Frequency selective surface based triple band multi slotted MIMO antenna for sub-6 GHz wireless applications

Abstract

The recent demand of high data rate and reliable wireless communication systems has accelerated the development of compact multiband antennas with better radiation performance. A multiband slotted Multiple-Input Multiple-Output (MIMO) antenna based on Frequency Selective Surface (FSS) has been presented in this work. The proposed antenna operates at 3.4 GHz, 6.68 GHz, and 8.4 GHz and encompasses significant frequency bands of sub-6 GHz 5G, C-band wireless systems, and X-band communication solutions. Multiband operation is accomplished through introducing well-designed slots in the radiating patch which adjust the surface current distribution and produce a variety of resonant paths. The MIMO antenna design is used to increase the channel capacity, diversity functionality, and link reliability while maintaining a compact design. In addition to this, a periodic FSS layer is added to the antenna structure to boost the gain of the antenna in the range from 12 to 14 dBi, radiation efficiency and also to avoid undesired surface waves. Complex simulations and parametric analyses are conducted to test the performance of the antenna in terms of reflection coefficient, radiation patterns, gain and MIMO diversity parameters including envelope correlation coefficient (ECC), diversity gain (DG), Channel Capacity Loss (CCL), Total active reflection coefficient (TARC), Mean Effective Gain (MEG). The proposed antenna exhibits good impedance matching, and better antenna elements isolation at the target frequencies, The achieved findings suggest the proposed multiband FSS-accompanied MIMO antenna is one of the potential solutions of the next-generation multiband wireless communication systems.

Introduction

The high pace of wireless communication systems development has posed an immense pressure on the desire to achieve high data rates, low latency, enhanced spectral efficiency, and a reliable connection^1,2. Various new technologies like fifth-generation (5G) and beyond-5G (B5G) communications, satellite communications, and advanced radar systems demand the use of antennas that can operate across multiple frequency bands with high gain and constant radiation properties³. With the development of smaller, more multifunctional wireless devices, the multiband antenna design has gained significant interest since it is now possible to make a single antenna system work at a wide range of frequencies rather than the multiple antennas, which reduces the complexity and cost of the system. The most popular choice of antenna type in modern wireless networks is the planar microstrip antennas, which has low profile, lightweight, easy fabrication, and can be used with printed circuit technology^4,5. Conventional microstrip antennas, however, are usually narrow band and low gain. In order to address these shortcomings, a number of methods have been explored over time in the recent years such as slot loading, fractal geometries, parasitic elements and defected ground structures (DGS). Slotted antennas are useful in the attainment of multiband characteristics⁶. When slots are introduced, the current distribution in the radiating patch introduces additional resonance paths that exhibit different operating frequencies. As a result, slot geometries in either the radiating patch or ground plane can be designed to achieve multiband operating frequencies⁷.

As the wireless services are continually increasing, more data need supported by the antenna systems and the reliability of the links should also increase. The MIMO technology has become one of the solutions to fulfill these needs. MIMO systems allow a spatial multiplexing and diversity gain in the channel capacity by using more than one antenna on the transmitter and receiver, which do not need more spectral resources^8,9. Besides, MIMO systems are able to reduce multipath fading and interference, which lead to more dependable wireless communication channels¹⁰. Despite of the above-mentioned advantages, incorporation of many antenna elements on a small platform brings challenges like mutual coupling of radiating elements^11,12. Mutual coupling is harmful to such key MIMO performance metrics such as ECC, DG, and total radiation efficiency¹³. The literature has suggested several methods of minimizing mutual coupling of compact MIMO antenna arrays. These include neutralization lines, parasitic structures, stubs, defected ground structures (DGS) and orthogonal antenna element placement. Also slot-based radiators and specially designed feeding mechanisms have been presented to offer a better isolation level with small dimensions^14,15.

High gain and effective radiative performance are another critical need of the modern wireless antennas. In order to address the above-mentioned issue of the compact planar antennas, researchers have investigated the application of artificial electromagnetic structures including metasurface and frequency selective surface (FSS)^16,17. It is possible to control the electromagnetic waves in these structures and therefore achieve better radiation properties, less back radiation, and higher gain in the antennas¹⁸. FSSs are periodic arrays of metallic structures which have frequency-dependent transmission and reflection properties. FSS structures when combined with antenna systems can serve as reflective surfaces, superstrates or filtering layers to enhance the antenna performance^19,20.

