Mutual Coupling Reduction of a Circularly Polarized MIMO Antenna Using Parasitic Elements and DGS for V2X Communications

In this article, a 4-port circularly polarized (CP) multiple-input multiple-output (MIMO) antenna at 5.9 GHz is presented for the vehicle to everything (V2X) communications. A novel hybrid decoupling structure consisting of circular ring parasites and defected ground structure (DGS) is used to reduce the mutual coupling between the MIMO elements. The circular ring parasites are incorporated between the antennas to reduce the mutual coupling between perpendicular antennas. Whereas, the DGS significantly reduces the mutual coupling between adjacent antennas. These circular ring parasites and DGS affect the electromagnetic field distribution and consequently reduce the mutual coupling. The effectiveness of the decoupling structure is explained through transmission coefficient and surface current distribution. The simulated and measured results show that the proposed MIMO antenna provides overlapping S11 (–10 dB) and axial ratio (3-dB) bandwidth of 2.04% (5.82 – 5.94 GHz) with a peak gain of 7.68 dBic. Moreover, the fabricated MIMO antenna offers excellent diversity performance, isolation between antenna elements is very high (>34dB), envelope correlation coefficient (ECC) is lower than 0.001, and diversity gain is 9.99 dB, very close to the ideal value of 10 dB. Furthermore, the link budget for the proposed MIMO system is calculated to check the suitability for V2X communications which shows that the antenna can communicate between them over a distance of 1.5 kilometers with a 21 dBm of transmit power. Owing to these qualities, the proposed MIMO antenna can be an excellent choice for V2X MIMO communications.


I. INTRODUCTION
Vehicular communication is gaining popularity in academics and industry for Intelligent Transportation Systems to enhance road safety and traffic efficiency [1]. The vehicleto-everything (V2X) communication technology system enables vehicles to communicate between vehicles, traffic, and the environment around them using short-range wireless signals [2]. For V2X communication, the Federal Communication Commission (FCC) recommended the 5.9 GHz frequency band [3]. Moreover, the 3rd Generation Partnership Project (3GPP) and 5G Automotive Association (5GAA) both organizations also targeted the 5.9 GHz frequency band for V2X communication [4], [5]. All over the world, the frequency band from 5.850 -5.925 GHz is mostly used for V2X communication shown in Fig.1 [1], [3]- [6].
The circularly polarized MIMO antennas are considered as a better solution for V2X communication systems. In terms of signal propagation, CP has numerous significant benefits over linear polarization (LP). For example, CP antennas propagate in two orthogonal directions, allowing transmitting and receiving antennas with the same signal power to be oriented freely [7]. Furthermore, CP antennas outperform LP antennas in terms of immunity to multipath distortions, fading, and interferences [8]. These are the reasons the CP is a popular choice for wireless and satellite coupling is the band-stop filter [28], [29]. In [28], a resonator-based band-stop filter is used as a decoupling network whereas a matrix-based band-stop filter is used to reduce the mutual coupling between antenna elements [29]. While a metamaterial absorber structure is used to achieve high isolation between two patch antennas [30]. However, the techniques reported in [27]- [30] are especially used for linearly polarized antennas and do not consider the situation where the patches are arranged orthogonally to achieve orthogonal polarization.
In this article, we propose a novel hybrid decoupling structure consisting of circular ring parasites and DGS to reduce the mutual coupling of a circularly polarized MIMO antenna. A truncated corner square patch is used to produce the circular polarization which is fed by a quarter-wave matching feedline. All the antenna parameters of the proposed MIMO antenna including the link budget make it promising for the V2X communication system. The main attribute of this work is a very low mutual coupling among the MIMO antenna parts while offering circular polarization and high gain. By creating a well-designed arrangement of the circular ring parasites and DGS, the mutual coupling is reduced significantly. The rest of the paper is organized as follows. Section-II illustrates single element antenna geometrical specifications, design method, CP generation techniques, and results. Also, the design procedure of the proposed MIMO antenna, and the proposed mutual coupling reduction techniques are explained in detail in Section II. While, in Section-III, simulated and measured results, diversity parameters, and link budget of the proposed circularly polarized MIMO antenna is presented along with a performance comparison of the proposed work with previous works. The proposed work is finally concluded in Section IV. Fig. 2(a) represents the geometry of the single element antenna. The antenna is made up of a square patch radiator which is connected with a quarter-wave matching feedline followed by a 50-Ω microstrip line, a ground plane, and a Rogers RT5880 substrate (ℇr = 2.2 and tanδ = 0.0009) with the thickness of h = 1.524 mm. A 50-Ω coaxial cable is used to feed the microstrip line. The opposite corner of the square patch is truncated to generate circular polarization. The antenna is designed to resonate at the targeted frequency fr ≈ 5.9 GHz. The preliminary size of the square patch is predicted as [31]:

