A Novel Low-Profile 5G MIMO Antenna for Vehicular Communication

. Te proposed work is a novel low-profle 5G MIMO antenna confguration to exhibit dual-band frequencies for 5G NR-n2 band (1.9GHz) and safety band (ITS-5.9 GHz) in vehicular communication. In the proposed antenna, it is quite difcult to achieve the lowest resonance frequency in a comparatively miniaturized dimension concerning its operating wavelength. Te designed antenna is a modifed square patch with the dual-band resonance achieved by the incorporation of slots for increasing the electrical length within the dimension. Hence, the design comprises ring and loop slots exhibiting resonance at 1.9 GHz and the loop U, and modifed-W slots for 5.9GHz. Te antenna achieves 1.9% and 0.64% impedance bandwidth and a peak gain of 1.944dBi and 6.06dBi at the resonant frequencies of 1.9GHz and 5.9GHz, respectively, with dimensions of 0.114 λ o × 0.114 λ o × 0.0016 λ o, where λ o is the wavelength of the lowest operating frequency. Te MIMO confguration is presented to assess the antenna’s suitability for large-scale applications. Te MIMO antenna presented here is deployed with the edge-to-edge distance between the single element radiators being 0.01 λ o by parametric sweep. Te presented MIMO antenna provides an isolation value greater than 19 dB because of reduced mutual coupling between the single element radiators in that MIMO structure due to the presence of a ground slot. Te ECC values are 1.659 × 10 − 9 and 0.000601 for frequencies of 1.9GHz and 5.9 GHz, respectively, and the diversity gain is relatively near 10dB, which is the acceptable value for MIMO antennas. Tis modifed square single-element and MIMO antenna provides a relatively higher gain and better performance in vehicular communication for GSM and safety applications. Te MIMO confgurations’ on-vehicle analysis is performed to check the reliability of the designed antenna in a vehicular environment.

