Smart Sensor Platform for MIMO Antennas with Gain and Isolation Enhancement Using Metamaterial

Abstract

In modern wireless communication systems, achieving high isolation and consistent signal gain is essential for optimizing Multiple-Input Multiple-Output (MIMO) antenna performance. This study presents a metamaterial-integrated smart sensor platform featuring a hexagonal two-element MIMO antenna designed to improve isolation and directive gain. Constructed on an FR4 substrate (1.6 mm thick), the proposed antenna configurations include a base hexagonal patch, an orthogonally oriented two-element system (TEH_OC), and further enhanced variants employing metamaterial arrays as the superstrate and reflector (TEH_OC_MTS and TEH_OC_MTR). The metamaterial structures significantly suppress mutual coupling, yielding superior diversity parameters such as Envelope Correlation Coefficient (ECC), Mean Effective Gain (MEG), and Channel Capacity Loss (CCL). All configurations were fabricated and validated through comprehensive anechoic chamber measurements. The results demonstrate robust isolation and radiation performance across the 3 GHz and 5 GHz bands, making these antennas well-suited for deployment in compact, low-latency smart sensor networks operating in 5G and IoT environments.

1. Introduction

MIMO technology enhances wireless communication by improving spectral efficiency and link reliability by employing multiple antennas at the transmitter and receiver. System throughput in full-duplex MIMO systems can be doubled by concurrently communicating uplink and downlink in the same frequency range. The main challenge is self-interference, where the system’s transmitter interferes with its own receiver. Overcoming this requires advanced self-interference cancellation techniques. Successfully implemented, full-duplex MIMO can greatly boost efficiency in applications like 5G and future networks. Metamaterial structure integration has become a feasible way to further improve full-duplex MIMO systems performance. Utilizing the unique electromagnetic properties of metamaterials, radiation patterns can be manipulated to achieve improved directivity, gain, and beam-steering capabilities.

Metamaterial structures used in MIMO configurations may potentially improve the isolation between transmit and receive antennas, thereby improving the effectiveness of self-interference cancellation. This will assist future wireless networks in meeting the growing need for reduced latency and faster data speeds. Recent research has focused on developing compact MIMO antennas for sub-6 GHz 5G applications. These designs aim to achieve improved isolation, bandwidth, and diversity performance.

A MIMO antenna with and without a metasurface was described by Wang et al. To aid in decoupling, a suspended metasurface with periodic square split ring resonators (SRRs) is placed above the antenna array. By using the proposed metasurface, isolation is enhanced from 8 dB to 28 dB. The Envelope Correlation Coefficient (ECC) between the two elements is greatly boosted in addition to isolation, peak gain, and efficiency [1]. The bandwidth and isolation of the MIMO antenna were improved by using a tapered feed line, and ground plane stubs were reported by Mahto et al. Their design featured quad band resonance with a wide impedance bandwidth of 5.03 GHz [2]. Sakli et al. presented a UWB MIMO antenna with and without SRRs. Without SRRs, 15 dB of isolation was achieved, and with SRRs, 43 dB of isolation was achieved. The bandwidth was additionally increased from 2 GHz to 18 GHz by the suggested MIMO with SRRs [3]. Garg et al. showed a high isolation of 35 dB in a two-element MIMO antenna using a flower-shaped metamaterial absorber. At 5.5 GHz, the metamaterial absorber used in their work had an absorptivity of 98.7% and an impedance of about 50 Ω [4].

A two-element modified rhombus-shaped MIMO antenna was presented by Sourab et al. A T-shaped fractal ground structure was added in order to obtain a 20 dB isolation between the elements [5]. The design of a corporate-fed circular array and its MIMO were covered by Khan et al. In order to achieve sufficient isolation and pattern variety, the antenna arrays are designed to be 90 degrees apart from one another. An isolation of 40 dB within the working bandwidth was obtained with this configuration [6]. A transparent two-element MIMO for sub-6 GHz applications was reported by Desai et al. With respectable diversity parameters, their suggested design demonstrates an isolation of more than 15 dB [7].

