Introduction

5G and millimeter-wave communication have recently become among the most discussed technologies. This growing interest is driven by advancements in various modern technologies, not limited to 5G alone. 5G, in particular, promises to meet end-user demands for high data rates and low latency1,2. To achieve these high data rates, conventional narrowband spectrums are no longer sufficient; therefore, the wide bandwidth available in the millimeter-wave (mm-wave) spectrum is being explored. The limited availability of microwave bands, coupled with spectrum overcrowding due to numerous existing technologies, has led to growing interest in mm-wave frequencies ranging from 30 to 300 GHz3,4. These higher-frequency bands are capable of delivering data rates in the multi-gigabit-per-second (Gbps) range. Among these frequencies, the 28 GHz band has gained significant attention from researchers and has emerged as one of the most attractive portions of the 5G spectrum. This is primarily due to its ability to maintain signal integrity in harsh environments, as it offers relatively low atmospheric attenuation5,6. Atmospheric attenuation is a critical factor at higher frequencies, as it significantly weakens the signal as it travels from transmitter to receiver in free space.

The growing importance of the 5G spectrum also influences system requirements for modern devices, as new technologies demand updated systems capable of operating over wider bandwidths with higher output performance7,8. The shift toward millimeter-wave communication necessitates compact components that can be easily integrated into device systems. As a result, emerging technologies and state-of-the-art devices have redefined the requirements for wireless components, including filters, reconfigurable surfaces, and especially antennas, which serve as the backbone of efficient wireless communication9,10,11. Antennas operating at 28 GHz, due to the need for miniaturization, must have simplified geometries to reduce fabrication complexity and measurement setup tolerance. Moreover, antennas designed for millimeter-wave applications must exhibit key characteristics such as compactness, high gain, and stable performance across the wide operational bandwidth. These antennas are well-suited for short-range, non-line-of-sight (NLoS) applications, and they can also support advanced technologies like IoT and smart devices that rely on line-of-sight communication12.

Several techniques have been explored by researchers to improve antenna performance in terms of bandwidth, gain, and diversity, thereby enhancing their potential for applications such as vehicular communication, remote sensing, and object detection13,14,15. Among these, the most common technique involves the use of metasurfaces to enhance both the gain and bandwidth of the antenna system16,17,18. For example, in16, a truncated corner along with a rectangular slot patch is initially employed to improve the antenna’s bandwidth, followed by the integration of a partially reflecting surface to enhance gain characteristics. Additionally, in17, a slit-loaded circular patch is used in combination with a pair of vias to increase bandwidth. A metasurface is then applied to achieve high gain. However, both of these designs share a common drawback: the presence of an air gap between the antenna and the metasurface. This gap is not only critical for performance but also challenging to maintain during fabrication and integration. In contrast18, investigates the use of a metasurface without any air gap. While this approach eliminates the alignment issue, the presence of the metasurface still increases the overall profile of the antenna, which limits its suitability for mid-sized and compact electronic devices.

Parasitic patches have also been used to improve antenna performance; however, they tend to increase structural complexity and make fabrication more challenging19,20. Moreover, such designs often suffer from narrow bandwidth19 or only moderate gain values20. An array antenna, on the other hand, consists of multiple individual antenna elements arranged in specific configurations to function collectively as a single unit21. Despite their advantages, many mm-wave array designs exhibit high backward radiation or operate with narrow bandwidth, and most are limited by their large physical size22. In addition to the previously discussed performance requirements, several other key parameters are essential for the seamless operation of 5G communication devices. These include high capacity, reliability, low latency, high data rates, improved coverage, efficiency, and spatial diversity. These requirements are effectively addressed by multiple-input multiple-output (MIMO) antenna systems23,24.

