DGS-based wideband MIMO antenna for on–off body communication with port isolation enhancement operating at 2.45 GHz industrial scientific and medical band

Abstract

A wide band, dual radiator Multiple input multiple output (MIMO) antenna resonating at 2.4 GHz Industrial Scientific and Medical (ISM) band is designed and analysed for the body-centric wireless communication. Defected ground structure (DGS) is used to reduce the antenna size and to achieve wide bandwidth. Moreover, a decoupling slot is also etched in the ground plane to improve the port isolation. Proposed 2 × 1 array MIMO structure has occupied the volume of 90 × 40 × 0.8 mm³. Rectangular and cylindrical shaped multi-layered tissue models are used for simulation and pork tissue is used for the measurement of antenna performance. Broad 10-dB bandwidth of 1250/900 MHz, port isolation of 21/20 dB, gain of 5.53/5.07 dBi with, SAR value of 0.628/0.627 W/kg is achieved for the flat/deformed state of antenna. Results show that the proposed DGS-based antenna is reliable to operate on different tissue environment for body area network.

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DGS-based wideband MIMO antenna for
on–off body communication with port isolation
enhancement operating at 2.45 GHz industrial
scientific and medical band
Anupma Gupta & Vipan Kumar
To cite this article: Anupma Gupta & Vipan Kumar (2021) DGS-based wideband MIMO antenna
for on–off body communication with port isolation enhancement operating at 2.45 GHz industrial
scientific and medical band, Journal of Electromagnetic Waves and Applications, 35:7, 888-901,
DOI: 10.1080/09205071.2020.1865209
To link to this article: https://doi.org/10.1080/09205071.2020.1865209
Published online: 26 Dec 2020.
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JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS
2021, VOL. 35, NO. 7, 888–901
https://doi.org/10.1080/09205071.2020.1865209
DGS-based wideband MIMO antenna for on–off body
communication with port isolation enhancement operating at
2.45 GHz industrial scientific and medical band
Anupma Guptaaand Vipan Kumar b
aDepartment of Electronics and Communication Engineering, Thapar Institute of Engineering and
Technology, Patiala, Punjab, India; bDepartment of Electronics and Communication Engineering, Sri Sai
College of Engineering and Technology, Badhani, Pathankot, India
ABSTRACT
A wide band, dual radiator Multiple input multiple output (MIMO)
antenna resonating at 2.4 GHz Industrial Scientific and Medical (ISM)
band is designed and analysed for the body-centric wireless com-
munication. Defected ground structure (DGS) is used to reduce the
antenna size and to achieve wide bandwidth. Moreover, a decou-
pling slot is also etched in the ground plane to improve the port
isolation. Proposed 2 ×1 array MIMO structure has occupied the
volume of 90 ×40 ×0.8 mm3. Rectangular and cylindrical shaped
multi-layered tissue models are used for simulation and pork tissue
is used for the measurement of antenna performance. Broad 10-
dB bandwidth of 1250/900 MHz, port isolation of 21/20 dB, gain of
5.53/5.07 dBi with, SAR value of 0.628/0.627 W/kg is achieved for the
flat/deformed state of antenna. Results show that the proposed DGS-
based antenna is reliable to operate on different tissue environment
for body area network.
ARTICLE HISTORY
Received 15 May 2020
Accepted 13 December 2020
KEYWORDS
ISM band; defected ground
structure; decoupling slot;
deformed state;
multi-layered tissue model;
medical diagnosis
1. Introduction
Health-care is the primary domain of wireless bio-telemetry. Real-time psychological
parameters such as ECG, EEG, temperature, blood pressure, glucose level and pulse rate are
transmitted from a patient to the doctor at remote location for medical diagnosis. There-
fore, a reliable transmission link is must to confirm patient’s safety. Dedicated antenna that
can efficiently operate on lossy and complex biological tissue as well as in high scattering
and reflecting indoor locations is required for establishing this link. Some of the electro-
magnetic power is absorbed by body tissue, it reduces antenna gain and efficiency. Data
transmission reliability of low efficiency antenna degrades significantly in multi-fading envi-
ronment. Besides this, antenna surface current is interfered by tissue conductivity thus
altering impedance matching and resonance frequency [1,2]. All these critical issues make
on-body antenna designing an inspiring task for the researchers.
Various research work has been presented to solve different issues of on-body anten-
nas including gain enhancement for better communication link range, miniaturization to
CONTACT Anupma Gupta anupmagupta31@gmail.com
© 2020 Informa UK Limited, trading as Taylor & FrancisGroup

