Millimeter-Wave Dual-Band MIMO Antennas for 5G Wireless Applications

This paper suggests a dual-band 28/38 GHz four-element MIMO array based on dual-mode planar monopole antennas for 5G wireless applications. The design structure contains four planar monopole antennas; located at the corners on a 20 × 20-mm2 size Rogers RO4003 substrate with a dielectric constant of 3.55. The proposed planar monopole antenna has the shape of a crescent. In order to achieve the desired behavior and perfor mance, we engraved two rectangular slots on both sides and also added a notch at the bottom. In addition, we used a partial ground plane to enhance the isolation. Significant isolation (> − 23 dB) is achieved between antenna elements by employing spatial and polarization diversity techniques. To validate the design concept, a prototype of the four-element MIMO array is designed, fabricated, and measured. The experimental results show that the proposed antenna can cover the 27.25–29 GHz and 34.5–41 GHz bands with good isolation and high efficiency. Furthermore, the radiation pattern, the realized gain, and the channel capacity are also studied. According to the reached results, the proposed MIMO antenna may be a suitable application-oriented design for 5G MIMO applications at the millimeter-wave range.


Introduction
The capacity of mobile devices to transfer data at extremely fast speeds is one of the fundamental requirements for the next generation of mobile communications. An antenna for a mobile phone should be easy to understand and have a small profile [1,2]. The antenna of a handheld device must support broadband operation, a high data rate, and low power consumption in order to comply with the requirements of long-term evolution (LTE) and fifth generation (5G) of mobile communications [3,4]. This suggests that in order to handle the necessary data rates for future applications, a mobile device antenna should be able to operate in the millimeter-wave region of the electromagnetic spectrum.
A mobile device antenna should also be able to operate well at several frequency bands in the millimeter-wave spectrum due to size restrictions in order to support the applications of the upcoming mobile generations [5,6]. In a variety of circumstances, the antenna structure may be intricate, with layers with high electromagnetic absorbance to produce radiation patterns of the desired shape, or it may have a three-dimensional shape to operate effectively in a single-frequency band and to serve a specific purpose in special applications [7,8]. However, because of the size and weight restrictions, planar antennas with simple structures, multi-band operations, and omnidirectional emission patterns are preferred for mobile phones [9,10].
There are numerous difficulties in designing a MIMO antenna system that can function in mobile phones for the upcoming generations of mobile communications. Due to a lack of space, a handset's MIMO antenna needs first be reduced in size. Despite the little area that can be used for the antenna on a mobile device, the second problem is to provide high isolation between the antenna parts. The MIMO antenna's ability to operate at several frequencies while providing adequate bandwidth at each frequency presents the third challenge (multiple-band operation). Producing the requisite form of the radiation patterns over the operational frequency bands is the fourth challenge [11][12][13][14].
Additionally, a MIMO antenna may be required to offer a variety of diversity, including spatial, polarization, and pattern diversity. The provision of low envelops correlation coefficient (ECC), and high diversity gain (DG) is one of the most significant problems in MIMO antenna design for mobile devices [15,16].
There are now many antennas available that are made to be used with fifth generation mobile phones. In [17][18][19][20][21], several planar single-band millimeter-wave antennas are presented. Four-band MIMO antenna system operating at 28, 43, 52, and 57 GHz is proposed in the work of [28]. Dual-band printed millimeter-wave antennas are described in [23][24][25][26][27][28][29]. Dual bands 28/38 GHz MIMO antenna with two elements, isolation greater than 22 dB, and peak gain about 5.2 dBi are introduced in [23]. [24] describes the introduction of a dual-band 27/39 GHz MIMO antenna with two components, isolation greater than 25 dB, and peak gain close to 5 dBi. A dual-band 28/38 GHz slot MIMO antenna with two elements and isolation higher than 27 dB is introduced [25]. In [26], a two-port MIMO antenna operating at 28/38 GHz is examined. [27] discusses a dual-band four-port antenna that operates at 28/38 GHz with isolation of about 20 dB and peak gain more than 7.58 dBi. Four-port dual-band 28/38 GHz antenna with peak gain more than 7.9 dBi is achieved in [28]. Tri-Band MIMO antenna system operating at 28, 38, and 575 GHz is proposed in the work of [30].
This research presents a dual-band 28/38 GHz four-element MIMO array for 5G wireless applications based on dual-mode planar monopole antennas. Four planar monopole antennas are part of the design structure; they are positioned at the corners of a 20 × 20-mm 2 Rogers RO4003 substrate with a dielectric constant of 3.55. The envisioned planar monopole antenna has a crescent-shaped design. We etched two rectangular slots on both sides and also added a notch at the bottom to get the desired performance. To improve the isolation, we also used a partial ground plane. Spatial and polarization diversity techniques are used to produce significant isolation (> 23 dB) between antenna parts. The antenna design makes use of industrystandard CST software.
The portions of this paper are as follows: The design of the single element antenna to obtain the desired 28/38 GHz frequency bands is first covered in Section. 2. Second, the MIMO antenna design processes are discussed in Section. 3. Third, the results of the MIMO tests (S-parameters and radiation characteristics) are extracted and given in Section. 4 together with its diversity parameters, including the envelop correlation coefficient (ECC), diversity gain (DG), and channel capacity loss (CCL). In Section. 5, the paper's conclusion is offered after a summary of the antenna performance. Figure 1 depicts the evolution of the recommended antenna. The Rogers RO4003 substrate, which has a thickness and dielectric constant of 0.203 mm and 3.55, was used to design the antenna. As a starting point, the traditional patch antenna (antenna 1) has the shape of a crescent with the dimensions Dc et Lc, as illustrated in Fig. 1a, and is intended to operate in fundamental mode at a frequency of about 29.2 GHz (see blue curve in Fig. 2). Subsequently, as shown in Fig. 1b (antenna 2), we engraved two rectangular slots on both sides with sizes Ls, Ws and Ln, Wn respectively. These slots allow both the improvement of the bandwidth and the appearance of a second band that is not well suited (see green curve in Fig. 2).