For 5G applications, in addition to the millimeter-wave band and the 28 GHz spectrum, numerous antenna designs have also been proposed for the lower band, commonly referred to as the sub-6 GHz spectrum. The sub-6 GHz band offers broader coverage, improved reliability, and cost-effectiveness25. Its ability to effectively penetrate obstacles ensures stable indoor and urban connectivity. This band also facilitates a smooth transition from 4G to 5G, providing a balance between low latency and high capacity for practical wireless applications26. In27, a MIMO array antenna is proposed for mobile terminal applications. The antenna features a simple structure and has overall dimensions of 150 mm × 75 mm × 0.8 mm. In28, a three-element array antenna with a four-port MIMO configuration is introduced. The antenna is self-decoupled and achieves an isolation of less than − 20 dB.

However, recent trends indicate that to achieve overall high-performance parameters, researchers have increasingly designed MIMO antenna arrays that combine multiple techniques to overcome the limitations of earlier approaches29,30,31,32,33. In29, a wideband millimeter-wave antenna is presented, offering high isolation and gain. A two-element array is constructed with a strip-line parasitic patch to enhance isolation between the antenna elements. A self-decoupled two-port MIMO array antenna operating at 28 GHz is reported in30. This design features a 2 × 12 element MIMO array with a compact structure and simple layout. It offers a bandwidth of 27.5–28.35 GHz and a gain greater than 13 dBi. In31, an orthogonally placed two-port MIMO antenna with four array elements is proposed. This antenna is notable for its self-decoupling feature, high gain, and acceptable isolation levels. A semi-ground plane MIMO antenna is introduced in32, where the ground plane is partially removed to enhance performance in terms of bandwidth, gain, and isolation. In addition to the use of defected ground structures (DGS) and parasitic patches, metasurfaces are also employed to further improve antenna characteristics. In33, a MIMO array antenna integrated with a metamaterial sheet is presented. While this design significantly improves isolation and overall performance, it also increases the structural complexity and fabrication cost.

In the literature, numerous MIMO antenna arrays have been designed for the millimeter-wave spectrum, particularly operating around 28 GHz. However, many of these designs face limitations in terms of size, geometry, narrow operational bandwidth, or low gain. Several reported designs also exhibit structural complexity due to the incorporation of metasurfaces, defected ground structures (DGS), and electromagnetic bandgap (EBG) structures to enhance performance.

To address these limitations, this paper proposes a self-isolated MIMO array antenna specifically designed for 28 GHz applications. The proposed design achieves acceptable performance without the need for additional structures or layers. The present research introduces an antenna that provides high gain and wide bandwidth while maintaining a simplified geometrical structure and compact size. The array configuration is achieved by arranging multiple instances of the single antenna element to optimize overall performance.

To meet the demands of future wireless technologies, a MIMO configuration of the antenna array is developed, which demonstrates wideband performance, high gain, and favorable MIMO parameter outcomes. This paper is organized into five sections: Sect. “Introduction” provides the introduction; Sect. “Design of compact millimeter wave antenna” discusses the design of the antenna operating at 28 GHz; Sect. “Design of compact millimeter wave array antenna” presents the array configuration and corresponding results; Sect. “Potential of proposed antenna for MIMO and Massive-MIMO applications” details the MIMO configuration along with a performance comparison with existing literature; and finally, Sect. “Conclusion” concludes the study, followed by references.

Fig. 1
figure 1

Geometrical Layout of Proposed antenna operating at 5G Spectrum (a) top-view (b) side-view.

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Design of compact millimeter wave antenna

The geometrical layout of the proposed antenna is depicted in Figure. 1. It can be analyzed from a given figure that antenna contains rectangular patch loaded with two semicircular stubs. Proposed work has a full ground plane which is placed at the back side of antenna. The antenna is excited by single coaxial feeding technique by using SMA connector. Proposed design offers overall size of L × W × H = 16 mm × 40 mm × 1.52 mm. The radiative part and ground of antenna are made of copper thickness of 0.035 mm. The copper patch is placed over a well-known substrate material Rogers TMM 4 which has relative permittivity = 4.5 and Loss tangent = 0.002. The simulation and analysis of design is carried out by using EM software tool High Frequency Structure Simulator (HFSSv9). The optimized parameters of antenna are given below: L = 16, W = 40, H = 1.52, LP = 3, R = 3, t = 2, LS = 9, WP = 5 (Unit = millimeter).

