This work proposes a compact integrated multi-input multiple-output (MIMO) metasurface (MS) wideband antenna for sub-6 GHz fifth generation (5G) wireless communication systems. The obvious novelty of the proposed MIMO system is its wide operating bandwidth, high gain, small intercomponent clearances, and excellent isolation within the MIMO components. The antenna’s radiating spot is truncated diagonally, partially grounded, and metasurfaces are used to improve the antenna’s performance. The proposed prototype integrated single MS antenna has miniature dimensions of 0.58λ × 0.58λ × 0.02λ. Simulation and measurement results demonstrate wideband performance from 3.11 GHz to 7.67 GHz, including the highest gain achieved of 8 dBi. The four-element MIMO system is designed so that each antenna is orthogonal to each other while maintaining a compact size and wideband performance from 3.2 to 7.6 GHz. The proposed MIMO prototype is designed and fabricated on Rogers RT5880 substrate with low loss and miniaturized dimensions of 1.05? 1.05? 0.02?, and its performance is evaluated using the proposed square closed ring resonator array with a 10 x 10 split ring. The basic material is the same. The proposed backplane metasurface significantly reduces antenna back radiation and manipulates electromagnetic fields, thereby improving the bandwidth, gain, and isolation of MIMO components. Compared with existing MIMO antennas, the proposed 4-port MIMO antenna achieves a high gain of 8.3 dBi with an average overall efficiency of up to 82% in the 5G sub-6 GHz band and is in good agreement with the measured results. Moreover, the developed MIMO antenna exhibits excellent performance in terms of envelope correlation coefficient (ECC) of less than 0.004, diversity gain (DG) of about 10 dB (>9.98 dB) and high isolation between MIMO components (>15.5 dB ). characteristics. Thus, the proposed MS-based MIMO antenna confirms its applicability for sub-6 GHz 5G communication networks.
5G technology is an incredible advancement in wireless communications that will enable faster and more secure networks for billions of connected devices, provide user experiences with “zero” latency (latency of less than 1 millisecond), and introduce new technologies, including electronics. Medical care, intellectual education. , smart cities, smart homes, virtual reality (VR), smart factories and the Internet of Vehicles (IoV) are changing our lives, society and industries1,2,3. The US Federal Communications Commission (FCC) divides the 5G spectrum into four frequency bands4. The frequency band below 6 GHz is of interest to researchers because it allows long-distance communications with high data rates5,6. The sub-6 GHz 5G spectrum allocation for global 5G communications is shown in Figure 1, indicating that all countries are considering sub-6 GHz spectrum for 5G communications7,8. Antennas are an important part of 5G networks and will require more base station and user terminal antennas.
Microstrip patch antennas have the advantages of thinness and flat structure, but are limited in bandwidth and gain9,10, so much research has been done to increase the gain and bandwidth of the antenna; In recent years, metasurfaces (MS) have been widely used in antenna technologies, especially to improve gain and throughput11,12, however, these antennas are limited to a single port; MIMO technology is an important aspect of wireless communications because it can use multiple antennas simultaneously to transmit data, thereby improving data rates, spectral efficiency, channel capacity, and reliability13,14,15. MIMO antennas are potential candidates for 5G applications because they can transmit and receive data over multiple channels without requiring additional power16,17. The mutual coupling effect between MIMO components depends on the location of the MIMO elements and the gain of the MIMO antenna, which is a major challenge for researchers. Figures 18, 19, and 20 show various MIMO antennas operating in the 5G sub-6 GHz band, all demonstrating good MIMO isolation and performance. However, the gain and operating bandwidth of these proposed systems are low.