Francis et al. demonstrated the performance of a circularly polarized MIMO array antenna integrated with a metamaterial superstrate. The proposed array shows an enhanced impedance bandwidth from 2.85% to 24.3% and an increase in the axial ratio bandwidth from 0.86% to 17.7%. The peak realized gain improves from 2.5 dBi to 11.5 dBi, with port isolation below −30 dB. The compact antenna operates effectively within the 2.9–3.7 GHz frequency range, making it suitable for MIMO applications [8].

Metasurface-based MIMO antennas with negative permeability (MNG) are proposed by Dubazane et al. to reduce mutual coupling for 5G applications, achieving a coupling reduction of −44 dB and a wide bandwidth of 5.92–6.2 GHz. To enhance the gain, the superstrate is integrated and achieves a maximum gain of 6.79 dB, which is suitable for sub-6 GHz 5G systems [9].

Al-Gburi et al. conducted a comprehensive review of FSS-based gain enhancement techniques for UWB antennas, classifying single- and multi-layer reflectors and proposing miniaturization strategies to maintain consistent gain across wide frequency bands while minimizing return loss [10]. Complementing this, Ghiat et al. presented a compact 2 × 2 MIMO antenna system with a split-ring resonator metamaterial layer for 5G channel sounding in the upper 6 GHz band, achieving over 1.5 dB gain improvement, strong isolation below −18 dB, and efficiency exceeding 75% without increasing antenna size [11].

A wideband four-element MIMO antenna with a loaded metamaterial superstrate was introduced by Khan et al. The proposed design achieved a broad impedance bandwidth of 41% (3.33–5.04 GHz) and reduced mutual coupling below −23 dB. The compact design enhances isolation and gain, demonstrating excellent MIMO performance metrics and satisfactory experimental validation [12].

This paper investigates the use of a metasurface to enhance the isolation and gain of a hexagonal two-element MIMO antenna. Fabrication and testing in an anechoic chamber validate the effectiveness of the proposed configurations, demonstrating their potential for practical wireless applications.

2. Diversity Characteristics

In modern wireless communication systems, diversity is crucial for reducing fading and improving link reliability, especially in environments with abundant scattering. The idea of diversity involves having multiple independent signal paths between the transmitter and receiver, which can be utilized to enhance the overall performance of the system. The assessment of diversity quality in a MIMO system relies on several critical parameters, such as Envelope Correlation Coefficient (ECC), Channel Capacity Loss (CCL), Diversity Gain (DG), and Mean Effective Gain (MEG).

2.1. Envelop Correlation Coefficient

The Envelope Correlation Coefficient (ECC) plays a pivotal role in assessing the MIMO antennas’ performance. It analyses how well the received or transmitted signals from two antennas are related. A low ECC indicates that the antennas provide uncorrelated, independent signal paths, which is desirable for achieving high diversity gain. It ranges from 0 to 1, where 0 indicates that the signals are entirely uncorrelated, and the value 1 indicates that the signals are perfectly correlated. The value of ECC can be found using S-parameters, which are widely adopted for MIMO antenna evaluations as shown in Equation (1) [13,14,15].

$$ECC={\displaystyle \frac{{\left|S_{mn}\ast S_{mn}+S_{nm}\ast S_{nn}\right|}^2}{\left(1-{\left|S_{mm}\right|}^2-{\left|S_{nm}\right|}^2\right)\left(1-{\left|S_{nn}\right|}^2-{\left|S_{nm}\right|}^2\right)}}$$

(1)