Figure. 2 represents the design stages of proposed millimeter wave antenna and its impact on s-parameter. The antenna final geometry with desired operational band is obtained after following three main designing stages. In the initial step, a coaxial feed rectangular patch antenna is designed by using standard equations to calculate length (LP) and width (WP) of patch, with central frequency of 25 GHz. The designed antenna offers 25.5 GHz resonate with return loss of − 17 dB. To improve the bandwidth and return loss of antenna, the electrical length of antenna should be increased. Due to this fact, in the second step two strips are loaded to antenna as given in Figure. 2(a). The step results shift in frequency towards 26 GHz along with improvement in return loss. In last stage, The semi-circular stubs are loaded at the top and bottom. This step further improves the electrical length of antenna due to which the impedance matching of antenna is improved. The resultant antenna operates at 25.5 GHz, covering wide band of 24.5–27 GHz, as depicted in Figure. 2 (b).

Fig. 2
figure 2

Antenna designing mechanism (a) Steps (b) S11 response.

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Fig. 3
figure 3

Measured and simulated (a) S-parameter (b) Gain along with radiation efficiency.

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The single element of millimeter wave antenna is further analyzed by S-parameter, gain, efficiency and radiation pattern. S-parameter of antenna given in Figure. 3(a) which shows that antenna offers wideband of 24.5–27 GHz with resonance frequency of 25.25 GHz. The antenna offers minimum return loss value of − 28 dB at 25.25 GHz. The antenna operates over high gain of > 7.5 dBi at operational region of 24.5–27 GHz with peak value of 8 dBi at 26 GHz. Moreover, the figure also shows the radiation efficiency of antenna. It can be seen from given figure that antenna offers > 80% radiation efficiency through operational region of 24.5–27 GHz, which maximum value of 92% at resonance frequency of 25.5 GHz. To verify the simulated results, a hardware prototype of antenna is fabricated and tested. The hardware prototype of antenna is given in Figure. 3 along with measurement procedure. The measured and simulated results show good agreement with each other as per the provided figure. The outcomes offered by proposed antenna in terms of wideband, high gain and radiation efficiency make proposed antenna good applicant for upcoming 5G devices.

Fig. 4
figure 4

Comparison among simulated and measured radiation patterns of unit element antenna.

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Figure. 4 shows the radiation pattern of proposed millimeter wave antenna operating over 5G band. The radiation pattern of antenna is analyzed at resonate frequency of 25.5 GHz, at both E and H plane. It can be noted from figure that antenna offers stable radiation pattern, which is directive and have minimum back and side lobes. The minor distortion in the pattern is due to antenna geometry by loading semi-circular stub to improve bandwidth of antenna. Moreover, the measure results support the simulated outcomes, which improve the worth of the antenna. This result makes antenna a potential candidate for 5G devices. Figure. 5 depicts the fabricated prototype of proposed antenna and antenna array along with the antenna measurement setup utilized to study the far-field characteristics. The far-field parameters are measured using the Compact range electromagnetic anechoic chamber with frequency range up to 40 GHz by C&G Microwave company having the model number CATR-40R.

Fig. 5
figure 5

Hardware prototype (a) unit element (b) array antenna (c) measurement setup.

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Design of compact millimeter wave array antenna

Array antennas are widely used in radar, satellite communications, wireless communication (such as 5G and Wi-Fi), radio astronomy, and military applications due to their ability to steer beams electronically, enhance directivity, and suppress interference34. The number of elements, their spacing, and the phase shift between them all affect an array antenna’s radiation pattern. The combined radiation pattern is defined by the array factor, which is provided by35:

$$\:AF={\sum\:}_{n=M}^{M}\text{W}\text{n}{e}^{-j(kndcos\theta\:+\beta\:n)}$$
(1)

Where Wn is the amplitude of the nth element, k\(\:=\frac{2\pi\:}{\lambda\:}\) is the wave number, “d”​ is the position of the element, θ is the observation angle, and βn is the phase shift.

