4-Port MIMO Antenna with Defected Ground Structure for 5G Millimeter Wave Applications

We present a 4-port Multiple-Input-Multiple-Output (MIMO) antenna array operating in the mm-wave band for 5G applications. An identical two-element array excited by the feed network based on a T-junction power combiner/divider is introduced in the reported paper. The array elements are rectangular-shaped slotted patch antennas, while the ground plane is made defected with rectangular, circular, and a zigzag-shaped slotted structure to enhance the radiation characteristics of the antenna. To validate the performance, the MIMO structure is fabricated and measured. The simulated and measured results are in good coherence. The proposed structure can operate in a 25.5–29.6 GHz frequency band supporting the impending mm-wave 5G applications. Moreover, the peak gain attained for the operating frequency band is 8.3 dBi. Additionally, to obtain high isolation between antenna elements, the polarization diversity is employed between the adjacent radiators, resulting in a low Envelope Correlation Coefficient (ECC). Other MIMO performance metrics such as the Channel Capacity Loss (CCL), Mean Effective Gain (MEG), and Diversity gain (DG) of the proposed structure are analyzed, and the results indicate the suitability of the design as a potential contender for imminent mm-wave 5G MIMO applications.


Introduction
In the modern era, the eminent increase of wireless devices, inadequate bandwidth, and limited channel capacity have substantially promoted efforts to develop advanced standards for communication networks. Subsequently, this has promoted the development of next-generation (5G) communication systems at the mm-wave spectrum featuring much greater channel capacity and higher data rates [1,2]. The forthcoming 5G technology not only provides greatly increased reliability, high data rate requirements, and low power consumption to meet the massive increase in linked devices, but also promises to increase the prospects of emerging technologies such as virtual reality and smart cities [3][4][5]. However, critical limitations at the mm-wave spectrum, such as signal fading, atmospheric absorptions, and path loss attenuations need to be resolved, which becomes more significant with approaching 10 dB is obtained for the exhibited antenna. Furthermore, ECC, DG, channel capacity, and TARC are also inspected.
The design of a 4-element antenna array with MIMO capabilities at mm-wave 5G frequency bands is demonstrated in this paper. The proposed design is a high gain and wideband antenna with good MIMO characteristics for future 5th generation devices, such as smartwatches and mobile WiFi, etc. The proposed MIMO antenna with a compact and simple geometry facilitates its assimilation into 5G smart devices. The good MIMO performance of reported antenna endorses the appropriateness of the design for future 5G wireless communication applications.

Proposed Antenna Design
This work proposes a 4 port MIMO antenna system with overall substrate dimensions of 30 × 35 × 0.76 mm 3 , as shown in Figure 1. The antenna is modeled and simulated in a commercially available EM simulator CST microwave studio suite.
Electronics 2020, 9, x FOR PEER REVIEW 3 of 14 dielectric resonator antenna (DRA) is suggested in reference [35]. The reported antenna covers the 27. 19-28.48 GHz band for 5G applications. A gain value approaching 10 dB is obtained for the exhibited antenna. Furthermore, ECC, DG, channel capacity, and TARC are also inspected. The design of a 4-element antenna array with MIMO capabilities at mm-wave 5G frequency bands is demonstrated in this paper. The proposed design is a high gain and wideband antenna with good MIMO characteristics for future 5th generation devices, such as smartwatches and mobile WiFi, etc. The proposed MIMO antenna with a compact and simple geometry facilitates its assimilation into 5G smart devices. The good MIMO performance of reported antenna endorses the appropriateness of the design for future 5G wireless communication applications.

Proposed Antenna Design
This work proposes a 4 port MIMO antenna system with overall substrate dimensions of 30 × 35 × 0.76 mm 3 , as shown in Figure 1. The antenna is modeled and simulated in a commercially available EM simulator CST microwave studio suite. The reported antenna system comprises of four MIMO elements placed on the center of each edge on the top layer, as shown in Figure 1a. Figure 1b depicts that the layer at the bottom side is composed of a Defected Ground Structure (DGS) with rectangular, circular, and zigzag-shaped slots to further enhance the performance of the proposed design. The design is integrated on the Rogers R04350B substrate with a thickness and permittivity (εr) of 0.76 mm and 3.66, respectively. The dimensional details of the design are provided in Table 1, and the progression of the design from a single element to MIMO configuration is comprehensively discussed in the subsequent sections.