Metamaterials (MMs) are new materials that do not exist in nature and can manipulate electromagnetic waves, thereby improving the performance of antennas21,22,23,24. MM is now widely used in antenna technology to improve the radiation pattern, bandwidth, gain, and isolation between antenna elements and wireless communication systems, as discussed in 25, 26, 27, 28. In 2029, a four-element MIMO system based on metasurface, in which the antenna section is sandwiched between the metasurface and the ground without an air gap, which improves MIMO performance. However, this design has a larger size, lower operating frequency and complex structure. An electromagnetic bandgap (EBG) and ground loop are included in the proposed 2-port wideband MIMO antenna to improve the isolation of MIMO30 components. The designed antenna has good MIMO diversity performance and excellent isolation between two MIMO antennas, but using only two MIMO components, the gain will be low. In addition, in31 also proposed an ultra-wideband (UWB) dual-port MIMO antenna and investigated its MIMO performance using metamaterials. Although this antenna is capable of UWB operation, its gain is low and the isolation between the two antennas is poor. The work in32 proposes a 2-port MIMO system that uses electromagnetic bandgap (EBG) reflectors to increase the gain. Although the developed antenna array has high gain and good MIMO diversity performance, its large size makes it difficult to apply in next-generation communication devices. Another reflector-based broadband antenna was developed in 33, where the reflector was integrated under the antenna with a larger 22 mm gap, exhibiting a lower peak gain of 4.87 dB. Paper 34 designs a four-port MIMO antenna for mmWave applications, which is integrated with the MS layer to improve the isolation and gain of the MIMO system. However, this antenna provides good gain and isolation, but has limited bandwidth and poor mechanical properties due to the large air gap. Similarly, in 2015, a three-pair, 4-port bowtie-shaped metasurface-integrated MIMO antenna was developed for mmWave communications with a maximum gain of 7.4 dBi. B36 MS is used on the backside of a 5G antenna to increase the antenna gain, where the metasurface acts as a reflector. However, the MS structure is asymmetric and less attention has been paid to the unit cell structure.
According to the above analysis results, none of the above antennas have high gain, excellent isolation, MIMO performance and wideband coverage. Therefore, there is still a need for a metasurface MIMO antenna that can cover a wide range of 5G spectrum frequencies below 6 GHz with high gain and isolation. Considering the limitations of the above-mentioned literature, a wideband four-element MIMO antenna system with high gain and excellent diversity performance is proposed for sub-6 GHz wireless communication systems. In addition, the proposed MIMO antenna exhibits excellent isolation between MIMO components, small element gaps, and high radiation efficiency. The antenna patch is truncated diagonally and placed on top of the metasurface with a 12mm air gap, which reflects back radiation from the antenna and improves antenna gain and directivity. In addition, the proposed single antenna is used to create a four-element MIMO antenna with superior MIMO performance by positioning each antenna orthogonally to each other. The developed MIMO antenna was then integrated on top of a 10 × 10 MS array with a copper backplane to improve emission performance. The design features a wide operating range (3.08-7.75 GHz), high gain of 8.3 dBi and high average overall efficiency of 82%, as well as excellent isolation of greater than −15.5 dB between MIMO antenna components. The developed MS-based MIMO antenna was simulated using 3D electromagnetic software package CST Studio 2019 and validated through experimental studies.
This section provides a detailed introduction to the proposed architecture and single antenna design methodology. In addition, the simulated and observed results are discussed in detail, including scattering parameters, gain, and overall efficiency with and without metasurfaces. The prototype antenna was developed on a Rogers 5880 low loss dielectric substrate with a thickness of 1.575mm with a dielectric constant of 2.2. To develop and simulate the design, the electromagnetic simulator package CST studio 2019 was used.
Figure 2 shows the proposed architecture and design model of a single-element antenna. According to well-established mathematical equations37, the antenna consists of a linearly fed square radiating spot and a copper ground plane (as described in step 1) and resonates with a very narrow bandwidth at 10.8 GHz, as shown in Figure 3b. The initial size of the antenna radiator is determined by the following mathematical relationship37:
Where \(P_{L}\) and \(P_{w}\) are the length and width of the patch, c represents the speed of light, \(\gamma_{r}\) is the dielectric constant of the substrate. , \(\gamma_{reff }\) represents the effective dielectric value of the radiation spot, \(\Delta L\) represents the change in spot length. The antenna backplane was optimized in the second stage, increasing the impedance bandwidth despite the very low impedance bandwidth of 10 dB. In the third stage, the feeder position is moved to the right, which improves the impedance bandwidth and impedance matching of the proposed antenna38. At this stage, the antenna demonstrates an excellent operating bandwidth of 4 GHz and also covers the spectrum below 6 GHz in 5G. The fourth and final stage involves etching square grooves in opposite corners of the radiation spot. This slot significantly expands the 4.56 GHz bandwidth to cover sub-6 GHz 5G spectrum from 3.11 GHz to 7.67 GHz, as shown in Figure 3b. Front and bottom perspective views of the proposed design are shown in Figure 3a, and the final optimized required design parameters are as follows: SL = 40 mm, Pw = 18 mm, PL = 18 mm, gL = 12 mm, fL = 11. mm, fW = 4 .7 mm, c1 = 2 mm, c2 = 9.65 mm, c3 = 1.65 mm.
(a) Top and rear views of the designed single antenna (CST STUDIO SUITE 2019). (b) S-parameter curve.
