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Inset-Fed Planar Antenna Array for Dual-Band 5G MIMO Applications

An inset-fed planar MIMO antenna array has been presented for dual-band 5G applications. The proposed MIMO array offers numerous advantages such as compact size, planar structure, and high isolation. The single element of the array comprises an inset-fed rectangular patch and open circuit stubs designed on the top side of the substrate, while the bottom layer consists of a partial ground plane. Simulated and measured results show that the proposed antenna offers dual-band characteristics at 28 GHz and 38 GHz frequency bands, respectively. It has also been observed from the results that the proposed inset-fed planar antenna offers good radiation characteristics, acceptable gain, and radiation efficiency for both bands. Furthermore, a four-element-based MIMO antenna array has been designed for its possible use in 5G-enabled communication devices. It has been demonstrated that the proposed MIMO antenna provides high isolation between array elements without disturbing the characteristics of the individual antenna element. The proposed MIMO antenna array has been fabricated and measured for the validation of simulation results, and it has been observed that both the results are in good agreement.


INTRODUCTION
As 4th generation (4G) communication systems have gained maturity, most of the societies are now concentrating on future 5th generation (5G) communication systems [1]. One of the most demanding expectations from the upcoming 5G communication system is to provide a stable data throughput experience over a wide range. Recent studies have demonstrated the feasibility of the millimeterwave (mm-wave) spectrum for cellular network applications [2]. With the allocation of an unlicensed spectrum, currently, at 28 GHz, 38 GHz, and 60 GHz frequency bands, it is possible to handle a high data rate with low-cost infrastructure due to increased bandwidth. However, radio propagation path loss models of lossy mm-wave channels show that a high-performance antenna system is required [3]. So, there is a need to design low-loss, compact, and low-cost antennas that can easily be integrated with mm-wave circuitry.
For mm-wave frequencies, designing an antenna is more demanding where efficiency, flexibility, and compatibility with Multi-input Multi-output (MIMO) systems are of main concern for realizing low multipath fading and high spectral efficiency [4]. With these requirements, the MIMO antenna array should provide high gain and isolation between closely placed antennas to minimize the effect of adjacent elements on the array's performance. One of the main advantages of mm-wave frequencies is that the antenna size is small, which helps to design multiple antennas in a limited space with high isolation.
In [5], a patch antenna array of 8 × 8 elements was reported for 28 GHz 5G applications. A pair of patch antennas were designed on both sides of the substrate, and orthogonal feed lines were used to feed the patch elements. From the presented configuration, the authors achieved a gain of 20 dBi with the isolation of 20 dB between adjacent antenna elements. Although the design offered high gain, the use of orthogonal feed lines increased back and side lobe radiations. In [6], a dual-polarized wideband aperture antenna array was presented for 5G MIMO applications. The presented array consisted of 4 antenna elements, and each element had a tapered aperture hybrid structure and the backed cavity design. It was reported that the isolation between antenna elements was ≈ 18 dB, which is usually high for MIMO applications. In [7], series-fed MIMO phased arrays were presented for 28 GHz 5G applications. They were the first planar antenna arrays that offered good characteristics at mm-wave frequencies with a compact size.
An eight-element phased array was presented in [8] for dual-band 5G MIMO applications. A leafshaped bow-tie antenna was designed on both sides of the substrate. The designed antenna provided a wideband response in the frequency range of 25-40 GHz. It was further described that the designed MIMO antenna array performed well and offered low mutual coupling between array elements. In [9], an eight-element patch array was presented for 28/38 GHz 5G communications. The presented array consisted of eight microstrip patch antennas placed in a rectangular pattern. The presented array design performed well in the reporting bands, but the isolation between antenna elements was very poor, which restricts its use for MIMO applications.
In [10], a 16-element linear array of inset-fed microstrip patch antenna was designed for 5G applications. To enhance the bandwidth, the authors utilized a parasitic patch on top of each insetfed patch. From the presented configuration, they demonstrated the bandwidth of 6.78 GHz starting from 24.35 to 31.13 GHz. The array elements were non-aligned to each other to achieve high isolation. However, the designed array was large, and it could not be easily integrated with portable devices. A coplanar waveguide (CPW)-fed linear antenna array was presented in [11] for 5G networks. The authors utilized a T-shaped radiator and the partial ground plane with defected ground structure (DGS) to achieve high gain, efficiency, and bandwidth. They also reported that the isolation between antenna elements was less than 22 dB, which tends to achieve high channel capacity. Furthermore, it was observed from the results that the directivity of the proposed antenna array got reduced in the broadside direction due to the use of DGS.
In [12], a tri-band 2×2 antenna array was presented for 5G applications. Two antenna elements were designed in the array, and one element was placed perpendicular to achieve better envelope correlation. Shuhrawardy et al. [13] designed a compact four-element wideband MIMO antenna for 5G services. The single antenna element consisted of a rectangular patch with an inverted-F-shaped slot, a chicken neckshaped feed line, and a slotted ground plane. To achieve low mutual coupling, an isolation network was designed between the ground planes, which complicated the design structure. In [14], a four-element MIMO antenna design was presented. The antenna elements were placed on each side of the substrate. Each element consisted of an array of a pair of elements. For improved radiation characteristics, DGS was used, and for enhanced isolation, the polarization diversity technique was employed between the radiators. From the above-presented designs, it has been observed that they were either large, consisted of complex structures, or did not offer better performance for 5G applications.
In this paper, an inset-fed planar antenna design with a four-element MIMO configuration is presented for 5G networks. It is described that the designed antenna offers dual-band characteristics in the mm-wave spectrum, and it has high gain and efficiency for the bands of interest. Furthermore, the designed MIMO antenna array has a compact size and provides good isolation between antenna elements. It is also described that the designed MIMO antenna offers almost equal or better characteristics than the designs presented in [5][6][7][8][9][10][11][12][13][14].