The geometrical structure of millimeter wave array antenna is given in Figure. 6. It can be seen that the array antenna is obtained by placing two more antenna elements at right and left side without any changing in parameters. The antenna has the same overall size as the single element of antenna (L × W × H = 16 mm × 40 mm × 1.52 mm) and designed on the same substrate material (Rogers TMM 4). To improve the radiation characteristics of antenna, an array antenna is constructed with three elements. The two additional elements of antenna are connected with main central element by using curved extended feedline. The central element is excited by using coaxial cable, while the other two elements get excitation current by these curved strips. The connected strip is designed curved to overcome the spacing between antenna array elements to reduce the overall size of antenna. The curved strips (feedline) have dimensions of A2 = 10 mm, which bent dimensions of A1 and A3 = 3 mm.

Fig. 6
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Geometrical structure of millimeter wave array antenna.

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Fig. 7
figure 7

Measured and simulated (a) S-parameter (b) Gain along with radiation efficiency.

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Measured and simulated S-parameter of proposed millimeter wave array antenna is given in Figure. 7 (a). It can be seen from figure that antenna offer wideband of 24.4–27.6 GHz. The bandwidth of antenna is improved and increased by 0.7 GHz as compared to single element. The antenna offers minimum return loss value of − 27.5 dB. Moreover, the gain and radiation efficiency of array antenna is also depicted in Figure. 7 (b). It is shown in figure that antenna offers gain > 8.5 dBi throughout operational region of 24.4–27.6 GHz, with maximum value of 11 dBi at resonance frequency of 25.2 GHz. Radiation efficiency of antenna is depicted in Figure. 7 (b). Proposed array antenna offers overall efficiency of > 88% throughout operational region with maximum value of 92% at 26 GHz. Furthermore, strong agreement can be noticed between measured and simulated results, which increase the worth of proposed work.

The parametric analysis of the key parameters is also carried out to understand the design methodology for both single element and array antenna. Firstly, for single element, the thickness of the stub plays a critical role in controlling the impedance bandwidth of the antenna, as shown in Fig. 8 (a). When the inner radius of the semicircular stub (R1) is decreased from the optimized value the bandwidth becomes narrow along with poor matching across coverage area. Contrary to that, when the inner radius is increased to 4 mm, the resonating frequency is shifted toward the higher frequency with improved matching at the cost of narrowband, as depicted in Figure. 8 (a). Thus, this validates the utilization of optimized value of 3 mm where a good combination of impedance matching and wideband is achieved.

Fig. 8
figure 8

Parametric analyses of (a) radius of stub (b) length of meandered stub.

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In case of an array antenna, the key parameter that plays a vital role in achieving a good impedance matching along with size miniaturization is the length of extended stub A3. When the size of A3 is reduced to 1 mm which is close to straight line feed among elements, the impedance matching becomes really poor which results in narrowband, as depicted in Figure. 8(b). On the other hand, if the value of 2 mm is selected the impedance matching improves, however, the bandwidth remains less as compared to antenna standalone. Thus, the optimized value of the 3 mm is selected where the antenna offers a good matching along with broad bandwidth, as depicted in Figure. 8(b).

Figure. 9 shows the radiation pattern of proposed array antenna designed for millimeter wave applications. It can be noticed from figure that antenna offers stable radiation patterns at both E and H plane. The radiation pattern given in figure is analyzed at resonance frequency of 25.2 GHz. The strong agreement between tested and predicated radiation patterns is noticed. The outcomes and tested result agreement makes proposed array antenna strong applicant for upcoming millimeter wave 5G devices.

Fig. 9
figure 9

Measured and simulated radiation pattern of array antenna.

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Potential of proposed antenna for MIMO and Massive-MIMO applications

In millimeter-wave communication, MIMO antennas are essential and play a vital role, especially in 5G and 6G. Because of the substantial path loss at millimeter wave frequencies, MIMO improves coverage and reliability by strengthening signals through beamforming and spatial multiplexing36. By effectively using spectrum resources, massive MIMO at mmwave allows for high data speeds, higher capacity, and low-latency communication. In order to enable ultra-fast wireless connectivity for applications such as augmented reality, high-speed internet, and future wireless networks, it is crucial to overcome propagation problems21,37.