Metasurface is a term that refers to a periodic array of unit cells located at a certain distance from each other. Metasurfaces are an effective way to improve antenna radiation performance, including bandwidth, gain, and isolation between MIMO components. Due to the influence of surface wave propagation, metasurfaces generate additional resonances that contribute to improved antenna performance39. This work proposes an epsilon-negative metamaterial (MM) unit operating in the 5G band below 6 GHz. The MM with a surface area of 8mm×8mm was developed on a low loss Rogers 5880 substrate with a dielectric constant of 2.2 and a thickness of 1.575mm. The optimized MM resonator patch consists of an inner circular split ring connected to two modified outer split rings, as shown in Figure 4a. Figure 4a summarizes the final optimized parameters of the proposed MM setup. Subsequently, 40 × 40 mm and 80 × 80 mm metasurface layers were developed without a copper backplane and with a copper backplane using 5 × 5 and 10 × 10 cell arrays, respectively. The proposed MM structure was modeled using 3D electromagnetic modeling software “CST studio suite 2019”. A fabricated prototype of the proposed MM array structure and measurement setup (dual-port network analyzer PNA and waveguide port) is shown in Figure 4b to validate the CST simulation results by analyzing the actual response. The measurement setup used an Agilent PNA series network analyzer in combination with two waveguide coaxial adapters (A-INFOMW, part number: 187WCAS) to send and receive signals. A prototype 5×5 array was placed between two waveguide coaxial adapters connected by coaxial cable to a two-port network analyzer (Agilent PNA N5227A). The Agilent N4694-60001 calibration kit is used to calibrate the network analyzer in a pilot plant. The simulated and CST observed scattering parameters of the proposed prototype MM array are shown in Figure 5a. It can be seen that the proposed MM structure resonates in the 5G frequency range below 6 GHz. Despite the small difference in bandwidth of 10 dB, the simulated and experimental results are very similar. The resonant frequency, bandwidth, and amplitude of the observed resonance are slightly different from the simulated ones, as shown in Figure 5a. These differences between observed and simulated results are due to manufacturing imperfections, small clearances between the prototype and the waveguide ports, coupling effects between the waveguide ports and array components, and measurement tolerances. In addition, proper placement of the developed prototype between the waveguide ports in the experimental setup may result in a resonance shift. In addition, unwanted noise was observed during the calibration phase, which led to discrepancies between the numerical and measured results. However, apart from these difficulties, the proposed MM array prototype performs well due to the strong correlation between simulation and experiment, making it well suited for sub-6 GHz 5G wireless communication applications.
(a) Unit cell geometry (S1 = 8 mm, S2 = 7 mm, S3 = 5 mm, f1, f2, f4 = 0.5 mm, f3 = 0.75 mm, h1 = 0.5 mm, h2 = 1 .75 mm) (CST STUDIO SUITE) ) 2019) (b) Photo of the MM measuring setup.
(a) Simulation and verification of the scattering parameter curves of the metamaterial prototype. (b) Dielectric constant curve of an MM unit cell.
Relevant effective parameters such as effective dielectric constant, magnetic permeability, and refractive index were studied using built-in post-processing techniques of the CST electromagnetic simulator to further analyze the behavior of the MM unit cell. The effective MM parameters are obtained from the scattering parameters using a robust reconstruction method. The following transmittance and reflection coefficient equations: (3) and (4) can be used to determine the refractive index and impedance (see 40).
The real and imaginary parts of the operator are represented by (.)’ and (.)” respectively, and the integer value m corresponds to the real refractive index. Dielectric constant and permeability are determined by the formulas \(\varepsilon { } = { }n/z,\) and \(\mu = nz\), which are based on impedance and refractive index, respectively. The effective dielectric constant curve of the MM structure is shown in Figure 5b. At the resonant frequency, the effective dielectric constant is negative. Figures 6a,b show the extracted values of effective permeability (μ) and effective refractive index (n) of the proposed unit cell. Notably, the extracted permeabilities exhibit positive real values close to zero, which confirms the epsilon-negative (ENG) properties of the proposed MM structure. Moreover, as shown in Figure 6a, the resonance at permeability close to zero is strongly related to the resonant frequency. The developed unit cell has a negative refractive index (Fig. 6b), which means that the proposed MM can be used to improve the antenna performance21,41.