Single Element Antenna
At first, a single element of the patch antenna is designed as shown in Figure 2a. The primary antenna structure resonating at 28 GHz is obtained by following the well-established mathematical equations provided in references [1-4). The optimized single element antenna consists of an inverted C-shaped patch enclosing a rectangular-shaped slit.
(1) The reported antenna system comprises of four MIMO elements placed on the center of each edge on the top layer, as shown in Figure 1a. Figure 1b depicts that the layer at the bottom side is composed of a Defected Ground Structure (DGS) with rectangular, circular, and zigzag-shaped slots to further enhance the performance of the proposed design. The design is integrated on the Rogers R04350B substrate with a thickness and permittivity (εr) of 0.76 mm and 3.66, respectively. The dimensional details of the design are provided in Table 1, and the progression of the design from a single element to MIMO configuration is comprehensively discussed in the subsequent sections.

Single Element Antenna
At first, a single element of the patch antenna is designed as shown in Figure 2a. The primary antenna structure resonating at 28 GHz is obtained by following the well-established mathematical equations provided in references [1][2][3][4]. The optimized single element antenna consists of an inverted C-shaped patch enclosing a rectangular-shaped slit. (1) 421h 264 where W p and L p are the patch's width and length, h is the height of the substrate, ε e f f and ε relative are the effective permittivity and relative permittivity of substrate respectively. c, f c, , and ∆L are the speed of light, central frequency, and the effective length, respectively.

Two Element Antenna Array
The design is processed further from a single element to the two-element array, as shown in Figure 2b. The parallel feed network is proposed as the array's excitation mechanism. The main feed is matched at 50 Ω impedance while the impedance of the branched network is matched at 100 Ω. Afterward, a rectangular and two symmetrically placed circular slots are incorporated in the bottom layer in order to further optimize the obtained results, as illustrated in Figure 2c. Consequently, an enhanced bandwidth and gain are achieved by the reported antenna array. Both elements in the array antenna are separated by λ, which is approximately 11 mm at 28 GHz. Hence a compact array structure with proved performance is achieved.
where Wp and Lp are the patch's width and length, h is the height of the substrate, and are the effective permittivity and relative permittivity of substrate respectively. c, fc,, and ∆L are the speed of light, central frequency, and the effective length, respectively.

Two Element Antenna Array
The design is processed further from a single element to the two-element array, as shown in Figure 2b. The parallel feed network is proposed as the array's excitation mechanism. The main feed is matched at 50 Ω impedance while the impedance of the branched network is matched at 100 Ω. Afterward, a rectangular and two symmetrically placed circular slots are incorporated in the bottom layer in order to further optimize the obtained results, as illustrated in Figure 2c. Consequently, an enhanced bandwidth and gain are achieved by the reported antenna array. Both elements in the array antenna are separated by λ, which is approximately 11 mm at 28 GHz. Hence a compact array structure with proved performance is achieved.

MIMO Configuration
After the attainment of the two-element array, the design is progressed further, and the 4-port MIMO antenna system is obtained. Each MIMO element consists of an antenna array obtained previously in this work and is placed at the center positions of the board sides, as shown in Figure  1a. The overall dimensions of the board are 30 × 35 mm 2 . The MIMO antenna configuration thus obtained exhibits acceptable performance, but to further improve the performance and to reduce mutual coupling among the MIMO antennas, a zigzag-shaped DGS is integrated, as illustrated in Figure 1b. As a result, the isolation for the MIMO configuration is increased.

MIMO Configuration
After the attainment of the two-element array, the design is progressed further, and the 4-port MIMO antenna system is obtained. Each MIMO element consists of an antenna array obtained previously in this work and is placed at the center positions of the board sides, as shown in Figure 1a. The overall dimensions of the board are 30 × 35 mm 2 . The MIMO antenna configuration thus obtained exhibits acceptable performance, but to further improve the performance and to reduce mutual coupling among the MIMO antennas, a zigzag-shaped DGS is integrated, as illustrated in Figure 1b. As a result, the isolation for the MIMO configuration is increased.

