Wideband Circularly Polarized Textile MIMO Antenna for Wearable Applications

In this paper, a wideband circularly polarized (CP) textile multiple-input-multiple-output (MIMO) antenna is presented for wearable applications. The designed MIMO antenna consists of two sickle-shaped resonating elements and a common ground plane. Each antenna element is excited by a microstrip line feed, and an inverted L-shaped strip is introduced in the ground plane to support circular polarization. The antenna covers impedance bandwidth (<inline-formula> <tex-math notation="LaTeX">$\text{S}_{11}\le -10$ </tex-math></inline-formula> dB) of 3.6–13 GHz and 3-dB axial ratio bandwidth (ARBW) of 5.2–7.1 GHz. The proposed textile MIMO antenna exhibits envelope correlation coefficient (ECC) < 0.02, diversity gain (DG) <inline-formula> <tex-math notation="LaTeX">$>9.96$ </tex-math></inline-formula>, total active reflective coefficient (TARC) <inline-formula> <tex-math notation="LaTeX">$ < -10$ </tex-math></inline-formula> dB, channel capacity loss (CCL) < 0.2 b/s/Hz, and mean effective gain (MEG) ratio within ±0.5 dB. The antenna offers dual-sense circular polarization and high isolation (>18) between the resonating elements. Also, the proposed antenna is investigated for different human body situations, and its specific absorption rate (SAR) for human tissues specimen is studied. The overall size of the proposed CP textile MIMO antenna is <inline-formula> <tex-math notation="LaTeX">$32.5 \times 42 \times 1$ </tex-math></inline-formula> mm<sup>3</sup>. The designed MIMO antenna could be useful for wearable applications due to its textile layers, reasonable on-body performance, and compact size.


I. INTRODUCTION
Recently, wearable gadgets have received considerable attention from designers due to their widespread applications in healthcare, entertainment, navigation, remote monitoring, rescue, and security [1], [2]. An antenna is an important element of a wearable transceiving system. Wearable/textile antennas are needed to radiate efficiently in a number of situations such as body gestures, bending, running, and movement. Textile antennas must be low-profile, conformal, light in weight, flexible, robust, and compact in order to be easily integrated into portable electronic devices or clothing [3]. Antennas designed for wearable applications are intended to operate in close proximity to the human The associate editor coordinating the review of this manuscript and approving it for publication was Debabrata Karmokar . body. Therefore, several issues must be considered during the textile antenna design process, such as structural deformation, antenna placement, and fabrication [4], [5]. Wideband circularly polarized (CP) antennas are attractive candidates for wearable applications due to their orientation flexibility, better mobility, and multipath interference suppression capability [6]- [8]. Multiple-input-multiple-output (MIMO)/diversity technology is also gaining popularity for improving link capacity, particularly in complex multipath environments. A multi-element antenna with polarization diversity is a good choice for encountering multipath fading and establishing reliable channels [9]- [11].
Many wearable MIMO antennas with high inter-element isolation have been presented in the literature [12]- [17]. In [12], a dual-band textile-based MIMO antenna was proposed for WLAN applications, where vias were used VOLUME 9, 2021 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ to modify the resonant mode of the waveguide cavity. A circular-shaped MIMO antenna with high impedance surface (HIS) was reported [13], where isolation larger than 15 dB was achieved between the ports. A solo-coat textile MIMO antenna was presented for wearable applications [14], where the ground plane worked as the radiator. In [15], a two-element wearable MIMO antenna was reported for ultra-wideband (UWB) applications, where a partially suppressed ground plane was used to obtain isolation >26 dB. A wideband rectangular-shaped textile MIMO antenna was presented in [16], where two I-shaped stubs were used to achieve high inter-element isolation. Over the last five years, a few CP MIMO antenna designs have been developed that can be used for WLAN, C-band, and satellite applications [17]- [21]. In [17], a coplanar waveguide (CPW)-fed square slot MIMO antenna was presented, where inverted-L planar strips were used in the ground plane to obtain wide axial ratio bandwidth (ARBW). A dual CP antenna was reported in [18], where a combination of L-shaped strips was used to obtain circular polarization. In [19], a CP MIMO antenna consisted of grounded stubs and a mirrored F-shaped defected ground structure (DGS) in the ground plane was proposed. In [20], a CP antenna with monopole extension of the microstrip line was presented, where the orthogonal fields are induced through the modified ground plane. In [21], a CP MIMO antenna was presented for wearable gadgets, where the phase difference between orthogonal modes can be controlled by a metal strip loaded in the ground plane. The MIMO antennas presented in [12]- [16] showed linearly polarized (LP) characteristics while the antennas in [17]- [21] showed CP performance. Most of the wearable/textile antennas reported in the available literature are LP single element configurations with narrow ARBW, and wearable CP MIMO antenna with wide ARBW is rarely reported.
In this article, a low-profile compact-sized two-element CP MIMO textile antenna is presented for wearable applications. The proposed MIMO antenna element consists of a microstrip line-fed sickle-shaped radiator and a modified ground plane. An L-shaped stub is integrated with the modified ground plane to introduce a quadrature phase shift between the horizontal and vertical electric field vectors. The two identical antenna elements are located in a mirrored-image fashion to obtain dual-sense radiation characteristics. Port-1 emits left-hand CP (LHCP) waves while port-2 emits right-hand CP (RHCP) waves. This property makes the proposed MIMO antenna suitable for polarization diversity operation.