The developed prototype of a single broadband antenna was fabricated to experimentally test the proposed design. Figures 7a,b show images of the proposed prototype single antenna, its structural parts and the near-field measurement setup (SATIMO). To improve the antenna performance, the developed metasurface is placed in layers under the antenna, as shown in Figure 8a, with height h. A single 40mm x 40mm double-layer metasurface was applied to the rear of the single antenna at 12mm intervals. In addition, a metasurface with a backplane is placed on the rear side of the single antenna at a distance of 12 mm. After applying the metasurface, the single antenna shows a significant improvement in performance, as shown in Figures 1 and 2. Figures 8 and 9. Figure 8b shows the simulated and measured reflectance plots for the single antenna without and with metasurfaces. It is worth noting that the coverage band of an antenna with a metasurface is very similar to the coverage band of an antenna without a metasurface. Figures 9a,b show a comparison of the simulated and observed single antenna gain and overall efficiency without and with MS in the operating spectrum. It can be seen that, compared with the non-metasurface antenna, the gain of the metasurface antenna is significantly improved, increasing from 5.15 dBi to 8 dBi. The gain of the single-layer metasurface, dual-layer metasurface, and single antenna with backplane metasurface increased by 6 dBi, 6.9 dBi, and 8 dBi, respectively. Compared with other metasurfaces (single-layer and double-layer MCs), the gain of a single metasurface antenna with a copper backplane is up to 8 dBi. In this case, the metasurface acts as a reflector, reducing the antenna’s back radiation and manipulating the electromagnetic waves in-phase, thereby increasing the antenna’s radiation efficiency and hence the gain. A study of the overall efficiency of a single antenna without and with metasurfaces is shown in Figure 9b. It is worth noting that the efficiency of an antenna with and without a metasurface is almost the same. In the lower frequency range, the antenna efficiency decreases slightly. The experimental and simulated gain and efficiency curves are in good agreement. However, there are slight differences between the simulated and tested results due to manufacturing defects, measurement tolerances, SMA port connection loss, and wire loss. In addition, the antenna and MS reflector are located between the nylon spacers, which is another issue that affects the observed results compared to the simulation results.
Figure (a) shows the completed single antenna and its associated components. (b) Near-field measurement setup (SATIMO).
(a) Antenna excitation using metasurface reflectors (CST STUDIO SUITE 2019). (b) Simulated and experimental reflectances of a single antenna without and with MS.
Simulation and measurement results of (a) the achieved gain and (b) the overall efficiency of the proposed metasurface effect antenna.
Beam pattern analysis using MS. Single-antenna near-field measurements were carried out in the SATIMO Near-Field Experimental Environment of the UKM SATIMO Near-Field Systems Laboratory. Figures 10a, b show the simulated and observed E-plane and H-plane radiation patterns at 5.5 GHz for the proposed single antenna with and without MS. The developed single antenna (without MS) provides a consistent bidirectional radiation pattern with side lobe values. After applying the proposed MS reflector, the antenna provides a unidirectional radiation pattern and reduces the level of the back lobes, as shown in Figures 10a, b. It is worth noting that the proposed single antenna radiation pattern is more stable and unidirectional with very low back and side lobes when using a metasurface with a copper backplane. The proposed MM array reflector reduces the back and side lobes of the antenna while improving the radiation performance by directing the current in unidirectional directions (Fig. 10a, b), thereby increasing the gain and directivity. It was observed that the experimental radiation pattern was almost comparable to that of the CST simulations, but varied slightly due to misalignment of the various assembled components, measurement tolerances, and cabling losses. In addition, a nylon spacer was inserted between the antenna and the MS reflector, which is another issue affecting the observed results compared to the numerical results.
The radiation pattern of the developed single antenna (without MS and with MS) at a frequency of 5.5 GHz was simulated and tested.
The proposed MIMO antenna geometry is shown in Figure 11 and includes four single antennas. The four components of the MIMO antenna are arranged orthogonally to each other on a substrate of dimensions 80 × 80 × 1.575 mm, as shown in Figure 11. The designed MIMO antenna has an inter-element distance of 22 mm, which is smaller than the nearest corresponding inter-element distance of the antenna. MIMO antenna developed. In addition, part of the ground plane is located in the same way as a single antenna. The reflectance values of the MIMO antennas (S11, S22, S33, and S44) shown in Figure 12a exhibit the same behavior as a single-element antenna resonating in the 3.2–7.6 GHz band. Therefore, the impedance bandwidth of a MIMO antenna is exactly the same as that of a single antenna. The coupling effect between MIMO components is the main reason for the small bandwidth loss of MIMO antennas. Figure 12b shows the effect of interconnection on MIMO components, where the optimal isolation between MIMO components was determined. The isolation between antennas 1 and 2 is the lowest at about -13.6 dB, and the isolation between antennas 1 and 4 is the highest at about -30.4 dB. Due to its small size and wider bandwidth, this MIMO antenna has lower gain and lower throughput. Insulation is low, so increased reinforcement and insulation are required;
Design mechanism of the proposed MIMO antenna (a) top view and (b) ground plane. (CST Studio Suite 2019).