Scattering Parameters
The working principle and radiation characteristics of the reported antenna system are analyzed. Figure 3a shows the analysis of the reflection coefficient curves of the design from a single element to MIMO configuration. It is observed that the single element of the proposed antenna is resonating in the mm-wave frequency spectrum ranging from 26.8-29.6 GHz with a 2.8 GHz bandwidth. The apparent increase in bandwidth is noticed when the antenna is developed from a single element to two-element arrays. The bandwidth obtained for the antenna array is 3.5 GHz covering the 26.2-29.7 GHz frequency band. Moreover, the reflection coefficient curve of the MIMO Ant.1 in Figure 3a demonstrates that the frequency band covered now ranges from 26.1-29.78 GHz with a slight increase in bandwidth to 3.68 GHz. Furthermore, Figure 3b depicts the reflection coefficient curves for the MIMO Ant1-Ant4 with and without DGS. It is observed that the four MIMO antennas cover nearly the same band. In addition, the bandwidth of the MIMO antennas has improved after the incorporation of DGS. The frequency band now covered by the MIMO antenna system is 26.1-30 GHz with a 3.9 GHz bandwidth.

Scattering Parameters
The working principle and radiation characteristics of the reported antenna system are analyzed. Figure 3a shows the analysis of the reflection coefficient curves of the design from a single element to MIMO configuration. It is observed that the single element of the proposed antenna is resonating in the mm-wave frequency spectrum ranging from 26.8-29.6 GHz with a 2.8 GHz bandwidth. The apparent increase in bandwidth is noticed when the antenna is developed from a single element to two-element arrays. The bandwidth obtained for the antenna array is 3.5 GHz covering the 26.2-29.7 GHz frequency band. Moreover, the reflection coefficient curve of the MIMO Ant.1 in Figure 3a demonstrates that the frequency band covered now ranges from 26.1-29.78 GHz with a slight increase in bandwidth to 3.68 GHz. Furthermore, Figure 3b depicts the reflection coefficient curves for the MIMO Ant1-Ant4 with and without DGS. It is observed that the four MIMO antennas cover nearly the same band. In addition, the bandwidth of the MIMO antennas has improved after the incorporation of DGS. The frequency band now covered by the MIMO antenna system is 26.1-30 GHz with a 3.9 GHz bandwidth.   It is observed that isolation between antenna 1 and antenna 2 is low. In addition, similar behavior is observed between Ant 3 and Ant 4. Meanwhile, significant isolation is obtained for the antenna pairs 1 and 3, 1and 4, 2 and 3, as well as 2 and 4. A zig-zag shaped DGS is incorporated at the bottom layer of the MIMO antenna structure to reduce mutual coupling effects. Figure 4b validates the isolation enhancement between the MIMO antennas after the assimilation of the DGS. Hence, the minimum isolation obtained for the suggested MIMO antennas is below −10 dB.  It is observed that isolation between antenna 1 and antenna 2 is low. In addition, similar behavior is observed between Ant 3 and Ant 4. Meanwhile, significant isolation is obtained for the antenna pairs 1 and 3, 1and 4, 2 and 3, as well as 2 and 4. A zig-zag shaped DGS is incorporated at the bottom layer of the MIMO antenna structure to reduce mutual coupling effects. Figure 4b validates the isolation enhancement between the MIMO antennas after the assimilation of the DGS. Hence, the minimum isolation obtained for the suggested MIMO antennas is below −10 dB.

Surface Current Distribution
The radiating mechanism of the reported MIMO antenna system was analyzed further by investigating the surface current density. This focused on investigating the antenna parts that are influencing the radiation characteristics and elucidating the amount of coupling between different MIMO antennas. Figure 5 shows the surface current distribution when port 3 is activated at 28 GHz. The current flow is mainly concentrated around the feedline and along the edges of the inverted Cshaped antenna. Moreover, the circular and rectangular slots in the ground exhibit significant current distribution. This determines the contribution of DGS in radiation behavior. In addition, the concentration of the coupling current between MIMO antennas is insignificant due to the DGS, as demonstrated in Figure 5.