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 889
integrate easily with wearable devices, bandwidth enhancement to avoid frequency detun-
ing and isolating antenna from body tissue for low SAR value. Multiple resonating paths
are generated by using ring-shaped ground coupling [3] and fractal patch with defected
ground structure are proposed for bandwidth enhancement [4]. Parasitic patches [5]and
metallic reflector surfaces [6], EBG [7]andAMC[8] structures are added for reducing back
radiation. PIFA antenna is integrated with an extra substrate to enhance antenna gain is
presented in [9]. Complementary split ring resonator [10] and shorting pins [11]areusedto
reduce antenna dimensions.
However, referred antennas suffer from low radiation efficiency [3,4,6,10,11], occupy
large volumes [5,7,8] and fabricated on textiles substrates much immune to the external
environment [5,8,9]. In addition to this, all of these structures are single radiator struc-
tures and may suffer from the multi-fading. In past few years, MIMO technology is used
to enhance transmission capability of on-body antennas [12–16]. Keeping large separation
gap between the radiators [12], use of inductive radiators in a loop antenna [13], shorting
of cavity edges in SIW technology-based wearable antenna [14], adding of HIS in a co-
radiator-based MIMO antenna [15], adding of inverted U-shaped decoupling stub [16]and
rectangular-shaped decoupling slits [17], use of defected ground structure [18] are differ-
ent decoupling phenomena presented by the researchers. Rigid dielectric substrates and
large planar geometry of these antennas create discomfort to the user.
Defected ground structure (DGS) is gaining much attention for mitigating the insuffi-
ciencies of conventional microstrip patch antenna like low gain, narrow bandwidth and
high cross polarization [19–27]. Etching of different shapes in the ground plane disturbs
the current distribution by changing the reactive impedance and adding of resistance with-
out required additional surface [19]. In [20] multiple square and triangular slots are etched
in partial ground plane to achieve ultra-wide frequency range from 2 to 21 GHz. Different
slot topologies including circular and rectangular dumbbell [21,22], “U”, “V” and “H” shaped
slots [23,24], partial concentric rings [25], meander lines [26] and fractals [27]. In [24]26%
size reduction of antenna is achieved by modifying the dumbbell shape into H-shape slot.
In [23] high inductance was obtained using defected ground structure. Better return loss
level was attained in [25–27] etching complex and asymmetrical slots in ground. How-
ever, defected ground structure is an effective solution for performance enhancement of
microstrip patch antenna.
On the basis of above said issues of on-body antennas, authors have presented a
defected ground structure-based MIMO antenna for on-body to off-body communica-
tion. The proposed antenna exhibits the following unique features: (i) novel DGS slot for
wideband coverage from 1.75 to 3.00 GHz, that enables antenna to overcome frequency
detuning effect when operated on different biological tissue and more immune to struc-
tural deformation; (ii) Broadside radiation pattern which normal to the body surface and
enhances transmission capability towards the remote monitoring device; (iii) designed on
thin substrate; stable resonance and gain when subjected to bending (iv) MIMO structure
with low mutual coupling and good transmission capability in dense indoor-environment
and (iv) SAR below the safety restriction that makes antenna safe for human exposure.
Thus, designed structure is a good solution to improve the stability of antenna perfor-
mance in biological environment along with interference mitigation in dense scattering
situation.

