3-D twelve-port multi-service diversity antenna for automotive communications

This paper presents a twelve-port ultra-wideband multiple-input-multiple-output (MIMO)/diversity antenna integrated with GSM and Bluetooth bands. The twelve-port antenna is constructed by arranging four elements in the horizontal plane and eight elements in the vertical plane. The antenna element, which is created using a simple rectangular monopole, exhibits a frequency range of 3.1 to 12 GHz. The additional Bluetooth and GSM bands are achieved by introducing stubs into the ground plane. The size of the MIMO antenna is 100 × 100 mm2. The antenna offers polarization diversity, with vertical and horizontal polarization in each plane. The diversity antenna has a bandwidth of 1.7–1.9 GHz, 2.35–2.55 GHz, and 3–12 GHz, the radiation efficiency of 90%, and peak gain of 2.19 dBi. The proposed antenna offers an envelope correlation coefficient of < 0.12, apparent diversity gain of > 9.9 dB, effective diversity gain of > 8.9 dB, mean effective gain of < 1 dB, and channel capacity loss of < 0.35 bits/s/Hz. Also, the MIMO antenna is tested for housing effects in order to determine its suitability for automotive applications.

www.nature.com/scientificreports/ information signal across multiple pathways. The combination of multiple-input-multiple-output (MIMO) and UWB technologies can improve system robustness by avoiding the effects of fading and multipath propagation. MIMO transmits and receives uncorrelated signals while increasing channel capacity by forming parallel resolvable channels. However, the main challenges in MIMO antenna design are high inter-element coupling and compact size suitable for integration with other high-frequency devices 2 . In 3 , a UWB antenna with GSM, WCDMA, and WLAN integrated bands was presented. The ground plane of the antenna was modified with capacitively loaded line resonators. The multiband operation was achieved without increasing the size of the antenna, but the antenna showed single polarization. In 4 , a rectangular patch antenna with multiple standards was reported, where an octagonal-shaped slot was used to integrate multiple bands. In 5 , slots were introduced in the ground plane to achieve multiple band resonance without increasing the physical size of the antenna. In 6 , a compact UWB monopole antenna with a notch and resonating strips was designed to achieve the quad-band performance. In 7 , a compact-sized UWB antenna with band-notched characteristics was developed. The antenna offered good isolation, but its polarization was limited. In 8 , a dual-polarized UWB MIMO antenna with integrated 1.9 GHz and 2.4 GHz was presented. In 9 , a MIMO antenna was designed with good isolation for IEEE 802.11 a/b/g/n applications, however, only single polarization was obtained. The band-notched multiband antennas were designed in [10][11][12] . In 13 , a UWB MIMO antenna with improved isolation and dual polarization was proposed. In 14 , a quad-port UWB antenna with an integrated GSM band was proposed without increasing the overall antenna size. The antenna offered horizontal and vertical polarization. In 15 , a uniplanar four-port differently driven UWB antenna was presented, where high isolation and low cross-polarization were achieved through different feeding mechanisms. In 16 , a UWB antenna integrated with Bluetooth and WLAN bands was presented, where ring slots were loaded in the patch for achieving multiband characteristics. However, the overall size of the antenna element was larger. In 17 , the antenna elements were located perpendicular to each other, and good isolation was obtained without any isolation technique. In 18 , an RF amplifier was integrated with the UWB MIMO antenna, but only one type of polarization was achieved. In 19 , a compact broadband MIMO antenna for indoor wireless communication systems was proposed. The antenna offered good isolation without the use of decoupling structures, but it was limited to two polarization vectors. In 20 , eight differentially-fed microstrip antenna elements with dual polarization were arranged. The antenna covered the N79 band for 5G, but it had a low efficiency. In 21 , a slit/slot antenna fed by a transmission line was proposed for tri-polarized MIMO applications. A tri-polarized single-layer MIMO antenna with vias, which allows the different modes to resonate at the same frequency, was reported in 22 . However, the antenna geometry in the majority of the above-mentioned designs was complex and difficult to integrate with other circuits. In this paper, a MIMO antenna with twelve resonators arranged in horizontal and vertical planes is proposed. The main features of the presented work are: 1. The antenna covers two narrow bands (GSM and Bluetooth) and the entire UWB. Numerous wireless services required in automobiles are integrated into a single radiator, eliminating the need for multiple patches. 2. The 3-D orientation of the radiators reduces the total area occupied by the antenna, allowing more elements to be incorporated into a small space. 3. The polarization diversity is achieved by arranging the radiators orthogonally to each other. 4. Placing the antenna elements in both the E-plane and the H-plane result in additional polarization. In comparison to other antennas in the literature, the proposed design generates additional polarization vectors, resulting in a more robust diversity scheme. 5. The link reliability and channel capacity are improved due to the increased degree of freedom offered by the proposed antenna. 6. Isolation greater than 20 dB is obtained, without the usage of any decoupling structures. 7. The housing effects are investigated for the reliability test of the antenna for automotive applications. The horizontal and vertical orientations of the proposed antenna are tested in the presence of conducting bodies. The housing effects results validated the stability of the antenna. 8. The far-field performance of the proposed antenna on the vehicle is investigated, and the results show that the antenna exhibits omnidirectional characteristics when placed on the car body.
First and second sections present the design of the antenna element and MIMO antenna, respectively. Third section presents the results and diversity characteristics of the antenna. The antenna housing effects are discussed in fourth section, and fifth section presents the conclusion.