Antenna Geometry
The proposed dual-band inset-fed planar antenna design is depicted in Figs Table 1. Design parameters of the proposed inset-fed planar antenna.

Design Principle
Initially, an inset-fed planar antenna has been designed with a partial ground plane, shown in the inset of Fig. 3, to check its response for the proposed application, and its reflection coefficient, S 11 , is shown in Fig. 3. From the figure, it has been observed that the inset-fed planar antenna resonates at 38 GHz frequency band. After that, two open-circuit stubs are placed with the feed line, and it has been observed that when the length of stubs is taken nearly quarter wavelength (at 28 GHz), it offers inductive or capacitive impedance around the resonant frequency of the patch antenna and realizes dual-band response [15]. So, it is clear from the result of Fig   mentioning here that the inset-fed technique with open-circuit stubs plays an important role in achieving dual-band characteristics. Figure 5 shows the equivalent circuit of the proposed inset-fed planar antenna. One can model the insetfed patch by using the transmission line model because it is simple to implement [16]. The problem with the transmission line model is that it does not take into account different structure behaviors that exist in the configuration of an inset-fed patch antenna. So, in this paper, an equivalent circuit model is designed by utilizing the technique presented in [17] as shown in Fig. 5. The radiating stubs can be modeled by using a combination of series and parallel RLCs [18] as shown in Fig. 5.

Equivalent Circuit Model
A comparison between EM and circuit model simulations is shown in Fig. 6. From the figure, one can observe that S 11 of the proposed equivalent circuit model is in good agreement with the simulated data. The optimized equivalent circuit parameters are: R s1 = 5 kΩ, L s1 = 0.27 nH, C s1 = 0.26 pF,

Parametric Analysis
A parametric analysis of design parameters g, d, and W S is conducted to observe their effects on the antenna's performance, and the results are shown in Fig. 7. It is observed from Fig. 7(a) that when the value of g is increased from 0.2 mm to 1 mm, both the resonant frequencies shift towards the lower bands, which means that the change in g will increase the electrical dimensions of the antenna. It is a known fact that for inset-fed patch antenna design, the antenna's impedance mainly depends upon inset length. This effect can be seen from the result of Fig. 7(b) where a parametric study has been conducted by changing the inset length d. When the value of d is changed in the range of 0.5-2.5 mm, a change in the antenna's impedance is observed. For d = 0.5 mm and 1 mm, the antenna provides a wideband response in the frequency range of 32.45-38.41 GHz and 30.44-40.91 GHz, respectively, as shown in Fig. 7(b). For values > 1.5 mm, only one resonance is observed around 25 GHz, which means that the antenna's impedance matches well for 25 GHz frequency band.
The stub length also plays an important role to get desired resonant frequencies; therefore, a parametric study has also been conducted to observe the effect of W S on antenna's performance. From  Fig. 6, it is observed that the equivalent circuit model of stubs is more like a band-stop filter design, which means that the stubs will behave as a band-stop filter. From Fig. 7(c), it can be observed that for W S < 2 mm, no resonance occurs in the band of interest, while for W S > 2.5 mm, a minor shift in the resonant frequencies is observed.

Results and Discussion
The results of the dual-band inset-fed planar antenna are described in the following subsections. Particular features such as reflection coefficient, gain, efficiency, and radiation characteristics are described to evaluate its performance.

Realized Gain and Efficiency
The results of the realized gain of the proposed antenna are depicted in Fig. 9(a). while the measured gain values are noted to be 5.25 dBi and 5.90 dBi. Fig. 9(b) illustrates simulated and measured radiation efficiencies of the proposed antenna. From the figure, it has been observed that an average efficiency (simulated) of 88% is observed for both the bands. On the other hand, the average measured radiation efficiency for both bands is 84%.

Radiation Characteristics
The radiation characteristics of the proposed inset-fed planar antenna for E-plane (φ = 0 • ) and H-plane

Array Geometry
The next step is to design a four-element-based MIMO antenna array for 5G-enabled communication devices. The four antenna elements are designed on a single substrate by keeping in mind the conventional array design methodology. According to the theory of conventional array design, the minimum distance between antenna elements should be equal to or greater than λ/2 to reduce coupling effects. Therefore, a gap about 0.5 mm is created between antenna elements, and each element's width is increased from 10 to 10.5 mm, which is also related to the gap between two antenna elements denoted as S. This design technique does not allow antenna elements to correlate and also improves isolation.

