Design and Analysis of a Dual-Band Semitransparent MIMO Antenna for Automotive Applications

. Tis paper presents the design and development of a semitransparent antenna that can be used in automotive applications to support multiple wireless services. Te proposed quad-port multiple input multiple output (MIMO) antenna is comprised of four orthogonally arranged identical semitransparent antenna elements on a transparent substrate. Te MIMO antenna covers the 2.4 GHz band and 3.2–15GHz band, supporting automotive/wireless applications such as Bluetooth, Wi-Fi, intelligent transport system (ITS), advanced driver assistance systems (ADASs), vehicle to infrastructure (V2I), vehicle to vehicle (V2V), vehicle to network (V2N), and vehicle to device (V2D). Te parameters such as envelope correlation coefcient (ECC), diversity gain (DG), total active refection coefcient (TARC), and channel capacity loss (CCL) are studied to evaluate antenna diversity performance. Te antenna has an ECC of less than 0.15, DG of more than 9.85dB, TARC of less than − 10dB, and CCL of less than 0.2 bits/s/Hz. Te developed antenna can be easily installed on the mirror and windshield of the vehicles. Also, the housing efect and on-vehicle performance of the antenna at various positions in a


Introduction
In modern automobiles, advanced transceiving systems are required to alert the driver about the vehicle and to ensure a safer and smoother journey. A transceiver module with an integrated antenna could be able to access multiple parameters of a vehicle, allowing for comfortable driving and passenger entertainment through infotainment services. To provide such amenities, the vehicular antenna should support multiple wireless bands such as GPS, GSM, Wi-Fi, and Bluetooth. Te automotive antenna connects vehicles in order to create an intelligent transportation system (ITS), with vehicle monitoring and tracking, and a smart trafc system [1]. Customarily, each wireless service requires a dedicated antenna, which takes up a lot of chip space due to the existence of multiple radiators [2]. Te installation of the antennas is also challenging due to the weight, volume, and aesthetic constraints. Tis problem can be solved by integrating multiple wireless services into a single antenna, eliminating the need for additional installation space.
An automotive antenna must efciently receive signals from all directions [3]. Terefore, a multiple input multiple output (MIMO) antenna system could be useful as it can overcome the efects of multipath fading, increasing uplink and downlink data speeds without requiring additional spectrum [3]. However, in MIMO antennas, poor isolation between antenna elements increases coupling, which degrades radiation performance [4]. Te coupling can be reduced by increasing the distance between the antenna elements, but this increases the size of the MIMO antenna [5]. Te decoupling structures [6,7], frequency selective surfaces/metamaterials [8,9], defected ground plane [10,11], and neutralization lines [12,13] can improve isolation, but they complicate antenna design. Another method for increasing isolation is to position the antenna elements orthogonally to each other. Also, orthogonal antenna placement achieves polarization diversity with horizontal and vertical vectors [14,15]. Te automotive antenna can be mounted on the roof using shark fn mounting [16][17][18], trunk, bumper, wiper, spoiler, side mirror, rear mirror, and front mirror [19]. Te mounting location is determined by the type of antenna and the available space. Te roof top antenna is an excellent choice for automotive applications as it can receive signals from all directions. Tese antennas can be designed on rigid and nontransparent substrates. In [20], a 3D antenna was designed on the FR-4 substrate for the roof top placement. In [21], an eight-port MIMO antenna was developed on the Roger substrate for the vehicle's roof. One of the challenges for automotive engineers is to deploy antenna systems without compromising the vehicle's aesthetics. Tis issue can be alleviated by designing antennas with transparent substrates such as soda lime glass and borosilicate glass.
Tis paper presents a quad-port partially transparent MIMO antenna fabricated on a soda lime glass substrate. Te antenna resonates at frequencies between 2.4 GHz and 3.2-15 GHz, assisting Bluetooth, Wi-Fi, intelligent transport system (ITS), advanced driver assistance systems (ADAS), vehicle to infrastructure (V2I), vehicle to vehicle (V2V), vehicle to network (V2N), and vehicle to device (V2D). Better isolation is achieved in the proposed transparent substrate-based MIMO antenna without the use of any decoupling structures. Section 2 presents the evolution of the antenna and the surface current distributions. Section 3 discusses the radiation performance of the antenna. Section 4 investigates the antenna housing efect and directivity after installation in the vehicle. Figure 1 depicts the schematic and dimensions of the semitransparent antenna. Te proposed antenna is constructed on a soda lime glass substrate of dielectric constant of 7.3 and thickness of 1.1 mm. Te soda lime glass is chosen to mimic the proposed antenna with the windshield glass, maintaining the vehicle's elegance. Te proposed antenna ofers transparency of 33% with the soda lime glass substrate. For future consideration in achieving complete transparency, the antenna can be developed with transparent conducting oxide [22][23][24] as a radiator, in replacement to the copper. Figure 2 depicts the evolution of the antenna, while Figure 3 depicts the associated S-parameters. Figure 2(a) shows a rectangular monopole antenna with a partial ground plane. Te goal is to design a small antenna with multiple frequencies. Te sides of the radiator and the feed line are truncated in antenna 2 and antenna 3 to improve impedance matching (see Figures 2(b) and 2(c)). Te ground plane in evolution-4 is modifed to achieve a wider bandwidth of 3.2-15 GHz, as shown in Figure 2(d). In the next step, a meandering slot is introduced in the radiator (see Figure 2(d)) to achieve additional 2.4 GHz (Bluetooth/Wi-Fi) resonance. Te length of the slot is calculated to be 0.48λ 0 (59.4 mm).