Recent studies have demonstrated that integrating FSS structures with MIMO antennas can significantly improve system performance. For instance, in²¹, An eight-port MIMO antenna integrated with a FSS is proposed for the 28 GHz millimeter-wave (n257/n261) bands. In similar work²², A compact CPW-fed UWB monopole antenna (20 × 25 mm²) is combined with a curved single-layer FSS reflector consisting of an 11 × 11 array of hexagonal-slotted unit cells to achieve a broad bandwidth of 2.66–17.98 GHz. Some of the researchers have presented the slotted MIMO antenna design. For instance, in²³ a dual-band microstrip patch antenna operating at 28 GHz and 38 GHz with a 4-port MIMO configuration to enhance channel capacity and spatial diversity is presented. Further²⁴ discusses a miniaturized slotted MIMO antenna array is given for 5G mm-wave communication and integrates machine learning regression models to predict antenna gain and performance and in²⁵ the authors introduce a compact quad-port MIMO antenna using an inverted L-shaped radiating element and hexagonal defected ground structure (DGS) operating in the 25–29.5 GHz band. In similar manner, various designs have been reported in the literature^26–30.

According to the above discussion, this paper proposes a multiband slotted MIMO antenna combined with a FSS for the modern wireless communication systems. The proposed antenna’s operating frequencies are 3.4 GHz, 6.68 GHz and 8.4 GHz which are significant frequencies in the frequency bands of sub-6 GHz 5G, C band and X-band satellite or radar. The 3.4 GHz band is part of the sub-6 GHz spectrum designated for 5G mid-band communications, which provides the balance of coverage and speed. The 6.68 GHz band is part of the upper C-band spectrum, which is emerging as a potential candidate for next-generation Wi-Fi (Wi-Fi 6E/7 extensions), fixed satellite communications, and modern radar technology. The 8.4 GHz band is in the X-band region, which is widely used for satellite communications, deep space communications and high-resolution radar systems because of its good propagation and bandwidth characteristics. Multiband operation is attained by introducing strategically designed slots to the radiating patch which changes the distribution of surface currents and forms multiple resonant paths. To achieve the desired performance of high-channel capacity and diversity, a MIMO configuration is adopted, in the form of compact size and high isolation between antenna elements. Moreover, periodic FSS structure is included as well to enhance radiation properties, reduce the surface wave propagation and increase the overall antenna gain. The antenna is studied using the detailed simulations and the parametric studies to examine the impedance matching, radiation patterns, gain performance of the antenna. The results indicate that the proposed antenna is an effective multiband antenna with better isolation and radiation characteristics, thus a good candidate to the multiband wireless communication systems.

Proposed antenna design

The single element proposed design is shown in Fig. 1. The proposed structure has been developed using 1.6 mm thick RT duroid/Roger 5880 substrates (ε_r = 2.2, tan δ = 0.0012). The isometric view of the design is shown in Fig. 1a, which depicts the size of the substrate having 40 mm × 40 mm × 1.6 mm. The design has been fed with the SMA connector and the impedance has been matched with the conventional microstrip line. The top view of the proposed design is depicted in Fig. 1b, which shows multiple slots to achieve the required frequency bands with excellent impedance matching. The impact of these slots has been discussed in next section. The rear view of the design is shown in Fig. 1c, which shows the full ground plane. All the dimensional parameters of the proposed single element design are tabulated in Table 1. Further, performance of the proposed single-port antenna in the frequency domain is depicted in Fig. 2. The reflection coefficient with the multiple deep resonant notches at about 3.44 GHz, 6.68 GHz and 8.42 GHz are well below the − 10 dB reference line and this confirms good impedance matching and multi-band operation at these frequencies. Figure 2b shows the related input impedance behavior, with the real part of the impedance approaching the desired impedance value of 50 Ω at the resonant frequencies, with the imaginary part of the impedance crossing zero, which reflects good reactive cancellation. The current distributions at 3.4 GHz, 6.68 GHz and 8.4 GHz show the mode evolution and verify the physical mechanism for the multiband operation as portrayed in Fig. 2c. At the lowest resonance frequency (3.4 GHz), the current is mainly confined to the central radiator and its surrounding conductive lines, implying the presence of a fundamental mode with an effective half-wavelength current path. At the second resonance frequency (6.68 GHz), the current path is spread to the outer parts and edges, which suggests the existence of higher-order modes with shorter effective current paths. At 8.4 GHz, the current is more concentrated and distributed in several parts, which suggests the presence of higher-order modes caused by the joint effects of the radiator and parasitic elements. Overall Fig. 2 shows that the proposed design has good matching and stable impedance characteristics at several operating bands.

