Introduction
The 21st century is marked by a transformative technological revolution reshaping global connectivity. Central to this transformation is wireless communication, which has a profound impact on daily life. To meet the pressing needs of modern wireless communication—such as managing increased data traffic, providing extensive connectivity, and ensuring low latency—Sub-6 GHz technology has emerged as an effective solution. The Sub-6 GHz frequency range supports various technologies, including WiMAX, WiFi, 3G, 4G, and the burgeoning 5G applications [Reference Pan, Lin, Xu, Zhu, Bian and Li1]. This technology has ushered in a new era of innovation, fostering advancements in diverse fields such as cloud computing, smart traffic systems, artificial intelligence, automated industrial processes, robotics, high-definition live streaming, virtual and augmented reality, space exploration, smart homes, smart transportation, the Internet of Things (IoT), remote education, and healthcare services, particularly in response to global challenges like the COVID- pandemic [Reference Li, Masouros, Swindlehurst and Yu2].
Recently, machine learning methods have been employed to develop affordable portable electronic devices for 5G, WiFi, WiMAX, and WLAN applications. Sub-6 GHz 5G technology offers vital benefits such as high data transfer rates, extensive connectivity, ultra-low latency, high reliability, broad coverage, and improved mobility [Reference Li, Zhang, Rong and Han3, Reference Nasir, Jamaluddin, Khalily, Kamarudin and Ullah4]. Similarly, WiMAX technology, known for its high peak data rates, excellent mobility, and multi-device connectivity, remains widely used. The 3.5 GHz band (3.3–3.6 GHz), a key segment of the Sub-6 GHz spectrum, is extensively used for cellular backhaul, broadband internet, VoIP, interactive gaming, and IP multimedia subsystems [Reference Nadeem and Choi5–Reference Li, Zhang, Rong and Han7]. While mm-wave 5G provides very high data rates and large bandwidth, Sub-6 GHz 5G is more practical due to its broader coverage and easier deployment. The 3.5 GHz band (3.3–4.2 GHz) is especially favored in Sub-6 GHz 5G implementations as shown in Table 1, with various countries either licensing or planning to use this frequency range [Reference Das, Sharma and Gangwar8].
Table 1. Sub-6 GHz 5G frequency bands by region
Antennas are critical components in wireless technology, enabling smart antenna systems to send and receive multiple spatial data streams simultaneously. Multiple-input-multiple-output (MIMO) antenna technology is gaining significant attention for maximizing data speed and minimizing errors by utilizing multiple antenna elements at both ends of a wireless communication link [Reference Singhwal, Kanaujia, Singh, Kishor and Matekovits9]. However, providing sufficient field and port isolation between closely positioned antenna elements remains challenging. Enhanced isolation characteristics can significantly improve MIMO system performance, leading to a wide range of MIMO antenna designs in the literature [Reference Tran, Hussain and Le10, Reference Jamal, Li and Yeung11]. For instance, ultra-wideband antenna elements arranged perpendicularly establish polarization diversity and high isolation. Additionally, various decoupling structures and techniques, such as metal strips, neutralization lines, and modified feed structures, are employed to achieve high isolation and desired performance [Reference Dwivedi, Sharma, Pandey and Singh12].
Circularly polarized (CP) antennas have been extensively studied over the past few decades for their flexibility in source and receiver orientation and stable communication links. Various CP antennas, including patch, loop, horn, slot, and dielectric resonator antennas (DRAs), are explored in the literature [Reference Illahi, Iqbal, Sulaiman, Alam, Su’ud and Khattak13–Reference Eslami, Nourinia, Ghobadi and Shokri15]. DR-based CP antennas [Reference Khalid, Iffat Naqvi, Hussain, Rahman, Mirjavadi, Khan and Amin16] are particularly noted for their high radiation efficiency and wide control over size and bandwidth. Versatility in DR shape [Reference Xu, Guo, Liu, Deng, Chen and Ma17] and excitation schemes allows antenna designers to achieve CP radiation with desired patterns. Notable examples include rotated-stair DRs [Reference Kollipara and Peddakrishna18], annular DRs [Reference Fang, Leung, Lim and Chen19], orthogonal mode (OM)-shaped DRs [Reference Long, Dorris, Long, Khayat and Williams20], inclined slit-loaded square DRs [Reference Leung and Ng21], and special-shaped DRs [Reference Leung22].
