Dual Polarized, Multiband Four-Port Decagon Shaped Flexible MIMO Antenna for Next Generation Wireless Applications

A compact, dual polarized, multiband four-port flexible Multiple Input Multiple Output (MIMO) antennae with the connected ground and high isolation is designed with computation and experimental measurement studies. All four monopole radiators are embedded decagon-shaped flexible FR-4 substrate with an outer radius of 10 mm in order to accomplish circularly polarized (CP) radiations, bandwidth enhancement, and compact size of only $45\times 38\times 0.2$ mm3 ( $0.375\lambda \times 0.316\lambda \times 0.0016\lambda $ , at lowest resonating frequency 2.5GHz). The interconnected ground structure is loaded with an Interlaced Lozenge Structure (ILS) to suppress the surface wave radiations resulting in low mutual coupling between the radiators. The proposed MIMO antenna demonstrates measured 10-dB impedance bandwidths of 9.63% (2.37–2.61 GHz), 28.79% (3.30–4.41 GHz), and 16.91% (4.98–5.90 GHz) in the LTE 38/40, Sub-6 GHz 5G NR n77/n78, WLAN and Wi-Fi bands, respectively. Furthermore, broad 3-dB Axial Ratio Bandwidth (ARBW) of 28.79% (3.30–4.41 GHz) with gain greater than 4 dBi and efficiency above 80% are achieved. Finally, the bending analysis of the proposed flexible MIMO antenna along the X- and Y- directions shows good performances in terms of scattering parameters, 3 dB ARBW, and MIMO diversity parameters.