Fig. 1.

Dimensional view of the proposed single port design (a) Isometric view (b) Top view (c) Bottom view.

Table 1.

Dimensional parameters of the proposed design.

Dimensional parameter	Value (mm)
Substrate length (SUBL)	40
Substrate width (SUBW)	40
Substrate thickness (SUBH)	1.6
Patch length (LP)	28
Patch width (WP)	28
Length of slot (S1)	8
Width of slot (S2)	1
Effective length of L-shaped slot (S3)	16
Width of L-shaped slot (S4)	1
Effective length of square (S5)	8
Width of the feedline (S6)	1
Length of the feedline (S7)	6

Fig. 2.

(a) S₁₁(dB) variations with frequency (b) Real and Imaginary part of the impedance (c) Current distribution.

Figure 3. shows how the proposed antenna geometry progressively evolved with simple radiating structure into a multi-resonant structure. In Step-I, a simple square microstrip patch fed by a microstrip line is used in which the patch size generates the fundamental resonance of the patch. In Step-II the edge slots are included to disrupt the distribution of current on the surface, effectively lengthening the path taken by current, adding more resonant modes and better impedance tuning. In Step-III, the patch is further modified by etching corner-shaped (L-shaped) slots that make many current loops and produce more resonances thus providing multi-band behavior. Lastly, in Step-IV, small square holes are cut into the structure, which adds more capacitance and inductance, and additional capacitive-inductive effects add to refine impedance matching, enhance gain and stabilize the multi-frequency operation. The system of progressive slot loading and structural perturbation provides a systematic approach of converting the original single-band patch into a multi-band antenna with better impedance properties and an enhanced level of resonance control. Also, the current distribution of each step is shown in Fig. 3e, In the preliminary step, the electric field is spread out, suggesting low resonance. With the addition of slots and other features, the confinement of field around edges and discontinuities shows stronger surface current concentration. These elements introduce capacitive (charge stored at slot edges) and inductive (increased current paths) effects, resulting in LC resonances. In the final step, more field confinement around the slots and radiators shows better coupling, impedance matching, and the introduction of multiple resonances, leading to better gain and bandwidth.

Fig. 3.

Design steps of the proposed antenna (a) Step-I (b) Step-II (c) Step-III (d) Step-IV (e) Current distribution.

Figure 4a indicates the vaiations in the reflection coefficient throughout the antenna design development between Step-I and Step-IV. In Step-I, the patch square in its basic form has only one major resonance in the lower frequency band (around 3.4 GHz) with S₁₁ less than − 10 dB which signifies single band operation and narrow bandwidth. In Step-II, with the introduction of slots, a second resonance emerges in the mid-frequency range (around 6–7 GHz) and the impedance matching is again slightly enhanced which shows the influence of slot perturbation in creating a second resonant mode. A distinct triple-band response is now achieved in Step-III with deeper notches at the frequencies 3.4 GHz, 6.7 GHz, and somewhere near 8 GHz. Finally, in Step-IV, the fully optimized structure shows the deepest and most stable resonances with well below − 10 dB at all three operating bands, along with improved bandwidth and sharper return-loss characteristics. Overall, the S-parameter development indicates that every structural change increases multi-band generation and impedance matching, which transforms the antenna into a single-band radiator to a triple-band design. The return loss variaitons with resonant frequencies for all steps has been tabulated in Table 2. Moreover, there is a significant improvement in the gain at 6.68 GHz from step-III to Step-IV as depicted in Fig. 4b and c.

Fig. 4.

(a) Step wise S-parameters (b) Gain improvement from Step-III to Step-IV in terms of 3D polar plots at 6.68 GHz.

Table 2.

Resonating frequencies in each step.