II. ANTENNA CONFIGURATION
A. ANTENNA ELEMENT DESIGN Fig. 1 shows the geometric layout of the proposed CP textile antenna element. The physical size of the antenna element is 20 × 30.5 mm 2 . The antenna element consists of a sickle-shaped radiating patch and a modified ground plane     Fig. 2(a), a microstrip line-fed sickle-shaped radiator is designed on the top of the textile-based substrate material, and a partial ground plane on the bottom of the substrate material. The Ant. 1 shows resonating bandwidth of 3.6-4.5 GHz and 7.5-11.3 GHz. In step-2, as shown in Fig. 2(b), a strip of λ/2 wavelength is introduced in the partial ground plane to improve impedance bandwidth. Ant. 2 also shows two resonating bands.

1) DESIGN PROCESS
Furthermore, as shown in Fig. 2(c), the impedance mismatching is encountered by introducing an L-shaped strip in the ground plane (Ant. 3). The L-shaped strip also induces quadrature phase difference between the two electric field vectors (E x and E y ). The impedance bandwidth and axial ratio bandwidth (ARBW) of the Ant. 3 are 5.5-10.4 GHz and 5.3-5.8 GHz, respectively. In step-4, the L-shaped strip is alienated into two parts (of dimensions L 2 × W 2 and L 1 × W 1 ), as shown in Fig. 2(d), which improves the axial ratio of the antenna element (Ant. 4). The antenna resonating band needs to shift to the left side for covering the lower frequency range. For this reason, a rectangular slot (of dimensions L 3 × W 3 ) is etched from the ground plane of the antenna element as shown in Fig. 2(e). Thus, the current path length increases and shifts the operating frequency band towards the lower side. Also, superior CP performance is obtained in the proposed Ant. 5. The simulated results (impedance bandwidth and ARBW) of the design stages are listed in Table 1.   is LP as the phase difference between the electric field vectors is not 90 • . Similarly, Ant. 2 is also LP. Further, L-shaped strip of different lengths and widths are connected to the rectangular ground plane (Ant. 3 and Ant. 4) to achieve 90 • phase difference between the electric field vectors. By using this method, the amplitude of E x and E y becomes almost equal with 90 • phase difference between them [17].