Unit Element.
Te designed antenna has a total area of 25 mm × 25 mm. Te electrical characteristic of the antenna can be studied using the surface current distribution shown in Figure 4. For 2.4 GHz, the current density is given in the Figure 5. For 4 GHz, the current density is found to be high around the feed line (Figure 4(a)). Te radiation intensity is found to be higher around the feed line and the truncated edges of the radiator between 6 GHz and 9 GHz (Figures 4(b) and 4(c)). Te surface current distribution of the proposed prototype with and without slot is shown in Figure 5 for the frequency of 2.4 GHz. Te result shows that the slot is highly responsible for the resonance of 2.4 GHz.

Parametric Analysis.
Te parametric analysis of the radiator for resonance of UWB and the slot for the operation of 2.4 GHz is performed. Figure 6 shows the parametric dimensional change (PDC) and its corresponding results with respect to UWB resonance. Table 1 shows the change in values of the parameter for UWB. Te refection coefcient characteristics with respect to the UWB ( Figure 6(e)) shows that the variation in the parameter "G" and "W" provides better impedance matching and covers lower frequency. Figure 7 shows the parametric analysis of the stub for the resonance of 2.4 GHz. As the length of the stub (H) increases, the resonance shifts towards the lower frequency. Table 2 shows the change in parameters for the 2.4 GHz. Te antenna is measured using the Anritsu MS 2037C vector network analyzer (VNA), as shown in Figure 8(d).

MIMO/Diversity
Te MIMO antenna has a −10 dB impedance bandwidth of 3.1-15 GHz and an additional resonance at 2.4 GHz (see Figure 9(a)). A better isolation can be achieved by incorporating decoupling structures into the antenna, increasing the space between the antenna elements, and orienting the antenna orthogonally. Te proposed semitransparent MIMO antenna achieves isolation of greater than 15 dB over the frequency range of 2.4-15 GHz, with a minimum spacing of 0.0192λ 0 . In the proposed design, no decoupling structures [25][26][27] are used to increase isolation, and only the orthogonal orientation of the element helps in increasing interelement isolation. Figure 10 shows the connected ground structure [28] and its corresponding refection coefcient characteristics are shown in Figure 11. Te results show that the antenna performance is not afected due to the addition of connected ground plane structure. Te grounds are connected to each other with a stub of width 0.25 mm.