Iterations	Resonating frequency	Return loss
Step-I	3.46	−20.7
	7.76	−11.4
Step-II	2.9	−10.03
	6.58	−11.3
Step-III	6.62	−13.02
	8.28	−14.8
Step-IV	3.44	−18.6
	6.68	−22.3
	8.42	−15.4

The parametrical analysis of the reflection coefficient of the proposed single element antenna with the variation of the slot dimensions is depicted in Fig. 5. Figure 5a indicates that change in effective length of the L-shaped slot (S3) parameter (i.e. S3 = 14 to 18 mm) greatly influences the impedance matching and the resonant frequencies, especially the mid and upper bands meaning that mostly slot controls the higher-order resonance and bandwidth tuning. Figure 5b is used to make minor adjustments to the matching characteristics by varying width of the L-shaped slot parameter S4 (i.e. S4 = 0.5 to 2.5 mm) to adjust the resonant dips, and to change the depth of S₁₁, which indicates that it can maximize coupling and improve impedance matching without radically changing the operating bands. Figure 5c showed that effective length of the square S5 (i.e. S5 = 6 to 10 mm) variation has a pronounced effect on the second and third resonant frequencies, leading to observable changes in frequencies and change in the return loss levels, indicating that the length of the square is the primary factor in controlling multiband behaviour and bandwidth. It can be observed that the values S3 = 16 mm, S4 = 1 mm, and S5 = 8 mm provide the best performance compared to other nearby variations. Any deviation from these values results in impedance mismatch, resonance shifts, or deterioration in multiband behaviour. Therefore, these dimensions were selected as the optimized parameters to meet the desired performance requirements. On the whole, the parametric study proves that with special consideration of slot dimensions, the resonance tuning, impedance matching, and bandwidth enhancement of the antenna could be accurately controlled.

Fig. 5.

Parametric variations of the proposed design (a) Effective length of L-shaped slot (S3) (b) Width of L-shaped slot (S4) (c) Effective length of square (S5).

Frequency selective surface array structure

A FSS plate is a planar structure consisting of several unit cells spaced on both sides. The frequency at which the FSS reflects incident waves depends on factors such as unit cell design, mutual spacing, substrate characteristics, and angle of incidence. Unit cells can be designed as either a Slot or a Patch as depicted in the Fig. 6a. Patch FSS acts as a low-pass filter, while Slot FSS acts as a high-pass filter. The unit cell used in this work features a simple design comprising a square slot at the four corners of the square patch, developed on a Roger/RT Duroid substrate with ε_r = 2.2 and thickness h = 1.6 mm. The overall dimensions of the unit cell are 20 mm × 20 mm. A square patch of 18 mm × 18 mm is placed at the center of the substrate, from which a 4 square slots of 7 mm × 7 mm is etched out as depicted in Fig. 6b and tabulated in Table 3. The unit cell was designed using HFSS software with two Floquet port configurations. The design parameters of the FSS unit cell are selected in such a way that it must cover the wide bandwidth of the corresponding radiating antenna for which it is intended to reflect incident waves. Therefore, the size of the patch has been optimized as corresponding results have been shown in Fig. 6c. Additionally, a substantial dip in the S21 curve within the bandwidth of interest is required. The proposed single unit FSS cell achieved the wide bandwidth of 4.7 GHz (4.5–9.2 GHz) centred around 6.67 GHz with a maximum dip in the S-curve of around − 48 dB as shown in Fig. 6d. Beyond bandwidth and S21 notch level, the reflection phase of the FSS surface is also a critical parameter; it should decrease linearly to enable the required phase reflection to the radiating antenna. The proposed FSS is now modelled into an equivalent lumped LC network, as shown in the Fig. 6e. The metal strips, current paths are modelled as inductance (L) and the gaps between elements are modelled as capacitances (C), with two ports as shown on both sides. The periodic LC structure thus controls the resonance of the FSS surface. The extracted values of L (L ≈ 1.2 nH) and C (C ≈ 0.079 pF) lead to a resonance at 6.6 GHz, which is in agreement with the full-wave analysis, as shown in Fig. 6f. The circuit simulation (ADS) and full-wave simulation (HFSS) were compared in terms of S11 and S12 which are found in good agreement and prove the proposed equivalent circuit model.

Fig. 6.

Unit cell FSS (a) Port excitation (b) Dimensional view (c) Parametric study (d) S-parameters (e) Equivalent circuit of FSS unit cell (f) Comparison of the S-parameters.

Table 3.

Dimensional parameters of the proposed design.

