Designs, developments, challenges, and fabrication materials for MIMO antennas with various 5G and 6G applications: a review

Abstract

The rapid expansion of digital media platforms and their growing user base in the wireless industry necessitate communication systems to provide information at high speeds with reliable connections. Therefore, wireless communication systems with a single antenna cannot accomplish these requirements. Consequently, the access and utilization of multi-input multi-output (MIMO) antennas are becoming more common in contemporary high-speed transmission systems. This article covers the fundamentals of MIMO antenna operation, the metrics for MIMO antenna performance parameters, and the design methodologies for specifying the three most commonly used antennas (two-port, quad-port, and eight-port). Additionally, it discusses their ability to improve channel capacity significantly. It focuses on designing MIMO antennas with ultra-wideband (UWB) for 5G systems operating between 1 and 27 GHz and millimeter-wave (mmWave) bands from 30 to 100 GHz. This article is valuable for researchers interested in developing MIMO antennas for diverse applications. It compiles advanced methods related to materials, advancements, challenges, and state-of-the-art technologies used in the design of high-performance MIMO antennas. We concluded that antennas that operate at mmWave frequencies have small dimensions and suffer from isolation problems in the MIMO formation. In contrast, antennas operating below 6 GHz are large and do not suffer from isolation problems.

Introduction

As technology has advanced to meet expectations, there have been requests for high-speed internet, high-definition video streaming, and fast data transfer rates [1, 2]. In addition, the widespread use of internet-based services has increased the demand for wireless communications systems with high rates of data and sufficient channel bandwidth. In most cases, single-input and single-output antennas cannot meet these demands [3]. Although these antennas are known as microstrips, they are extensively utilized and have several advantages, the most prominent of which are their low cost, suitable shapes, lightweight design, and flexibility via hybrid and monolith microwave circuits [4]. As a result, multi-input–multi-output (MIMO)-manufactured antennas, a new antenna approach, have become a viable choice for fast-speed technology for communication [5]. These antennas use coplanar shapes or strip lines to feed multiple radiating components individually to transmit and receive data [6, 7].

5G promises to enable large-scale events with thousands of users in smart cities, residences, healthcare, transportation, and infrastructure [8]. A 5G network utilizes low-, mid-, and high-frequency bands, enabling antennas to support multiple bands, handle wide bands, and adapt to various use cases for great coverage and connection [9, 10]. To support high data rates and accommodate large user counts, the deployment of multiband MIMO systems is expected. The fields of augmented reality, artificial intelligence, the Internet of Things, and three-dimensional media are examples of new technologies that are rapidly advancing communication. Because these technologies demand faster data rates, a rapid transition from 5G to 6G communications will be essential [11]. 6G wireless communication mostly uses the 0.1–10 THz frequencies [11]. The primary benefits that 6G offers to wireless manufacturing, healthcare, self-driving vehicles, intelligent cities, and renewable energy systems include larger capacity, more security, wider coverage, and very little latency [12].

In addition, these applications incorporate Wi-Fi (wireless fidelity), Bluetooth, global positioning system technology, wireless local area network, and other technologies to achieve a tiny multipurpose antenna [13]. The advent of 5G and 6G networks has sparked a wave of groundbreaking ideas put forth by researchers and rigorously evaluated [14]. As shown in Fig. 1, multiband MIMO permits the use of numerous frequency bands concurrently to cover the intended applications with decreased size and interference. These versatile antennas are perfect for wireless systems, offering a broader frequency range [15]. While working over many bands might raise the danger of interference across various frequency bands, needing careful design of the antenna and integration considerations, multiband antennas enable more effective use of the available spectrum of frequencies. The literature predominantly discusses wideband antennas and their effectiveness in 5G. However, there is a lack of characterization for multiband MIMO antennas in 5G and 6G [16]. Moreover, the 3rd Generation Partnership Project debuted 5G-Advanced in Release 18, laying the groundwork for its future evolution. Release 19 will concentrate on commercial deployment requirements and prepare for 6G [17].

Figure 1. Frequency bands assigned to 5G and 6G wireless technologies.