I. INTRODUCTION
The Quality of Service (QoS) of wireless communication can be characterized in terms of high data rate, broad bandwidth, low latency, and reliability for indoor and outdoor applications. The MIMO antennas (incorporating multiple The associate editor coordinating the review of this manuscript and approving it for publication was Bilal Khawaja . transmitting and receiving antennas) are gaining considerable attention in WLAN applications as they enhance the channel capacity and data rate without surrendering extra spectrum or transmitted power in a rich multipath fading environment.
Therefore, to enhance the transmission capacity and mitigate the multipath fading effect, the technique of deploying MIMO antennas in a restricted space is a preferred solution. The MIMO antennas design with various geometries (with or without decoupling structures) are reported in [1] and [5], and amid these designs, [1] has reported a two-port MIMO antenna design with space diversity functions, while [2], [5] has implemented a four-port multiband MIMO antenna to enhance the channel capacity. Even though the works in [2] and [5] can cover the 5G NR n77/n78, they are of linearly polarized (LP) senses, and due to polarization mismatch as well as uncorrelated fading multipath, the receiving antennas may achieve an unequal power amplitude that affects the diversity gain (DG) and signal-to-noise ratio (SNR). Notably, the polarization mismatch effect can be minimized by employing a CP MIMO antenna, instead of an LP MIMO antenna. Because MIMO antenna with CP radiation can further mitigate multipath effects, it can therefore improve the channel capacity and QoS.
To generate CP MIMO/diversity radiations for C-band, 5G sub-6 GHz, WLAN, and K-band (for satellite applications), several two-port [6], [13] and four-port [14], [17] MIMO antennas designed with different approaches such as modifying the patch structure, the use of offset feeding line, and slot-loading in the ground plane have been reported. In [6], a corner-truncated square patch of size 56 × 32×3 mm 3 was reported to generate a desirable CP bandwidth (or 3-dB ARBW) of 15.3% (5.08-5.92 GHz) with a 10-dB impedance bandwidth of 21% (5.12-6.32 GHz). In [7], a dual port, dual polarized MIMO antenna (150×100 × 0.8mm 3 ) with two radiators and good isolation has been reported with an operating band of (2.47-2.55 GHz). A dual-polarized MIMO antenna with pattern diversity designed-body communication is reported in [8]. The reported design has applied two stubs extended from the left and right ground plane to generate a desirable CP radiation with 100% overlapping of 10-dB impedance bandwidth and 3-dB ARBW of 19.13% (5.2-6.3 GHz). In [9], a dual-polarized CP MIMO antenna was achieved by using a branch line coupler and dual-feed probes to excite a stacked square patch (with multilayer structure and vias) for the K band applications. Nevertheless, the antennas in [6] and [9] are designed using an expensive Roger substrate that substantially increases the manufacturing cost of dual-polarized due to the manufacturing cost of building a two-port MIMO antenna, many antenna engineers have relied on a typical low-cost substrate, known as the FR-4 [10], [13]. In [10], a dual-polarized two-port planar slot MIMO antenna fabricated on a low-cost FR-4 substrate with Split Ring Resonators (SRR) as an isolating element for C-band applications has been reported. In this case, the antenna dimension is 105×50 × 1.6mm 3 , and to yield a desirable 3-dB ARBW of 8.97% (4.245-4.635) GHz, a rectangular slot is loaded into the ground plane, followed by introducing a perturbed T-shaped ground stub into the slot. A coplanar waveguide (CPW) fed wideband two-port CP MIMO antenna with a pieshaped decoupled structure is introduced in [11] and occupies an area of 100 × 70 mm 2 . In dual-polarized CP MIMO antenna with a lined patch to reduce the mutual coupling between the adjacent antenna elements has been introduced with broad 10-dB impedance bandwidth of 36% and 3-dB ARBW of 23% across the bands of interest. However, the MIMO antenna units do not share a common ground plane, and hence, they may not be suitable for practical realization when integrated into a compact-size wireless device. By further observing the CP MIMO antenna design in [10] and [12], they have exhibited very large dimensions that may restrict the feasibility of integrating into a low-profile wireless device. Therefore, a compact size (97 × 27.69 mm 2 ) two-port CP MIMO antenna for 5.8 GHz WLAN applications is reported in [13], and good CP radiation is achieved by loading a dual-slot 90 • apart on a truncated corner rectangular patch. Even though [13] has reduced size, it has also exhibited narrow 10-dB bandwidth and 3-dB ARBW of 1.5% (5.77-5.86 GHz), which limits its application in many modern wireless devices. Nevertheless, the implementation of a 2-port MIMO antenna design configuration is not a desirable solution for accommodating a much greater number of users, and thus the development of compact-size MIMO antenna types (especially those with CP radiation) is presently the research hotspot in the area of Wi-Fi MIMO antenna design.
Recently, many different types of CP MIMO antenna design structures have been introduced [14], [17]. Amid these designs, [14] has introduced a 4×4 pentagonal microstrip element CP MIMO antenna with a size of 186×188 × 1.6 mm 3 . In [15], a dual CP stacked patch antenna based on a reflector for a 4 × 4 MIMO configuration operating at a 2.4 GHz WLAN band is investigated However, due to the use of two stacked FR-4 substrates (each has a thickness of 1.53 mm), it has a very large overall antenna size of 157 × 96×39 mm 3 which limits its feasibility of integrating into a low-profile wireless device. To reduce the antenna size to 60×60 × 1.6 mm 3 , a wideband four-port CP MIMO antenna composed of four G-shaped monopole radiators with decoupling structure is investigated in [16]. The antenna operates in the Sub-6 GHz band and an I-shaped feeding strip is utilized to generate CP radiation with 3-dB ARBW of 6.70% (3.46-3.7 GHz). A dual CP MIMO antenna comprising of four G-shaped monopole elements, in which two CP elements are left-hand CP and the other two are right-hand CP are reported in [17]. To achieve a very wide 3-dB ARBW of 67.7% (4.2-8.5 GHz) with good isolation of > 18dB between any two adjacent CP elements, a cross-shaped decoupling structure was applied to interconnect the ground plane of each CP element, but at the cost of increasing the overall size of this MIMO antenna to 70 × 68 × 1.6 mm 3 .
Apart from the above CP MIMO antenna design, dielectric resonator (DR) based antennas have also been studied as they can yield a much wider CP bandwidth [18], [23]. Amid these designs, desirable CP radiations are generated by applying the following techniques, such as the L-type feeding network [18], 45 • truncation of diagonally placed DR element [19] for 5G applications, Y-shaped DR [20] for WLAN and WiMAX, offset feeding and asymmetrical cut [21] for 5 GHz WLAN, alignment of DR material with zero distance [22] for (7.72-8.08 GHz), and parasitic patch with DR element [23] for WiMAX applications. However, it is notable that the VOLUME 10, 2022 DR-based antennas are bulky in construction and exhibit a high profile due to the thickness of DR material.
An antenna that is built on a flexible substrate [24], [28] can aid in reducing the overall antenna dimension (especially the profile) as well as the ability to bend the antenna for specific unique applications (such as wearable devices), therefore, it is imperative to study further the performances of antennas using a flexible substrate. Similar to the antenna design that has applied rigid substrate, flexible antenna design can also be applied to many wireless applications, such as the UWB [25], MIMO wearable [26], [27], and MIMO reconfigurable type [28]. However, the above reported flexible antennas are twoport LP antennas with undesirable isolation levels between the antenna elements.
In view of the above investigation, this research work has proposed a novel, compact, multiband, and flexible four-port dual-polarized antenna for wireless application.