Dimensional parameter	Value (mm)
Substrate length (Sx)	20
Substrate width (Sy)	20
FSS length and width (Px)	18
Slot length and width (SLx and SLy)	7
Central strip thickness (T1)	2
Outer strip thickness (T2)	1

Figure 7 is used to demonstrate the FSS array design as the extension of single element explained in the previous section and its impact on the gain characteristics of a single port antenna. 4 × 4 FSS structure as shown in Fig. 7a depicts that it is a periodic pattern of unit cells etched on a dielectric substrate and the total size is L_FSS = 80 mm × W_FSS = 80 mm. The FSS is made to be a superstrate layer and could be overlaid on the antenna to improve its radiation properties. The side view of the assembly has been given in Fig. 7b, and the air gap G created between the antenna and the FSS layer is very important, which is an essential parameter that determines the electromagnetic coupling and the performance of the system overall. In Fig. 7c, the entire assembly of the antenna in which the single port antenna is mounted on the 4 × 4 FSS array. The antenna is centrally located above the FSS structure whereby the periodic surface will get to interact with the radiated electromagnetic waves and either redirect or focus it in the broadside direction to enhance the directivity and gain of the antenna. The comparison has been made on the gain of the antenna with and without the FSS in a frequency range between 3.0 and 8.5 GHz. As can be seen, the antenna with the FSS superstrate (red curve) has a much higher peak realised gain than the antenna without the FSS (black curve). With FSS the gain is about 7.5 to 8dBi at 3.5 GHz and 8 GHz, but the antenna alone can hardly go above 3dBi much of the band as depicted in Fig. 7d. In addition, the figure also shows the effect of gap (G), which is an important design parameter as it determines the phase of the reflected wave from the FSS surface with respect to the radiated wave from the antenna. When G = 1 mm, the gain is relatively lower and has more pronounced nulls, especially in the higher frequency band. This is because the near-field effect is significant and the phase mismatch results in destructive interference. When G = 1.5 mm, the gain is significantly enhanced across all frequency bands and the curve has the highest and most consistent gain. This suggests that the reflected waves are closer to in-phase (constructive interference) with the radiation of the antenna. For G = 2 mm, the gain is still enhanced but has a slight decrease and ripples compared to 1.5 mm. This is because the spacing is beginning to move away from the in-phase condition, causing phase errors. This enhancement shows that the FSS is a good reflective surface acting as a partially reflective resonant cavity reinforcing radiation in the preferred direction; a significant enhancement of the gain over the entire frequency band.

Fig. 7.

Structure of FSS array (a) 4 × 4 FSS structure (b) Side view (c) Single port antenna mounted on FSS array (d) Gain comparison.

MIMO antenna configuration

The two configurations of a 2-element MIMO antenna array and their corresponding S-parameter performance has been explained in this section and illustrated in Fig. 8. The single port antenna element has been scaled to a 1 × 2 MIMO antenna array by placing two identical antenna elements on a common substrate in two distinct spatial configurations to assess the behaviors of isolation and impedance matching. Figure 8a displays linear arrangement, in which both antenna elements are arranged side-by-side along an identical axis on the substrate with the size of 40 mm × 80 mm. In this design, the two components are co-polarized oriented in the same direction and the feeding ports are placed on the same side of the substrate. This configuration is more prone to increased mutual coupling between the elements, because the elements are oriented parallel and close together. Further, Fig. 8b represents the Orthogonal arrangement in which one antenna element is turned 90° with regard to another element on the same substrate. This orthogonal arrangement brings polarization diversity between the two elements, which in effect reduces mutual coupling and elevates port isolation, turn out to be more effective arrangement to implement MIMO. The S-parameter comparison of the two configurations in the frequency range of 1 to 10 GHz is provided Fig. 8c. S₁₁ and S₂₂ reflection coefficients of the two configurations exhibit excellent impedance matching at three different frequency ranges around 3.44 GHz, 6.88 GHz, and 8.44 GHz, which proves the existence of the tri-band nature of the antenna. the linear configuration has isolation of −18 dB and the orthogonal configuration has isolation of −51 dB at 3.44 GHz. Similar improvement pattern can be observed at other two resonating frequencies. Generally, the orthogonal setup will provide a greater isolation in the high-frequency bands because of the polarization variety added by the rotated element arrangement and thus is more appropriate in practice to MIMO applications where low mutual coupling is of utmost importance to enhance channel capacity and system functionality. Further, the improvement in the isolation has been verified by the current distribution as depicted in Fig. 8d. For the linear configuration, the surface currents clearly show that significant current is flowing from the excited element to the adjacent element, which suggests strong mutual coupling via near-field effects. This leads to increased energy leakage and consequently poor isolation. However, the surface current distribution for the orthogonal configuration indicates that the currents on the adjacent element are significantly reduced when the first element is excited. The orthogonal configuration changes the current flow and reduces the field coupling between the two elements.

Fig. 8.

