Design and Analysis of UWB MIMO Antenna for Smart Fabric Communications

Tis paper presents a fexible multiple-input, multiple-output (MIMO) antenna with ultrawideband (UWB) performance for smart clothing applications. Te MIMO antenna is comprised of four octagonal-shaped radiators with several slots loaded into them, and it ofers a frequency range of 2.9–12 GHz. Te unit cell has a size of 0.26 λ 0 × 0.164 λ 0 × 0.014 λ 0 and the MIMO antenna has a size of 0.48 λ 0 × 0.48 λ 0 × 0.014 λ 0 , where λ 0 corresponds to the lowest operating frequency. Te radiation and diversity performances of the antenna are evaluated, and the obtained metrics are envelope correlation coefcient (ECC) < 0.045, diversity gain (DG) > 9.9dB, total active refection coefcient (TARC) < − 14dB, and channel capacity loss (CCL) < 0.13 bits/s/Hz. Te bending analysis of the MIMO antenna is performed. Te specifc absorption rate (SAR) of the MIMO antenna is also investigated, and the obtained values are 0.229W/Kg (4GHz), 0.253W/Kg (7GHz), and 0.463W/Kg (10GHz).


Introduction
In recent years, smart clothing has gained popularity for wearable electronic gadgets such as watches, buttons, military berets, sunglasses, and so on [1]. Wearable electronics attached to textiles should be fexible and conformal for onbody applications. Tis inspires the development of smart clothing, where wearable antennas are used to transmit/ receive data [2]. Wearable antennas are widely used in the medical, military, and healthcare industries, as well as in fashion design and aesthetics [3]. In [4], a 2.4 GHz planar inverted-F antenna (PIFA) was presented for security applications. However, it has the disadvantage of a narrow bandwidth. Te development of a compact, wide-coverage antenna is essential for wearable applications. In [5], a semicircular slot antenna was designed on a lower dielectric substrate material for ultrawideband (UWB). Te substrate material used in antenna design is important for wearable applications, and a low-loss substrate material can improve antenna efciency. Another important factor is the relative permittivity of the substrate, and a substrate with a lower dielectric constant reduces surface wave losses and improves wave propagation [6]. In [7], a textile-based wearable antenna was designed with a polyester substrate, and the impact of the weaving process was evaluated. A conformal spiral antenna made of conductive elektrisola e-threads that operates at 1-3 GHz frequency was presented in [8]. In [9], a dual-port embroidery textile antenna was designed for military applications. Te literature [10][11][12][13] reported antenna designs using a variety of fexible substrates such as polydimethylsiloxane (PDMS), denim, jeans, cotton, jute, and felt. Tese antennas are inexpensive and simple to integrate into fashionable clothing.
It is difcult to create a wideband textile antenna that can reliably transmit data even when the human body's posture changes. Multiple-input, multiple-output (MIMO) antennas could be used to address such issues [14]. MIMO antennas have several advantages over a single antenna in terms of increased data rate, improved system reliability, and quality [15]. Te only drawback of MIMO systems is the mutual coupling between antenna elements. Mutual coupling degrades the antenna's performance by interfering with nearby signals. Mutual coupling can be reduced by using periodic structures, such as an electromagnetic bandgap (EBG), an artifcial magnetic conductor (AMC), and a frequency selective surface (FSS) [16][17][18]. In [17], a metamaterial-based AMC structure was developed on a polyester substrate to improve bandwidth and interelement isolation. Tese aspects, however, increase the complexity of the antenna structure [19]. Tere are some simple strategies for improving isolation, such as adding slits and slots or decoupling structures [20], introducing parasitic elements between the radiators [21], and a defected ground structure [22]. Furthermore, mutual coupling can be efectively reduced by adding meander lines to the ground plane [23]. Te orthogonal architecture of antennas leads to minimization of mutual coupling, as reported in [24]. Also, keeping a suffcient spacing between the antenna elements can also help to reduce mutual coupling without the use of any decoupling mechanisms.
In this study, the design and development of a UWB MIMO fexible antenna for smart fabric applications are presented. Smart fabric refers to integration of antennas to the body outfts that track the movements and collect biometric data, such as body temperature, heart rate, and pulse rate which could be helpful for patient monitoring, location tracking, and security applications [25]. Te proposed work uses cotton fabric for a textile antenna, and the proposed MIMO antenna's efectiveness is assessed using surface current distribution and diversity metrics. Te proposed quad-port MIMO antenna provides an impedance bandwidth of 9.1 GHz (2.9-12 GHz) and a maximum gain of 4.84 dBi. Te bending study is used to evaluate the fexibility of the proposed antenna, which can be bent up to a bending angle of 143.3°. SAR analysis is performed to assess its impact on the human body, and the obtained SAR values are well below 1.6 watts/kg. Te fexibility of the developed antenna makes it potentially useful in applications such as patient monitoring and healthcare. Figure 1 shows the design of the proposed octagonal-shaped UWB monopole antenna. Te antenna is designed on the cotton textile (dielectric constant � 1.6; loss tangent � 0.04) substrate material with a thickness of 0.014λ 0 [6]. Cotton is preferred for antenna design because of its strength and durability. Also, it stretches easily and is weather-resistant and water-repellent. It is highly recommended for people with skin allergies due to its hypoallergenic properties. Cotton material is widely used in hospitals for curtains, pillows, and patient clothing [26]. Te designed antenna is made of cotton material and works as a textile antenna when integrated with a patient cloth. Te size of the unit cell is 0.26λ 0 × 0.164λ 0 × 0.014λ 0 .