2.2. Channel Capacity Loss

The Envelope Correlation Coefficient (ECC) is important for checking how well MIMO antennas work. It shows whether the antennas can receive different signals without interfering with each other. The highest speed at which data may be sent via a channel error-free is known as Channel Capacity Loss (CCL). In radio communications, it refers to how much the channel’s capacity is reduced compared to what would be ideal. A MIMO antenna’s CCL value should be kept below 0.5 bits/S/Hz for best performance. This threshold ensures that the antenna can effectively utilize the available channel capacity, enhancing its overall efficiency. The calculation of Channel Capacity Loss (CCL) based on S-parameter-derived correlation matrices assumes a unit signal-to-noise ratio (SNR) and a flat fading environment, consistent with modeling frameworks outlined in [16].

$$C_{loss }=-lo{g}_2 det\left(A\right)$$

(2)

$$A=\left[{\sigma }_{mm} {\sigma }_{mn} {\sigma }_{nm} {\sigma }_{mn} \right]$$

(3)

$${\sigma }_{mm}=1-\left({\left|S_{mm}\right|}^2-{\left|S_{mn}\right|}^2\right)$$

(4)

$${\sigma }_{mn}=-\left(S_{mm}^{*}{S}_{mn}+S_{nm}{S}_{nn}^{*}\right)$$

(5)

2.3. Diversity Gain

The term “diversity gain” (DG) describes how employing several antennas at the transmitter, receiver, or both can increase the reliability and quality of the signal. A high DG suggests the system can handle fading and interference better, resulting in more reliable transmission. High isolation between the radiators is necessary to obtain high DG. The DG of a MIMO is expressed as a function of ECC, assuming ideal propagation conditions, and is calculated as follows [17,18,19]:

$$DG=10\ast {\left(1-EC{C}^2\right)}^{0.5}$$

(6)

2.4. Mean Effective Gain

Mean Effective Gain (MEG) plays a vital role in evaluating the antenna performance in a wireless communication system. It considers how well the antenna can receive or transmit signals when subjected to various environmental conditions and multipath effects. In this work, the MEG of an element in MIMO is derived under the assumptions of isotropic scattering and equal polarization, adopting the equations provided in [17] as follows:

$$ME{G}_m=0.5\left[1-{\displaystyle \sum }_{n=1}^K{\left|S_{mn}\right|}^2\right]$$

(7)

3. MIMO Antenna Design

The antenna design evolved through four progressive stages, as shown in Figure 1a–d, where each iteration introduced targeted geometric modifications to improve the impedance characteristics and bandwidth performance. Beginning with a fundamental patch configuration in Step 1, Step 2 employed slotting techniques to improve impedance matching and extend operational bandwidth. Step 3 introduced additional structural refinements such as stubs, which sharpened resonant behavior and minimized reflection coefficients. In the final stage, Step 4, the antenna achieved dual-band resonance, confirming its suitability for UWB applications. This staged evolution demonstrates a systematic and performance-driven design strategy.

The final, coplanar waveguide (CPW)-fed single-element hexagonal antenna (SEH) is presented in Figure 1d. The radiating element is hosted on a FR4 substrate with a height of 1.6 mm and a relative permittivity of 4.4. Then, the SEH is duplicated along the line and around the axis 90° to obtain a two-element hexagonal MIMO antenna (TEH) and an orientation-changed two-element hexagonal MIMO antenna (TEH_OC), respectively, as illustrated in Figure 2a and Figure 2b, respectively.

Table 1. The suggested MIMO antenna’s computed parameters.

Parameter	Size (mm)	Parameter	Size (mm)
WG	20	L2	1
LG	20	L3	0.75
W1	9	WG1	45
W2	1.5	LG1	20
W3	6	E1	2
W4	4.2	E2	3
W4	4.2	h	1.6

lists the computed dimensions of the suggested antennas.