Antenna design
Evolution of the UWB antenna element. The proposed UWB monopole antenna element is depicted in Fig. 1. The overall size of the antenna element is 30 × 30 mm 2 . The antenna element is designed on the FR-4 substrate with relative permittivity of 4.4, loss tangent of 0.025, and thickness of 1.6 mm. The design equation for the UWB planar monopole antenna is given as 23,24 where f l is the lowest resonating frequency of the antenna and p is the distance between the patch and the ground plane, and the empirical constant k is calculated as where 0.335π[(a + b)] corresponds to the expression (l + r), and the semi-length and semi-width are denoted by a and b, respectively. The design parameters of the UWB monopole antenna are given in Table 1. The evolution of the proposed UWB antenna element is depicted in Fig. 2. The length and width of the monopole radiator are optimized to achieve the UWB specifications. The gap between the patch and the ground plane is important for improving radiator performance. The lower corners of the monopole are truncated to improve impedance matching. A hexagonal-shaped defect is introduced in the ground plane to improve impedance matching. The simulated reflection coefficients of the design steps are shown in Fig. 3.

Integration of bluetooth and GSM bands.
The ground plane of the antenna element is modified to integrate Bluetooth and GSM bands with the UWB, as shown in Fig. 1b. A stub of length 'b' is added to the ground plane for Bluetooth (2.4 GHz) resonance. Also, a stub of length (s = t + u) is added to the ground plane for the GSM frequency band. The widths of the stubs are adjusted to improve impedance matching. It is also   Equivalent circuit of the proposed antenna. The equivalent circuit is used to investigate the physical mechanism of the antenna 25 . The equivalent circuit is calculated using the impedance characteristics, shown in Fig. 5. The two maximum impedance points (3.83 GHz and 9.86 GHz) are selected from the reflection coefficient characteristics, and the corresponding circuit for UWB is derived. When the impedance curve moves from low (negative) to high (positive), a series resonant circuit is drawn, and when the curve moves from high (positive) to low (negative), a parallel resonant circuit is drawn 26 . The equivalent circuit of the antenna is shown in Fig. 6, and the corresponding RLC parameters are shown in Table 2. The two parallel resonant circuits correspond to 1.8 GHz and 2.4 GHz, respectively, and the two series resonant circuits correspond to UWB.  Fig. 7. Figure 7a, b show the surface current at 1.8 GHz and 2.4 GHz, respectively. The longer stub has a higher current density at 1.8 GHz, while the shorter stub has the highest current density at 2.4 GHz. The surface current distribution for UWB shows that truncation of patch edges aids in higher current density.