Experimental Results
The reported MIMO antenna system was fabricated on a Rogers RO4350B substrate using the photolithography process, and measurements were performed in order to endorse the antenna capabilities for practical utilization. The fabricated prototype and AUT i.e., antenna under test in the anechoic chamber, is shown in Figure 6. The detailed discussion and comparative analysis of measured results are provided in the subsequent section.

Surface Current Distribution
The radiating mechanism of the reported MIMO antenna system was analyzed further by investigating the surface current density. This focused on investigating the antenna parts that are influencing the radiation characteristics and elucidating the amount of coupling between different MIMO antennas. Figure 5 shows the surface current distribution when port 3 is activated at 28 GHz. The current flow is mainly concentrated around the feedline and along the edges of the inverted C-shaped antenna. Moreover, the circular and rectangular slots in the ground exhibit significant current distribution. This determines the contribution of DGS in radiation behavior. In addition, the concentration of the coupling current between MIMO antennas is insignificant due to the DGS, as demonstrated in Figure 5.

Surface Current Distribution
The radiating mechanism of the reported MIMO antenna system was analyzed further by investigating the surface current density. This focused on investigating the antenna parts that are influencing the radiation characteristics and elucidating the amount of coupling between different MIMO antennas. Figure 5 shows the surface current distribution when port 3 is activated at 28 GHz. The current flow is mainly concentrated around the feedline and along the edges of the inverted Cshaped antenna. Moreover, the circular and rectangular slots in the ground exhibit significant current distribution. This determines the contribution of DGS in radiation behavior. In addition, the concentration of the coupling current between MIMO antennas is insignificant due to the DGS, as demonstrated in Figure 5.

Experimental Results
The reported MIMO antenna system was fabricated on a Rogers RO4350B substrate using the photolithography process, and measurements were performed in order to endorse the antenna capabilities for practical utilization. The fabricated prototype and AUT i.e., antenna under test in the anechoic chamber, is shown in Figure 6. The detailed discussion and comparative analysis of measured results are provided in the subsequent section.

Experimental Results
The reported MIMO antenna system was fabricated on a Rogers RO4350B substrate using the photolithography process, and measurements were performed in order to endorse the antenna capabilities for practical utilization. The fabricated prototype and AUT i.e., antenna under test in the anechoic chamber, is shown in Figure 6. The detailed discussion and comparative analysis of measured results are provided in the subsequent section.

Scattering Parameters
The Scattering parameters of the proposed prototype are measured using the Rohde & Schwarz ZVA 40 VNA. Figure 7a,b illustrate the simulated and measured scattering parameter curves for the reported MIMO antenna. It is observed from the reflection coefficient curve in Figure 7a that MIMO Ant1 is covering the 25.5-29.6 GHz, frequency band. Likewise, the other MIMO antennas exhibit nearly similar reflection coefficient curves with slight shift in bands. The maximum measured bandwidth thus achieved for the proposed antenna is 4.1 GHz. The transmission coefficient analysis is shown in Figure 7b. The minimum measured isolation obtained is −17 dB between the Ant3 and Ant4. Simulated and measured results are exhibiting good coherence. However, insignificant differences are due to fabrication losses or unavoidable use of coaxial cables during the measurement [36,37]. Hence, the obtained measured results possess the suitability of the reported MIMO antenna for future mm-wave 5G applications.

Scattering Parameters
The Scattering parameters of the proposed prototype are measured using the Rohde & Schwarz ZVA 40 VNA. Figure 7a,b illustrate the simulated and measured scattering parameter curves for the reported MIMO antenna. It is observed from the reflection coefficient curve in Figure 7a that MIMO Ant1 is covering the 25.5-29.6 GHz, frequency band. Likewise, the other MIMO antennas exhibit nearly similar reflection coefficient curves with slight shift in bands. The maximum measured bandwidth thus achieved for the proposed antenna is 4.1 GHz. The transmission coefficient analysis is shown in Figure 7b. The minimum measured isolation obtained is −17 dB between the Ant3 and Ant4.