II. GEOMETRY AND LAYOUT OF SINGLE DECAGON-SHAPED FLEXIBLE MONOPOLE ANTENNA
The top and breakdown views of a single decagon-shaped flexible monopole antenna are illustrated in Figures 1a to 1c. The antenna element is printed on a flexible, cost-effective and decagon shaped FR-4 substrate (relative permittivity r = 4.4, loss tangent tan δ = 0.025, thickness h = 0.2 mm) with physical dimensions of 20×19×0.2mm 3 (0.166λ× 0.158λ× 0.001λ, at 2.5 GHz).
By solving equations (1) and (2) and using optimization technique in 3-D electromagnetic Computer Simulation Technology Microwave Studio Suit (CST MWS)˙, a e is achieved as 10 mm for better performance of antenna elements and compactness. For this corresponding radius of 10 mm, the optimized side length of the decagon FR-4 substrate is 6.18 mm.
As mentioned earlier, the proposed monopole antenna is composed of a U-shaped radiator with a middle-protruded diamond-shaped ring radiator top-loaded by a horizontal strip. Due to the decagon-shaped substrate, the two ground planes used to support the coplanar waveguide (CPW) fed technique with beveled corners. Here, a 50 CPW-fed microstrip transmission line with length L b and width W c is sandwiched between the two beveled corners ground planes, leaving an air gap width of W a on both sides to achieve good impedance matching. A commercially available SMA connector (Type 10) is utilized in this case, and it is also included during the simulation process, as depicted in Figures 1b and 1c. Notably, the two ground planes are beveled at an interior angle of α 1 = 144 • without disturbing the SMA connector to achieve good CP radiations across the desired operating band.
At the top of the microstrip transmission line, a U-shaped radiator of equal arm length L g and width W f is loaded to generate a resonance at 2.5 GHz. The air gap L c between the U-shaped radiator and ground planes reduces the inductive impedance by producing an equal amount of capacitive reactance, thus achieving good impedance matching at 2.5 GHz, and it is maintained throughout the desired operating band. To further induce a second resonance at 3.6 GHz, a diamondshaped ring radiator with side length L h and ring-corner width W h protruded from the middle section of the U-shaped radiator via a short connecting strip of length L d . By further observing Figure 1, one can also see that the two corners of the diamond-shaped ring radiator have an air gap distance of W g away from the two vertical arms of the U-shaped radiator. Here, the middle square slot used to form the diamond-shaped ring radiator has a slot size of L e × L e for improving the impedance matching at 3.6 GHz. Finally, to generate a third resonance at 5.5 GHz, a horizontal strip of length W i and width L i is affixed at the top of the diamond-shaped radiator at an angle of α 2 . The angle α 2 and horizontal strip result in bandwidth enhancement by producing a longer path for surface current at resonating frequency of 5.5 GHz. The optimized parameters of the designed antenna are listed in Table 1.