Development of the MIMO antenna
The proposed twelve-port MIMO antenna configuration is depicted in Fig. 8a. The antenna is created by arranging four elements in the horizontal plane and eight elements in the vertical plane. The two vertical planes, each with four elements, are arranged in a cross configuration with the horizontal plane. The overall size of the antenna is 100 × 100 mm 2 . Inter-element isolation can be improved by increasing the distance between the antenna elements or by using a decoupling structure between them 27 . The spacing between the resonating elements is 0.24λ 0 to achieve better isolation. In comparison to the conventional 2-D arrangement, the 3-D orientation of the radiators provides polarization flexibility. When the radiators are oriented in opposite directions, the correlation between them decreases, and the isolation increases. As a result, the MIMO antenna prototype provides polarization diversity while also increasing reliability.

Fabrication and measurement
The antenna element and MIMO antenna are fabricated in order to test their performance. The Anritsu MS2037C VNA is used to test the S-parameters of the twelve-port MIMO antenna.  The S-parameters (S 11 , S 66 , and S 1212 ) are measured at port-1 in the horizontal plane, and port-6 and port-12 in the vertical planes. The S ii characteristics show that the antenna has a good impedance over the UWB, GSM, and Bluetooth frequencies.
The mutual coupling characteristics of the proposed twelve-port MIMO antenna are depicted in Fig. 10. The S ij characteristics illustrate that the antenna elements offer isolation greater than 20 dB. Radiation performance. The measured radiation patterns of the twelve-port MIMO antenna at 1.8 GHz, 2.4 GHz, 3.1 GHz, 5 GHz, 6.8 GHz, and 8.5 GHz are depicted in Fig. 11. The radiation performance of the fabricated prototype is measured in an anechoic chamber as depicted in Fig. 8b. Figure 12 presents the measured gain and efficiency of the prototype antenna. The gain and efficiency of the proposed antenna are greater than 1.6 dBi and 90%, respectively. Diversity performance. The diversity performance of the twelve-port MIMO antenna is estimated using metrics such as envelope correlation coefficient (ECC), diversity gain (DG), mean effective gain (MEG), total active reflection coefficient (TARC), and channel capacity loss (CCL). The ECC value should ideally be zero, but www.nature.com/scientificreports/ in practice it is < 0.5. ECC can be calculated using the S-parameter or the far-field, respectively, using Eqs. (4) and (5).
where S ij denotes the S-parameter of antenna i in relation to antenna j, F i is the field radiated by the antenna. The calculated ECC values show that the antenna elements are less correlated, as shown in Figs. 13 and 14.
The two types of diversity gain are apparent diversity gain (ADG) and effective diversity gain (EDG), which are calculated using the Eqs. (6) and (7), respectively. They differ in the way that EDG includes efficiency while ADG does not. The practical limit for DG is > 9.9 dB. The ADG and EDG are calculated using the far-field and S-parameters, and they meet the practical limit for DG. Tables 3 and 4 present the ADG and EDG of the proposed MIMO antenna in relation to port-1 and port-12, respectively.  Ideally, the MEG difference should be less than 3 dB. The proposed MIMO antenna has a MEG difference of less than 1 dB.
TARC is another metric used to determine the impact of one antenna element on another. TARC is defined as the square root of the total reflected power divided by the total incident power, as shown in Eq. (9).
where a i is the incident signal and b i is the received signal. Figure 15 depicts the TARC of the MIMO antenna in relation to port-1 and port-12. The calculated results show that the lower the TARC value, the lower the mutual coupling.  S1j (dB) Frequency (GHz) S1,2 S1,3 S1,4 S1,5 S1,6 S1,7 S1,8 S1,9 S1,10 S1,11 S1,12  S12j (dB) Frequency (GHz) S12,1 S12,2 S12,3 S12,4 S12,5 S12,6 S12,7 S12,8 S12,9 S12,10 S12,11 www.nature.com/scientificreports/ CCL is used to investigate capacity loss due to correlation in MIMO channels. The CCL of a MIMO system can be calculated as Figure 16 depicts the CCL of the MIMO antenna in relation to port-1 and port-12. The correlation matrix of the receiving antenna is given by www.nature.com/scientificreports/ ρ 12 = − S * 11 S 12 + S * 21 S 12 , and ρ 21 = − S * 22 S 21 + S * 12 S 21 . The practical limit of CCL is 0.4 bits/s/Hz, and the proposed antenna offers CCL less than 0.35 bits/s/Hz. Maximal ratio combining (MRC) and selection combining (SC) are diversity combining techniques that combine the signals received from the antenna to increase the mean signal to noise ratio (SNR) and yield reliability in fading environments. The Eq. (12) can be used to calculate the cumulative distribution function (CDF) of the MIMO antenna under the rayleigh condition 28 . Figure 17 shows that the twelve-port configuration outperforms the two-element case in terms of diversity performance. www.nature.com/scientificreports/ where λ is the eigen value obtained from the signal covariance matrix (Λ MRC ) and K is the number of antenna elements. The covariance matrix is given by Eq. (13).