Despite considerable advancements, the integration of circularly polarized (CP) radiators within a single antenna system for MIMO applications has not been extensively studied. Most prior research has focused on two-element dielectric resonator antenna (DRA) arrays and two-port MIMO-DRAs that exhibit CP radiation. In this paper, we present a novel CP MIMO DRA design. The configuration arranges two identical dielectric resonators (DRs) in a diagonal layout on a compact ground plane, aiming to significantly mitigate mutual coupling. A strategically placed conformal metal strip is employed to stimulate the higher-order degenerate mode pair, thereby generating circularly polarized (CP) waves. The antenna is capable of achieving both broadband circular polarization and a wide impedance-matching bandwidth over the same frequency range. The antenna’s performance is rigorously validated through comprehensive simulations and measurements, including return losses, axial ratio (AR), radiation patterns, envelope correlation coefficient (ECC), and diversity gain (DG). The strong agreement between simulated and measured results demonstrates that the proposed CP MIMO DRA is a promising candidate for 5G Sub-6 GHz and WiMAX applications.
Design evolution steps
A single unit of the proposed MIMO Dielectric Resonator Antenna (DRA) has already been published [Reference Illahi, Iqbal, Sulaiman, Alam, Su’ud and Jamaluddin23]. The geometry of the Circularly Polarized (CP) MIMO DRA is shown in Figure 1. The design features two identical dielectric resonators (DRs) constructed from alumina (
$\epsilon_{r} = 9.9$, 
$\tan \delta$ = 0.0001). These DRs are excited by a unique H-shaped feeding strip and are arranged diagonally on a compact and flexible ground plane (see Figure 1a). The underlying principles and geometry of the CP DRA were detailed in our earlier research [Reference Li, DeJean, Laskar and Tentzeris24]. The dimensions of the DRA were optimized using transcendental equations derived from the dielectric waveguide model (DWM) [Reference Mongia and Ittipiboon25]:
\begin{equation}
k_{x}=\frac{\pi}{D}, \quad k_{y}=\frac{\pi}{W}, \quad k_{x}^{2}+k_{y}^{2}+k_{z}^{2}=\epsilon_{r}k_{0}^{2},
\end{equation}
\begin{equation}
k_{z}\tan \left ( k_{z}H/2 \right )=\sqrt{\left ( \epsilon _{r}-1 \right ) k_{0}^{2}-k_{z}^{2},}
\end{equation}The antenna’s resonance frequency is related to the wavenumber kz, which depends on the DRA parameters such as D, W, H, and ϵr. Resonance occurs when the wavenumbers kx, ky, and kz along the x-, y-, and z-axes fulfill the criteria outlined in equations 1 and 2.
Figure 1. (a) Proposed circularly polarized multiple-input-multiple-output DRA. (b) Optimized design configuration.
From these calculations, the dimensions of the DRA have been optimized to 
$\text{D}=7\,\mathrm{mm}$, 
$\text{W}=12\,\mathrm{mm}$, and 
$\text{H}=13\,\mathrm{mm}$. The CP MIMO design was developed using CST Microwave Studio with two main goals: maintaining a compact structure and achieving circular polarization through higher mode excitation to enhance gain for 5G Sub-6 GHz and WiMAX applications [Reference Mongia and Ittipiboon25]. The first goal was met by utilizing the same compact, flexible ground plane (
$350\times350~\text{mm}^2$) previously reported [Reference Li, DeJean, Laskar and Tentzeris24].
The two identical DRAs were positioned with an optimized edge-to-edge spacing to ensure that the antenna remains compact. The second goal was accomplished by using an H-shaped feeding strip, consisting of three metallic strips, designed to produce wideband circular polarization via higher-order mode excitation. The optimal positioning of the strip was determined to be 
${d}_{1} = {d} - 0.75\lambda_0$, specifically 
${d}_{1} = 6\,\mathrm{mm}$. The optimized feed parameters are 
${d}_{1}=6\,\mathrm{mm}$, 
${d}_{2}=5\,\mathrm{mm}$, 
${d}_{3}/{w}_{1}=1\,\mathrm{mm}$, 
${l}_{1}/{l}_{3}=11\,\mathrm{mm}$, 
${l}_{2}=10\,\mathrm{mm}$, and 
${w}_{2}/{w}_{3}=1.5\,\mathrm{mm}$. Extensive simulations were carried out to verify that the design is both compact and suitable for 5G Sub-6 GHz and WiMAX applications.
Examination, functionality, and optimization of circularly polarized MIMO dielectric resonator antennas
This section outlines the methods used to optimize the antenna’s bandwidth and reduce mutual coupling by evaluating four different placement scenarios for the dielectric resonators (DRs) to determine the most effective configuration. The proposed CP MIMO DRA is then simulated under various conditions to ensure its suitability for 5G Sub-6 GHz and WiMAX applications.