A. WORKING MECHANISM OF THE PROPOSED DECAGON-SHAPED FLEXIBLE MONOPOLE ANTENNA
The stepwise design stages of the proposed decagon-shaped flexible monopole antenna with five different layouts are depicted in Figure 2, and their corresponding reflection coefficient (S 11 ) and (AR) dB diagrams are illustrated in Figure 3. As shown in Figure 2a, the U-shaped radiator designed in Step-1 induces a resonance at 2.5 GHz (as defined in Figure 3), and it is printed on a square-shaped flexible substrate. The length of the U-shaped radiator (2L g +W e ) is a quarter-wavelength long at a resonance frequency of 2.5 GHz. In Step 1, due to the CPW-fed technique applied to the U-shaped radiator, excellent impedance matching is achieved, and it can produce a 10-dB impedance bandwidth of 0.24 GHz (2.38-2.62 GHz) covering the LTE 38/40, as shown in Figure 3a. However, as seen in Figure 3b, the AR values are around 40 dB, so the antenna in Step-1 is an LP radiator. In Step-2, as shown in Figure 2b, a diamond-shaped radiator protrudes from the U-shaped radiator to induce a resonance at 3.6 GHz without disturbing the antenna defined in Step-1. As the air gap distance of W g reduces the inductive reactance (produced due to the addition of a diamondshaped radiator), it maintains the impedance matching at 3.6 GHz. Hence, Step-2 can successfully resonate at 3.6 GHz with an impedance bandwidth of 0.82 GHz (3.40-4.22 GHz) covering a partial 5G Sub-6 GHz band n77/n78. To further enhance the antenna's bandwidth introduced in Step-2, a diamond slot is loaded into the diamond-shaped radiator, forming a diamond-shaped ring radiator that produces a higher-order mode of 4.44 GHz, as shown in Figure 2c. As this higher-order mode has overlapped with the fundamental mode at 3.6 GHz, the antenna in Step-3 can now yield wider impedance bandwidth of 1.14 GHz (3.30-4.44 GHz) covering the 5G Sub-6 GHz band n77/78, as shown in Figure 3a. The horizontal symmetricity of the diamondshaped ring structure perturbs the current distribution on the radiator and generates the two electric vector field components, namely, E x and E y with equal amplitude (across 3.30-4.18 GHz) frequency band, as shown in Step-2 and Step-3 of Figure 3b (with AR < 3 dB).
To achieve resonance at 5.5 GHz further, in Step-4, a horizontal strip whose length is a quarter-wave long at 5.5 GHz is loaded onto the top-corner section of the diamond-shaped ring radiator in Step-3, as shown in Figure 2d. Here, the antenna in Step-4 can yield 10-dB impedance bandwidth of 0.75 GHz (5.20-5.95 GHz) with LP radiations (AR > 30 dB), as shown in Figures 3a and 3b, respectively, and it cannot cover the 5.2 GHz, WLAN band. To further enhance the impedance bandwidth at a higher frequency and to increase the E x and E y electric field vectors with equal amplitude (0 dB or ∼1) and phase difference ( ∅ = π /2) across (3.30-4.44 GHz), Step-5 is developed, as shown in Figure 2e, in which the entire flexible substrate was reduced to a decagon shape (from a square-shaped type), and the two CPW-fed ground planes are now with a beveled corner. This transformation enhances the impedance bandwidth of the proposed monopole (Step-5) to 0.91 GHz  at the low band (f L ) 2.5 GHz, middle band (f M ) 3.6 GHz, and high band (f H ) 5.5 GHz, respectively. In Figure 4a, the U-shaped radiator has exhibited a half-wavelength current distribution path at f L , with an operating band of (2.38-2.62 GHz). As shown in Figure 4b, a quarterwavelength current distribution path f M is also demonstrated along with the diamond-shaped ring radiator, resulting in the excitation of the (3.30-4.44 GHz) band. Finally, as visualized in Figure 4c, a half-wavelength current distribution path at f H is depicted across the top-loaded horizontal strip, showing a broad operational band of (5.02-5.93 GHz).

C. CP MECHANISM OF THE PROPOSED DECAGON-SHAPED FLEXIBLE MONOPOLE ANTENNA
To fully comprehend the CP excitation of the proposed monopole antenna element (or the antenna in Step-5) across the band of interest (3.30-4.44 GHz), Figure 5 depicts its corresponding calculated amplitude ratio (E x /E y ) and PD of the two orthogonal electric field components. Here, one can observe that the amplitude ratio is closer to 0 dB with PD of near 90 • across the band of interest.
The surface current distribution of the antenna in Step-5 is analyzed from Figures 6a to 6d at f M for the variation of phase angle at 0 • , 90 • , 180 • , and 270 • , respectively. Initially, at the 0 • phase, the maximum current vectors are in the +y direction, whereas at the 180 • phase, the current flows in