Procedures for Single-Antenna Design
By adding a notch in the crescent that has the size We and Le, Antenna 3 in Fig. 1c is recommended as the antenna to create the intended 28/38 GHz frequency bands based on the prior design processes (see red curve in Fig. 2).
It should be noted that the proposed antenna is fed by a microstrip line La × Wa and the ground plane is partially used of length Lg as shown in Fig. 1d.
The dimensions of the patch are given in Table 1.
The results of the simulations are shown in Fig. 2.
As a result, the suggested antenna configuration in Fig. 3a is built, and Fig. 3b shows a prototype photo of it along with a launcher connector. The suggested single-element dual-band antenna's frequency is used to simulate and evaluate the S11 results, which are shown in Fig. 4. The achieved frequency bands are 27.25 to   29 GHz in the first band and 34.5 to 41 GHz in the second, and the simulated results are tested using the vector network analyzer. The tested findings, however, were only successful in the first band's frequency range of 27.25 to 28.9 GHz and the second band's frequency range of 34.5 to 40.5 GHz. The manufacture and measurement tolerance, which cannot be eliminated, is the cause of the slight difference between the two values.

MIMO Antenna
Following the completion of the constituting antenna element, the design is evolved to produce the dual-band MIMO antenna. As shown in Fig. 5, the four antennas are arranged orthogonally with a spacing of "d" units. This orthogonal arrangement would allow for polarization diversity and could be used to reduce mutual coupling between different antenna components without the use of any complex decoupling structure. The total dimension of the board is L MIMO × W MIMO which is equal to 20 mm × 20 mm. Figures 6 and 7 depict the effect of antenna element separation (d) on antenna performance. When the separation (d) is increased from 3 to 5 mm, the isolation between elements improves. As a result, the optimized distance (d) equals 4 mm to achieve the desired performance. Figure 8 depicts the simulated surface current distributions for the proposed MIMO antenna at f = 28 GHz and f = 38 GHz. When port 1 is radiated and ports 2, 3, and 4 are connected to 50, the results are extracted. At 28 GHz, current density is collected around the crescent horns, etched slots, and notch, indicating that all of these elements are radiation sources, whereas at 38 GHz, current density is collected around both slots and the notch. Second, the current is concentrated around the operated port, with only a small amount of current flowing to the other ports, confirming the ports' high isolation.