A systematic approach was employed to optimize the orientation and placement of the DRs, as depicted in Fig. 2. Four distinct scenarios were tested, with results shown in Fig. 3. A key challenge was to maintain a compact ground plane to ensure the antenna’s effectiveness for 5G Sub-6 GHz and WiMAX applications. In every scenario, the edge-to-edge distance was maintained at 0.25λ 0 (8 mm) to effectively reduce mutual coupling while utilizing a compact ground plane [Reference Petosa26]. Here, λ 0 denotes the free-space wavelength corresponding to the central frequency of the operational band.
Figure 2. Antenna optimization and E-field distribution. (a) Case I. (b) Case II. (c) Case III. (d) Proposed.
Figure 3. Case study of the proposed CP MIMO DRA. (a) 
$\text{S}_{11}$, (b) 
$\text{S}_{21}$, and (c) axial ratio.
In Case I (Fig. 2a), the two DRAs were arranged parallel to each other but flipped. While the impedance bandwidth was satisfactory at 4.45 GHz, the antenna suffered from poor matching at this frequency, significant mutual coupling (see Fig. 3a and b), and limited circular polarization bandwidth (Fig. 3c). In Case II (Fig. 2b), both DRs were placed in a parallel configuration, but this arrangement did not yield any significant improvement (Fig. 3).
In Case III (Fig. 2c), the DRs were positioned back-to-back in line, resulting in improved impedance matching bandwidth (Fig. 3a and b) but degrade isolation. However, a bit of improvement in the AR passband was observed, but it was not in the desired band (Fig. 3c), because of these results there is a need for further design modifications to achieve the desired results.
In the final optimized design (Fig. 2d), the dielectric resonators (DRAs) were positioned diagonally. This configuration resulted in an impressive impedance-matching bandwidth of about 37.5% (ranging from 3.40 to 4.75 GHz). The orthogonal degenerate modes, 
$\text{TE}_{\delta13}^x$ at 3.5 GHz and 
$\text{TE}_{1 \delta 3}^{y}$ at 4.1 GHz, were successfully excited to produce circularly polarized (CP) waves. The electric field distribution of each DRA, illustrating this effect, is shown in Fig. 4. The proposed CP MIMO DRA offered an AR bandwidth of approximately 23.3% (3.4–4.2 GHz) with high isolation exceeding −25.3 dB throughout the passband.
Figure 4. E-field distribution on each DRA. (a) 
$\text{TE}_{\delta13}^x$ at 3.49 GHz. (b) 
$\text{TE}_{1 \delta 3}^{y}$ at 4.15 GHz.
Validation of results through prototype
A prototype has been constructed for evaluation, as depicted in Fig. 5. To eliminate any potential air gaps between the dielectric resonators (DRAs) and the ground plane, double-sided adhesive copper tape was utilized. This tape, fashioned into H-shaped feeding strips, was used to firmly attach the DRs to the ground plane. Each feeding strip includes an SubMiniature version A (SMA) connector soldered at its end to enable the excitation of the wideband DRAs.
Figure 5. Photograph of the proposed CP MIMO DRA. (a) Front view. (b) Top view
In the experiment, S-parameters were measured in free space using a vector network analyzer, while the axial ratio (AR), radiation patterns, and gain were assessed in an anechoic chamber. The measured results for the proposed MIMO DRA, depicted in Fig. 6a–c show good agreement between measured and simulated values. The antenna demonstrates a broad impedance matching bandwidth of 35.52% (3.4−4.75 GHz) with mutual coupling below −37.5 dB. The CP MIMO DRA achieves a measured 3-dB AR bandwidth of 24.6% (3.4–4.2 GHz). The resonance frequencies were predicted using the DWM mathematical equations [Reference Petosa26] [Reference Mukherjee, Patel and Mukherjee27], and these predictions were validated through simulation and measurement, as illustrated in Fig. 6c. The predicted, simulated, and measured values for 
$\text{TE}_{\delta13}^x$ and 
$\text{TE}_{1 \delta 3}^{y}$ are detailed in Table 2, indicating close agreement. Any discrepancies between measured and simulated results can be attributed to experimental factors such as cable losses, connector losses, and other measurement limitations.
Figure 6. Measured and simulated results of the proposed CP MIMO DRA. (a) 
$\text{S}_{11}$, (b) 
$\text{S}_{21}$, and (c) axial ratio.