IV. DESIGN OF THE FOUR-PORT DECAGON SHAPED DUAL-POLARIZED FLEXIBLE MIMO ANTENNA WITH ILS
The geometry and breakdown view of the proposed fourport decagon-shaped dual-polarized flexible MIMO antenna is shown in Figures 9a and 9b, respectively. The proposed MIMO antenna is composed of four identical decagon-shaped monopole radiators arranged in a mirror image fashion with each other, as portrayed in Figure 9.  The antenna radiators are arranged such that the Ant. 1 and Ant. 2 are separated by a distance of W • , whereas the Ant. 1 and Ant. 4 are separated by an edge-to-edge distance of L p . A trapezium-shaped patch is utilized to interconnect the disconnected ground planes between Ant. 1 and Ant. 2, as well as between Ant. 3 and Ant. 4. Here, a decoupling structure (ILS) has been deployed in the antenna elements' interconnected ground planes to suppress the radiating waves and obtain high isolation among adjacent antenna elements. The hexagon-shaped slot formed in the middle of the flexible FR-4 substrate further minimized the mutual coupling between Ant.1 and Ant. 4, as well as between Ant. 2 and Ant. 3. The mirrored pattern of the monopole radiators supports the polarization diversity in the respective operative  Table 1. For any MIMO antenna, the incurred mutual coupling can be minimized by leaving sufficient space (spatial diversity) among the radiators. Hence, proper spacing is arranged between the radiators to preserve a good 3 dB ARWB of the individual antenna element. However, this results in a disconnected ground plane between Ant. 1 and Ant. 2, as well as between Ant. 3 and Ant. 4. The S 11 , S 12 , and AR of this configuration are shown in Figures 11a-c (Blue dashed line). It is observed that there are negligible effects on S 11 and AR, and good isolation larger than 20 dB is also achieved. However, for practical realization, the ground planes of the four antenna elements must be interconnected. Therefore, a trapezium-shaped patch is loaded between Ant. 1 and Ant. 2, as well as between Ant. 3 and Ant. 4, so as to interconnect the ground planes, as shown in Figure 10b. From Figures 11a-c (green dashed  line), it is observed that the 3 dB ARBW and mutual coupling between antenna elements degrade significantly. However, it is a fact well-known that it is quite challenging to preserve a wide ARBW of the antenna elements and to maintain good isolation when the antenna elements are closely deployed in a restricted space with interconnected ground planes. To improve the isolation between adjacent antenna elements, an ILS (decoupling structure) [31], [35] is loaded between the antenna elements, as displayed in Figure 10c. The proper location of the ILS is anticipated in such a way that the minimal coupling between the antenna elements and wide 3 dB ARBW can be preserved. With the addition of ILS, the ARBW of the designed antenna element increases significantly. It is clearly noted from Figures 11a-c (Brown line) that the ILS has a negligible effect on the 10-dB impedance bandwidth but considerably enhances the isolation between the antenna elements and the 3 dB ARBW.
To further verify the design steps of the proposed decagonshaped dual polarized flexible MIMO antenna, the surface current distributions when only port-1 is activated are depicted in Figures 12a-c. It is easily noted from Figure 12a that Ant. 1 and Ant. 2 are not mutually coupled with each other due to disconnected ground resulting in suppressing the radiating waves from Ant. 1, whereas, in Figure 12b, Ant. 1 and Ant. 2 are strongly coupled with each other due to coupled ground and Ant. 2 is getting affected by the electric field of Ant. 1 resulting in co-related signals in a rich multipath fading environment. As seen from Figure 12c, after the loading of ILS, it is able to suppress the radiating To validate the proposed design, the proposed decagon-shaped dual polarized flexible MIMO antenna (loaded with two ILS) was fabricated as shown in Figure 14(a) and its radiation performance including radiation patterns, gain and efficiency were measured in anechoic chamber as shown in Figure 14(b). The simulated and measured results are well agreed with each other except for minimal deviation that may be due to fabrication error or soldering tolerances.