2-element MIMO Configurations (a) Linear (b) Orthogonal (c) S-parameters (d) Current distribution.

Now, a further extension of the previously discussed 2-element MIMO array is now scaled up to incorporate four antenna elements on a single common substrate for enhanced MIMO functionality and spatial diversity. Figure 9a shows the physical configuration of 4 element MIMO antenna. The four elements are symmetrically placed on all the four corners of a square substrate and each element is orthogonally tilted to its neighbors. The orthogonal placement strategy directly expands the orthogonal configuration in the 2-element MIMO case where, by simply rotation of neighboring elements by 90°. It was observed that port isolation was enhanced by polarization diversity. All its elements are fed on the outer edge of the substrate, and the general structure provides a spatial distance and orthogonality of polarization between neighboring ports, which act together to reduce mutual coupling between all element pairs. The S-parameter results of the 4-element MIMO array at a frequency range of 2–9 GHz is shown in Fig. 9b. The reflection coefficients S₁₁, S₂₂, S₃₃, and S₄₄ overlap almost perfectly, which proves that all four elements are well-matched in the three operating frequency bands. As far as the mutual coupling is concerned, the transmission coefficients S₁₂, S₁₃ and S₁₄ are the isolation between adjacent elements, diagonally opposite elements and other combinations of ports. The S₁₃ and S₁₄ parameters are smaller than − 50 dB over the majority of the band, which is a great sign of isolation between non- adjacent and diagonally opposite ports. The S₁₂ parameter, which is an indicator of a coupling between adjacent ports, indicates a bit higher in terms of the level of coupling, but at a sufficiently low level, especially due to the orthogonal nature of the elements next to each other. The 4-element MIMO system is effective in ensuring good impedance matching and high inter-element isolation in all three frequency bands, which makes it effective in the requirement of modern wireless communication networks which require high data throughput and spatial multiplexing.

Fig. 9.

4-element MIMO Configuration (a) Antenna elements arrangement (b) S-parameters.

MIMO antenna configuration with FSS

The above discussed 4-element MIMO antenna design is mounted on the FSS plate as shown in Fig. 10. The layered structure shown in Fig. 10a is the stack of top radiating patch on Substrate 1, ground plane, FSS layer and finally Substrate 2 at the bottom with the total board size of 80 mm × 80 mm with G = 1.5 mm between FSS and antenna substrate. The fabricated protype of the design is shown in Fig. 10b. The simulated and measured S₁₁ results are compared as depicted in Fig. 10c, where three resonant dips appear below the − 10dB level at about 3.4 GHz, 6.6 GHz and 8.4 GHz, indicating triple-band operation with good simulation and measurement agreement. The simulated and measured isolation S12, S13, S14 over the 2–9 GHz operating band are plotted in Fig. 10d with the isolation values being mainly below − 40 dB over both operating bands, which demonstrates a high level of port-to-port isolation between the MIMO elements, due to the use of the FSS layer as a shield that suppresses the mutual coupling between the antenna elements.

Fig. 10.

4-element MIMO with FSS Configuration (a) stacked arrangement (b) Fabricated prototype (c) Return loss comparison (d) Isolation comparison.

Figure 11 presents the simulated and measured radiation patterns of the proposed MIMO antenna at three operating frequencies 3.4 GHz, 6.68 GHz, and 8.4 GHz for both the ϕ = 0° and ϕ = 90° planes, comparing co-polarization (Co-P) and cross-polarization (X-P) components. At 3.4 GHz (Fig. 11a), the radiation pattern exhibits a nearly omnidirectional, broad Co-P pattern in both planes with relatively low X-P levels, and good agreement between simulated and measured results. At 6.68 GHz (Fig. 11b), the pattern becomes more directional with a cleaner main lobe visible, particularly in the ϕ = 90° plane, and the X-P levels remain well suppressed compared to the Co-P, indicating good polarization purity at this frequency. At 8.4 GHz (Fig. 11c), the pattern shows increased complexity with minor lobes and slight asymmetry, which is typical at higher frequencies due to surface wave excitation and higher-order modes, and while the measured results follow the general trend of the simulated patterns except minor discrepancies are observed likely due to fabrication tolerances and measurement environment effects. Overall, the Co-P levels consistently dominate over X-P across all three frequencies in both planes, confirming the antenna’s stable radiation characteristics and acceptable polarization discrimination throughout its operating bandwidth. The cross-polarization levels are significantly lower (generally below − 15 to − 20 dB in the main radiation direction), confirming minimal undesired radiation components. Additionally, the simulated and measured patterns show close agreement, validating the design accuracy and indicating negligible losses, which implies high radiation efficiency. The patterns maintain similar shapes across the operating bands, demonstrating good pattern stability with only minor variations at higher frequencies due to increased higher-order mode effects. The comparison among simulated and measured peak realized gain with radiation efficiency of the proposed antenna at different operating frequencies is illustrated in Fig. 12, which shows a consistent increase in gain with the integration of FSS. Specifically, the gain improves from approximately 6 dBi around 3.4 GHz to about 12 dBi at same frequency, indicating enhanced radiation performance with the integration of FSS. Similarly, The efficiency remains high across all operating bands, improved from about the range 81%−85% to 94%−97%.

