = 90°). There is a good agreement betweenthe measurement and the simulation.

However, these results are not sufficient to concludethat the radiation pattern is reconfigured. An importantparameter to evaluate the diversity performances of a systemis the envelope correlation coefficient (ECC). Indeed, it eval-uates the correlation between the signals received by the twoantennas and defines if the diversity antenna system couldproperly operate. The ECC can be calculated using eitherthe farfield radiation patterns or S-parameters [33, 34]. How-ever, using S-parameters for the ECC calculation is valid onlyfor the antenna having a very high efficiency [35]. In thisantenna design, using S-parameters could cause nonaccurateresults particularly around 800MHz (efficiency around 50%).Therefore, in this study, ECC values are obtained using thefarfield radiation patterns. Figure 22 plots the ECC on eachLTE frequency bands.

The ECC value range is between 0 and 1 with 1 corre-sponding to similar radiation patterns. The necessary diver-sity requirement is an ECC value lower than 0.5. For theantenna system, the ECC is lower than 0.25 meaning thatthe radiation pattern is well reconfigured for the four LTE

Maximum realized gain on the vehicle roo�opMaximum realized gain on 200 mm × 200 mm ground plane

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(dBi

)

0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.70.7Frequency (GHz)

Figure 16: Comparison of maximum realized gains according to the frequency.

10 International Journal of Antennas and Propagation

bands. In [26], authors indicated that the distance betweenthe antenna elements and their respective orientations inthe system affect both ∣S21∣ and ECC values. Therefore, thedistance of the antenna elements in MIMO system has beenoptimized, and the antenna orientation is selected to beorthogonal as presented in Figure 12. Finally, a low ECCvalue is obtained, and the efficiencies of both antenna ele-ments are higher than 50% on all the LTE frequency bands.

4. Conclusions

A compact and multiband MIMO system has been designedfor automotive applications. The proposed antenna is exhi-biting all the advantages of using a dielectric resonatorantenna by covering LTE 800DD, LTE 1800+, LTE 2100,and LTE 2600. The DRA design satisfies the −6 dB imped-ance bandwidth for the four distinct frequency bands. A total

Table 2: Comparison with other studies.

Reference Electrical size Matching bands (MHz) Maximum total efficiency (dB) Maximum realized gain (dBi)

[22] λ0/15 × λ0/38 × λ0/94790–862 −4 −1.52300–2500 −0.9 5

3200–3600 −0.9 6.5

[25] λ0/6 × λ0/5 × λ0/9828–880 −0.56 1.6

1600–2200 −0.23 4.3

[26] λ0/7 × λ0/7 × λ0/12828–880

N/A

1

1710–2170 1

2490–2690 1

[27] λ0/5 × λ0/5 × λ0/12 780–2350 N/A4

6

[28] λ0/5 × λ0/17 × λ0/325698–960

N/A

4

1710–2170 5.5

2490–2690 2.5

[29] λ0/9 × λ0/11 × λ0/8698–960 −1.3 5

1470–2690 −0.7 6

[30] λ0/8 × λ0/8 × λ0/8698–960

N/A

4.5

1470–2690 4.5

5850–5925 4.5

[31] λ0/6 × λ0/13 × λ0/13

700–960

N/A

4

1700–2170 5.2

2500–2700 7

3400–3800 8.1

[32] λ0/9 × λ0/13698–960

N/A4

1710–2690 4

Proposed antenna λ0/8 × λ0/17 × λ0/29790–894 −0.97 2

1710–2170 −0.46 4

2490–2690 −0.97 4

MIMODRA

18 mm

40 mm

Main DRA

Y

X

Y

XMain DRA

MIMO DRA

Figure 17: MIMO antenna system in CST and prototype.

11International Journal of Antennas and Propagation

efficiency better than 50% is achieved in the LTE 800DDband and 75% on the other frequency bands while beingminiature. It has global dimensions of λ0/8 × λ0/17 × λ0/29at 790MHz.

0.7

0.8

0.9 1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.8 2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

Measured |S11|Simulated |S11| = |S22|Measured |S22|

−35

−30

−25

−20

−15

−10

−5

0

|S11

| and

|S22

| (dB

)

(a)

MeasuredSimulated

0.8

0.9 1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.8 2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

0.7

Frequency (GHz)

−35

−30

−25

−20

−15

−10

−5

0

|S21

| = |S

12| (

dB)

(b)

Figure 18: MIMO system: (a) measured and simulated ∣S11∣ and ∣S22∣ parameters and (b) measured and simulated ∣S21∣ parameters.

Simulated-efficiency

MIMO DRA measured-efficiency

Main DRA measured-maximum gain

Main DRA measured-efficiency

Simulated-maximum gain

MIMO DRA measured-maximum gain

0102030405060708090

100

Effici

ency

(%)

−4.00−2.000.002.004.006.008.0010.0012.0014.0016.00

Max

imum

real

ized

gai

n (d

Bi)

0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.70.7Frequency (GHz)

Figure 19: Measured and simulated maximum realized gains andtotal efficiencies of the MIMO system.

Frequency Main DRA MIMO DRA

800 MHz

1575 MHz

1800 MHz

2150 MHz

2600 MHz

7.0 dB

−2.3 dB

−11.5 dB

−20.8 dB

−30.0 dB

X

Z

Y

Figure 20: Radiation patterns of the MIMO system in 3Dmeasurement.

12 International Journal of Antennas and Propagation

Then a MIMO system has been proposed and iscomposed by two DRAs placed within a small area. It is pre-senting a low ECC value (under 0.25) while having goodradiating performances. All the measurements are wellmatched with the simulations. Moreover, the proposedantenna has a simple rectangular shape with only one sur-face printed with copper which can be easily fabricated.With these features, this antenna is very suitable for practicalautomotive applications.

Data Availability

All the data used to support the findings of this study areincluded in the article.

Conflicts of Interest

The authors declare that there is no conflict of interestregarding the publication of this paper.

Acknowledgments

This work is supported by Ethertronics Inc. and XLIMLaboratory.

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