890 A. GUPTA AND V. KUMAR
Figure 1. Topology of single element antenna (a) front view (b) back view (c) simulation setup.
Tab le 1. Electrical properties of body tissue at 2.45GHz.
Tissue layers Electric permittivity (r) Conductivity (σ) in S/m Loss tangent Thickness of tissue layers
Skin layer 38 1.46 .28 2 mm
Fat layer 5.2 0.1 0 .14 5 m m
Muscle layer 52.7 1.8 .24 10 mm
Bone layer 18.6 0.78 .31 7 mm
2. Antenna topology and the simulation setup
Initially a single element structure with a rectangular patch and a defected ground with
wide asymmetrical slot is designed. Antenna is built on RT/Duroid 5880 substrate (εr=2.2,
loss tan =0.0009) with a height of 0.8 mm. To match the antenna performance in practi-
cal on-body application, a multi-layered tissue model consists of skin, fat, muscle and bone
layers is designed using CST microwave studio. Electrical properties of equivalent body tis-
sue are specified in [28] are briefed in Table 1. Planar dimension of tissue model is 70 ×
70 mm2. Single radiator antenna topology and prospective view of the simulation setup is
represented in Figure 1.
Further, antenna design process is explained in two subsections; first single on-body
antenna is designed to achieve the desired resonance frequency (in Section 2.1) and for
MIMO performance, two identical antennas are placed parallel on the common ground
plane, which is further modified to enhance port isolation (in Section 2.2).
2.1. Design evaluation of single element defected ground antenna
Three cases are analysed to explain antenna design process and shown in Figure 2. Com-
parison of return loss plot for all the three cases is drawn in Figure 3.
Case 1: To begin with the design process, a quarter wavelength conventional microstrip
patch antenna with full ground plane of size 45 ×40 mm2operating in free space is consid-
ered. Initial length and width of the patch is taken as 22 ×25 mm2; which causes antenna
to resonate at 4.9 GHz and justified the design equations ((1)–(4)) given in [29]. As body
tissue is added below the antenna, resonance frequency decreases to 3.8 GHz with poor
impedance matching (Figure 3). Thus, antenna taken in case 1 is modified through defected

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 891
Figure 2. Antenna topologies during design process (a) front view (b) back view case 1 (c) back view
case 2 (d) case 3.
Figure 3. Return loss plot for three design topologies.
ground structure to achieve targeted operating frequency.
Ereff =Er+1
2+Er−1
21+12 h
Wp−0.5
(1)
where Er=dielectric constant of substrate; his the thickness of substrate; Ereff is the
effective dielectric constant due to fringing field.
Wp =c
2fr2
Er+1(2)
Lp =c
2fr√Ereff −2L(3)
L=h×0.412(Ereff +0.3)w
h+0.264
(Ereff −0.258)w
h+0.8(4)

892 A. GUPTA AND V. KUMAR
Figure 4. Input impedance plot for three simulation cases.
where Wp is the width and Lpis the length of the patch; Lis the increase in length due to
fringing field.
Case 2: DGS technique is preferred here, due to its advantages of wide bandwidth and
high gain. A wide rectangular slot of size 30 ×24 mm2is etched in the ground plane; this
increases the inductance of antenna. Impedance plot to study reactance for all three cases is
shown in Figure 4. Imaginary impedance becomes more positive depicting high inductance
in impedance. This increased value of inductance tunes antenna to resonate at 3.2 GHz. |S11|
of −20.48 dB with 50-ohm impedance matching is achieved in case 2. Etching of rectangular
slot generates multiple resonating path which in turn enhances antenna bandwidth.
Case 3: Two slots are merged with the wide ground slot; a small rectangular slot of size
5×6.25 mm2at left side and another semi-circular slot of radius 10.87 mm at the top side.
Combined effect of three slots helps antenna to operate at 2.45 GHz. Small rectangular slot
helps to lower the frequency and the semi-circular slot contributes to optimum impedance
matching. Two slots have changed antenna reactance and resistance that contributes to
achieve resonance at desired band without increasing antenna planar dimensions. In case
3, near-zero reactance with 50-ohm impedance at 2.45 GHz can be observed in Figure 4.
Value of geometrical parameters of single element antenna is mentioned in Table 2.
2.2. MIMO structure design with isolation enhancement
In order to design the MIMO structure two self-complementary DGS-based antenna struc-
tures are placed parallelly. Due to connected ground significant current is flowing from port
1 to port 2. Therefore, to reduce the port-coupling a rectangular slot of 13 ×5mm
2is etched
at the centre of ground plane (along X-axis). Topology of the MIMO structure with isolation

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 893
Tab le 2. Dimensions of the antenna topology.
Parameter Value (mm) Parameter Value (mm)
Wp 25.14 Lp 22.2
W45 L40
Wf 1.8 Lf 11
Wc 30.5 Lc 14.59
Ws 6.25 Ls 5
R10.87 – –
Figure 5. Topology of dual element MIMO antenna (a) front view (b) back view without slot (c) back
view with decoupling slot.
Figure 6. S-parameters plot of MIMO antenna with and without decoupling slot.
slot is shown in Figure 5. S-parameters of MIMO antenna with and without slot are rep-
resented in Figure 6. Etching of slot has reduced the coupling from −12.13 to −20.67 dB.
Addition of decoupling slot has also reduced the value of |S11|from−43.89 to 25.86 dB. Still
it can efficiently cover the targeted bandwidth for the ISM band.
Design mechanism for the etching of the decoupling slot can be defined with the help
of surface current distribution shown in Figure 7. Maximum current flowing from port 1
to port 2 can be observed at the centre of lower edge, thus, a rectangular slot is etched
between the ground of two elements to change the current path. From the surface current
plot (Figure 7) it can be found that current flowing from port 1 to port 2 is offsets by the
ground slot which enhances the isolation.