The millimeter wave antenna design discussed in Sect. 2 and the array antenna design discussed in Sect. 3 further progressed to obtain the MIMO antenna. The study is carried out to investigate the potential of the proposed array antenna for the 5G MIMO and massive MIMO applications. Massive MIMO antenna is constructed and studied due to its vital role in future 5G technology. It has numerous applications in forthcoming devices used for connectivity in IoT and smart cities due to enhancing data rates, improving network reliability, supporting high mobility applications, and offering low latency rates38.

Fig. 10
figure 10

Structure of millimeter wave array MIMO antenna.

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For said purpose, the four elements of the MIMO antenna are designed by placing them along opposite edges. The overall size of the MIMO antenna is ML × MW × H = 40 mm × 85 mm × 1.52 mm. The four-port MIMO configuration of the antenna is constructed on the same substrate material (Roger 6002) used for single and array elements. The size of the antenna is increased, after placing 4 array elements to make a massive MIMO antenna. The gap between two elements, the adjacent element, and the opposite element is given by G1 = 10 mm, G2 = 18.1 mm, and G3 = 5.3 mm. The MIMO configuration of an antenna is self-decoupled and contains a full ground plane made of copper with a thickness of 0.035 mm. The geometrical structure of the four-port MIMO array antenna is given in Figure. 10.

  1. A.

    Transmission and Reflection Coefficient

In Figure. 11, the reflection and transmission coefficient of the proposed four-port MIMO array antenna is given. It can be noticed from the figure that the proposed 4-Port MIMO antenna design offers a wideband of 24.4–27.9 GHz with a resonant frequency of 25.3 GHz. The proposed work offers a minimum return loss value of − 29 dB at 25.3 GHz, as given in Figure. 11 (a). The S-parameter of the proposed MIMO antenna is studied for each element, which shows good agreement and similarity. A minor distortion is noted which is due to the placement of the MIMO antenna and also the effect of connectors.

Fig. 11
figure 11

MIMO antenna performance (a) S-parameters (b) Current distribution at 26 GHz.

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The transmission coefficient of the antenna is depicted in Figure. 11 (a). As the given figure shows the antenna offers isolation of less than − 27 dB at the operational region between all possible cases. The minimum value of isolation is obtained without placing any decoupling structure, which reduces the complexity of the design and provides free space for other circuit elements. The isolation between adjacent and diagonal antenna elements is given in the depicted figure. In the case of two adjacent elements, the minimum isolation at the operational region is − 28 dB at 25.3 GHz, while the maximum isolation value is observed around − 35 dB at 25.3 GHz between two diagonal antenna elements. The isolation is minimal in diagonal elements as they are placed far away from each other, while the isolation is higher in adjacent elements due to near placement. Overall, MIMO antenna isolation is under acceptable range.

The current surface graph of the proposed MIMO array antenna is provided in Figure. 11 (b). It can be seen that when one port is excited the current induced in the other elements remain very low, almost zero in case of diagonal elements and the value is less than 1 A/m for side-by-side elements. This current distribution graph further provides the insight of low mutual coupling without utilizing any additional decoupling structure.

  1. B.

    Gain and Radiation Efficiency

Figure 12 shows the proposed MIMO array antenna’s gain vs. frequency curve. It is evident that the suggested MIMO antenna array provides gain greater than 10 dBi within the 24.4–27.9 GHz operating range. At resonant frequencies of 25.3 GHz and 27 GHz, respectively, the highest value gain of 11.85 dBi and 11.7 dBi is observed. The suggested MIMO antenna provides a minimum gain of 9.8 dBi at 26 GHz and 8.5 dBi at 28.5 GHz in the operational range. This work can be regarded as a good contender for 5G devices working over high data rates because of the high value of gain provided by the suggested MIMO antenna.

Fig. 12
figure 12

MIMO antenna gain along with radiation efficiency.

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The radiation efficiency of the suggested MIMO array antenna is also displayed in the figures. The given figure shows that the antenna’s radiation efficiency is greater than 95% in the operational zone, reaching a high of 98% at 26.5 GHz. At 25.25 GHz, the lowest radiation efficiency rating is 90%. The suggested antenna is effective and advantageous for upcoming small 5G devices because of the excellent radiation efficiency results.