Unit Cell Design.
Te designed antenna operates in the frequency range of 3.1-12 GHz.
Te proposed antenna design began with an octagonalshaped patch and a partial ground plane of length 0.038λ 0 , as shown in Figure 2(a). Figure 3 depicts the refection coeffcient plots at four stages of evolution. It covers an impedance bandwidth of 0.7 GHz (3.1-3.8 GHz) and 4.75 GHz . In evolution stage 2, the ground plane is lengthened to 0.07λ 0 and truncated on both sides by 0.0048λ 0 (Figure 2(b)). As a result, the refection coefcient curve covers the frequency of 3.1-3.8 GHz in the lower frequency region and extends from 6.7 to 12 GHz in the higher frequency region. In the next stage, three sides of the octagonal radiator are truncated (Figure 2(c)) to lower the impedance in the lower frequency range. In the fourth evolution stage, a rectangular stub of length 0.029λ 0 and width 0.048λ 0 is introduced (Figure 2(d)) between the octagonal radiator and feedline. It increases the electrical length of the radiating element and results in a broad bandwidth of 8.9 GHz (3.1-12 GHz).
Te current distribution is investigated in order to better understand the radiation performance of the antenna. Figure 4 depicts the surface current distribution at 4 GHz and 10 GHz. Te current is distributed in the feedline to achieve the lowest operating frequency as shown in Figure 4(a). Te current is highly dispersed in the circular slot, two octagonal slots, feedline, and around the radiator's circumference at 10 GHz, as shown by the refection coeffcient curves.
Te monopole antenna's lowest operating frequency (f l ) is determined by using the following equation [27]: where l m and b m are the length and width of the monopole radiator and the distance between the monopole radiator and the ground plane is denoted by p m which is equivalent to 0.25 cm. k is related to the efective dielectric constant to the power of 0.5. Rewrite equation (1) as (2) to equate it to the perimeter of the octagonal radiator.
Te term (l m + b m ) is rewritten as 1.46π(s l + s w ), which is the perimeter of an octagonal radiator. s l and s w are the semilength and semiwidth of the octagonal radiator.
For better understanding the physical characteristics of the proposed antenna, equivalent circuit is illustrated in Figure 5. Te analogous circuit is made up of the R, L, and C components. Te series and parallel arrangements of the R, L, and C components are determined from the real and imaginary curves shown in Figure 6. Serial connection is made if the real and imaginary curves move from low to high, and parallel connection is made if these curves travel from high to low [28]. Te simulated S11 curve shows the deep resonance at 3.6, 7.8, and 10.8 GHz frequencies throughout the entire UWB. Terefore, the serial and 2 International Journal of Antennas and Propagation parallel arrangements of R, L, and C components are constructed at these three frequencies. At 3.6 and 10.8 GHz frequencies, the real and imaginary impedance curves move from high to low, resulting in a parallel confguration of R, L, and C components. However, at 7.8 GHz, the real and impedance curves shift from low to high, resulting in a serial confguration of R, L, and C components. Te S11 curve obtained from the equivalent circuit is compared with the simulated S11 curve as depicted in Figure 7. Te simulated refection coefcient curve shows an impedance bandwidth of 8.9 GHz (3.1-12 GHz), whereas the refection coefcient obtained from the equivalent circuit shows an impedance bandwidth of 8.8 GHz (3.1-11.9 GHz). Figure 8 depicts the layout of the MIMO antenna. Te octagonal-shaped antenna elements are transformed into a quad-port MIMO antenna, with each element positioned orthogonally to the other. Te spacing between the antennas is retained as 0.07λ 0 for good isolation. Te MIMO antenna occupies a total area of 0.48λ 0 × 0.48λ 0 × 0.014λ 0 . Te designed antenna is fabricated, as shown in Figure 9, and its S-parameters are measured using an Agilent MS 2037C vector network analyzer.   refection coefcient curves. Without the use of decoupling structures, an isolation of >17 dB is achieved across the UWB range.