compact size [2], and requiring wide operating bandwidth [3][4][5], which are added requirements nowadays [6][7][8][9]. Hence, designing an antenna that includes the above-mentioned prominent features is to be designed. In vehicular communication, both the sub-6 GHz [10,11] and millimeterwave frequencies [12] of 5G have become renowned bands of operation as all other technologies move towards 5G technology for communications. Tis vehicular communication radiator uses an Unlicensed National Information Infrastructure (U-NII-4) of 5.9 GHz for establishing communication in a vehicle-to-network (V2N) platform, which increases road safety and trafc efciency. Also, the 5G NR-n2 band is specially allocated for personal communication services used for vehicle-to-network communication.
A regular microstrip patch antenna can be used for 5G, but it exhibits a narrow bandwidth due to the reduction in antenna size. To improve the bandwidth of the antenna, some of the techniques such as stacking substrate, meta-surface [13], thick, low permittivity substrate, shorting pins [14,15], slots and slits [15][16][17], crossing stub structure [16], and folded patch [17], are used but with a lateral increase in size, volume, and cost of fabrication. One of the methods to improve bandwidth is using microstrip line and proximity feeding techniques with the meta-surface antenna [13] for 5 GHz frequency band excitation with an improved bandwidth, but the antenna size is comparatively large for a single-band antenna. Coupled microstrip line patch is also used with the gap loading technique [14] to reduce the antenna size and obtain low-frequency [18]. Another method for improving bandwidth is etching slots of diferent shapes, such as V-slot [15], elliptical slot [19], and U-slot [17], in the antenna. In [20], the substrate integrated irregular ground (SIIG) structure with connected vias is designed using a technique of plated-through hole, providing a reduction in its footprint of 75.6% and also to stabilize the frequency, the patch is sliced into small square patches [21].
In vehicular communication, vehicles will play a crucial role in the communications of the modern era, which promises to ofer ubiquity, ultra-reliability, and cheap cost [22][23][24]. Instead of attempting to alter the current communications architecture, 5G ofers a platform that unifes all currently used and future-proofed technologies to supply clients with a wide range of services [25]. Te antenna to be placed on the surface of the vehicle should be compact. Hence, miniaturization of the antenna is proposed by using techniques such as using high permittivity substrate and irregular ground plane [20], but it is expensive and complex. Because of these consequences, low-profle and single-layer antennas are proposed [3,13,14,[19][20][21]. Te automotive application supports various services at diferent frequency ranges and requires a separate antenna for operation, which leads to the crowding of antennas on the vehicle surface [26]. Tis reduces the antenna performance on account of interference. To subdue this drawback, dual-band or wideband [27] antennas are proposed. Te number of antennas performing diferent services is reduced. Te techniques used for dual-band are structure sharing, shared aperture, coupled microstrip lines [14], and stacking of substrate, and liquid crystal polymer [28] is used as the substrate to obtain multiband.
Other than the above constraints in vehicular communication, the gain and efciency of the antenna also play a vital role, which can be improved by implementing MIMO [29,30]. Te MIMO confguration of the antenna exhibits increased mutual coupling due to its spacing between antenna elements, which reduces isolation. Traditionally, the distance between the elements is kept at half the wavelength. In recent periods, various studies of MIMO isolation augmentation have been conducted. Numerous decoupling methods like defective ground structures, parasitic scatterers, wideband neutralization lines, beamforming networks, metamaterials [31][32][33], reactively loaded elements, characteristic modes, and others are available for 2 or 4 MIMO antennas. Some of the techniques that are used to improve isolation in the literature include separate isolation structures [34], etching slits [35,36], and slots [37,38] on the ground and adding decoupling devices [39,40]. Te isolation structure used in [34] consists of a two-proximity coupled fed microstrip square ring patch antenna and two quarter-wavelength microstrip slot antennas, which are printed on diferent sides to reduce mutual coupling, which in turn reduces the isolation. Te MIMO antenna in [39] uses decoupling devices such as a winding branch of resonance, a modifed serpentine structure [41], and a reversed T-slot etched in-ground for better isolation, but the size is larger for compact antennas.
In this paper, a compact low-profle dual-band antenna operating at the frequencies of 1.9 GHz 5G NR-n2 band and 5.9 GHz safety band for vehicular communication is proposed. Te dimension of the antenna proposed is 0.114λ0 × 0.114λ0 × 0.0016λ0 and the antenna size is reduced to 0.125λ0. Te single-layer substrate of Rogers RT-5870 dielectric of permittivity (εr) 2.33 is used. Te impedance bandwidth is 1.9% and 0.64% and the antenna gains are 1.94dBi and 6.06dBi for the operating frequencies of 1.9 GHz and 5.9 GHz, respectively. Te MIMO confguration for the proposed antenna is designed and its characteristics are analyzed. In this manuscript, the sections are organized as follows: the presented antenna design is in Section 2. In Section 3, the results and discussions for the designed antenna are stated. Te onboard analysis for the vehicular application is described in Section 4, followed by the conclusion and references in Section 5.

Antenna Design
Te construction of the proposed single-layer antenna is shown in Figure 1. Te radiating patch is embedded above the thin dielectric substrate Rogers RT-5870 of permittivity εr � 2.33, loss tangent 0.0012, and thickness 0.254 mm. Te overall dimensions of the proposed antenna are 0.114λ0 × 0.114λ0 × 0.0016λ0, where λ0 is the free-space wavelength at the lowest resonating frequency of 1.9 GHz. Te radiating patch is fed with a coaxial probe along the surface parallel to the y-axis, which is represented as a feed line in Figure 1

Design of Dual-Band Antenna.
Te fundamental parameter to obtain the resonance at a lower frequency is the electrical length of the stubs in the radiator, i.e., by increasing the inductive efect. For a better understanding of the designed patch antenna, the labels and physical dimensions are shown in Figure 1(c). Hence, various techniques are used to increase this inductive efect. Te detailed structural evolution process of the antenna is explained in the next section. Here, the proposed antenna exhibits the refection coefcient characteristics below −10 dB obtained from simulation as pictured in Figure 2. Te low-frequency resonance is because of the increased coupling efect of the outer slots, whereas the higher frequency resonance is obtained by the coupling efect of the slots in the midpart of the radiator. Te bandwidth of each band depends on the overall size of the slots and stubs used in the patch antenna. Te higher frequency covers a band of (5.90 GHz-5.94 GHz) 0.64% and the lower frequency covers a band of up to (1.88 GHz-1.92 GHz) 1.9%.