Table 2. Comparison between predicted, computed, and measured mode frequencies
Figure 7 presents the measured and simulated radiation patterns for the two ports at 3.5 GHz. They are analyzed for both 
$\Phi=0^{\circ}$ and 
$\Phi=90^{\circ}$. In the boresight direction, the measured co-polar fields (left-hand circular polarization) are more than 18 dB stronger than the cross-polar fields (right-hand circular polarization). This substantial difference indicates robust performance, making the antenna well-suited for a diverse array of practical applications.
Figure 7. Radiation patterns of the proposed CP MIMO DRA at 3.5 GHz. (a) Port 1. (b) Port 2.
MIMO diversity analysis
The effectiveness of the MIMO antenna is validated through the analysis of the ECC and DG. Ideally, the ECC should be zero [Reference Mukherjee, Patel and Mukherjee27], but in practical applications, a value under 0.5 is generally deemed acceptable. The ECC is a vital measure used to assess the similarity between signals received by different antenna ports, reflecting the level of mutual coupling between them, with values ranging from 0 (no coupling) to 1 (complete coupling). For the outdoor testing conducted, the ECC results are exceptionally positive, consistently staying below 0.01, which indicates robust performance [Reference Abdulmajid, Khalil and Khamas28]. Figure 8 depicts the ECC values for the CP MIMO antenna, showing how the ECC for Port 1 and Port 2 is calculated from far-field radiation patterns according to Equation 1. Over the full operating frequency range, the measured ECC for the wideband CP MIMO antenna remains below 0.04. These results underscore the antenna’s effectiveness in minimizing mutual coupling and ensuring consistent, reliable performance.
Figure 8. ECC and DG of the CP MIMO antenna.
Diversity gain (DG) is a critical metric used to evaluate the performance of MIMO antenna systems. Ideally, a DG value of 10 is preferred; however, in real-world applications, a value above 6 is generally deemed acceptable [Reference Alieldin, Huang, Stanley, Joseph and Lei29]. As shown in Fig. 8, the newly developed wideband CP-MIMO antenna achieves a measured DG exceeding 8 dB. The DG for the MIMO antenna can be calculated using the following formula [Reference Iqbal, Illahi, Sulaiman, Alam, Su’ud and Mohd Yasin30]:
\begin{equation}
\rho_e = \frac{4\pi F_1(\theta, \phi) \cdot F_2(\theta, \phi) \, d\Omega}{24\pi F_1(\theta, \phi)^2 \, d\Omega + 4\pi F_2(\Theta, \phi)^2 \, d\Omega},
\end{equation}
\begin{equation}
\text{DG} = 10\sqrt{(1-{\rho_e}^2).}
\end{equation}Furthermore, Fig. 9 illustrates the efficiency and gain of the proposed antenna design. It shows a consistent efficiency of approximately 90% within the targeted frequency range and a stable gain of 5.95 dBic. These results highlight the antenna’s effectiveness and reliability in converting input power into radiated energy, confirming its practical application viability.
Figure 9. Efficiency and gain of the CP MIMO antenna.
Table 3 provides a detailed comparison of the proposed CP MIMO antenna with other existing CP MIMO systems. This evaluation covers various performance metrics such as size, operating bandwidth, AR bandwidth, isolation, and ECC. The proposed antenna exhibits notable advantages, particularly in terms of compact size, broad operating bandwidth, and AR bandwidth. Its DRA configuration ensures that extensive impedance and AR bandwidth overlap, adequately covering the Wi-Fi 6E band. Additionally, the antenna demonstrates excellent ECC, enhancing its suitability for MIMO applications. When compared to cutting-edge CP MIMO antennas, the proposed design stands out due to its superior performance across these key parameters.
Table 3. Comparison with the other paper in the literature
Conclusion
An investigation into a CP MIMO DRA tailored for 5G Sub-6 GHz and WiMAX applications has been conducted. The design incorporates a unique conformal metal strip to achieve circular polarization, combined with a straightforward geometric configuration. The dielectric resonators (DRAs) are positioned diagonally, and the approach for optimizing mutual coupling while enhancing bandwidth is detailed. The antenna demonstrates effective overlapping bandwidth, satisfactory isolation, and consistent gain performance in both free space and anechoic chamber environments. A prototype was constructed for experimental validation, and the measured results align well with the simulations. Notably, this research represents the first instance of a CP MIMO DRA, contributing a novel advancement to the field of wireless technology. The performance of the MIMO antenna is evaluated through its ECC and DG metrics. The findings show that the antenna’s MIMO capabilities meet acceptable standards, affirming its potential for future Wi-Fi 6E applications. In summary, the proposed two-port CP MIMO antenna offers several notable benefits, including circular polarization, advantageous radiation properties, and a design based on dielectric resonators. These features make it a highly promising choice for upcoming Wi-Fi 6E deployments.