894 A. GUPTA AND V. KUMAR
Figure 7. Surface current distribution at 2.4 GHz (a) without slot (b) with decoupling slot.
Figure 8. Photograph during measurement (a) antena on flat surface (b) |S11| on VNA flat state (c)
antenna on bent surface (d) |S11| on VNA bent state (e) antenna in anechoic chamber.
Figure 9. S-parameters of antenna on flat surface.
3. Result analysis
Based on the simulated data a model of designed structure is fabricated. Results are
verified by measuring the antenna performance on a pork tissue. Using Agilent N5247A pro-
grammable network analyser and anechoic chamber is used for experiment. Photographs
of the experimental setup are shown in Figure 8.

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 895
Figure 10. S-parameters of antenna on bent surface.
Figure 11. 2-D radiation plot (a) on flat tissue (b) on-bent tissue.
3.1. S-parameters
Comparison of simulated and measured s-parameters in flat state is represented in Figure
9. Simulated reflection coefficient (S11) and transmission coefficients (S21 )areinwell
agreement with the measured. Overlapping 10-dB impedance bandwidth is 1.25 GHz
(1.75–3.00 GHz). Large bandwidth of antenna avoids frequency detuning and makes it
robust to operate on different types of biological tissues. Minimum coupling of 22 dB is
achieved at 2.45 GHz and for the whole operating bandwidth coupling is less than 15 dB.
Further to confirm the antenna reliability to work on non-planar body structure, it is bent
along x-axis. While simulation, a cylindrical muscle tissue of radius Rx=25 mm is designed.
For the measurement, antenna is wrapped around pork lion and hold with a letter tape
(shown in Figure 8(c)). S-parameters for bent condition is plotted in Figure 10. On compari-
son with flat structure bandwidth is reduced from 1250 to 900 MHz (1.75– 2.65 GHz) in bent

896 A. GUPTA AND V. KUMAR
Figure 12. Gain plot (a) simulated and measured gain (b) 3-D gain on flat tissue (c) 3-D gain on-bent
tissue.
state. Negligible effect of bending can be observed on mutual coupling. It confirms that
antenna has stable response for varying tissue properties.
3.2. Radiation characteristics
Simulated and measured E-plane (φ=0°) and H-plane (φ=90°) radiation pattern for both
the flat and bent state are shown in Figure 11. In E-plane antenna has similar behaviour for
the flat and bent state. For the h-plane, antenna shows dipole like characteristics (on flat
tissue) and broadside type characteristics (on bent tissue). For both the working conditions
maximum radiated power is oriented towards normal to the body surface confirming the
desired behaviour of the proposed antenna designed for on–off body communication.

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 897
Figure 13. Radiation efficiency plot of proposed antenna.
SimulatedandmeasuredgainiscomparedinFigure12(a) and 3-D gain plots for flat and
bent state (Figure 12(b,c)) are shown to confirm the antenna gain. Antenna retains more
than 4 dBi gain over the whole bandwidth for both the operating scenarios. At 2.45 GHz
gain value of 5.55 and 5.07 dBi is achieved on flat and bent tissue respectively.
On-body and free space radiation efficiency of the antenna structure is shown in
Figure 13. The free space antenna has more than 90% efficiency and it falls to 34% at
2.45 GHz. Free space antenna shows that very small power is attenuated due to port
impedance mismatch and loss in dielectric substrate. Low on-body efficiency is due to the
power absorption in complex body tissue.
3.3. Envelope correlation coecient and diversity gain
ECC and diversity gain are the important diversity parameters of a MIMO antenna. Higher
value of ECC signifies high correlation of radiation between the radiators operating in near
surface. ECC value should be limited to 0.5 for better MIMO performance. High value of
DG shows the accomplishment of diversity performance. Value of ECC and DG from the
measured S-parameters is calculated using Equations (5) and (6) and plotted in Figure 14.
ECC is less than 0.03 and diversity gain is more than 9.35 over the operating bandwidth.
ρe=|S∗
11S12 +S∗
21S22 |2
(1−(|S11|2+|S21|2))(1−(|S22 |2+|S12|2)) (5)
DG =10(1−|ECC|)(6)