  1. C.

    Radiation Pattern

Figure 13 shows the radiation pattern of the suggested MIMO array antenna intended for millimeter wave applications. The given graphic shown that the antenna provides steady radiation patterns at the E and H planes. The figure’s radiation pattern is examined at 25.3 GHz, which is the resonance frequency. With the primary beam orientated towards θ = 0°, the suggested 4-port MIMO antenna array provides a broadside radiation pattern in the H-plane (ϕ = 90°) in concept. However, a broadside radiation pattern is obtained in the E-plane (ϕ = 0°) in principle. Due to multiple stubs loading and the impact of other MIMO elements on the targeted element, a slight distortion in the radiation pattern is observed.

The outcomes obtained from a single antenna element, array antenna, and four-port MIMO antenna are given in Table 1. The antenna single element offers a wideband of 24.5–27 GHz, which is further increased in the case of array antenna 24.4–27.6 GHz and MIMO array antenna 24.4–27.9 GHz. Similarly gain and radiation efficiency of the antenna also increased, as the antenna single offers 8 dBi, while the array and MIMO array antennae offer 11 dBi and 11.2 dBi, respectively. Same case for radiation efficiency. The single element of the antenna offers 92% radiation efficiency while the array antenna offers 95%. The MIMO antenna offers a maximum radiation efficiency of 98%. The size of the antenna remains the same in the case of a single antenna element and array antenna, while in the case of MIMO, the size of the antenna increased due to the placement of four elements.

Fig. 13
figure 13

Measured and simulated radiation pattern of array MIMO antenna.

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Table 1 Comparison between various antenna types and outcomes.
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  1. D.

    MIMO Parameters

The MIMO parameters of the suggested MIMO array antenna are displayed in Fig. 14 in terms of the envelope correlation coefficient (ECC), mean effective gain (MEG), channel capacity loss (CCL), and diversity gain (DG). The equations that are utilized to extract the parameters are also given in Fig. 14, the detailed discussion on these equations can be found in39.

  • The Envelope Correlation Coefficient (ECC) is a key metric in MIMO antenna systems that evaluates the correlation between antenna elements based on their radiation patterns and impedance characteristics. It determines how independently the antenna elements operate in a multipath environment, directly impacting the system’s diversity gain and overall performance. A lower ECC value signifies better isolation between elements, reducing interference and improving spatial multiplexing efficiency.

For optimal MIMO operation, ECC values should ideally be close to zero, ensuring minimal correlation between antenna elements. As shown in Fig. 14(a), the proposed MIMO antenna achieves an ECC value of less than 0.001, which is well below the acceptable threshold. This extremely low ECC value indicates that the antenna elements are highly isolated, allowing for efficient spatial diversity and enhanced system capacity. The strong isolation ensures reduced signal degradation, making the proposed MIMO antenna suitable for high-performance wireless communication applications.

  • Mean Effective Gain (MEG) is a crucial parameter in MIMO antenna systems that determines the ratio of received power to incident power in a multipath environment. It provides insight into how an antenna element receives signals compared to the total available power in its surroundings. MEG plays a significant role in evaluating the diversity performance of MIMO systems, ensuring that each antenna element contributes effectively to overall system efficiency. Ideally, for optimal operation, the MEG value should range between − 3 dB and − 12 dB. A value outside this range may indicate an imbalance in signal reception, leading to performance degradation.

As illustrated in Fig. 14(b), the proposed MIMO antenna achieves an MEG of approximately − 6 dB across the entire operational frequency range. This value falls within the acceptable limits, indicating that the antenna elements receive power efficiently while maintaining a good balance between diversity gain and signal strength. The stable MEG performance suggests that the antenna is well-optimized for MIMO applications, ensuring reliable communication with minimal signal fading and enhanced overall system capacity.

Fig. 14
figure 14

Antenna MIMO parameters (a) Envelop correction co-efficient (b) Mean effective gain (c) Channel capacity loss (d) Diversity gain.