The review in this paper is a study that categorizes several MIMO antenna designs and their properties. This study mainly concentrates on MIMO antenna design strategies using particular methodologies to obtain the necessary antenna efficiency. Additionally, the comparison tables included in the paper will assist readers in putting the approaches presented for improved MIMO antenna performance into practice and modifying them as necessary. Moreover, this review can offer a better understanding of future research directions.

This article presents six sections, organized as follows: The “Introduction” section provides a general introduction to MIMO antennas. The “Basic parameters of MIMO antennas” section introduces the criteria and parameters that determine the performance and efficiency of multiport MIMO antennas. The “Design categories for MIMO antennas” section presents and discusses the latest work related to the designs of MIMO antennas for frequencies from 1 to 27 GHz and those based on millimeter-wave (mmWave) frequencies. The “Importance of equivalent circuits for antennas” section discusses the importance of equivalent circuits for antennas. The “Challenges, developments, and future directions discussion” section discusses challenges, trends, and future developments in the fabrication and design of multiport MIMO antennas. Finally, the “Conclusions” section presents conclusions and suggestions for future MIMO antenna fabrication challenges.

Basic parameters of MIMO antennas

In addition to the S-parameter and the radiation features, various diversity parameters are employed to verify the overall performance of a MIMO antenna. For real-world scenarios, MIMO antennas must adhere to predetermined diversity parameters. As a result, this section provides some fundamental diversity parameters for MIMO antennas.

Envelope correlation coefficient (${\rho _{ECC}}$)

The diversity measure that shows how the neighboring MIMO antenna components correlate is $({{{\varepsilon }}_r}$. It may be estimated using the S-parameters or radiation patterns. It’s essential to evaluate its value using the far-range radiation pattern, as ${\rho _{ECC}}$ explains the unique radiation patterns of different radiating parts in MIMO systems. Additionally, it is evident that the majority of planar antennas experience loss. It’s best to avoid using the S-parameters to calculate ${\rho _{ECC}}$. The allowable limit of envelope correlation coefficient (ECC) in a realistic situation must be less than 0.5. Equations (1) and (2) provide the formulas for utilizing information about the radiation pattern of a MIMO system. In contrast, Equation (3) provides the formula for ${\rho _{ECC}}$ utilizing information about the S-parameters [18, 19].

(1)\begin{align}{\rho _{ECC}} = \frac{{{{\left| a \right|}^2}}}{{a \times a}}\,\end{align}

(2)\begin{align}a = \int\limits_0^{2\Pi } \int \limits_0^\Pi \left( {{\varrho ^*}_{{{\theta }}p}{\varrho _{{{\theta }}q}}{P_{{\theta }}}{X_{{\text{PR}}}} + {\varrho ^*}_{\varphi p}{\varrho _{\varphi q}}{P_\varphi }} \right)d\Omega \end{align}

(3)\begin{align}{\rho _{ECC}} = \frac{{\left( {{{\left| {{S_{\left( {ii} \right)}}\,{S_{\left( {ij} \right)}} + {S_{\left( {ji} \right)}}\,{S_{\left( {jj} \right)}}} \right|}^2}} \right)}}{{\left( {1 - \left( {{{\left| {{S_{\left( {ii} \right)}}} \right|}^2} + {{\left| {{S_{\left( {ji} \right)}}} \right|}^2}} \right)} \right)\left( {1 - \left( {{{\left| {{S_{\left( {jj} \right)}}} \right|}^2} + {{\left| {{S_{\left( {ij} \right)}}} \right|}^2}} \right)} \right)}}\end{align}

Where ${X_{{\text{PR}}}}$ is the cross-polarization level, it can be a percentage of the mean power across the φ & θ directions. $S$ is the S-parameter (reflection coefficient) for each port at different frequencies, while $i$ and $j$ represent the number of ports in the MIMO array.

Diversity gain (DG) (G_Diversity)

In wireless networks, ${G_{Diversity}}$ represents the quality and dependability of MIMO antennas. As a result, the ${G_{Diversity}}$ for the MIMO antenna within the permitted frequency spectrum has to be high (10 dB). Equation (4) can be utilized to calculate the ${G_{Diversity}}\,$used the value of ${\rho _{ECC}}$ [20].