A. SIMULATED AND MEASURED REFLECTION COEFFICIENT
As shown in Figure 15  The equation to calculate the S-parameters in terms of reflection coefficient (S11) at three different resonating frequencies is as follows S 11 = magnitude of the reflection coefficient = (VSWR-1)/(VSWR+1) The magnitude of S11 in terms of dB is expressed as follows:

B. SIMULATED AND MEASURED TRANSMISSION COEFFICIENT
The simulated and measured transmission coefficients (S 12 , S 13 , S 14 ) of the proposed four-port decagon shaped dual polarized flexible MIMO antenna are displayed in Figure 16. It is very well noted that the isolation is larger than 20 dB throughout the desired bands of interest. At the time of measurement, one antenna element is active, while the remaining other antenna elements are terminated using a 50 load impedance.

C. SIMULATED AND MEASURED AXIAL RATIO
The simulated and measured 3 dB ARBW of the proposed decagon shaped dual polarized flexible MIMO antenna is depicted in Figure 17. It is easily visualized that the MIMO antenna possesses a very wide simulated 3 dB ARBW of 29.45% (3.30-4.44 GHz), and its corresponding measured result was 28.79% (3.30-4.41 GHz). Thus, the measured CP bandwidth can well cover the 5G Sub-6 GHz NR band n77/78 with 100% overlapping the measured 10-dB impedance bandwidth of the antenna element working in the f M .

D. SIMULATED AND MEASURED GAIN AND EFFICIENCY
The simulated and measured gain and efficiency curves of the proposed decagon shaped dual polarized flexible MIMO antenna are depicted in Figure 18. The measured gain was above 4 dBi, and the measured efficiency was well above 80% throughout the three operating bands. Notably, the measured outcomes are well-validated with the simulated results.

E. SIMULATED AND MEASURED RADIATION PATTERNS
The simulated and measured radiation patterns at f M (3.6 GHz) in the E-plane (xy plane) and H-plane (xz plane) of the proposed decagon shaped dual polarized flexible MIMO   antenna are depicted in Figure 19 when all the ports are excited simultaneously. From Figure 19, it is noted that in both the planes (ϕ = 0 • , ϕ =90 • ), the Ant. 1 and Ant. 3 illustrate RHCP radiation patterns with 3-dB beamwidth of 78.90 • ±0.5 in the +z-direction. Likewise, Ant. 2 and Ant. 4 demonstrate the LHCP radiation patterns with 3-dB

F. ENVELOPE CORRELATION COEFFICIENT (ECC) AND DIVERSITY GAIN (DG)
The envelope correlation coefficient (ECC) of the proposed decagon shaped dual polarized flexible MIMO antenna is calculated from the far-field patterns using Equation (4) as follow: where 'F i (θ , φ)' is the radiated field of the i th antenna. Figure 21 shows the ECC curves between Ant. 1-2, Ant. 1-3, and Ant. 1-4. It is observed that the calculated ECC values are less than 0.04 across the three bands of interest. Therefore, from the ECC values, it is confirmed that the proposed decagon shaped dual polarized flexible MIMO antenna has achieved high inter port isolation and better diversity performance in a rich multipath fading environment.
The diversity gain (DG) is calculated by using the below Equation (5) and it is depicted in Figure 21.
It is noted that the DG is very closer to 10 dB throughout the functioning bands.

G. TOTAL ACTIVE REFLECTION COEFFICIENT (TARC)
The Total Active Reflection Coefficient (TARC) between Ant. 1 and Ant. 2 is calculated using the below Equation (6), and its experimental set-up is shown in Figure 22a.
where θ is the input phase angle that is varied from 0 0 to 120 0 with step increment of 30 0 , S ii and S jj are the reflection coefficients of the port i and port j, respectively. Figure 22b depicts the TARC values for polarized antenna elements, where it is visualized that under the variation of phase angle, the performances of the proposed MIMO antenna remain undisturbed in the scattering environment.