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  • Channel Capacity Loss (CCL) is a critical parameter in MIMO antenna systems that quantify the loss in channel capacity due to mutual coupling and correlation between antenna elements. It directly affects the efficiency of MIMO communication by indicating how much capacity is reduced due to imperfections in the antenna design. Lower CCL values imply better MIMO performance, as they ensure minimal degradation in data transmission efficiency.

For an efficient MIMO system, the acceptable CCL value should be below 0.5 bits/s/Hz. As shown in Fig. 14(c), the proposed 4-port MIMO array antenna achieves a CCL of approximately 0.15 bits/s/Hz, which is well within the acceptable range. This indicates that the antenna system experiences minimal capacity loss, ensuring efficient spatial multiplexing and high data throughput. The low CCL value confirms that the antenna design is optimized for MIMO applications, providing reliable performance with reduced interference and enhanced communication efficiency.

  • Diversity Gain (DG) is a crucial performance parameter in MIMO antenna systems that quantifies the improvement in signal reliability by leveraging multiple antenna elements. It represents the reduction in transmission power needed to achieve a specific signal quality in a multipath environment. A higher DG value indicates better diversity performance, leading to improved signal reception and reduced fading effects.

Ideally, a MIMO antenna should provide a DG of 10 dB to ensure optimal diversity performance. However, in practical scenarios, a slightly lower value is still acceptable. As depicted in Fig. 14(d), the proposed MIMO array antenna achieves a DG of approximately 9.99 dB, which is very close to the ideal value. This indicates that the antenna system effectively mitigates signal fading and enhances communication reliability. The near-ideal DG value confirms the efficiency of the proposed design in ensuring stable performance, making it suitable for high-reliability MIMO applications. Thus, the strong performance of the proposed antenna and strong comparison among simulated and measured radiation pattern make it a potential candidate for wireless communication systems and heterogeneous applications like GNSS and v2X40,41,42.

Table 2 Comparison between proposed antenna and recent published work.
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  1. E.

    Comparison with Literature work

In Table 2, a comparison between the proposed antenna and recently published work is provided. The comparison is based on antenna size, operational bandwidth, number of ports used, gain, minimum isolation, ECC, and CCL. It can be seen that29 has a compact size and offers a wide bandwidth; however, it also exhibits a lower isolation value (–15 dB) compared to the proposed design. Similarly, the proposed antenna provides a wider bandwidth compared to30,31,32,33. Although30,31 offer high gain, they operate over a narrow bandwidth. Furthermore, the structural complexity of the referenced designs is also examined, and it is observed that the proposed antenna has low structural complexity, whereas most of the existing works exhibit moderate to high complexity. This comparison highlights that the proposed antenna is compact, operates over a wide bandwidth, and offers high gain and low isolation, along with low ECC and CCL. These advantages further enhance the value of the proposed four-port MIMO antenna array, making it a suitable candidate for future 5G devices.

Conclusion

A four-port MIMO antenna array is designed and investigated in this paper. Each element of the MIMO antenna comprises of three element arrays. Initially, a unit element consisting of rectangular patch loaded with two semi-circular stubs was designed to achieve broadband behavior in millimeter-wave band-spectrum allocated for 5G applications. Moreover, stable radiation patterns and high gain are achieved throughout the band of interest. This antenna is further utilized to design a three-element array which significantly improves the radiation characteristics. While the array is extended to form a four-pot MIMO antenna that has the potential to be used for pattern diverse applications require high channel capacity. A fabricated prototype of the unit element and array antenna is utilized to verify the simulated results, which yields results strong comparison and stable performance by covering the band spectrum of 24.4–27.9 GHz with a peak gain of 11.5 dBi. While the MIMO antenna also offers peak isolation of 48 dB having low value of ECC and CCL of 0.16 with in operational bandwidth. Additionally, a comparison is made with published work which signifies the potential of proposed work for present- and modern-day applications requiring compact size antenna having high gain and broadside radiation pattern. Thus, making the proposed work a strong candidate for 5G devices operating in mm-wave band spectrum.