(4)\begin{align}{G_{Diversity}} = 10*\,\sqrt {1 - {{\left| {{\rho _{ECC}}} \right|}^2}} \end{align}

Channel capacity loss (CCL)

It denotes the volume of data that can be conveyed across a communication link, accounting for potential channel loss. A specified MIMO system’s predetermined channel capacity loss (CCL) value is 0.4 bits/s/Hz. Equation (5) provides the formula for CCL through S-parameters [21].

(5)\begin{align}& CCL =\nonumber\\ & \quad - {\log _2}\left[ det\left[ \begin{array}{@{}cc@{}} {1 - \left[ {{{\left| {{S_{11}}} \right|}^2} + {{\left| {{S_{12}}} \right|}^2}} \right]}& { - \left[ {{S^*}_{11}{S_{12}} + {S^*}_{21}{S_{12}}} \right]} \\ { - \left[ {{S^*}_{22}{S_{21}} + {S^*}_{12}{S_{21}}} \right]}& {1 - \left[ {{{\left| {{S_{22}}} \right|}^2} + {{\left| {{S_{21}}} \right|}^2}} \right]} \end{array} \right] \right]\end{align}

Average effective gain (AEG)

It is a crucial diversity parameter for MIMO antennas, indicating the additional power received compared to an isotropic antenna. The average effective gain (AEG) ratio has to be <3 dB for a MIMO antenna to function better at equal power output. Using Eq. (6), the evaluation value of AEG can be determined [21].

(6)\begin{align}AEG = \frac{{AE{G_i}}}{{AE{G_j}}} = \frac{{0.5 \times \left[ {1 - {{\left| {{S_{ii}}} \right|}^2} - {{\left| {{S_{ij}}} \right|}^2}} \right]}}{{0.5 \times \left[ {1 - {{\left| {{S_{ij}}} \right|}^2} - {{\left| {{S_{jj}}} \right|}^2}} \right]}}\end{align}

Overall active reflection coefficient (OARC)

The overall active reflection coefficient (OARC) of a MIMO system is calculated by dividing the total reflected power by the total incident power. The ratio of the total power incident upon the patch is compared to the total reflecting power resulting from the radiating components. Equation (7) provides the equation for the expanded OARC for multi-port MIMO antennas. If we want to calculate the OARC for two ports, it will be according to the formula shown in Eq. (8) [22].

(7)\begin{align}OARC = \frac{{\sqrt {\mathop \sum \nolimits_{i = 1}^N {{\left| {{b_i}} \right|}^2}} }}{{\sqrt {\mathop \sum \nolimits_{i = 1}^N {{\left| {{a_i}} \right|}^2}} }}\end{align}

(8)\begin{align}OARC = \frac{{\sqrt {{{\left| {{S_{11}} + {S_{12}}{e^{j{{\theta }}}}} \right|}^2} + {{\left| {{S_{21}} + {S_{22}}{e^{j{{\theta }}}}} \right|}^2}} }}{{\sqrt 2 }}\end{align}

Where ${a_i}$ and ${b_i}$ represent the coefficients of the scattering matrices, and is the number of ports in the MIMO configuration.

Design categories for MIMO antennas

In this section, there are three subsections. The first discusses contemporary designs of dual-port MIMO antennas based on the ultra-wideband (UWB) band, covering frequencies from 1 to 27 GHz, including the L, S, C, X, Ku, and K bands. In addition to those based on mmWave frequencies of 28 GHz and above, as shown in Fig. 2. The second subsection discusses recent studies of quad-port antenna designs. The third subsection presents and discusses proposed works for antenna designs of eight ports and above.

Figure 2. A schematic of multiband MIMO antenna designs.