Antenna housing effects
The location of the antenna in the vehicle has a significant impact on its performance. The proposed antenna can be mounted on the roof of a car using a shark fin mount or integrated into the existing printed circuit board. The proposed automotive antenna can be installed on the roof of a car through the chassis cavity 29 . For automotive communications, the antenna housing effect is discussed in order to evaluate antenna performance in the presence of metallic conductors [30][31][32] . A metal plate is used to mimic the car roof to investigate the effects of antenna housing. The size of the metal plate ranges from 40 × 40 × 5 cm 3 to 80 × 80 × 5 cm 3 .
Two scenarios are considered when studying the effects of antenna housing. The antenna is positioned in the xz-and yz-planes as shown in Fig. 18. In the xz-plane, the antenna is perpendicular to the metal conductor, while in the yz-plane, the antenna is to the side of the metal conductor. The omnidirectional characteristic is influenced if the antenna is placed at the top of the yz-plane. Figure 19 depicts the simulated reflection coefficients of the twelve-port antenna when antenna housing effects are taken into account. The simulation results show that the presence of a metal conductor has no significant effect on the antenna characteristic in either scenario. The presence of a metal plate has no effect on the xz-plane. Even in the presence of a metal plate, the antenna maintains its omnidirectional behavior.
The asymptotic solver in CST is used to estimate the far-field performance of the proposed antenna when integrated with a vehicle. An open-source CAD model of the Volkswagen Touareg is used for estimating the far-field characteristics. The on-car performance of the proposed antenna is depicted in Fig. 20. The results imply that the antenna exhibits omnidirectional characteristics when placed on the body of the vehicle. The directivity is greater than 6 dB for all observed frequencies. Table 5 compares the reported and proposed MIMO antenna designs. The main advantages of the proposed antenna are: 1. In comparison to the antenna structures 7,14,20,33-59 , the proposed antenna geometry has twelve-elements, and covers two narrow bands (GSM and Bluetooth) and the entire UWB.  14,20,34,[36][37][38][39][40][41][42][43][44][45][46][47][48][50][51][52][53][54][55][56][57][58][59] , the proposed MIMO antenna configuration occupies less area while having a larger number of resonating elements. The antennas in 7,33,35,47 occupied an equivalent/ smaller area but had fewer elements. 4. The proposed MIMO antenna outperforms in terms of ECC, DG, MEG, TARC, and CCL, whereas all of these diversity factors were not investigated in the majority of reported papers 14,20,33-38,40,41,43-45,47,49,51-55,58,59 . 5. The housing effect and on-car body performance of the proposed 3-D MIMO antenna are investigated, whereas they were previously studied only for single-element/two-element/2-D MIMO antenna designs 1,31,32,45,55 .
Thus, it can be concluded that the proposed design has packed more elements in a smaller space while maintaining a high degree of isolation between them. Further, the distinct orientation of the antenna elements offers a wider range of polarization vectors, which is highly desirable in a rich scattering and deep fading environment.

Conclusion
In this work, a MIMO antenna that operates in the UWB, Bluetooth, and GSM bands is presented. The antenna is made up of twelve elements that are arranged in horizontal and vertical planes. The antenna diversity performance is investigated, and the values are within the limits. The proposed antenna achieves high gain and efficiency. The antenna housing effect is investigated to determine the consistency of the radiator when it is installed in a vehicle. The reflection coefficients and directivity investigated from the antenna housing effect are satisfactory. The antenna can be installed in automobiles for automotive applications such as V2V communication and ITS.