2) CP PERFORMANCE
The surface current distributions of the proposed textile antenna (at ωt = 0 • , ωt = 90 • , ωt = 180 • , and ωt = 270 • ) are shown in Fig. 4. A 1 and A 2 symbolize the orthogonal current vectors, and A 3 represents their sum. At ωt = 0 • , the surface current density on the upper part of the patch (A 1 ) and the edge of the L-shaped strip (A 2 ) increases, and the sum (A 3 ) of these two vectors is heading towards the upper right, shown in Fig. 4(a). At ωt = 90 • , as displayed in Fig. 4(b), the vector sum A 3 is heading towards the lower right, which illustrates that the current vectors are rotating clockwise as time progresses. Similarly, at ωt = 180 • and 270 • , the sum (A 3 ) travels in the clockwise direction as displayed in Figs. 4(c) and (d), respectively. Therefore, the proposed textile antenna demonstrates LHCP operation in the broadside direction.
The |E x /E y | and phase difference plots of the proposed Ant. 5 and Ant. 1 are shown in Figs. 5(a) and (b), respectively. The curves reveal that the L-shaped strip on the ground plane balances the magnitude of horizontal and vertical electric field vectors and introduces a 90 • phase difference between them. The current path increases by etching a rectangular slot from the ground plane, hence shifting the resonating frequency band towards the left side.

1) DESIGN PROCESS
Due to the mirrored-image arrangement, the L-shaped strips of the antenna elements unite to form a T-shaped stub at the middle of the MIMO Antenna A as shown in Fig. 7(a). Without any decoupling element between the antenna elements, the S 12 parameters of the presented MIMO antenna are stable. The S-parameters and axial ratio curves of the    Table 2. Fig. 8(c) shows the simulated axial ratio beamwidth of the proposed MIMO antenna at 6 GHz in the φ = 0 • and φ = 90 • planes. It can be seen that the 3-dB axial ratio beamwidth ranges from −63 • to 72 • for φ = 0 • and −68 • to −62 • for φ = 90 • . The simulated and measured efficiency of the proposed antenna are shown in Fig. 8(d), and the peak efficiency is about 78% at 9.5 GHz. The efficiency is low due to the dielectric loss, smaller surface area, and compact size of the antenna.

2) DUAL-SENSE CP PERFORMANCE
The proposed two-port textile MIMO antenna demonstrates dual-sense radiation characteristics. The surface current distributions of the textile antenna (at ωt = 0 • , ωt = 90 • , ωt = 180 • , and ωt = 270 • ) are shown in Fig. 9. Here, A 1 and A 2 symbolize the orthogonal current vectors, and A 3 represents their sum. By changing the port excitation, either LHCP or RHCP behavior can be generated in the broadside direction of the antenna. Figs. 9(a) and (b) illustrate clockwise and anti-clockwise movement of the electric field vectors at port-1 and port-2, respectively.
As shown in Fig. 9(a), the resultant (A 3 ) is heading towards the upper right at ωt = 0 • , while the vector sum (A 3 ) is heading towards the lower right at ωt = 90 • . On the contrary, in Fig. 9(b), the vector sum (A 3 ) is heading towards the upper left at ωt = 0 • , while the sum (A 3 ) is heading towards the lower left at ωt = 90 • . When port-1 is excited, port-2 is terminated with a load of 50 and vice versa. The simulated surface current distribution of the proposed MIMO antenna at 8.5 GHz is shown in Fig. 10. It is found that the current distribution is uniform throughout the patch area, with the exception of the patch element in the middle, and this validates the gain of the antenna.