A. SINGLE ELEMENT ANTENNA
(1) The simulated and measured results of the single element antenna are shown in Fig. 3. A good agreement is observed between the simulated and measured results. The single element antenna offers -10 dB impedance bandwidth of 7.74 % (5.59 -6.04 GHz), and 3-dB axial ratio bandwidth of 120 MHz (5.82 -5.94 GHz) as seen in Fig. 3(a). The offered bandwidth of the single element antenna covers the whole desired V2X communication spectrum. In Fig. 3(b), the gain and radiation efficiency of the single element is illustrated. The antenna offers a maximum stable gain of 7.68 dBic and radiation efficiency of more than 94% at the entire operating frequency. The measurement value of gain and radiation efficiency is slightly lower than the simulated values due to the measurement equipment losses.

B. PROPOSED MIMO ANTENNA
The geometry and configurations of the proposed 4-port (2×2) MIMO antenna are shown in Fig. 4, which is made up of four single-element antennas that are arranged in an orthogonal pattern on a single substrate. The minimum inter-element spacing between the MIMO antenna elements is Es = 8.65 mm. The proposed hybrid decoupling structure is the combination of a novel DGS structure in the ground plane and a novel circular ring parasitic structure between the antenna elements. The proposed DGS structure significantly reduces the mutual coupling between adjacent antennas. Whereas the proposed circular ring parasitic structure reduced the mutual coupling between perpendicular antennas. And their combined effect offers very high isolation between the MIMO elements. Those results are shown in the later sections and the step-by-step design procedure of the proposed MIMO antenna are explained in the following sub-section. The optimum values for the proposed MIMO antenna parameters are listed in Table  1.  This antenna offers high mutual coupling between antenna elements, which is expressed by the transmission coefficient, shown in Fig. 5. The transmission coefficient for the adjacent antennas, S12, S14, S23, and S34 show identical curves. Similarly, the perpendicularly located antennas, S13, and S24 curves are the same. The mutual coupling of the MIMO antenna elements reduces the MIMO performances and lowers the data rate. The isolation level of 20 dB might be enough for general communications. However, seamless connectivity with a high data rate is crucial for V2X communication systems. To ensure high data and uninterrupted communication, modulation transmission requires about an SNR of 30 dB in an additive white Gaussian noise (AWGN) channel and, therefore, the mutual coupling should be below −30 dB in V2X communications [1], [18].

2) MIMO ANTENNA WITH DGS
Defected ground structure (DGS) is a very effective way to reduce mutual coupling between printed MIMO antennas. In literature, various types of DGS are used to establish a high level of coupling separation between the MIMO antenna elements [24]- [26]. In this work, a novel defected ground structure is designed to reduce the mutual coupling. The proposed DGS structure is designed by etching out the copper layer from the ground plane, two circular rings at the center, and four boomerang-shaped stubs connected to the bigger ring.
Using the proposed DGS, the mutual coupling between adjacent antennas (S12, S14, S23, and S34) is reduced significantly at the higher frequency shown in Fig. 5.
However, the mutual coupling between perpendicular antennas (S13 and S24) is not improved. Therefore, another method is investigated, which is explained in the later section. The individual effect of circular rings and boomerang-shaped stubs in the ground plane is shown in Fig. 6, only for the adjacent antennas (S12, S14, S23, and S34), as these structures do   not show any significant effect on perpendicular antennas (S13 and S24).

3) MIMO ANTENNA WITH PARASITIC ELEMENTS
In [27], the effects of parasitic elements on the mutual coupling of MIMO antenna are investigated by adding multiple square parasitic elements in close proximity to MIMO elements. In this work, a novel circular ring parasitic structure is proposed. The inner ring is designed by the inner radius and outer radius of r3 and r4, respectively. The inner ring is separated on four places with a distance of g1. Seven smaller rings are placed in close proximity to the inner ring with the inner radius of r5, and outer radius of r6, while they maintain a distance of g3 between them. In detail dimensions and parameters of the proposed circular ring parasites are presented in Fig. 4 and Table. 1, respectively. Fig. 7 shows the transmission coefficient of the MIMO antenna with parasitic structure. The proposed circular ring parasitic structure reduces the mutual coupling between the perpendicular antennas (S13 and S24). Also offers better performances for the adjacent antennas (S12, S14, S23, and S34), than the MIMO antenna without decoupling structure. The independent effect of different components of the proposed circular ring parasitic structures for perpendicular antennas (S13 and S24) is illustrated in Fig. 8.