Results and Discussion
In the following subsections, the performance of the proposed MIMO antenna array is described.
The performance is investigated in terms of reflection coefficient, isolation between antenna elements, radiation characteristics, envelop correlation coefficient, diversity gain, and mean effective gain.

Transmission Coefficient
The simulated and measured transmission coefficient results of the proposed MIMO antenna array are shown in Fig. 14. A good agreement is observed between the simulated and measured transmission coefficients. One can also observe from the results of Fig. 14 that the isolation between two adjacent antenna elements is less than 25 dB, which ensures low mutual coupling between array elements. The reason behind reduced isolation is that each array element has an independent ground plane, which does not allow the current to move from one element to another. This effect can also be observed in Fig. 15, where the surface current distribution of the proposed MIMO antenna array is presented. The plot of Fig. 15 is extracted by exciting only antenna 1 of the MIMO array. These results also validate the high performance of the proposed dual-band MIMO antenna array.

Radiation Characteristics
The simulated and measured radiation characteristics of the proposed MIMO antenna array for E-(φ = 0 • ) and H-planes (φ = 90 • ) for resonant frequencies 28.44 GHz and 39.49 GHz are depicted in Figs. 16(a)-(d). The antenna is tested in an anechoic chamber using a standard procedure. A horn antenna is used as a reference antenna, and the proposed MIMO antenna array is placed on the other side. To measure the radiation pattern in both the planes, two different measurements are carried out where the proposed array is placed horizontally and vertically for φ = 0 • and 90 • , respectively. For 28.44 GHz, in the case of E-plane, omnidirectional radiation characteristics are observed (shown in Fig. 16(a), while for H-plane, directional radiation characteristics are observed as shown in Fig. 16(b). For 39.49 GHz (Fig. 16(c) and 16(d)), the antenna demonstrates an approximately directional radiation pattern in E-plane and nearly omnidirectional pattern in H-plane. The cross-polarization component is less at some angles. However, for some angles, the level of cross-polarization is high, shown in Figs. 16(a)-(d), which is due to the excitation of hybrid modes at higher frequencies [19]. Moreover, the thickness of the substrate is less than the quarter wavelength therefore, and a 180 • phase difference is introduced between the reflected surface waves and the waves that impinge on the substrate. Due to this phenomenon, destructive interference will be produced, which results in the increment of the cross-polarized field [19,20].

Envelope Correlation and Diversity Gain
The envelope correlation coefficient (ECC) is an important factor to evaluate the MIMO system's performance. It measures the degree of similarity among the received signals. In an ideal scenario, the MIMO system requires an ECC equal to zero. The correlation between the radiation patterns of MIMO elements can also be evaluated using ECC. So, ECC can be calculated by using the radiation characteristics of the MIMO antenna as [21] where E i and E j represent far-field radiation characteristics for ports i and j.
For the proposed array configuration, the value of ECC is approximately equal to zero for the desired frequency bands as shown in Fig. 17. This result also demonstrates good isolation between antenna elements, which is an important factor for simultaneous operation. Diversity gain (DG) is another important parameter to assess the performance of MIMO antennas. It can be calculated as [22]: Figure 18 shows the diversity gain of the proposed MIMO antenna array. It is observed from the figure that the proposed MIMO antenna has DG > 9.96 dB for both bands.

Mean Effective Gain
In a fading environment, mean effective gain (MEG) is the measure of the amount of power received by the antenna elements compared to an isotropic antenna. One can compute the MEG as [22]:  The MEG 1 and MEG 2 can be written as [22]: Also, MEG 3 and MEG 4 can also be evaluated using Equation (3). For our proposed MIMO configuration, the value of MEG 1 and MEG 2 is less than −3 dB, which is also clear from the result of Fig. 19. It can  be observed from the figure that the difference between MEG 1 and MEG 2 is less than 3 dB. A comparison between the proposed and previously reported MIMO antenna arrays is presented in Table 2. The comparison is done in terms of array size, substrate used, dielectric constant of the substrate, isolation between antenna elements, gain, and radiation efficiency of the single antenna element. It is observed from the data of Table 2 that the proposed MIMO antenna array offers good characteristics with compact size.

CONCLUSION
An inset-fed dual-band planar antenna design has been presented for 5G applications along with MIMO configuration. The proposed antenna design consists of an inset-fed rectangular patch element, open circuit stubs, and a partial ground plane. It is shown that the inset-fed configuration with open-circuit stubs provides dual-band characteristics. It is also shown that the proposed antenna offers average realized gain and radiation efficiency of 5.5 dBi and 84% for both the bands, respectively. Furthermore, a four-element antenna array is designed to demonstrate the performance of the proposed antenna for 5G MIMO applications. The results of the MIMO configuration demonstrate low mutual coupling and good isolation characteristics between adjacent antenna elements. From the presented results, it is noted that the proposed antenna is suitable for 5G MIMO applications.