Scattering Parameters
The Scattering parameters of the proposed prototype are measured using the Rohde & Schwarz ZVA 40 VNA. Figure 7a,b illustrate the simulated and measured scattering parameter curves for the reported MIMO antenna. It is observed from the reflection coefficient curve in Figure 7a that MIMO Ant1 is covering the 25.5-29.6 GHz, frequency band. Likewise, the other MIMO antennas exhibit nearly similar reflection coefficient curves with slight shift in bands. The maximum measured bandwidth thus achieved for the proposed antenna is 4.1 GHz. The transmission coefficient analysis is shown in Figure 7b. The minimum measured isolation obtained is −17 dB between the Ant3 and Ant4. Simulated and measured results are exhibiting good coherence. However, insignificant differences are due to fabrication losses or unavoidable use of coaxial cables during the measurement [36,37]. Hence, the obtained measured results possess the suitability of the reported MIMO antenna for future mm-wave 5G applications. Simulated and measured results are exhibiting good coherence. However, insignificant differences are due to fabrication losses or unavoidable use of coaxial cables during the measurement [36,37]. Hence, the obtained measured results possess the suitability of the reported MIMO antenna for future mm-wave 5G applications.

Radiation Patterns
To understand the radiational behavior of the proposed design, the 2D radiation patterns of antennas were measured using the commercial ORBIT/FR far-field measurement system in an anechoic chamber, as shown in Figure 6b. The far-field measurements were performed in the xz and yz planes with theta range of −90 • to 90 • . The horn antenna with standard gain of 24 dBi was used for signal transmission. In Figure 8 the simulated and measured 2-D radiation patterns are shown at 27.5 and 28 GHz for Ant1. In the xz plane, the maximum radiation is observed at −35 • while in the yz plane, the main beam is directed at −25 • . The antenna exhibits overall good performance for both simulated and measured data. However, inconsistencies are observed between simulated and measured results due to fabrication errors and unavoidable cable losses. Also, these sorts of measurement systems are not the most appropriate ones for measuring small antennas, especially in mm wave frequency range, and the effect of the measurement system could affect the results, which in fact creates a discrepancy between simulated and measured results.
Electronics 2020, 9, x FOR PEER REVIEW 8 of 14 To understand the radiational behavior of the proposed design, the 2D radiation patterns of antennas were measured using the commercial ORBIT/FR far-field measurement system in an anechoic chamber, as shown in Figure 6b. The far-field measurements were performed in the xz and yz planes with theta range of −90° to 90°. The horn antenna with standard gain of 24 dBi was used for signal transmission. In Figure 8 the simulated and measured 2-D radiation patterns are shown at 27.5 and 28 GHz for Ant1. In the xz plane, the maximum radiation is observed at −35° while in the yz plane, the main beam is directed at −25°. The antenna exhibits overall good performance for both simulated and measured data. However, inconsistencies are observed between simulated and measured results due to fabrication errors and unavoidable cable losses. Also, these sorts of measurement systems are not the most appropriate ones for measuring small antennas, especially in mm wave frequency range, and the effect of the measurement system could affect the results, which in fact creates a discrepancy between simulated and measured results.

Gain and Percentage Efficiency
The proposed design demonstrates a simulated peak gain value of 8.45 dB while peak measured gain is 8.3 dBi for Ant.1. Similarly, the peak antenna efficiency is about 82% as shown in Table 2. Moreover, the antenna exhibited a nearly stable gain with 3dB gain bandwidth from 26-29.97 GHz.

Gain and Percentage Efficiency
The proposed design demonstrates a simulated peak gain value of 8.45 dB while peak measured gain is 8.3 dBi for Ant.1. Similarly, the peak antenna efficiency is about 82% as shown in Table 2. Moreover, the antenna exhibited a nearly stable gain with 3dB gain bandwidth from 26-29.97 GHz.

MIMO Performance Parameters
To ensure the proposed antenna's multi-channel performance was high, the key performance metrics such as ECC, DG, CCL, and MEG were analyzed. Detailed discussion of the parameters is provided below.