Dual-port MIMO antenna designs for various applications

In a recent study [23], researchers proposed the design of a two-port compact antenna with its middle layer (substrate) made of RO5880, which has a thickness of 0.787 mm. The overall dimensions of this antenna are 52 × 28 $m{m^2}$, as shown in Figs. 3(a) and (b). This antenna is designed for operation in a resonant frequency range of 2.3–11.5 GHz, as shown in Figs. 4(a), (b) and (c), making it suitable for UWB applications. The suggested antenna shows significant isolation improvement, reaching up to −16 dB, as shown in Fig. 4(a), achieved using shared radiators with small rectangular slots. This feature reduces interference, boosts overall performance, and demonstrates its capabilities through a detailed analysis of MIMO performance factors in Table 1. A good range of results was identified. The simulations and measurements indicate that the antenna design is feasible and successful. Its broad bandwidth, small size, and improved isolation qualities make it an attractive option for UWB imaging with microwave systems of the future.

Figure 3. Antenna structural shapes on both sides: (a) simulation design; (b) realistic manufacturing design [23].

Figure 4. The reflection coefficient parameter of the antenna in simulation and realistic measurements. (a) The simulation side; (b) The simulation and practical side of S11; (c) The simulation and practical side of S22 [23].

Table 1. An overview of the latest research into the advancement of wideband two-port antennas

In addition, the authors presented a two-element MIMO antenna that operates at mmWave frequencies (25.2–29.5 GHz). This antenna is characterized by its rectangular outer shape, as shown in Figs. 5(a) and (b), with geometric dimensions (W = 50 mm, L = 12 mm, H = 0.787 mm). The material used for its substrate layer is Rogers 5880, which has a permittivity (${{{\varepsilon }}_r}\,$ = 2.2) and loss tangent (δ = 0.02). The authors clarified that the antenna resonates at 28.4 GHz and that the isolation ratio between the first and second ports is lower than −32 dB, as demonstrated in Fig. 6(a). They added that because the antenna offers acceptable performance parameters, it is appropriate for communications systems based on mmWave frequencies. For example, the gain rate reached 11.4 dBi, the ECC is <0.00025, and the diversity gain (DG) is >9.996 dB, as shown in Figs. 6(b) and (c), respectively [24].

Figure 5. The fabrication geometry of the proposed antenna is (a) front view and (b) back view [24].

Figure 6. The antenna performance curves for (a) reflection coefficient (S11 and S22) and isolation (S12 and S21), (b) ECC, and (c) DG [24].

Other scholars have recently published numerous studies on the design of two-port MIMO antennas, which are well detailed in Table 1. Table 1 showcases the comparative data of diverse antenna designs tailored for various purposes. Because mutual coupling is crucial in MIMO antennas, researchers in reference [25] found that MIMO elements have a notable isolation ratio of −65 dB. Although the remaining designs were satisfactory and produced similar results, one stood out above the others.

The remaining recent works have been compiled in Table 2 using the same methodology. Researchers present a dual-port MIMO antenna operating in mmWave bands. It has been discovered that a significant number of the designs utilized similar materials to fabricate the antennas. In addition, there is a discrepancy in the results: the researchers in reference [26] achieved good results for isolation ratio, efficiency, ECC, DG, CCL, and gain. While the other proposed designs also produced good results, they showed varied performance due to the different dimensions and geometries suggested by each author for the antenna design.

Table 2. A detailed comparison and summary of recent research papers introducing dual-port antennas in the mmWave bands

Four-port MIMO antenna designs for various applications

Recently, researchers presented in reference [66] a foldable MIMO antenna for smart apparel applications with UWB capability. The MIMO antenna operated in the 2.9–12 GHz band, as shown in Fig. 7, and consisted of four octagonal radiators with empty holes built into them, as shown in Figs. 8(a), (b) and (c). The dimensions and thickness of this antenna are 50 × 50 × 1.6 mm³. The antenna’s radiation and diversification performances are analyzed, and the metrics obtained include ECC < 0.045, DG > 9.9 dB, OARC < − 14 dB, and CCL < 0.13 bits/s/Hz. Additionally, the suggested antenna has a 20 mm bend radius, making it appropriate for applications in wearable devices. This antenna is suitable for applications involving patient monitoring.

Figure 7. The S-parameter curves versus the different frequencies [66].

Figure 8. (a) Prototype antenna with four ports on the front; (b) antenna bending model at 20 mm; and (c) antenna performance measurement using a vector network analyzer device [66].