The geometric arrangement and excitation method of the proposed metasurface MIMO antenna are shown in Figure 13a. A 10x10mm matrix with dimensions of 80x80x1.575mm is designed for the back side of a 12mm high MIMO antenna, as shown in Figure 13a. Additionally, metasurfaces with copper backplanes are intended for use in MIMO antennas to improve their performance. The distance between the metasurface and the MIMO antenna is critical to achieve high gain while allowing constructive interference between the waves generated by the antenna and those reflected from the metasurface. Extensive modeling was performed to optimize the height between the antenna and the metasurface while maintaining quarter-wave standards for maximum gain and isolation between MIMO elements. The significant improvements in MIMO antenna performance achieved by using metasurfaces with backplanes compared to metasurfaces without backplanes will be demonstrated in subsequent chapters.
(a) CST simulation setup of the proposed MIMO antenna using MS (CST STUDIO SUITE 2019), (b) Reflectance curves of the developed MIMO system without MS and with MS.
The reflectances of MIMO antennas with and without metasurfaces are shown in Figure 13b, where S11 and S44 are presented due to the almost identical behavior of all antennas in the MIMO system. It is worth noting that the -10 dB impedance bandwidth of a MIMO antenna without and with a single metasurface is almost the same. In contrast, the impedance bandwidth of the proposed MIMO antenna is improved by dual-layer MS and backplane MS. It is worth noting that without MS, the MIMO antenna provides a fractional bandwidth of 81.5% (3.2-7.6 GHz) relative to the center frequency. Integrating the MS with the backplane increases the impedance bandwidth of the proposed MIMO antenna to 86.3% (3.08–7.75 GHz). Although dual-layer MS increases throughput, the improvement is less than that of MS with a copper backplane. Moreover, a dual-layer MC increases the size of the antenna, increases its cost, and limits its range. The designed MIMO antenna and metasurface reflector are fabricated and verified to validate the simulation results and evaluate the actual performance. Figure 14a shows the fabricated MS layer and MIMO antenna with various components assembled, while Figure 14b shows a photograph of the developed MIMO system. The MIMO antenna is mounted on top of the metasurface using four nylon spacers, as shown in Figure 14b. Figure 15a shows a snapshot of the near-field experimental setup of the developed MIMO antenna system. A PNA network analyzer (Agilent Technologies PNA N5227A) was used to estimate scattering parameters and to evaluate and characterize near-field emission characteristics in the UKM SATIMO Near-Field Systems Laboratory.
(a) Photos of SATIMO near-field measurements (b) Simulated and experimental curves of S11 MIMO antenna with and without MS.
This section presents a comparative study of the simulated and observed S-parameters of the proposed 5G MIMO antenna. Figure 15b shows the experimental reflectance plot of the integrated 4-element MIMO MS antenna and compares it with the CST simulation results. The experimental reflectances were found to be the same as the CST calculations, but were slightly different due to manufacturing defects and experimental tolerances. In addition, the observed reflectance of the proposed MS-based MIMO prototype covers the 5G spectrum below 6 GHz with an impedance bandwidth of 4.8 GHz, which means that 5G applications are possible. However, the measured resonant frequency, bandwidth, and amplitude differ slightly from the CST simulation results. Manufacturing defects, coax-to-SMA coupling losses, and outdoor measurement setups can cause differences between measured and simulated results. However, despite these shortcomings, the proposed MIMO performs well, providing strong agreement between simulations and measurements, making it well suited for sub-6 GHz 5G wireless applications.