4) PROPOSED MIMO ANTENNA WITH HYBRID DECOUPLING STRUCTURE
For the designed MIMO antenna, the DGS structure offers better performances for adjacent antennas, whereas the proposed parasitic structure provides better isolation for perpendicularly placed antennas. Hence, both techniques are combined, and a hybrid decoupling mechanism is proposed as illustrated in Fig. 9, to achieve very low mutual coupling between the MIMO elements. Fig. 10 highlights the mutual coupling reduction by using the proposed hybrid decoupling configuration. It can be seen that, in the operating frequency band, the proposed decoupling structure (DGS and parasitic elements) significantly reduced the coupling effect of the MIMO elements and established high isolation among the adjacent antennas. For the adjacent antennas (|S12|, |S14|, |S23|, and |S34|) the maximum isolation for MIMO antennas without decoupling configurations is 28 dB, whereas, with the proposed hybrid decoupling configuration for the proposed MIMO system, this level is increased to 65 dB. The best isolation value for perpendicularly situated antenna elements (|S13 and |S24|) without any decoupling   Frequency (GHz) 0 (w/o decoupling structure) (with parasitic structure) S12, S14, S23, S34 S13, S24 S12, S14, S23, S34 S13, S24 mechanism is 21 dB, however, this value is enhanced to 34 dB after using the hybrid decoupling structure.
The decoupling structure's efficacy can also be determined by observing the surface current distribution [26]. The surface current distribution for the different exciting ports of the proposed MIMO antenna at frequency 5.9 GHz is shown in Fig. 11. While port-1 is activated, as shown in Fig. 11(a), the excited patch induced significant current flow to the nearby elements for the MIMO antenna without a decoupling mechanism, resulting in a very high mutual coupling. On the contrary, the excessive induced current to the surrounding patches is prevented by the proposed decoupling mechanism and reducing mutual coupling among MIMO antenna parts and providing high isolation. Fig. 11(b) shows the surface current distribution for the excited port-3 to confirm that the proposed decoupling structure has the same effect.

III. PROPOSED MIMO ANTENNA RESULTS
To evaluate the proposed design, a prototype of the proposed MIMO antenna is manufactured and measured. In Fig. 12, the fabricated antenna photograph and its setup for measurement are shown. All the MIMO antenna elements are connected with a K-connector (50-Ω 2.92mm). A PNA network analyzer(Agilent Technologies E8364B) is used to measure the S-parameters in open-air environments. A commercial institution [33] measures and characterizes the far-field radiation performance of the antenna in an anechoic facility. A well-calibrated regular gain horn antenna is employed as a transmit antenna, while the prototype antenna is used as a receiver. Amplifiers are used to provide constant power reception. During testing, the antenna is rotated to detect the radiation intensity at various angles. The reliability of the proposed MIMO antenna system is determined by examining the diversity performance in the perspective of the transmission coefficient, ECC, diversity gain, mean effective gain (MEG), and channel capacity loss (CCL).

A. REFLECTION COEFFICIENT AND AXIAL RATIO
Reflection coefficient curves of the proposed MIMO antenna, both simulated and measured are shown in Fig. 13. The proposed hybrid structure is included between the MIMO components to reduce mutual coupling, resulting in MIMO antennas with a slightly different S11 (-10 dB) bandwidth than single-element antennas. The fabricated prototype offers a -10 dB impedance bandwidth of 7.05% (5.61-6.02 GHz). The simulated and measured findings show a high level of agreement. As shown in Table 2