Radiation Performance.
Te antenna is kept inside the anechoic chamber, and its radiation characteristics, such as gain and patterns, are measured. When port-1 is enabled, the adjacent ports (ports-2, -3, and -4) are matched with a 50ohm impedance. When port-2 is excited, all other ports are matched, and vice-versa. Te radiation patterns of the proposed antenna are plotted at three diferent frequencies in the E-plane (ϕ � 90°) and H-plane (ϕ � 0°). Te radiation pattern curves appear bidirectional in the E-plane and omnidirectional in the H-plane. Te radiation pattern is plotted for both free-space and on-body conditions. As illustrated in Figure 12, the radiation performance of the antenna is reduced in the presence of a human body when compared to the free-space condition due to the lossy behavior of the human tissues. Figure 13 depicts the efciency and gain curves of the proposed antenna. Te observed simulated gain values are 2.83 dBi for 3 GHz, 3.54 dBi for 6 GHz, and 4.84 dBi for 10 GHz whereas the measured gains at 3 GHz, 6 GHz, and 10 GHz are 2.5 dBi, 3.2 dBi, and 4.5 dBi, respectively. Te maximum efciency achieved is 91.6%. International Journal of Antennas and Propagation

Bending Analysis.
Textile antennas should be fexible and capable of working in bending situations [29]. To meet this requirement, the proposed MIMO antenna is tested in a variety of bending scenarios. Figure 14 depicts the simulated bending analysis of the proposed antenna with diferent bending radius (r � 25 mm, r � 20 mm, and r � 15 mm). Figure 15 depicts the measured bending analysis of the MIMO antenna. Te developed antenna performs well in both simulated and measured scenarios, covering the entire UWB spectrum, as shown in Figure 16. Te performance of the proposed MIMO antenna begins to degrade at bending radius r � 15 mm. Te critical bending angle is the bend angle at which the antenna performance starts to degrade.
Bending angle (θ) of the antenna is calculated using the following equation [30]: where br is the bending radius and h y is the length of the antenna in the y-plane. Te bending angles obtained at three diferent bending radiuses (25 mm, 20 mm, 15 mm) are 114.64°, 143.3°, and 191.08°. Tese bending angles reveal that the proposed MIMO antenna can be bent up to 143.312°without altering its working performance. However, at the bending angle of 191.08°, the performance of the proposed MIMO antenna starts to deteriorate, which is the critical angle. Terefore, the critical angle of the proposed MIMO antenna is 191.08°.

Diversity Performance.
Te evaluation of diversity metrics is important for MIMO antenna. One of them is the envelope correlation coefcient (ECC), which provides correlation information between adjacent unit cells [31]. It can be calculated using the following equation [32]: where F 1 and F 2 are the radiated felds obtained from the radiation patterns and (θ, φ) are elevation and azimuth angles varying between 0 ≤ θ ≤ π and 0 ≤ φ ≤ 2π. Another diversity parameter is diversity gain (DG). It defnes how well a signal is transmitted with the minimum loss during transmission [33]. It can be calculated using the following equation [34]: (5) Figure 17 depicts the DG and ECC graphs of the MIMO antenna using the far feld. Te total active refection coefcient (TARC) is also used to calculate the diversity performance of the antenna. It is the square root of the sum of the total refected waves (b i ) divided by the square root of the total incident waves (a i ).
TARC can be calculated for a two-port antenna using the following equation: Channel capacity loss (CCL) defnes transmission loss in high-data-rate transmission [35]. CCL is calculated using the following equation: where |ψ| R � ρ 11 ρ 12 ρ 21 ρ 22 is the correlation matrix and (9) Figure 18 depicts the TARC and CCL plots of the MIMO antenna.

SAR Analysis.
Specifc absorption rate (SAR) is the ratio of absorbed power to unit mass. S12 (Sim) S12 (Meas) S13 (Sim) S13 (Meas) S14 (Sim) S14 (Meas) where W denotes the amount of energy absorbed by human tissue, V denotes the sample volume, and ρ is the mass density [36]. Te proposed antenna is kept on 100 mm × 100 mm rectangular human body tissue model, as shown in Figure 19(a). Table 1 lists the tissue properties of the human body [37]. Te input power is set at 1 watt. As shown in Figure 19

International Journal of Antennas and Propagation
parts of the open-source imported model [38], and the SAR values are shown in Figure 20. Figure 21 depicts the refection coefcient curves for the on-body scenario of the MIMO antenna. It covers an impedance bandwidth of 8.5 GHz (3.1-11.6 GHz). Te antenna is tested for on-body performance by placing it on the human body. Figure 22 depicts the measured S-parameter results, which show an impedance bandwidth of 9.9 GHz (2.1-12 GHz). Figure 23 depicts the refection coefcient curves plotted in free space and on the human body. Te refection coefcient curves do not change when the antenna is placed on the human body. Table 2 compares the performance of the developed antenna to the existing literature.

. Conclusion
In this work, a quad-port UWB MIMO antenna is presented for smart clothing applications. Te antenna is made of cotton fabric that has been engineered to function as a textile antenna when combined with a patient cloth. Te antenna is constructed using four similar antenna elements positioned orthogonally to improve isolation without adding decoupling structures. Over the UWB range, isolation is greater than 17 dB, and the calculated diversity characterization is comparable. Also, the proposed antenna can bend up to 20 mm, making it suitable for wearable applications. Te SAR analysis of the MIMO antenna shows satisfactory values that are signifcantly lower than 1.6 W/Kg, making the proposed antenna suitable for patient monitoring applications.

Data Availability
Te data used to support the fndings are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.