The simulated and observed MIMO antenna gain curves are shown in Figures 2 and 2. As shown in Figures 16a,b and 17a,b, respectively, the mutual interaction of MIMO components is shown. When metasurfaces are applied to MIMO antennas, the isolation between MIMO antennas is significantly improved. The isolation plots between adjacent antenna elements S12, S14, S23 and S34 show similar curves, while the diagonal MIMO antennas S13 and S42 show similarly high isolation due to the greater distance between them. The simulated transmission characteristics of adjacent antennas are shown in Figure 16a. It is worth noting that in the 5G operating spectrum below 6 GHz, the minimum isolation of a MIMO antenna without a metasurface is -13.6 dB, and for a metasurface with a backplane – 15.5 dB. The gain plot (Figure 16a) shows that the backplane metasurface significantly improves the isolation between MIMO antenna elements compared to single- and double-layer metasurfaces. On adjacent antenna elements, single- and double-layer metasurfaces provide minimum isolation of approximately -13.68 dB and -14.78 dB, and the copper backplane metasurface provides approximately -15.5 dB.
Simulated isolation curves of MIMO elements without MS layer and with MS layer: (a) S12, S14, S34 and S32 and (b) S13 and S24.
Experimental gain curves of the proposed MS-based MIMO antennas without and with: (a) S12, S14, S34 and S32 and (b) S13 and S24.
The MIMO diagonal antenna gain plots before and after adding the MS layer are shown in Figure 16b. It is worth noting that the minimum isolation between diagonal antennas without a metasurface (antennas 1 and 3) is – 15.6 dB across the operating spectrum, and a metasurface with a backplane is – 18 dB. The metasurface approach significantly reduces the coupling effects between diagonal MIMO antennas. The maximum insulation for a single-layer metasurface is -37 dB, while for a double-layer metasurface this value drops to -47 dB. The maximum isolation of the metasurface with a copper backplane is −36.2 dB, which decreases with increasing frequency range. Compared to single- and double-layer metasurfaces without a backplane, metasurfaces with a backplane provide superior isolation across the entire required operating frequency range, especially in the 5G range below 6 GHz, as shown in Figures 16a, b. In the most popular and widely used 5G band below 6 GHz (3.5 GHz), single- and dual-layer metasurfaces have lower isolation between MIMO components than metasurfaces with copper backplanes (almost no MS) (see Figure 16a), b) . The gain measurements are shown in Figures 17a, b, showing the isolation of adjacent antennas (S12, S14, S34 and S32) and diagonal antennas (S24 and S13), respectively. As can be seen from these figures (Fig. 17a, b), the experimental isolation between MIMO components agrees well with the simulated isolation. Although there are minor differences between the simulated and measured CST values due to manufacturing defects, SMA port connections and wire losses. In addition, the antenna and MS reflector are located between the nylon spacers, which is another issue that affects the observed results compared to the simulation results.
studied the surface current distribution at 5.5 GHz to rationalize the role of metasurfaces in reducing mutual coupling through surface wave suppression42. The surface current distribution of the proposed MIMO antenna is shown in Figure 18, where antenna 1 is driven and the rest of the antenna is terminated with a 50 ohm load. When antenna 1 is energized, significant mutual coupling currents will appear at adjacent antennas at 5.5 GHz in the absence of a metasurface, as shown in Figure 18a. On the contrary, through the use of metasurfaces, as shown in Fig. 18b–d, the isolation between adjacent antennas is improved. It should be noted that the effect of mutual coupling of adjacent fields can be minimized by propagating the coupling current to adjacent rings of unit cells and adjacent MS unit cells along the MS layer in antiparallel directions. Injecting current from distributed antennas to MS units is a key method for improving isolation between MIMO components. As a result, the coupling current between MIMO components is greatly reduced, and the isolation is also greatly improved. Because the coupling field is widely distributed in the element, the copper backplane metasurface isolates the MIMO antenna assembly significantly more than single- and double-layer metasurfaces (Figure 18d). Moreover, the developed MIMO antenna has very low backpropagation and side propagation, producing a unidirectional radiation pattern, thereby increasing the gain of the proposed MIMO antenna.
Surface current patterns of the proposed MIMO antenna at 5.5 GHz (a) without MC, (b) single-layer MC, (c) double-layer MC, and (d) single-layer MC with copper backplane. (CST Studio Suite 2019).