Evolution of Antenna with S-parameters.
Te proposed antenna design is initiated with a square patch as shown in Figure 3(a) (STAGE-1). A ring-shaped slot is added in the square patch to provide the resonance at 8 GHz with less impedance matching than in STAGE-2. In STAGE-3, the meandering technique is incorporated by inserting a loopshaped slot to increase the electrical length of the designed antenna, shifting the frequency to 1.9 GHz. Tis is due to the inductive efect caused by the electrical length enhancement by each curve of the loop. To counteract this efect of the antenna, a coupled U-shaped slot is added in between the loop and the ring for coupling purposes, which introduces another band around frequency 2.6 GHz. Tis coupled U-slot is the combination of an asymmetrical U-slot and a thin U-slot, which helps in increasing the coupling between the slots by the step-impedance matching technique. Two inverted U-slots are added at the center, as shown in STAGE-4. To attain the desired excitation frequencies, the inverted U-slots are converted into a modifed W-slot in STAGE-5, which resonates at 1.9 GHz and 5.9 GHz. To prevent that capacitance efect at a higher frequency, a stub that induces the inductance efect is added. Finally, at STAGE-6, two vertical rectangular stubs are added to the inverted U-slots and a horizontal rectangular stub to the bottom side of the ring. Normally, a patch antenna exhibits a narrow bandwidth when the size of the antenna is reduced. Here, the bandwidth of the frequencies is improved by those stubs. Te simulated refection characteristic of each stage of the proposed antenna is depicted in Figure 3(b). Te dual-band response and bandwidth of the proposed antenna are controlled by the respective slots incorporated in the modifed square patch as mentioned in the evolution.

Proposed MIMO Antenna Design.
To improve the performance characteristics of the proposed antenna design, MIMO confguration is implemented. In this, the separation of antenna elements is fxed by using the parametric sweep International Journal of Antennas and Propagation 3 technique as shown in Figure 4(b). With this, the performance characteristics of various spacing values are simulated, and s � 1.5 mm is fxed based on the refection and isolation characteristics exhibited by the antenna. Te designed MIMO antenna and its isolation characteristics are shown in Figures 5(a) and 5(b). Tis MIMO antenna isolation value exceeds 19 dB as there is defective ground structure [37,38] between the two single-element radiators placed adjacent to one another. Te gain of the MIMO antenna is relatively larger compared to that of the single element antennas. Te MIMO antenna gains are 2.93 dBi and 6.18 dBi for frequencies of 1.9 GHz and 5.9 GHz, respectively.

Results and Discussions
Te proposed antenna is fabricated and measured to validate the results obtained from the CST Studio simulation. Te measurement setup of the fabricated prototype contains Agilent's Vector Network Analyzer which is shown in Figure 6(a) and the radiation pattern measurements are carried out in an anechoic chamber shown in Figure 6(b) with the input power of 20 dBm.  cover the required operating frequencies as depicted in Figure 7(a). Te disparity between the simulated and measured S11 in the resonant frequencies is due to the soldering of the SMA connector and tolerance of manufacture [39]. In a two-port MIMO antenna design, an isolation value of greater than 19 dB is exhibited by both the simulated and measured characteristics, which are shown in Figure 7(b). Te radiation pattern of the designed antenna structure exhibits a directional radiation pattern for the operating frequencies. Te simulated and measured radiation patterns for the YZ-plane and XZ-plane are shown in Figure 8.

Single Element Antenna-Performance Analysis.
Te surface current density determines the region of maximum current fow for that specifc resonant frequency. Te induced current distribution over the radiator is illustrated in Figure 9. Te current fow in the region where the loop and ring slot are coupled exhibits a lower frequency (1.9 GHz), and the current fow at the coupled U-slot, loop, and modifed W-slot exhibits the higher frequency (5.9 GHz). Te gain and total efciency value of the proposed antenna are shown in Figure 10. Te simulated gain is 1.94 dBi for 1.9 GHz and 6.06 dBi for 5.9 GHz, respectively. Te radiation and total efciencies of the proposed antenna for the frequencies are 80.5% and 88.3% for the 5G NR-n2 band and 83.3% and 90.1% for the safety band, respectively.