Fig. 11.

Simulated and measured radiation patterns (a) 3.4 GHz (b) 6.68 GHz (c) 8.4 GHz.

Fig. 12.

Simulated and Measured Gain with Radiation Efficiency.

MIMO diversity parameters

The Diversity analysis of the presented triple band MIMO antenna system is required to ensure the reliable performance and enhanced diversity gain especially in multipath signal environments. Also, the adjacent radiating elements for the discussed MIMO antenna must be perfectly isolated to validate good diversity performance. The diversity performance is evaluated for the proposed MIMO antenna design in this section. The evaluated parameters are ECC, DG, and CCL, TARC and MEG to validate its utilisation multiband applications.

Envelope correlation coefficient (ECC)

ECC determines the relation between the distinct elements of the MIMO antenna, in terms of the crucial diversity parameter, ECC, using S-parameters, as detailed in calculated using Eq. (1).

1

The ECC for simulation and measurement results are calculated and plotted as indicated in Fig. 13a. It is depicted that the designed MIMO antenna demonstrates favourable ECC performance in the desirable resonating bands. The ECC value remains well below the desirable limit of 0.5, confirming uncorrelated operation with robust isolation among various antenna elements, satisfying the desired condition of less than 0.5 for MIMO antenna applications, and validating consistently low ECC values across all three operational bands.

Fig. 13.

Simulated and Measured Diversity Parameters (a) ECC (b) DG (c) CCL (d) TARC (e) MEG.

Diversity gain (DG)

DG is another crucial parameter to determine the diversity performance of MIMO antenna. It evaluates the Signal-to-Noise (S/N) ratio of the MIMO antenna system in comparison to the single-radiating element and is evaluated using Eq. 2. The diversity gain has an ideal value of 10. Further, measured results for the DG of designed triple element MIMO antenna are plotted and compared with simulated values in Fig. 13b. The simulated and measured value in the desired resonant bands are observed to range between 9.999 dB and 10 dB, fulfilling the desired condition for MIMO applications. The value of DG is aligned to the simulated value of 10 dB at the desired bands, including resonant frequencies confirming the significant contribution of the presented MIMO antenna design in overcoming the multipath fading and enhancing the reliability of the wireless link and validating its practical utility for better performance in wireless communication systems.

2

Channel capacity loss (CCL)

The channel capacity as specified in Shannon’s theorem, determines the maximum information transmission rate relying majorly on the performance of other diversity parameters increasing linearly with the increase in the number of antenna elements in MIMO system configuration. Thus, the channel capacity loss (CCL) is the crucial parameter to evaluate the performance of MIMO antenna compared to that of single element and assess the degradation in the system performance calculated using Eqs. (3)-(6) below

3

4

5

6

The ideal value of CCL must be 0 bits/s/Hz and generally below 0.5 bits/s/Hz to be considered acceptable for high-performance systems. The measured CCL for the designed MIMO antenna compared to its simulated value as shown in Fig. 13c are below 0.05 b/s/Hz and 0.0025 b/s/Hz, respectively satisfying the requirement of maintaining CCL below 0.5 bits/s/Hz across the desired resonating bands. Additionally, the designed antenna exhibits CCL = 0.0 bits/s/Hz at resonant frequency of 3.4 GHz, 6.6 GHz and 8.4 GHz, justifying a state of maximum spectral efficiency directly corresponding to a near-zero ECC value, signifying high port-to-port isolation, ensuring the system’s high data throughput potential is maintained within its theoretical limit.