In another recent work in reference [67], a compact UWB four-element MIMO antenna design with band rejection is presented, as shown in Figs. 9(a) and (b). The recommended antenna can function at 3–12 GHz and has S11 ≤ −10 dB thanks to the four components’ rectangle radiators with curving sides and partially grounded planes with an engraved slot. The antennas were arranged orthogonally without decoupling features, simplifying the engineering process and ensuring high isolation between the components. On a cheap FR4 substrate, the recommended design dimension and thickness are 47 × 47 × 1.6mm³. The antenna operates within the operational bands with a maximum gain of 4.8 dBi, as shown in Fig. 10(a). It has an ECC of less than 0.005, as shown in Fig. 10(b). The antenna has a DG of 9.98 dB, as shown in Fig. 10(c). The CCL of the antenna is less than 0.4 bit/s/Hz, as shown in Fig. 10(d). The results were consistently positive, which allowed the recommended antenna to be employed in UWB MIMO communications systems.

Figure 9. An antenna manufacturing prototype (a) on the front side and (b) on the back side [67].

Figure 10. The basic parameters to determine the efficiency of the proposed antenna in reference [67] are (a) gain curves, (b) ECC curves, (c) DG curves, and (d) CCL curves.

A recent research article [68] presented a quad-port broadband metamaterial (MM) antenna to achieve a high gain in new radio communications operating at sub-6 GHz, as shown in Figs. 11(a) and (b). The suggested four MIMO antennas are arranged orthogonally to the neighboring antennas. It achieves the compact size and properties of 55.2% bandwidth with a low interelement edge-to-edge length of 0.19 ${{-}\mkern-10mu\lambda _{min}}$ at 3.25 GHz. The intended MIMO system is implemented using an inexpensive FR-4 printed material, measuring just 0.65 ${{-}\mkern-10mu\lambda _{min}}$×0.65 ${{-}\mkern-10mu\lambda _{min}}$×0.14 ${{-}\mkern-10mu\lambda _{min}}$, as shown in Figs. 12(a), (b) and (c). A high peak output gain of about 7.1 dBi between −9 and −50 dB isolation is displayed by the developed miniature MIMO system with an MM reflector, as shown in Fig. 11(a). Furthermore, the proposed broad-spectrum MM increases MIMO’s various perspectives and radiation properties, with an average overall efficiency of 68% throughout the target bands. The specified antenna for the MIMO system has good multiplex efficiency, with a value of more than −1.4 dB, an acceptable CCL of less than 0.35b/s/Hz, an exemplary DG of 9.96 dB, and an exceptional ECC of less than 0.045. The values of the rest of the parameters that determine antenna performance are listed in Table 3. In the end, the researchers explained that the proposed antenna is a potential approach to the 5G system.

Figure 11. The return loss curves versus the various frequencies for (a) simulation side curves and (b) comparisons between simulation side curves and manufacturing measurements [68].

Figure 12. The design of the proposed antenna shapes (a) CST simulation, (b) practical design on the front side, and (c) practical design on the back side [68].

Table 3. A summary of recent studies on the design of a four-port antenna that operates at frequencies below 27 GHz

In another recent study [69], researchers proposed a four-port MIMO antenna with dimensions of 90 × 90 × 1.6 mm³, as shown in Figs. 13(a), (b), (c) and (d). The researchers explained that the antenna is proposed for wide-band communications systems. Based on the results achieved by the antenna, it was good in most parameters to determine performance and efficiency. It worked in a wide frequency range that reached 9.33 GHz because the antenna operates at frequencies from 2.67 to 12 GHz, as shown in the reflection coefficient (return losses) curves in Fig. 14(a). In addition, it achieves an isolation ratio between ports of less than −15 dB, as shown in Fig. 14(b), an ECC of less than 0.1, and a DG of 9.97 dB. While the gain and efficiency for the antenna reached 5 dBi and 75%, respectively, the researchers concluded that the antenna is well-suited for UWB applications.

Figure 13. Antenna designs for simulation and manufacturing: (a) simulation front side, (b) simulation back side, (c) fabrication front side, and (d) fabrication back side [69].

Figure 14. Curves of S-parameters versus different frequencies from 2 to 12 GHz (a) return loss curves and (b) isolation curves between ports [69].