MIMO Antenna-Performance Analysis.
Diversity gain determines the signal strength of the MIMO antenna by comparing the signal gathered by the desired antenna element with the signal gathered by the combination of other antenna elements, which is used to evaluate the performance of the MIMO antenna. By the postprocessing method, the simulated diversity gain of the proposed MIMO antenna is analyzed and found to be nearly equal to 10 dB, which is the required value. Another major performance characteristic of the MIMO antenna is the envelope correlation coefcient (ECC), which determines how the antenna pattern is independent of one another. Te acceptable ECC value for the MIMO antenna is less than or equal to 0.3. Te ECC values of the proposed antenna are also calculated using the postprocessing method in CST, where the value is 1.659 × 10 −9 and 0.000601 for frequencies of 1.9 GHz and 5.9 GHz, respectively. On comparing the proposed model with the previous works of literature, the MIMO confguration exhibits better isolation characteristics as expected because some of the previous models use separate isolation PEC structures [34] and decoupling devices [39], but this model provides good isolation than the literature without any additional isolation structures in the radiator. A link budget assesses a communication system's performance while considering all the losses that a communication link experiences. Here the MIMO antenna link budget is calculated for the operating frequencies of 1.9 GHz and 5.9 GHz. Te received power is calculated using the method in [42]. However, for metropolitan areas, the Federal Communication Commission and the World Health Organization advise antenna power of up to 30 dBm. V2X communication is subject to a 21 dBm transmission power limit set by the 5G Automotive Association (5GAA). Hence, the values are plotted for 21 dBm and 30 dBm power in the resonant frequencies of 1.9 GHz and 5.9 GHz. As per the noise margin obtained as mentioned in [42], the proposed antenna provides a 1.25 km and 1.15 km link for the transmit power of 21 dBm as represented in Figure 11, which is better than the 0.3 km requirement set by the 5G Automotive Association.

. Onboard Analysis
To study more about the antenna performance after its placement in the car, computational modeling is carried out as shown in Figure 12. Some of the factors that inhibit the  International Journal of Antennas and Propagation respective operating frequencies of the presented antenna are analyzed. Te simulated far-feld patterns of the onvehicle analysis for 1.9 GHz and 5.9 GHz are shown in Figure 12.
Te proposed antenna design has the following prominent features contributing to its novelty: (1) An undemanding antenna structure with modifed slots in the basic square patch is proposed. Tis requires a very simple fabrication procedure and is less expensive. (2) Te presented antenna does not involve highly stacked layers as in [20] and other structures such as shorting vias and active components.
(3) Te designed radiator is layered over a dielectric Rogers RT5870 substrate, which ofers low electric loss and very low thickness and is highly ideal for high-moisture environments compared to the other substrates used in [20,27,34]. (4) Tis slot incorporated square patch antenna provides a dual-band for the 5G new radio n2 band and a safety band for vehicular communications unlike the other complex structures for single-band antenna [14,16,21].  x Theta E-Vector Phi International Journal of Antennas and Propagation (6) Te onboard analysis signifes that the designed antenna does not have any noticeable performance reduction in real-time applications. Table 1 compares the proposed work with previous literature.

Conclusion
Tus, a low-profle antenna operating at dual-band 1.9 GHz of 5G NR-n2 band and 5.9 GHz of safety band is proposed. Te slots and stubs are used to achieve the lower resonant frequency and to improve bandwidth. Te presented antenna, of dimensions 0.114λo × 0.114λo × 0.0016λo, achieves 1.9% and 0.64% impedance bandwidth and peak gains of 1.944 dBi and 6.06 dBi at the resonant frequencies of 1.9 GHz and 5.9 GHz, respectively. Te S-parameter and radiation pattern of corresponding frequencies are simulated and compared with measured results obtained from the fabricated prototype. To improve performance, the single elements in the MIMO antenna design are spaced 0.01λo, which exhibits better isolation of 18 dB in the two-port confguration. Te diversity gain of 10 dB and ECC values of less than 0.3 are obtained for the proposed design with the requirements. Te antenna size is reduced by about 0.125λ0 from 0.5λ0 when compared with the previous kinds of literature, which can be suitable for the various vehicular communication services. Te overall size reduction, performance enhancement, and MIMO deployment make the proposed antenna reliable and versatile in vehicular communication areas.

Data Availability
Te data used or analysed during the study can be obtained from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.  10 International Journal of Antennas and Propagation