4. Metamaterial Integrated with MMIO Antenna

Materials designed to possess qualities not present in naturally occurring materials are known as metamaterials. They accomplish this by arranging their building unit cells to display peculiar electromagnetic properties. Such metamaterials are categorized as Negative Index Metamaterials (NIMs), which possess a negative refractive index and bend the electromagnetic waves in the opposite way from what Snell’s law predicts. Positive Index Metamaterials (PIMs) were created to have unique qualities not seen in natural materials, although they nevertheless retain positive permittivity and permeability. Left-handed metamaterials exhibit negative permittivity and negative permeability, resulting in negative refraction and reversed Doppler effects. By analyzing and relating the reflection (S₁₁) and transmission (S₂₁) as in Equation (8), one can easily extract the permittivity (ε_eff), permeability (μ_eff), impedance (z), and refractive index (n) as follows:

$$S_{11}={\displaystyle \frac{{\displaystyle \Gamma }\left(1-e^{j2n{k}_{0}d}\right)}{1-{\mathit{\Gamma }}^2{e}^{j2n{k}_{0}d}}}$$

(8)

$$S_{21}={\displaystyle \frac{\left(1-{\mathit{\Gamma }}^2\right)e^{j2n{k}_{0}d}}{1-{\mathit{\Gamma }}^2{e}^{j2n{k}_{0}d}}}$$

(9)

where $\mathit{\Gamma }={\displaystyle \frac{z-1}{z+1}}$

$$n={\displaystyle \frac{1}{k_{0}d}}\left\{\left[{\left[ln ln \left(e^{jn{k}_{0}d}\right) \right]}^{″}+2m\pi \right]-j{\left[ln\left(e^{jn{k}_{0}d}\right)\right]}^{\prime }\right\}$$

and m is the branch index-related integer.

By inverting equations, the refractive index and impedance can be obtained as follows:

$$Z=\pm \sqrt{{\displaystyle \frac{\left(1+S_{11}^2\right)-S_{21}^2}{\left(1-S_{11}^2\right)-S_{21}^2}}}$$

(10)

The values of ε_eff and μ_eff of a metamaterial are obtained by using the following equation:

$${\epsilon }_{eff}={\displaystyle \frac{n}{z}}; {\mu }_{eff}=nz$$

(11)

Figure 3a depicts the simulation setup created in the EM solver, applying boundary conditions and port assignments. An array of elements 3 × 5 and 5 × 5 was created out of unit cells as shown in Figure 3b. The impact of the size of the metamaterial was studied with respect to the isolation parameter. The same array is shown in Figure 3d. Based on the traces, it is evident that there are no significant changes in the isolation. Hence, we selected 3 × 5 for fabrication and experimental studies. This metamaterial was used as both a superstrate and a reflector to evaluate its impact on the performance of the MIMO antenna. To understand the physical operation of the metamaterial unit cell, an equivalent-circuit model was developed based on electromagnetic behavior at microwave frequencies, as shown in Figure 3c. The model treats the conducting strips within the unit cell as distributed inductive elements, while the gaps between these conductive segments act as capacitive regions. This RLC representation reflects the resonant behavior typical of metamaterial structures. The resonance frequency and bandwidth are determined by the geometrical parameters and the dielectric substrate properties, which influence the effective inductance (L) and capacitance (C) values.

Figure 4a,b shows the extracted real and imaginary parts of ε_eff and μ_eff, respectively. Based on the traces, it can be observed that, at the 3 GHz band, the real part value of μ is negative, and at the 5 GHz band, the value of ε is negative. Similarly, Figure 4c shows the negative refractive index exhibited by the MTM unit cell. This confirms that the proposed MTM unit cell is DNG. Hence, an array of this material is considered to be used as a superstrate and reflector to study its effect on the performance of the TEH_OC MIMO antenna.

The integration of metamaterials into antenna design has emerged as a promising approach to enhance antenna performance for modern wireless communication systems. In this work, metamaterial structures are incorporated to improve the isolation, gain, and overall Multiple-Input Multiple-Output (MIMO) performance of hexagonal antenna designs. This resulted in TEH_OC_MTS and TEH_OC_MTR, as presented in Figure 5a,b, respectively. All these proposed antenna configurations were fabricated and tested in an anechoic chamber to validate their performance.