Author contributions
J.I. and K.C. designed the proposed model, and M.U. and G.I. performed the simulations and fabrication. All authors contributed equally to analyzing data and reaching conclusions, answering the review, and writing the paper.
Competing interests
The authors report no conflict of interest.
Dr. Javed Iqbal holds a B.Sc. degree in Telecommunication Engineering from N.W.F.P University of Engineering & Technology, Peshawar, Pakistan (2007), an M.S. in Electronic Communication and Computer Engineering from the University of Nottingham, Malaysia campus (2013), and a Ph.D. in Electrical and Electronic Engineering from Universiti Kuala Lumpur, Gombak, Malaysia (2019). He is currently an Assistant Professor in the Faculty of Engineering and Technology at Gomal University, Pakistan. His research interests include Circularly Polarized MIMO Dielectric Resonator Antennas, 5G NR Band Applications, Millimeter Waves, and Wireless Body Area Networks. His contributions to the field have been recognized with several awards, including the Best Paper of the Year (2019) from the IEEE Malaysia Chapter and the ANUGERAH SANGGAR SANJUNG USM 2021 award for a high-impact journal publication in 2022.
Lway Faisal Abdulrazak received the B.Sc. degree (Hons.) in electronics and communication engineering from Omar Al-Mukhtar University, Libya, in 2005, and the master–s and Ph.D. degrees in telecommunication and electrical engineering from the Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia, in 2007 and 2011, respectively. He is currently an Associate Professor with the Department of Computer Science, Cihan University of Sulaimaniya, Iraq. He has published several scientific articles in high impact factor journals and conferences. His research interests include mobile communication, networking, interference analysis techniques, mathematical modeling for coexistence analysis in wireless networks, wave propagation, free space optics, and optical communication.
Dr. Kayhan Celik was born in Kayseri, Turkey, in 1990. He received a B.S. degree in Electrical and Electronics Engineering from Erciyes University, Kayseri, Turkey, in 2011. He earned his M.S. and Ph.D. degrees from Gazi University, Ankara, Turkey, in 2015 and 2021, respectively. In 2023, he was assigned to the position of Assistant Professor at the Electrical and Electronics Engineering Department, Faculty of Technology, Gazi University in Ankara. His research interests include energy harvesting, antenna design, chaotic circuits, and image encryption algorith
Muhammad Usal Ali received MS, Electronic and Electrical Systems from The University of Lahore, Punjab Pakistan, in 2020 from 2023 and also Bachelor of Science in Electrical Technology from The University of Lahore, Punjab Pakistan, in 2013, respectively doing PhD in Electrical Engineering From Gomal University, KPK, Pakistan. Since 2014, he has been a Senior Electrical Engineer in Electrical Power & Control Systems switchgears PVT. Ltd, Punjab Pakistan. His research interests include Electrical cable insulation,antenna design for 5G Sub GHz Application.
Ghaffer Iqbal Kiani (Member, IEEE) received the B.Sc. degree (Hons.) in Electrical and Electronic Engineering from the Islamic University of Technology, Dhaka, Bangladesh, in 1997, the M.Sc. degree (Hons.) in Electronic Engineering from the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, Pakistan, in 2003, and the Ph.D. degree in electronic engineering from Macquarie University, Sydney, Australia, in 2009. From 2009 to 2012, he was a Postdoctoral Fellow in very-high throughput wireless communication systems with the Commonwealth Scientific and Industrial Research Organization ICT Centre, Sydney. Since 2013, he has been with the Department of Electrical and Computer Engineering, King Abdulaziz University, Jeddah, Saudi Arabia, where he is currently an Associate Professor. He has published many high-quality research papers in the field of antennas and propagation. His current research interests include frequency selective surfaces, metamaterials, electromagnetics, antenna design, microwave polarizers, Micro-Electro-Mechanical Systems (MEMS), Nanoelectromechanical Systems (NEMS), Radio Frequency Identifications (RFID), and THz modulators. He received the Best Student Paper Award from the 2008 Workshop on Applications of Radio Science Conference, Gold Coast, Australia, for the paper “Transmission Improvement of Useful Signals through Energy Saving Glass Windows Using Frequency Selective Surfaces.”
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