In addition, in a recent article [70], academics proposed manufacturing a four-port MIMO antenna, as shown in Figs. 15(a) and (b). This antenna is made of copper for both the patch and ground layers. While its substrate layer is made of FR4, the design dimensions of the antenna are 50 × 20. This antenna operates at two resonant frequencies: 38 and 60 GHz, as shown in Fig. 16(a). The proposers confirmed that the antenna is suitable for 5G system applications. The antenna provides satisfactory results for the isolation ratio between the four ports, which reached −42 dB at 38 GHz and −47 dB at 60 GHz (Fig. 16(b)). Moreover, the value of ECC is <0.05, and the DG is >9.98. The gain values reach 6.5 at 38 GHz and 5.5 dBi at 60 GHz.

Figure 15. Practical aspects of the proposed antenna include (a) manufacturing the antenna and (b) measuring the antenna’s performance using an analysis device (Rohde & Schwarz) [70].

Figure 16. (a) S-parameter curves for the simulation and measurement sides, and (b) isolation curves between ports [70].

Other recent articles summarize the designs of quad-port (2 × 2) MIMO antennas based on frequencies below 27 GHz in Table 3, while designs based on mmWave bands are listed in Table 4. It has been found that antennas operating at sub-6 GHz frequencies have larger dimensions and do not experience isolation problems between the ports. In contrast, mmWave antennas are small in dimensions and suffer from isolation problems between antenna elements in MIMO configuration.

Table 4. A summary of the most recent research on quad-port antennas designed for mmWave frequency bands

Six and more ports MIMO antenna designs for various applications

In the recent manuscript [120], the researchers designed a multi-port antenna (six ports), as shown in Figs. 17(a) and (b). The researchers focused on providing this antenna to operate at mmWave frequencies, so the antenna worked in two bands, the first band from 27.7 to 28.1 GHz and the second band from 36.92 to 39.5 GHz, as shown in Figs. 18(a) and (b).

Figure 17. The structural manufacturing design of the proposed antenna for (a) the front face and (b) the back face [120].

Figure 18. Antenna performance measurement curves for (a) and (b) reflection coefficient and (c) transmission coefficient [120].

The researchers were able to obtain good results in light of the challenges in the field of antenna manufacturing. So, the antenna achieved satisfactory outputs for the isolation value that reached less than −20, as shown in Fig. 18(c). The highest gain value for the first band reached 13.3 dBi, and the second band reached 10.09 dBi, as shown in Fig. 19(a). While the ECC value is <0.01, the DG value is >9.988, and the CCL value was less than 0.4 bits/s/Hz. In the end, the antenna achieved an overall efficiency of 92% and 94% for the two bands, respectively, as shown in Fig. 19(b).

Figure 19. The complementary results achieved by the antenna are (a) gain curves for both sides of CST simulation and actual measurements, and (b) total efficiency curves for simulation and manufacturing [120].

Moreover, in the manuscript [121], the authors presented a new geometry for an eight-port antenna with geometric dimensions (150 × 80 × 1.6 mm³), as shown in Fig. 20. The authors focused on presenting an antenna composed of four elements, each containing two ports. Note that the antenna was designed with both simulation aspects using the HFSS software (version 2020), as shown in Fig. 20, and actual fabrication using practical laboratories, as shown in Figs. 21(a) and (b). The materials for manufacturing the antenna are copper for the patch and ground layers and FR4 (permittivity of 4.4 and tangent loss of 0.02) for the substrate layer. This antenna operates at sub-6 GHz frequencies ranging from 3.4 to 3.6 GHz, as shown in Fig. 22(a). The antenna achieved the lowest isolation value between the two ports (first and second) of −14 dB and the highest value between the two ports (first and fourth) of −43 dB, as shown in Fig. 22(b). It also achieved good performance values for the ECC parameter <0.065, with the highest gain reaching 6.24 dB. The efficiency for the simulation side ranges between 75% and 85%, and the practical side ranges between 60% and 75%, as shown in Fig. 23.

Figure 20. A geometric design of the proposed octa-port MIMO antenna using the HFSS simulation program [121].