Envelope Correlation Coefficient (ECC)
ECC is one of the key performance parameters of MIMO systems, and it is calculated using Equation (5) [10]. Figure 9a shows the ECC curve of the proposed antenna over frequency, relatively larger values of ECC are shown between antennas 1 and 2 as well as 3 and 4. The overall antenna module ensures that there are correlation values below the practical standard of 0.5.

Diversity Gain (DG)
Diversity gain demonstrates "the loss in transmission power when diversity schemes are performed on the module" for the MIMO configuration. The diversity gain is calculated by using Equation (6) given in reference [10]. Figure 9b describes the DG to be approximately 10 dB throughout the band, which ensures good diversity performance of the antenna. DG = 10 1 − ρ eij 2 (6) 28. 5 8.22 8.1 85 82

MIMO Performance Parameters
To ensure the proposed antenna's multi-channel performance was high, the key performance metrics such as ECC, DG, CCL, and MEG were analyzed. Detailed discussion of the parameters is provided below.

Envelope Correlation Coefficient (ECC)
ECC is one of the key performance parameters of MIMO systems, and it is calculated using Equation (5) [10]. Figure 9a shows the ECC curve of the proposed antenna over frequency, relatively larger values of ECC are shown between antennas 1 and 2 as well as 3 and 4. The overall antenna module ensures that there are correlation values below the practical standard of 0.5.

Diversity Gain (DG)
Diversity gain demonstrates "the loss in transmission power when diversity schemes are performed on the module" for the MIMO configuration. The diversity gain is calculated by using Equation (6) given in reference [10]. Figure 9b describes the DG to be approximately 10 dB throughout the band, which ensures good diversity performance of the antenna.

Channel Capacity Loss (CLL)
CCL was enlisted among the MIMO performance parameters, thereby providing details of channels capacity losses of the system during the correlation effect. The CCL is calculated numerically by Equations (7)-(10). Figure 10 illustrates that for the proposed MIMO antenna, the obtained CCL is less than the practical standard of 0.4 bit/s/Hz [38] for the entire operating band, which ensures the proposed system's high throughput

Channel Capacity Loss (CLL)
CCL was enlisted among the MIMO performance parameters, thereby providing details of channels capacity losses of the system during the correlation effect. The CCL is calculated numerically by Equations (7)-(10). Figure 10 illustrates that for the proposed MIMO antenna, the obtained CCL is less than the practical standard of 0.4 bit/s/Hz [38] for the entire operating band, which ensures the proposed system's high throughput where a is the correlation matrix, a = σ 11 σ 12 σ 21 σ 22 (8) σ ij = − S ii * S ij + S ji S jj * (10) where a is the correlation matrix,

Mean Effective Gain(MEG)
For diversity, performance analysis mean effective gain is an important parameter and is defined as the mean received power in the fading environment. The mean effective gain is calculated using Equation (11) provided below, and the numerically estimated values are tabulated in Table 3.
In this equation, K is the number of antennas, i represents antenna under observation, and μ is the radiation efficiency. For good diversity performance, the practical standard followed is that MEG should be −3 ≤ ( ) < −12, which is therefore validated for the obtained MEG values of all MIMO antennas of the proposed design.

Mean Effective Gain(MEG)
For diversity, performance analysis mean effective gain is an important parameter and is defined as the mean received power in the fading environment. The mean effective gain is calculated using Equation (11) provided below, and the numerically estimated values are tabulated in Table 3.
MEG i = 0.5 µ irad = 0.5 1 − K j=1 S ij (11) In this equation, K is the number of antennas, i represents antenna under observation, and µ irad is the radiation efficiency. For good diversity performance, the practical standard followed is that MEG should be −3 ≤ MEG (dB) < −12, which is therefore validated for the obtained MEG values of all MIMO antennas of the proposed design. Table 3. The mean effective gain of the reported antenna.

Comparison with Related Work
A comparison with related works reported in literature is tabulated in Table 4. The comparative analysis with other works shows that the proposed design demonstrates better performance in terms of compactness, bandwidth and gain. Moreover, MIMO performance analysis is provided in detail for the proposed design, and it is observed that the MIMO antenna proposed in this work exhibits