C. RADIATION PATTERNS
The simulated and measured radiation patterns of the proposed MIMO antenna at 5.9 GHz in xoz-plane and yozplane for all antenna ports are presented in Fig. 15. For all of the ports tested, the antenna provides steady radiation patterns with low back and side-lobe levels. The antenna offers the RHCP sense of polarization because the LHCP is very negligible as compared to the RHCP showing high polarization isolation. The XPD (cross-polarization discrimination) in CP antennas is the difference between the RHCP and the LHCP levels. The XPD of the proposed CP-MIMO antenna is high (more than 15 dB), as can be seen in the radiation pattern plot in Fig. 15. The antenna port-1 shows a maximum gain of 7.68 dBic with a half-power beamwidth of 91° and 70° at xoz-plane and yoz-plane, respectively. All the other antenna ports offer the same value of half-power beamwidth, but the planes are different because of the orientation of the elements. As the DGS is very sensitive to the ground plane, the antenna is simulated with a car model to check the antenna performance at practical uses. The antenna is placed 5 mm above the car body. The proposed MIMO antenna shows almost identical performance with the car model. The 3D-radiation pattern of the proposed MIMO antenna with the car model is shown in Fig. 16 while port-1 is excited.

D. RADIATION EFFICIENCY AND GAIN
The proposed MIMO antenna elements offer practically identical radiation efficiency and gain to the proposed single element antenna which is explained earlier and shown in Fig.  3(b). The proposed MIMO antenna offers a radiation efficiency of more than 94% at the entire operating frequency. The simulated and measured gains of each antenna element in the proposed CP-MIMO assembly are presented in Table 2.
Only port 1 is excited while the remaining ports were terminated with a 50-Ω load, during the measurement of antenna element 1. Similarly, the rest of the antenna elements measurements are made by merely exciting the corresponding antenna port and connecting the 50-Ω load to other ports. The gain values for all antennas are nearly identical, and they are similar to the single element CP-antenna studied in Section II.

E. ENVELOPE CORRELATION COEFFICIENT (ECC)
The ECC measures how independent MIMO elements are in terms of performance, such as polarization and radiation patterns [34]. The ECC (ρeij) of the proposed MIMO antenna system is computed based on S-parameters and far-field radiation by using the equation (2) and (3), respectively [26].   (2) and (3) are less than 0.001 within the operating frequency. Fig. 17(a) shows this outstanding diversity performance of the proposed MIMO antenna.

F. DIVERSITY GAIN (DG)
Another important parameter is the DG, which indicates how the diversity scheme of MIMO antennas affects the radiated power. For the proposed MIMO antenna the DG is calculated from the value of S-parameters using equation 4 [26] and shown in Fig. 17(b).
The proposed MIMO antenna offers a diversity gain of more than 9.99 dB for all the antenna elements, which value is almost similar to the ideal value.

G. MEAN EFFECTIVE GAIN (MEG)
The MEG, which is a ratio of the mean received power to the mean incident power of the antenna, is another significant statistic to quantify diversity performance in MIMO systems [36]. The acceptable practical value of MEG should be within -3 dB to -12 dB range [37]. For our proposed MIMO system, the MEG value is calculated by using the equation 5 [36], [37]: In this equation, i stand for the observation port, and the antennas number in the MIMO system is represented by N.   MIMO system. The MEG values are less than -7 dB within the operating frequency band.

H. CHANNEL CAPACITY LOSS (CCL)
The CCL is a metric that estimates the maximum amount of channel loss that allows a message to be successfully transmitted over a communication channel. For reliable communication, the CCL should not exceed 0.4 bits/s/Hz. Equations (6) and (7) provide the formulas for calculating the CCL [7], [36]. Where, correlation matrix of the MIMO antenna elements is MANT, and ρii = 1 − (|Sii| 2 − |Sij| 2 ) and ρij = −(Sii * Sij + Sji * Sij) for i, j = 1, 2, 3 or 4. Fig. 19 shows the CCL value at the intended transmission frequency was significantly less than 0.15 bits/s/Hz.