Figure 21. The fabrication geometry of the proposed MIMO antenna for (a) the front view and (b) the back view [121].

Figure 22. (a) The reflection coefficients for the simulation and fabrication aspects, and (b) the mutual coupling between all ports [121].

Figure 23. Overall antenna efficiency for all simulations and fabrication measurements [121].

Furthermore, several recent studies have introduced MIMO antennas with eight or more ports. Summaries of designs operating at sub-27 GHz and mmWave frequencies are provided in Tables 5 and 6, respectively. In these studies, increasing the number of ports while reducing the antenna size has increased mutual coupling between MIMO antenna elements and deteriorated the results in some works.

Table 5. A detailed comparison of the latest articles proposing eight-port antennas operates at frequencies sub-27 GHz

Table 6. A comparison of recent research on eight-port or more antennas relying on mmWave bands

Importance of equivalent circuits for antennas

In recent years, an equivalent circuit representation of antennas has gained popularity. A wide variety of studies have utilized the model to either build a custom antenna or analyze and isolate the lost parts of the antenna [149]. On the other hand, multi-antennas, like these in the MIMO system, have received less attention. Adding more antennas to the system with MIMO technology may significantly increase the data capacity and performance of the system. To improve communication capacity in MIMO systems, researchers implemented the space decoupling approach, also known as the network decoupling technique. As a result, a robust equivalent circuit simulation is crucial for designing and evaluating separation techniques [150]. Additionally, equivalent circuit models, also known as network models, have attracted attention for their ability to facilitate the study of circuit effects such as amplifier noise, matching, and reconfigurability.

These models also allow the simulation of combined antenna arrays and multiuser MIMO systems [151]. These particular types of models are appealing from a computational standpoint because both transmit and receive arrays may be expressed as comparable circuits. This means that a small number of full-wave computations or measurements are needed, and then circuits with different levels of complexity can be examined through effective circuit-level modeling. As officially shown in reference [152], a similar impedance matrix (apart from a transpose) can be used for both modes of operation. This implies that network analysis can be used to model an antenna array in both transmission and reception modes. An equivalent model was used to study how mutual coupling affects adaptive arrays. It created a beamformer to improve the signal-to-interference-to-noise ratio (SINR) and showed its relationship with loaded voltages and open circuits in the receiving array [153].

Challenges, developments, and future directions discussion

Massive MIMO (M-MIMO) antennas outperform traditional multi-antenna systems. MIMO and 5G technologies might revolutionize wireless networking. Nevertheless, various issues persist that impede the actual application of M-MIMO. For each type of application, hardware components confront several challenges, including material selection, size, cost, and characteristic properties (bandwidth, efficiency, gain, mutual coupling, and so on) [154]. Many design issues will arise due to the wide variety of devices, and the 5G frequency range will exacerbate them. The spectrum must be adaptable to accommodate devices that operate on different spectral bands. There has been an increasing focus on the sub-6 GHz band for 5G communication to address these challenges, offering an effective solution. Base station approaches in 5G sub-6 GHz employ single and multiband designs over several kinds of bands of frequencies, which provide specific challenges. Utilizing various array geometries, such as patch sub-arrays, multimode-slotted designs, and other configurations [155], can lead to high gain and effective performance. Because the current concerns are about antenna placement, number requirements, and mutual interference prevention – especially in light of the new desire for a 5G network – frequencies do not provide a wide range of issues [156].

An antenna has a fixed number of components arranged in symmetrical and asymmetrical array designs. As an illustration of base station approaches, symmetric and asymmetric planar structures are rectangular arrays, such as the (4 × 4) and (4 × 1) designs of elements. Regarding the radiation patterns’ effectiveness, gain, and directivity, they examined the effects of one of the elements in addition to the arrays through antenna configuration. In the design of smartphones, antennas are positioned at the edges in a symmetrical or nonsymmetrical manner for two sides or one side, as in the (8 × 8) model of components, where each of the four is located at an edge. These results are excellent with set element spacing and complete isolation [157].