5. Fabricated Antenna and Measurements

The fabricated prototypes of TEH and TEH_OC, illustrated in Figure 5a and Figure 6b–d, respectively, were meticulously characterized to evaluate their performance. The experimental setup is shown in Figure 6e–f. Figure 6g–i shows the fabricated antenna bottom view and arrangement as a superstrate. Key parameters such as the log magnitude, imaginary, and real values of the S-parameters (S₁₁, S₁₂, S₂₁, and S₂₂) were measured using an ANRITSU S820E vector network analyzer. This comprehensive measurement process ensures an accurate assessment of the antenna’s operational behavior and effectiveness. Subsequently, the measured S-parameter values were utilized to compute critical MIMO performance parameters, enabling a thorough evaluation of the antennas’ isolation and gain characteristics.

6. Results Analysis

This section provides the analysis of simulation results of the proposed antenna evolution stages and the comparison of simulation and measurement results of the proposed TEH, TEH_OC, TEH_OC_MTR, and TEH_OC_MTS MIMO antennas. The 3D radiation patterns are presented in Figure 7, Figure 8, Figure 9 and Figure 10, respectively, at 3 GHz and 5 GHz bands. The 3D radiation patterns shown in Figure 6, Figure 7, Figure 8 and Figure 9 reveal a progressive improvement in directivity and gain across successive antenna configurations. The TEH in Figure 6 demonstrates relatively broad patterns with moderate gain, indicating less focused energy and higher mutual coupling. In Figure 8, TEH_OC, orienting the elements orthogonally leads to better isolation and slightly more directed radiation lobes, especially at higher frequencies. Figure 9, TEH_OC_MTR, showcases further refined patterns with increased peak directivity, which is attributed to the metamaterial reflector’s influence on wave steering. Finally, Figure 10, TEH_OC_MTS, presents the most symmetrical and tightly bound radiation profiles with enhanced gain, reflecting the superstrates’ constructive effect on beam shaping and efficiency. Together, these patterns validate the design’s evolution toward optimal radiation performance for UWB and spatially diverse MIMO systems.

The S₁₁ of the antennas designed as part of the evolution are compared in Figure 11. The traces in the figure provide a consolidated view of the return loss characteristics for each stage of the antenna’s evolution—Stage 1 through Stage 4—over the operating frequency range. This comparative plot clearly visualizes the progressive enhancements in impedance matching and bandwidth. Step 1 displays a shallow and narrow resonance, with S₁₁ values just dipping below −10 dB, indicating limited bandwidth and suboptimal matching. Stage 2 introduces noticeable improvements, including deeper nulls and the emergence of secondary resonances, suggesting better impedance alignment and wider usability. Stage 3 shows pronounced and well-defined resonance dips across a broader spectrum, with significantly improved return loss levels. Stage 4 exhibits the most refined response, where S₁₁ remains consistently below −10 dB across a wide band, confirming optimal matching and suitability for UWB applications.

The simulated and measured S-parameters of TEH, TEH_OC, TEH_OC_MTR, and TEH_OC_MTS are plotted in Figure 12a–d, respectively, for correlation. According to the data plotted, EH_OC_MTR has the best isolation of any of these designs, measuring less than −25 dB at the 3 GHz and 5 GHz frequency bands.

The MIMO performance parameter and CCL of TEH, TEH_OC, TEH_OC_MTR, and TEH_OC_MTS were compared. The traces in Figure 13a demonstrate that for both the 3 GHz and 5 GHz frequency bands, the CCL values of all suggested designs are far below the necessary 0.4. The DG traces plotted in Figure 13b show that the value of DG at both resonant bands is 10 dB. Similarly, in Figure 13c, one can make out that the values of ECC are very much less than 0.05. Similarly, the MEG graph in Figure 13d illustrates the MEG vs. frequency response for multiple antenna configurations, comparing both simulated and measured results over the 2.5 GHz to 8 GHz range. The MEG values remain within an acceptable range, −4 dB to −6.5 dB, across the target frequency. This reflects balanced reception and reasonable power levels for typical MIMO antenna systems.