I. LINK BUDGET
A link budget is a method of quantifying the performance of a communication link while accounting for all the power, gains, and losses that a communication signal experiences in a telecommunication system, from the transmitter to the receiver, via a communication medium such as a waveguide, radio waves, optical fiber, or cable. The proposed MIMO antenna's link budget is calculated to determine its suitability for V2X communication by using the following equation [37]: Where, PRX and PTX are the received and transmit power in dBm, respectively, GTX is the transmitter antenna gain in dBic, LTX is the entire system degradation in decibels (dB) at the transmitter, LFSP is the total propagation losses in dB between the transmit and receive antennas, LM is miscellaneous losses (fade margin, polarization misalignment, etc.) in dB, GRX is the receiver antenna gain in dBic, and, LRX is the overall system loss at the receiver side in dB.
Here, only free space path loss (LFSP) varies with distance (d) and frequency (f), and all other parameters are constant for a specifically designed system. The free space path loss (LFSP) can be calculated as [38]: MATLAB is used as a tool to calculate the link budget of the proposed MIMO antenna by using the equation (8) and (9) Where, MDS stands for minimum detectable signal, T is the temperature (290) in Kelvin and, k is Boltzmann's constant (−228 dBW/(kHz)). If we use a 70 MHz channel bandwidth with a 13.8 dB noise figure, then the proposed antenna will offer a minimum of -81.8 dBm of sensitivity.
Since the FCC and WHO (World Health Organization) suggests antenna power up to 1 W in urban areas. Moreover, 5GAA suggests 21 dBm transmit power at the time of antenna testing for the V2X communication system [40], to ensure the antenna's effectiveness in actual usage. The link budget of the proposed MIMO antenna is calculated for different transmit power from 126 mW to 1W shown in Fig. 16. From the link budget graph, it can be seen that the antenna can communicate between them over the distance of 1.5 kilometers with the  minimum Tx power of 21 dBm, whereas the 5GAA asks for a minimum communication distance of 300 meters between the antennas [40]. Table 3 compares the proposed CP-MIMO antenna to other similar MIMO microstrip patch designs with significant isolation that have recently been published in the literature. In terms of several performance metrics, the comparison is made including antenna ports, overall antenna volume, center frequency, antenna polarization, mutual coupling reduction techniques, minimum inter-element spacing, maximum gain, radiation efficiency, minimum and maximum isolation, and envelope correlation coefficient (ECC). It can be seen that a better maximum value of isolation can be achieved by using a decoupling structure, particularly [20], [21], [22], [26], [30]. However, these designs suffer from either low gain and lower number of ports, or cannot offer circular polarization. Our proposed design is superior in terms of ports, radiation efficiency, and the value of ECC with a strong isolation capability. The proposed design is superior in terms of the number of ports, radiation efficiency, and the value of ECC with a strong isolation capability. Also, this antenna offers the lowest inter-element spacing and occupies the lowest overall space among the 4-port MIMO antennas. Furthermore, the proposed MIMO antenna provides high gain and circular polarization while preserving good isolation among the MIMO antenna radiators.

IV. CONCLUSION
A high gain circularly polarized MIMO antenna is presented for V2X communications. A truncated corner square patch is used to produce the circular polarization which is fed by a quarter-wave matching feedline. The low mutual coupling between MIMO antenna elements is the key differentiating characteristic of the proposed design. The overall compact size of the proposed circularly polarized single element antenna design is 0.62λ0 × 1.0λ0 × 0.03λ0. S11 (-10 dB) bandwidth of the single antenna element is 7.74% (5.59 -6.04 GHz), according to simulations and measurements. The antenna also offers a peak gain of 7.68 dBic with a stable and symmetrical radiation pattern and high radiation efficiency (>94%). Using the proposed single element, the MIMO antenna is created by positioning all single elements orthogonally. A novel hybrid decoupling structure consisting of circular ring parasites and defected ground structure is proposed to reduce the mutual coupling between the circularly polarized MIMO elements. The electromagnetic field dispersion is affected by the proposed hybrid decoupling structure, resulting in the high dispersion of induced current to surrounding patches being stopped and good isolation between MIMO antenna elements is accomplished. A prototype of the proposed 2×2 (4-port) MIMO antenna is fabricated and measured to validate the design concept. The overall compact size of the proposed MIMO antenna is 1.4λ0× 1.4λ0 × 0.03λ0. The simulation and measurement result shows a good agreement. The proposed CP-MIMO antenna has a wide -10 dB impedance bandwidth of 7.05% (5.61 -6.01 GHz) and a 3-dB axial ratio bandwidth of 2.04% (5.82 -5.94 GHz) with a stable peak gain of 7.68 dBic. All the diversity performances of the proposed CP-MIMO antenna are excellent, between all the MIMO antenna elements high isolation is achieved (>33 dB), ECC is less than 0.001, diversity gain is almost similar to the ideal value (9.99 dB), and acceptable channel capacity loss (0.15 bits/s/Hz) and mean effective gain (-7 dB). The link budget of the proposed MIMO antenna is also calculated based on the measured results to check its suitability for V2X communication which shows that the antenna can communicate between them over the distance of 1.5 kilometers with 21 dBm of Tx power. All these features of the proposed antenna system make it a suitable candidate for V2X MIMO communications.