A. S-PARAMETERS AND AXIAL RATIO
The performance of the proposed textile MIMO antenna is measured using an Anritsu MS2038C vector network analyzer. The measured and simulated reflection coefficients of the textile MIMO antenna are shown in Fig. 11(a). The measured and simulated −10 dB impedance bandwidths are 113% (3.6-13 GHz) and 121% (3.3-13.6 GHz), respectively. As shown in Fig. 11(a), the measured isolation between port-1 and port-2 is >17 dB while the simulated isolation is >19 dB. Since coupling is stronger at lower frequencies, the decoupling structure is designed for a frequency range of 4-6 GHz.
The measured and simulated axial ratio plots (in the broadside direction) of the textile MIMO antenna are illustrated in Fig. 11(b). The measured 3-dB ARBW is 30% (5.2-7.1 GHz) and the simulated 3-dB ARBW is 37% (5-7.3 GHz). The minimum (measured) value of the axial ratio is 1.2 dB at 5.2 GHz.
The measured and simulated gain plots of the proposed textile antenna are shown in Fig. 11(c). The measured peak gain is 5.7 dB at 8.5 GHz. The simulated and measured outcomes of the textile antenna are in good agreement. A small difference exists due to the fabrication error and the adhesive used to join the textile materials and the copper part. Fig. 12 illustrates the measured and simulated radiation patterns of the proposed CP textile MIMO antenna at 5.3 GHz and 6.3 GHz. The MIMO antenna shows LHCP characteristics when port-1 is excited and port-2 is matched with a load of 50 . In the same way, the MIMO antenna shows RHCP behavior when port-2 is excited and port-1 is matched with a load of 50 . The radiation patterns in Figs. 12(a)-(d) validate the dual-sense behavior of the proposed CP textile MIMO antenna.

C. MIMO PERFORMANCE
To support the proposed textile antenna diversity performance, MIMO parameters such as envelope correlation coefficient (ECC), diversity gain (DG), total active reflection VOLUME 9, 2021

1) ECC AND DG
In MIMO systems, ECC demonstrates the correlation between antenna ports. The following relation can be used to evaluate the ECC [25]. The simulated and measured ECC curves of the presented textile MIMO antenna are displayed in Fig. 13(a). The ECC between antenna elements-1 and -2 is less than 0.02.
Another important MIMO parameter is DG, which can be calculated using the following relation.
The simulated and measured DG curves of the proposed textile MIMO antenna are shown in Fig. 13(a). The DG of the textile antenna is greater than 9.96 dB.

2) TARC
When the antenna elements in a multi-port antenna system operate concurrently, they affect each other's performance. TARC takes into account this effect, which is defined as the square root of the ratio of total incident power to reflected power in the overall MIMO system. The following equation can be used to compute the TARC of the proposed two-port MIMO antenna [26]. The simulated and measured TARC curves are presented in Fig. 13(b). Here, the measured and simulated TARC values are less than −10 dB for the entire operating band.

3) CCL AND MEG
CCL is the knowledge of the maximum cut-off on the message transmission rate over a communication channel, and it can be evaluated as [25].
The measured and simulated CCL curves of the proposed textile MIMO antenna are presented in Fig. 14(a). The CCL values are less than 0.2 b/s/Hz for the entire operating band.
MEG reflects gain behavior of the MIMO antenna. It demonstrates that the impact of the wireless environment on diversity has been considered. The following equations can be used to compute the MEG [22].
The measured MEG graphs of the proposed textile MIMO antenna are plotted in Fig. 14(b). It is noticed that the difference between MEG 1 and MEG 2 is ±0.5 dB.