Within the operating frequency, Figure 19a shows the simulated and observed gains of the designed MIMO antenna without and with metasurfaces. The simulated achieved gain of the MIMO antenna without metasurface is 5.4 dBi, as shown in Figure 19a. Due to the mutual coupling effect between MIMO components, the proposed MIMO antenna actually achieves 0.25 dBi higher gain than a single antenna. The addition of metasurfaces can provide significant gains and isolation between MIMO components. Thus, the proposed metasurface MIMO antenna can achieve high realized gain of up to 8.3 dBi. As shown in Figure 19a, when a single metasurface is used at the back of the MIMO antenna, the gain increases by 1.4 dBi. When the metasurface is doubled, the gain increases by 2.1 dBi, as shown in Figure 19a. However, the expected maximum gain of 8.3 dBi is achieved when using the metasurface with a copper backplane. Notably, the maximum achieved gain for the single-layer and double-layer metasurfaces is 6.8 dBi and 7.5 dBi, respectively, while the maximum achieved gain for the bottom-layer metasurface is 8.3 dBi. The metasurface layer on the back side of the antenna acts as a reflector, reflecting radiation from the back side of the antenna and improving the front-to-back (F/B) ratio of the designed MIMO antenna. In addition, the high-impedance MS reflector manipulates electromagnetic waves in-phase, thereby creating additional resonance and improving the radiation performance of the proposed MIMO antenna. The MS reflector installed behind the MIMO antenna can significantly increase the achieved gain, which is confirmed by experimental results. The observed and simulated gains of the developed prototype MIMO antenna are almost the same, however, at some frequencies the measured gain is higher than the simulated gain, especially for MIMO without MS; These variations in experimental gain are due to measurement tolerances of the nylon pads, cable losses, and coupling in the antenna system. The peak measured gain of the MIMO antenna without the metasurface is 5.8 dBi, while the metasurface with a copper backplane is 8.5 dBi. It is worth noting that the proposed complete 4-port MIMO antenna system with MS reflector exhibits high gain under experimental and numerical conditions.
Simulation and experimental results of (a) the achieved gain and (b) the overall performance of the proposed MIMO antenna with metasurface effect.
Figure 19b shows the overall performance of the proposed MIMO system without and with metasurface reflectors. In Figure 19b, the lowest efficiency using MS with backplane was over 73% (down to 84%). The overall efficiency of the developed MIMO antennas without MC and with MC is almost the same with minor differences compared to the simulated values. The reasons for this are measurement tolerances and the use of spacers between the antenna and the MS reflector. The measured achieved gain and overall efficiency across the entire frequency are almost similar to the simulation results, indicating that the performance of the proposed MIMO prototype is as expected and that the recommended MS-based MIMO antenna is suitable for 5G communications. Due to errors in experimental studies, differences exist between the overall results of laboratory experiments and the results of simulations. The performance of the proposed prototype is affected by impedance mismatch between the antenna and the SMA connector, coaxial cable splice losses, soldering effects, and the proximity of various electronic devices to the experimental setup.
Figure 20 describes the design and optimization progress of the said antenna in the form of a block diagram. This block diagram provides a step-by-step description of the proposed MIMO antenna design principles, as well as the parameters that play a key role in optimizing the antenna to achieve the required high gain and high isolation over a wide operating frequency.
The near-field MIMO antenna measurements were measured in the SATIMO Near-Field Experimental Environment at the UKM SATIMO Near-Field Systems Laboratory. Figures 21a,b depict the simulated and observed E-plane and H-plane radiation patterns of the claimed MIMO antenna with and without MS at an operating frequency of 5.5 GHz. In the operating frequency range of 5.5 GHz, the developed non-MS MIMO antenna provides a consistent bidirectional radiation pattern with side lobe values. After applying the MS reflector, the antenna provides a unidirectional radiation pattern and reduces the level of the back lobes, as shown in Figures 21a, b. It is worth noting that by using a metasurface with a copper backplane, the proposed MIMO antenna pattern is more stable and unidirectional than without MS, with very low back and side lobes. The proposed MM array reflector reduces the back and side lobes of the antenna and also improves the radiation characteristics by directing the current in a unidirectional direction (Fig. 21a, b), thereby increasing the gain and directivity. The measured radiation pattern was obtained for port 1 with a 50 ohm load connected to the remaining ports. It was observed that the experimental radiation pattern was almost identical to that simulated by CST, although there were some deviations due to component misalignment, reflections from terminal ports, and losses in cable connections. Additionally, a nylon spacer was inserted between the antenna and the MS reflector, which is another issue affecting the observed results compared to the predicted results.
The radiation pattern of the developed MIMO antenna (without MS and with MS) at a frequency of 5.5 GHz was simulated and tested.
It is important to note that port isolation and its associated characteristics are essential when evaluating the performance of MIMO systems. The diversity performance of the proposed MIMO system, including envelope correlation coefficient (ECC) and diversity gain (DG), is examined to illustrate the robustness of the designed MIMO antenna system. The ECC and DG of a MIMO antenna can be used to evaluate its performance as they are important aspects of the performance of a MIMO system. The following sections will detail these features of the proposed MIMO antenna.