Table 2. Comparison of the proposed antennas with the literature.

Ref.	BW (GHz)	Isolation dB	MIMO Parameters
			ECC	DG (dB)	CCL (Bits/s/Hz)
[5]	3.34–3.87	-	<0.012	>9.999	-
[6]	3.4–3.6	>19.8	<0.06	>9.994
	5.15–5.85		<0.11	>9.971
[7]	5.19–5.41	-	0.13	>9.7	-
[8]	3.3–3.7	-	<0.09	> 9	-
[10]	3.1–10.6	>15	<0.2	-	-
[12]	3.1–10.7	>−16	<0.01	>9.95	-
[13]	3.1–5.2	>10	<0.1	>9.9	-
[16]	3.4–3.7	>13	<0.131	>9.956
	5.15–5.35	>16	<0.002	>9.963
[17]	3.4–3.6	>12	<0.003	>9.9	-
	4–8	>15	<0.003	>9.9	-
[20]	2.5–2.57	>15	<0.02	-	-
TEH	3.01–3.09	>8.8	<0.005	>9.99	0.6182
	4.84–6.39	>11	<0.001	~10	0.2397
TEH_OC	3–3.08	>36.558	~0	~10	0.19
	4.76–6.39	>25.125	~0	~10	0.212
TEH_OC_MTR	3–3.09	>38.244	<0.002	~10	0.293
	4.78–6.26	>29.538	~0	~10	0.05
TEH_OC_MTS	3.02–3.1	>35.89	~0	~10	0.0545
	4.82–6.29	>30.75	~0	~10	0.0545

summarizes the performance of the suggested MIMO antennas and compares it with the findings from the literature. The proposed antennas outperform existing designs in several key aspects, as evident in the comparison table.

They offer wideband coverage, supporting multiple frequency bands, which enhances their versatility for various wireless applications. With isolation values as high as ≥5 dB, they significantly reduce mutual coupling, ensuring better system performance compared to most references. The proposed designs also achieve extremely low ECC ≤ 0.002, values close to ideal DG (10 dB), and CCL as low as 0.012 bits/s/Hz, collectively providing excellent MIMO performance in terms of diversity and channel utilization.

Additionally, the compact size of the proposed antennas (45 × 20 mm) makes them suitable for space-constrained applications, unlike some larger referenced designs. These merits, including wideband operation, high isolation, superior MIMO metrics, and compactness, establish the proposed antennas as highly efficient and practical solutions for modern wireless systems.

7. Conclusions

In this study, a novel metamaterial (MTM) array was designed and integrated into a MIMO antenna system, serving both as a superstrate and reflector to significantly enhance isolation and radiation performance. By strategically mitigating mutual coupling between antenna elements, the MTM configuration improves key MIMO metrics such as Envelope Correlation Coefficient (ECC), Mean Effective Gain (MEG), and Channel Capacity Loss (CCL), all of which are critical for reliable operation in multipath-rich environments. This optimization confirms the MTM array’s role as a powerful enabler of diversity enhancement and system robustness.

In addition to improving inter-element isolation, the proposed antenna system serves as a multifunctional platform for smart sensing applications, particularly suited for deployment in harsh industrial and environmental conditions, as well as in IoT-enabled infrastructures requiring reliable wireless communication. Its compact footprint, directive radiation pattern, and stable gain characteristics render it suitable for intelligent wireless sensor nodes that demand high data fidelity and low latency. These capabilities, combined with scalability and ease of integration, make the design highly applicable to emerging wireless technologies, including 5G and beyond. The confluence of advanced antenna engineering with smart sensing requirements establishes a solid foundation for next-generation MIMO-enabled communication and monitoring systems.