H. MEAN EFFECTIVE GAIN (MEG)
The Mean Effective Gain (MEG) is another essential parameter for the estimation of diversity performance of the proposed flexible four-port dual CP MIMO antenna. The MEG is achieved using below Equation (7): where in Equation (6), K is the number of antennas, i is the active antenna and η irad is the radiation efficiency of the i th antenna. Using Equations (8)(9)(10)(11), the MEG values for Ant. 1, Ant. 2, Ant. 3, and Ant. 4 are calculated and depicted in Table 2: From Table 2, it is confirmed that the MEG values for all four antennas are between the limit −3 ≤ MEG (dB) < −12 defined by industry and standards. It is also visualized that the ratio of MEG1/MEG2 and MEG3/MEG4 is approximately equal to one, which confirms that the proposed decagon    shape MIMO antenna system has attained better diversity performance in a multipath fading environment.

I. CHANNEL CAPACITY
The ergodic channel capacity of the proposed decagon shape dual polarized flexible MIMO antenna across the operating bands is analysed in Figure 23 using the equation mentioned in [30]. It is validated that the capacity varies between 20 and 21 bps/Hz, and it is 1.86 times greater than the maximum limit of 2 × 2 MIMO antenna.

VI. BENDING ANALYSIS OF PROPOSED FLEXIBLE FOUR PORT DUAL CP MIMO ANTENNA
To validate the bending effects of the proposed flexible MIMO antenna and make sure that it can still operate well with conformal surfaces, the effects on various typical antenna performances when bending the proposed MIMO antenna are performed unlike [36] and [38]. The simulated and fabricated bending scenarios are performed on a semi-cylindrical Styrofoam (ε r = 1.03), in which the proposed flexible MIMO antenna is attached to the prescribed bending radius. Notably, the scattering parameters, AR curves, and ECC performances are considered for the validation of flexible operation by exciting port 1.
Here, the proposed MIMO antenna is bent for a radius of 15 mm along the X-axis and Y-axis. The case of a 15 mm radius along both X and Y-axis is selected by assuming the worst-case scenario under the maximum bending capability of the proposed MIMO antenna as shown in Figure 24 (a) and (b).

A. BENDING ANALYSIS OF PROPOSED DECAGON SHAPED DUAL POLARIZED FLEXIBLE MIMO ANTENNA ALONG X-AXIS
The proposed flexible MIMO antenna is bent from the center along the X-axis, as depicted in the simulated prototype (see Figure 25a) and the fabricated prototype (see Figure 25b). The simulated and measured S-parameter, AR, and ECC obtained after bending the MIMO antenna along the X-axis are compared with the ones without bending, as shown in Figures 25 c-e. From Figure 25c, it is visualized that there are negligible effects on the S 11 across the f L (2.5 GHz) and f M (3.6 GHz), whereas a small decrease in the operational bandwidth was observed in the f H (5.5 GHz). Notably, even though the f H has suffered from such a decrease in the bandwidth, it still covers the entire 5 GHz WLAN band. The simulated and measured transmission coefficient of the bending MIMO antenna is well below -20 dB throughout the three operating bands, which conforms well with the simulation results without bending. From Figures 25d-e, it can be observed that there is a negligible deviation in the AR and ECC curves.

B. BENDING ANALYSIS OF THE PROPOSED DECAGON SHAPED DUAL POLARIZED FLEXIBLE MIMO ANTENNA ALONG THE Y-AXIS
The MIMO antenna is bent from the center along the Y-axis, as seen in the simulated prototype (see Figure 26a) and the fabricated prototype (see Figure 26b). The simulated and measured S-parameter, AR, and ECC obtained after bending the MIMO antenna along the Y-axis are compared with the simulated ones without bending, as shown in Figures  26 c-e. From Figure 26c, it is observed that there is a negligible effect on the bandwidth of 2.5 GHz band and 3.6 GHz band when bending the MIMO antenna, whereas a small decrease in the operational bandwidth was observed in the f H (5.5 GHz), and the f H is also slightly shifted towards higher spectrum. Even though the desired f H has suffered from a VOLUME 10, 2022 small decrease in the bandwidth and minor shift in frequency, it can still cover the entire 5 GHz WLAN band. The simulated and measured transmission coefficient of the bending MIMO antenna is well below -20 dB throughout the three operating bands, and they conform well with the simulation results of the one without bending. From Figures 26d-e, a negligible deviation in the AR and ECC curves is observed, and they are acceptable within the required range.