Envelope Correlation Coefficient (ECC). When considering any MIMO system, ECC determines the degree to which the constituent elements correlate with each other regarding their specific properties. Thus, ECC demonstrates the degree of channel isolation in a wireless communication network. The ECC (envelope correlation coefficient) of the developed MIMO system can be determined based on S-parameters and far-field emission. From Eq. (7) and (8) the ECC of the proposed MIMO antenna 31 can be determined.
The reflection coefficient is represented by Sii and Sij represents the transmission coefficient. The three-dimensional radiation patterns of the j-th and i-th antennas are given by the expressions \(\vec{R}_{j} \left( {\theta ,\varphi } \right)\) and \(\vec {{R_{ i } }} Solid angle represented by \left( {\theta ,\varphi } \right)\) and \({\Omega }\). The ECC curve of the proposed antenna is shown in Figure 22a and its value is less than 0.004, which is well below the acceptable value of 0.5 for a wireless system. Therefore, the reduced ECC value means that the proposed 4-port MIMO system provides superior diversity43.
Diversity Gain (DG) DG is another MIMO system performance metric that describes how the diversity scheme affects the radiated power. Relation (9) determines the DG of the MIMO antenna system being developed, as described in 31.
Figure 22b shows the DG diagram of the proposed MIMO system, where the DG value is very close to 10 dB. The DG values of all antennas of the designed MIMO system exceed 9.98 dB.
Table 1 compares the proposed metasurface MIMO antenna with recently developed similar MIMO systems. The comparison takes into account various performance parameters, including bandwidth, gain, maximum isolation, overall efficiency, and diversity performance. Researchers have presented various MIMO antenna prototypes with gain and isolation enhancement techniques in 5, 44, 45, 46, 47. Compared with previously published works, the proposed MIMO system with metasurface reflectors outperforms them in terms of bandwidth, gain, and isolation. Additionally, compared to similar antennas reported, the developed MIMO system exhibits superior diversity performance and overall efficiency at a smaller size. Although the antennas described in Section 5.46 have higher isolation than our proposed antennas, these antennas suffer from large size, low gain, narrow bandwidth, and poor MIMO performance. The 4-port MIMO antenna proposed in 45 exhibits high gain and efficiency, but its design has low isolation, large size, and poor diversity performance. On the other hand, the small size antenna system proposed in 47 has very low gain and operating bandwidth, while our proposed MS based 4-port MIMO system exhibits small size, high gain, high isolation and better performance MIMO. Thus, the proposed metasurface MIMO antenna can become a major contender for sub-6 GHz 5G communication systems.
A four-port metasurface reflector-based wideband MIMO antenna with high gain and isolation is proposed to support 5G applications below 6 GHz. The microstrip line feeds a square radiating section, which is truncated by a square at the diagonal corners. The proposed MS and antenna emitter are implemented on substrate materials similar to Rogers RT5880 to achieve excellent performance in high-speed 5G communication systems. The MIMO antenna features wide range and high gain, and provides sound isolation between MIMO components and excellent efficiency. The developed single antenna has miniature dimensions of 0.58?0.58?0.02? with a 5×5 metasurface array, provides a wide 4.56 GHz operating bandwidth, 8 dBi peak gain and superior measured efficiency. The proposed four-port MIMO antenna (2 × 2 array) is designed by orthogonally aligning each proposed single antenna with another antenna with dimensions of 1.05λ × 1.05λ × 0.02λ. It is recommended to assemble a 10×10 MM array under a 12mm high MIMO antenna, which can reduce back-radiation and reduce mutual coupling between MIMO components, thereby improving gain and isolation. Experimental and simulation results show that the developed MIMO prototype can operate in a wide frequency range of 3.08–7.75 GHz, covering the 5G spectrum below 6 GHz. In addition, the proposed MS-based MIMO antenna improves its gain by 2.9 dBi, achieving a maximum gain of 8.3 dBi, and provides excellent isolation (>15.5 dB) between MIMO components, validating the contribution of MS. In addition, the proposed MIMO antenna has a high average overall efficiency of 82% and a low inter-element distance of 22 mm. The antenna exhibits excellent MIMO diversity performance including very high DG (over 9.98 dB), very low ECC (less than 0.004) and unidirectional radiation pattern. The measurement results are very similar to the simulation results. These characteristics confirm that the developed four-port MIMO antenna system can be a viable choice for 5G communication systems in the sub-6 GHz frequency range.
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Post time: Oct-10-2024