IV. BENDING ANALYSIS
The wearable antenna may bend when mounted in garments worn on the human body, such as the arms and thighs. To ensure the structural integrity of the antenna, simulations were run to test its ability to bend along the E-plane or H-plane at different radii, 15 mm, 25 mm, 35 mm, and 45 mm. The simulated S 11 and axial ratio results of the textile MIMO antenna at different bending radii are shown in Figs. 15 and 16, respectively. For better realization, the MIMO antenna is analyzed in two states: bending along the E-plane and bending along the H-plane. Figs. 15(a) and (b) show the simulated results for the E-plane and H-plane bending with radius varying from 15 mm to   The MIMO antenna performs well under bend conditions and exhibits a similar bandwidth, however, as the bending radius decreases, the S 11 results deteriorate due to impedance mismatching between the port and feed line. The same trend can be seen in the H-plane bending analyses, with the resonating band shifting to the right as the bending radius decreases. Fig. 16 shows the simulated axial ratio, for the E-plane bending, with radius varying from 15 mm to 45 mm in comparison to the original antenna. The 3-dB ARBW shifts to the lower side as the bending radius increases, and it degrades at 6 GHz due to the feed line offset. Figs. 17(a) and (b) show the measured S 11 and axial ratio of the proposed textile antenna in the E-plane, respectively, and the simulated and measured results are found to be in good agreement.
Also, the diversity performance of the MIMO antenna is investigated in terms of ECC, DG, MEG, TARC, and CCL in various bending situations, as shown in Table 3. It has been found that as the bending radius increases, the performance of the antenna slightly decreases due to changes in the current distribution on the ground plane of the antenna.
Furthermore, the on-body analysis of the antenna is also studied. Table 4 shows the intrinsic values of conductivity, permittivity, density, and loss tangent of skin, fat, muscle and bone of a four-layer human arm shown in Fig. 18(a).  The simulated S 11 results for different bending radius are shown in Fig. 18(b), and it is noticed that the curves are shifted slightly towards the right side due to the lossy nature of the human arm and the reduction in the current path length.

V. SPECIFIC ABSORPTION RATE CHARACTERISTICS
The specific absorption rate (SAR) can be defined by the following relation [27].
where dm is the incremental mass, dE is the time derivative of the incremental energy, dA is the volume element, and ρ is  - [30]. For 1 W of input power, the maximum SAR value obtained for 10 g of tissue is 5.434 W/kg. According to the new guidelines, the majority of devices for human body applications require mW power range.
The calculated maximum input power of the antenna is 367.86 mW for 10 g of tissue at 2 W input power, which is less than the maximum standard limit. Thus, the proposed antenna will operate within allowable limits. Table 5 compares the proposed textile MIMO antenna and recently reported textile/wearable antennas. The parameters compared are antenna size, substrate material, operating bandwidth, fractional bandwidth, gain, ARBW, sense of polarization, and isolation. The wearable antennas presented in [12]- [16] were LP. The antenna proposed in [11] showed circular polarization behavior, and [17]- [21] were CP with dual sense, but they showed a small operating bandwidth. In contrast to the reported antenna designs, the proposed textile MIMO antenna exhibits small size, wider axial ratio and impedance bandwidths, and dual-sense (LHCP/RHCP) behavior.

VI. CONCLUSION
In this paper, a wideband CP two-port textile MIMO antenna is proposed. The overall size of the textile antenna is 32.5 × 42 × 1 mm 3 . The proposed MIMO antenna exhibits an impedance bandwidth of 113% and ARBW of 30%. The antenna shows DG >9.96 dB, ECC <0.02, and CCL <0.2 b/s/Hz. The isolation obtained is greater than 18 dB without the use of any additional decoupling elements. SAR analysis of the proposed antenna is also studied for human tissue models and it is found within the acceptable range. The presented antenna is useful for off-body and on-body WLAN, Wi-MAX, and C-band uplink/downlink applications.  Ltd., 2009) and around 100 scientific papers published in international journals and conferences. His research interests include wireless communications and digital signal processing, with a particular focus on smart antennas, cooperative communications, signal classification for cognitive radio systems, synchronization and channel estimation, multiple-input multiple-output antenna systems, multicarrier systems, and error-correcting codes. He was a recipient of the Young Scientist Award from the International Union of Radio Science in 1999. He is currently a Professor with the School of Computational and Integrative Sciences, Jawaharlal Nehru University, New Delhi, India. He has been credited to publish more than 350 research articles with more than 2900 citations and an H-index of 25 in several peer-reviewed journals and conferences. He had supervised 50 M.Tech. and 15 Ph.D. scholars in the field of RF and microwave engineering. He is a member of the Institution of Engineers, India, the Indian Society for Technical Education, and the Institute of Electronics and Telecommunication Engineers of India. He had successfully executed five research projects sponsored by several agencies of the Government of India, such as DRDO, DST, AICTE, and ISRO. He is currently on the editorial board of several international journals. VOLUME 9, 2021