VII. PERFORMANCE COMPARISON OF PROPOSED FLEXIBLE FOUR PORT DUAL CP MIMO ANTENNA
In order to validate the novelty and suitability of the proposed decagon shaped dual polarized flexible MIMO antenna, it is compared with the existing state of art as shown in Table 3.
From the above table it can be concluded that the proposed decagon shaped dual polarized MIMO antenna is smallest in size as compared to all the reported state of arts, covers multiple bands as compared to [5], [7], [10], [16], [18], [21], and [22] is cost-effective as compared to [6], [7], [18], and [26] as well as uses low-cost flexible substrate apart from [1] and [22]. Thus, the proposed decagon shaped dual polarized flexible MIMO antenna is novel is design with multiple bands, compact in size, cost-effective, easy to manufacture and is thus a suitable for small form factor next-generation wireless devices.

VIII. CONCLUSION
A very low-profile decagon shaped dual polarized flexible MIMO antenna with pattern diversity has been successfully studied. The proposed flexible MIMO antenna is cost-effective, easy to design, occupies a volume size of 1.896 × 10 −4 λ 3 , and operates at LTE 38/40, 5G Sub-6 GHz NR band n77/n78, and WLAN/Wi-Fi frequency bands for next-generation wireless devices. Due to the bevelled ground plane embedded on the decagon shaped substrate and the small diamond-shaped ring radiator protruding from the U-shaped radiator, desired CP bandwidth of 28.79% (3.30-4.41 GHz) was excited across the Sub-6 GHz frequency band. Furthermore, the proposed flexible MIMO antenna has exhibited good radiation characteristics with VOLUME 10, 2022 nearly omnidirectional radiation patterns, gain larger than 4 dBi with nearly and stable efficiency above 80% across the three desired bands of interest. From the bending analysis performed on the proposed flexible MIMO antenna, desirable performances in terms of scattering parameters, 3 dB ARBW, and ECC are demonstrated with very minor detrimental effects. The above characteristics make the proposed flexible MIMO antenna a suitable candidate for small form factor next-generation wireless devices.  was also assigned to Fractus in South Korea to head up the research team. One of his main tasks was to provide training, education, and development of the team's core competency and provide a research and development vision to address the rapidly growing mobile device market. Under his leadership, the company had secured major contracts with companies, such as Samsung, LG, and Bellwave, to name a few. He published a book about ''Korean Experiences,'' in 2015. Since 2017, he has been with Ignion in the role of CTO. He leads the company's research and development activity to create new products, envisage new technologies and technical evangelism, foster synergies with partners, and provide technology strategy to scale the company's business. He holds more than 150 granted invention patents (USA, Asia, and Europe) in the antenna field, many of which have been licensed to antenna companies. Among his most outstanding contributions is that of the inventor of antenna booster technology, a technology that fostered the creation of Ignion. The wireless industry has adopted many of these products worldwide to allow wireless connectivity to the IoT devices through a miniature component called an antenna booster that is ten times smaller than conventional antennas. He is the author of more than 270 journals and international and national conference papers (H-index=52 with more than 8000 citations based on Google-Scholar). He has taught more than 50 antenna courses worldwide (USA, China, South Korea, India, U.K., France, Poland, Czech Republic, Tunisia, Perú, Brazil, Canada, and Spain). He has directed over 150 bachelor's, master's, and Ph.D. theses. He has authored seven books. He has participated in over 20 national/international projects and research grants valued at over e13 million, of which he was the principal researcher in many of them. His current research interests include antenna boosters, multiband and small antennas, broadband matching networks, diversity antenna systems/MIMO, electromagnetic dosimetry, genetically optimised antennas, and antennas for wireless handset devices.
Dr. Anguera was a member of the fractal team that in 1998 received the European Information Technology Grand Prize for the Applied Science and Engineering for the fractal-shaped antenna application to cellular telephony. In 2017, he was appointed as a Professor of mobile communications at the University of South Wales, U.K. He has served as a Project Co-ordinator for major international research projects, including Eureka LOOP, FP7 C2POWER, and H2020-SECRET, whilst serving as a Technical Manager for FP7 COGEU and FP7 SALUS. He is the author of more than 500 scientific works, including 11 book editorials. He is a fellow of the IET (FIET) and a Senior Fellow of the Higher Education Academy (SFHEA). His professional affiliations include Chartered Engineer (C.Eng.).