Diversity Characteristics.
Multiple antennas are required in the vehicular environment to receive signals from all directions. Te parameters such as envelope correlation coefcient (ECC), diversity gain (DG), total active refection coefcient (TARC), and channel capacity loss (CCL) are studied to evaluate the diversity functioning of the antenna [29]. ECC denotes how an antenna element in a system is independent of other antennas. Te ECC is calculated using the following equation, and the practical limit of ECC is less than 0.5.
where F 1 and F 2 are the radiated felds. Te term DG refers to an improvement in the signal-tointerference ratio without sacrifcing quality, and it is also related to ECC.
Te CCL gives transmission rate and signal reliability information. It is calculated using the following equation: where correlation matrix is given as |ψ| R � ρ 11 ρ 12 ρ 21 ρ 22 . International Journal of Antennas and Propagation TARC (Γ t a ) is the ratio of the total refected power b i divided by the total incident power a i in an N-port system, where i varies from port-1 to port-4. Te ECC, DG, CCL, and TARC curves are plotted in Figure 12.
(5) Table 3 shows the mean efective gain (MEG) of the semitransparent MIMO antenna. Te MEG ratios of antenna 1 and antenna 2 are found to be close to unity for the desired frequencies.  International Journal of Antennas and Propagation 2.5. Radiation Characteristics. Figure 13 depicts the radiation plots of the semitransparent antenna at 2.4 GHz, 4 GHz, 6 GHz, and 9 GHz frequencies. In copolarization, the transmitter and receiver have the same polarization, whereas in cross-polarization, the transmitter and receiver have diferent polarizations. Te radiation patterns in the elevation plane (E-plane, φ � 90°) are bidirectional, whereas the radiation patterns in the azimuth plane (H-plane, φ � 0°) are omnidirectional. Te gain and efciency plots of the proposed semitransparent antenna are shown in Figure 14. Te maximum values of the simulated and measured gain are 2.24 dBi and 2.14 dBi, respectively. Te efciency obtained is greater than 75%.   (Figure 15(a)), and the refection coefcient characteristics are studied. Te plot in Figure 15(b) suggests that the antenna's performance is unafected even after installation in the windshield. Te far-feld characteristics of the transparent substratebased antenna imported into the car model are depicted in Figure 16. A Volkswagen Touareg open-source car model is used for the far-feld study. Te semitransparent antenna has near omnidirectional characteristics and improved directivity, as illustrated in Figures 16(a)-16(c).

Efects of Surrounding Conductors.
A metal plate that acts as a large ground plane is used to study the efect of other conductors near the semitransparent MIMO antenna. Te metal plate imitates the roof of the vehicle. Te proposed semitransparent MIMO antenna is positioned vertically (case-1) and horizontally above metal (case-2) (see Figures 17(a) and 17(b)). Te metal plate measures 40 cm × 40 cm × 5 cm. Te gap between the metal plate and the semitransparent antenna element is 10 mm.             [33][34][35][36], the proposed semitransparent antenna provides dual-band without any reconfgurability techniques. (iv) Unlike previous work [30,33], the designed semitransparent antenna operates at integrated frequencies between 2.4 GHz and 3.2-15 GHz. (v) Te vehicular environment necessitates the use of multiple antennas to receive signals from all directions. Terefore, by arranging the semitransparent antenna elements orthogonally, the antenna provides better isolation without any decoupling structures and also has two polarization vectors (horizontal and vertical). (vi) Te CCL is found to be less than 0.2 bits/s/Hz unlike [31,32,34].

Conclusion
A quad-port semitransparent MIMO/diversity antenna is presented for automobile applications. Te transparent substrate does not afect the aesthetic appearance of the vehicle. Te antenna supports the 2.4 GHz band and UWB. Te proposed transparent substrate-based antenna is mounted in the car model and its refection coefcient characteristics and diversity parameters are investigated. Te results indicate that the diversity parameters are within the practical range. Te semitransparent antenna is also tested for housing efects and on-vehicle performance. Te proposed antenna can be integrated into a vehicle for automotive applications such as ITS, ADAS, trafc management, and vehicular communications.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.