It can be challenging to assess how closely spaced MIMO antenna components negatively affect mutual coupling [158]. However, the small size and decoupling techniques have significantly resolved the issue by improving isolation between the antennas, thereby positively impacting the characteristics. The antennas’ small size and compact design could have contributed to their excellent spectrum efficiency and minimal mutual coupling. It should be mentioned that M-MIMO base stations are now supported through both 2D and 3D positioning of antenna components. Nevertheless, applying 2D or 3D antenna arrays can significantly increase energy efficiency and improve coupling effects. Utilizing decoupling methods to expand the separation of array elements is a feasible approach that enhances spectral efficiency. For improved results, a small 3D array M-MIMO antenna can be employed using or without decoupling techniques like hexagonal, triangular, and cylindrical models. One of the primary characteristics of 5G MIMO antennas is decoupling techniques, which are necessary due to the size of smartphones and the design requirements for the massive methods [159, 160].

During the comprehensive review of many studies and recent works presented in this article, we concluded many points that will serve researchers in the future when they provide an ideal antenna, the most important of which are:

1. 1. It was concluded that choosing the ideal materials involved in manufacturing the antenna plays an important role in improving the results of the antenna.

2. 2. It has been discovered that material properties such as dielectric constant and loss tangent also play an important role in achieving excellent results if chosen appropriately.

3. 3. It has been noted that the thickness of the antenna layers has a major role in improving or deteriorating the antenna results.

4. 4. It was also concluded that increasing the antenna ports in the MIMO configuration increases the mutual coupling between the antenna elements, and this will cause an increase in noise and interference between the electromagnetic signals fed to each port. Thus, the antenna’s performance will degrade.

5. 5. It was also discovered that the ideal MIMO antenna design consists of three stages, as shown in the flowchart in Figs. 24(a), (b) and (c).

Figure 24. The proposed antenna design stages are (a) the first stage, (b) the second stage, and (c) the third stage.

Furthermore, the small size and ease of combining with other components are inherent advantages of planar antenna architectures, such as microstrip antennas. Generally, these antennas have a lower bandwidth and gain than 3D antennas. Likewise, they may be more susceptible to interference and coupling between MIMO ports. In contrast, the major challenges for antenna designers in creating an ideal multiband antenna are summarized in Fig. 25.

Figure 25. The primary challenges for designers in creating multiband MIMO antennas.

Conclusions

An extensive study of design techniques, advancements, fabrication materials, difficulties, and MIMO antenna applications was provided in this article. The design of the antennas in this study was divided into two parts: the first part included broadband antennas, and the second part included mmWave antennas. In the first part, we presented a comprehensive study on the three most important types of MIMO antennas (two-, four-, and eight-port) that operate at frequencies between 1 and 27 GHz. In the second part, we discussed the designs of MIMO antennas based on the mmWave bands between 30 and 100 GHz for the three most in-demand types in the market (two-, quad-, and eight-port). In both parts, we compared the latest works presented by researchers in previous studies, and we focused in this comparison on the parameters that determine the ease of understanding the designs by the reader. These parameters are the geometric structure of the antenna, the number of ports, fabrication materials, the dimensions of the antenna (width × length × thickness), antenna operating frequencies, gain, port isolation techniques, overall efficiency, ECC, DG, and CCL.

Therefore, we have drawn several conclusions that will serve future researchers when manufacturing ideal antennas. The most important of which is that fabrication materials play a major role in improving the performance of antennas. It was also noted that the mutual coupling between the ports in the MIMO configuration is greatly improved thanks to the use of many modern technologies simplified in this article. In addition, we concluded that antennas that operate at mmWave frequencies have small dimensions and suffer from isolation problems between the antenna elements in the MIMO formation. Unlike the antennas that operate at frequencies below 6 GHz, which have larger dimensions and do not suffer from isolation problems between the ports, this gives the best results, so it has become used in various modern wireless application systems. Furthermore, we concluded that all the work and comparisons presented will help all researchers provide high-performance MIMO antenna designs to meet the rapid development requirements in modern wireless communications and applications for the current 5G or future 6G systems. In the future, a MM technique with particular properties (isolating materials) will be used to address hardware challenges, component characteristics, modification, and enhancement. This will effectively lead to improvements in size, efficiency, gain, bandwidth, and several other aspects.