Total active reflection coefficient (TARC)

Finally, the TARC parameter measures the diversity performance in terms of ratio of total reflected power to that of total incident power across the designed MIMO antenna and is evaluated using Eq. (7):

7

Here, a_k and b_k, represents the incident and the reflected wave at the k_th port respectively with N denoting the total number of ports. The TARC parameter ideally should be of less than 0 dB value and preferably below − 10 dB in the operating region, and must ensure the reflection of 10% of the active input power. The measured TARC (τ) value compared to its simulated value as shown in Fig. 13d w.r.t. frequency (GHz). Also, the designed MIMO antenna exhibits TARC < −10 dB at the desired resonant frequencies, indicating efficient power transmission. Thus, the proposed triple band element MIMO antenna depicts optimal TARC performance in the desired frequency bands ensuring minimal reflections.

Mean effective gain (MEG)

Another crucial diversity performance metric for MIMO antenna characterisation in wireless channels is the MEG. When multipath fading signals are present, MEG assesses the antenna element’s capacity to receive electromagnetic signals. The ratio of the mean received power to the mean incident power of antenna elements is thus shown in Fig. 13e. Every MEG ratio in the operating bands is getting closer to 1 dB.

8

9

10

11

MEG(1,2)=MEG1/MEG2 12

MEG(1,3)=MEG1/MEG3 13

MEG(1,4)=MEG1/MEG4 14

The proposed design demonstrates several notable advantages over the referenced state-of-the-art works, as summarized in the comparison Table 4. Unlike most prior works that support only single-band^19,25,28 or dual-band^26,27 operation, the proposed antenna uniquely achieves multiband operation spanning three distinct frequency bands at 3.4 GHz, 6.68 GHz, and 8.4 GHz, making it well-suited for next-generation multi-standard wireless systems. Furthermore, the proposed design achieves an isolation of > 40 dB, which is substantially higher than²⁵(> 25 dB) and²⁷(> 20 dB), while several comparative designs^19,26,28 do not report isolation at all, underscoring the superior port decoupling of the proposed structure. In terms of gain, the proposed antenna achieves 13.8 dBi, which is the highest among all compared works, outperforming¹⁹(9.5 dBi)²⁶, (9 dBi)²⁷, (7.5 dBi)²⁸, (7 dBi), and²⁵(6.32 dBi) reflecting its enhanced radiation efficiency. The proposed work demonstrates clear novelty by achieving triple-band MIMO operation (3.4, 6.68, and 8.4 GHz) with an integrated FSS layer, offering significantly higher gain (~ 13.8 dBi) and isolation (> 40 dB) compared to existing designs. Additionally, it ensures compactness with an optimized FSS size and maintains a simple, fabrication-friendly structure.

Table 4.

Comparison of the proposed design with state-of-the-art literature.

Reference	Number of ports	Dimensions of the antenna (mm2)	Operating frequency (GHz)	MIMO antenna gain (dBi)	FSS array	Isolation (dB)
19	1	40 × 30	3.9–9.5	9.5	58 × 28	N.A.
24	8	60 × 60	14–18	6.32	N.A.	> 25
25	1	80 × 80	2.3–2.65 4.9–6.45	9	100 × 70	N.A.
31	4	16 × 29.4 (Monopole)	2.35–2.7 3.3–3.6	7.5	100 × 100	> 20
32	1	50 × 50	3.6–3.9	7	61 × 61	N.A.
Prop.	4	80 × 80	3.4, 6.68 & 8.4	13.8	80 × 80	> 40

Conclusion

A compact multiband slotted MIMO antenna with FSS has been introduced in this paper for the modern wireless communication. The antenna has the capability of working at 3.4 GHz, 6.68 GHz, and 8.4 GHz, and is therefore suitable in sub-6 GHz 5G, C-band wireless networks, and X-band communication and radar applications. The proposed design exhibits the improved peak realized gain in the range from 12 to 14 dBi and radiation efficiency ranges from 94 to 97% with the integration of the FSS structure beneath 4 element MIMO antenna array. The diversity parameters have also been analyzed to verify the enhanced channel capacity in the MIMO configuration. Finally, the simulated and measured results of the proposed design show good agreement, and future research may focus on incorporating advanced features such as beam steering, enhanced gain, and improved spectral efficiency in massive MIMO systems.

Author contributions

A.K. formulated the conceptA.K, K.G. and P.S. wrote the main manuscript textR.K. prepared figures and tablesAll the authors have reviewed the manuscript.

Funding

Open access funding provided by Manipal University Jaipur. This work was conducted with no external source of funding. The research was funded by the authors and conducted independently.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.