Results and Discussion
To validate the proposed antenna's characteristics, a prototype was built and experimentally measured, as shown in Fig. 9. Standard PCB technologies were used in the fabrication process. Finally, as detailed in the following sections, an investigation of measured and simulated results was conducted, taking into account various performance metrics such as reflection coefficient, transmission coefficient, broadside gain, and radiation patterns.   Figure 11 depicts the total measured and simulated antenna efficiencies and realized antenna gains. It is worth noting that we have chosen the performances of Ant 1 and Ant 2; this is the most critical case due to their close proximity. The measured total antenna efficiency and antenna gain agree well with the simulation, as shown in Fig. 11. The total measured antenna efficiency on the two operating bands is greater than 80%, and the measured and simulated antenna gains is 9.7 dBi in band 28 Ghz and is 11.5 dBi in band 38 Ghz. The above measured results confirm that the proposed 4-element MIMO antenna has both good isolation and good total antenna efficiency. Figure 12 depicts the simulated and measured radiation pattern results (E-plane and H-plane) at 28 GHz and 38 GHz when the antenna is excited at port 1 and terminated with a 50 load at port 2 to port 4. At the two frequency bands, the antenna has a semi-omnidirectional pattern, and there is a consistent trend between the simulated and tested results, with minor differences due to the measurement setup.

Diversity Performance
The diversity performance of the proposed MIMO antenna is investigated in this section using various performance metrics such as the envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity loss (CCL). ECC is a significant parameter for MIMO systems that determines antenna element independence in terms of their individual characteristics. Using the equations below, the ECC characteristics of each antenna element can be calculated from the complex measured and simulated results [12,13]: Figure 13 depicts the simulated and tested ECC results for the suggested two-port dual-band MIMO antenna. The ECC value is less than 0.0001 in the first band and less than 0.00001 in the second band with good matching between the simulated and measured results.
To evaluate the MIMO performance, the ECC and DG can be coupled by Eq. (2) [24]: The results of the simulated and measured DG are shown in Fig. 14 along with the suggested MIMO antenna's frequency. The DG value is approximately 9.99 in both bands, with a strong correlation between the two outcomes.
The design of MIMO antennas also requires the evaluation of the channel capacity loss (CCL). This parameter can be calculated as a function of the S-parameters [12,13] using the formulas below: where Figure 15 displays the results of the CCL simulated and measured at the frequency of the recommended MIMO antenna. Within the operating frequency bands, the CCL is equal to a value less than 0.4 bit/s/Hz. Table 2 compares the required task with other relevant recent publications in the literature. It is clear that the majority of the MIMO antenna designs described in [15][16][17][18][19][20][21][22][23][24] are larger than the intended antenna, which restricts their suitability for use with contemporary small communication devices. However, only a small 14 The simulated and measured DG results with a frequency of the suggested MIMO antenna number of designs have been able to achieve a compact structure with low gain.

Performance Comparison
Additionally, compared to the suggested MIMO antenna, the isolation obtained for these MIMO designs is poor. It is also important to note that, in contrast to this work, the majority of previous studies solely examined ECC and DG for MIMO performance. As a result, the antenna design suggested in this work outperforms existing designs and establishes its suitability for present and future communication systems. It has a relatively small size, high gain, MIMO capabilities, and good diversity performance.

Conclusıon
A miniaturized dual-frequency band (28/38 GHz) microstrip patch antenna for 5G mobile phones is provided in this work. The proposed antenna has been engineered to function in frequency ranges between 27 and 29 GHz and between 34.5 and 41 GHz, with isolation less than − 24 dB through the two operated bands. To confirm the antenna design and demonstrate the superior quality of the recommended two-element MIMO antenna, MIMO parameters such as ECC, DG, and CCL have been extracted from simulation and measurement data. The findings from the simulation and testing show a positive trend within the two operational bands, which points to the suggested structure being used for 5G communications.