898 A. GUPTA AND V. KUMAR
Figure 14. Measured ECC and diversity gain.
Figure 15. Simulated SAR (a) on flat tissue (b) on bent tissue.
3.4. Eect of near eld radiation on body tissue in terms of SAR
Due to the close proximity of body tissue and antenna, near field power is absorbed by
tissue and causes heating effect. Thus, the limit of heat exposure in terms of specific absorp-
tion rate is required to consider. SAR value of 2 W/kg for 10gram of tissue is limited by IEEE
C95.1-2005 standard. Figure 15 depicts the simulated SAR impact at 2.45 GHz for 100 mW of
input power. Antenna has 0.687/0.627 W/kg of SAR for the flat/bent conditions. SAR value
is attained under the standard limit that ensures the safety of user.
Finally, a list of comparison between proposed structure and previously published on-
body MIMO antenna is given in Table 3. It shows that the proposed structure has highest
gain of all, and broadest bandwidth except [12]. Port isolation is also better than other struc-
tures except [16]. Most of the antennas are not analysed for bending effect which limits

JOURNAL OF ELECTROMAGNETIC WAVES AND APPLICATIONS 899
Tab le 3. Comparison of proposed antenna with existing on-body MIMO antennas.
Frequency Size Gain Bandwidth SAR Port Isolation Bending Efficiency
Ref. (GHz) (mm3) (dBi) (MHz) (W/kg) (dB) effect (%)
[3] 2.45, 5.2 38 ×38 ×0.6 2, 1.1 dB 520/620 0.459, 0.523 Notapplicable Not analysed 21/26
[4]2.4539×39 ×0.503 −0.57 190 1.56 Not applicable Frequency
detuned by
15 MHz
36
[5]2.4572×78 ×2 0.6dB 45 0.22 Not applicable Not analysed 68
[6]2.4440×58 ×3.5 −0.63 dBi 285 0.864 Not analysed Not analysed 19.37
[7] 2.45, 5.2 70 ×70 mm23.49, 7.7 dBi 1.2, 1.3GHz 0.647 Not applicable Stable to bending
effect
Not given
[9]2.4550×16 mm21.98 dBi 250 0.522 Not applicable Not analysed 29.1
[11]2.45Radius=20 mm,
thick-
ness =1mm
−12.03 dB 170 0.4 Not applicable Stable to frequency
detuning but
impedance
matching
reduces
14
[12] 2019 2.45 54.98 ×76mm22.83 1900 1.2 12 Not analysed Not given
[13] 2018 2.45 30 ×30 ×0.5 NA 140 NA 20 Not analysed 3 dB
[14] 2015 2.45,5 101.9 ×92.3 ×3 NA 160, 671 0.056/0.067 20 Negligible
detuning
56
[15] 2018 2.45 Radius =22.9mm,
h=3.2 mm
4.2 dBi 90 0.55 15 Not analysed 60.7
[16] 2020 2.45 28 ×25 ×0.5 0.5dB 300 0.512 30 Rightward shift of
frequency
40
Proposed
work
2.45 40 ×90 ×0.8 5.55 1250 0.687 22 Stable to both
frequency and
impedance
34
their practical utilization as compared to the proposed structure. Hence, designed antenna
shows promising performance for on to off body communication.
4. Conclusion
Authors have explained the behaviour of a defected ground structure-based MIMO antenna
for on to off body communication mode of wireless body area network. Important parame-
ters such as S-parameters, gain and radiation patterns are measured on a pork tissue in the
flat and the bent position. Antenna shows good impedance matching and radiation charac-
teristics to be operated on biological tissue. Mutual coupling between the two radiators is
also reduced from −12.13 to −20.67 dB after adding of decoupling slot. Less than 0.03 value
of ECC and more than 9.3 dB value of DG confirms antenna diversity for the MIMO applica-
tion. Reliable performance is obtained when antenna is analysed for structural deformation
along x-axis. Antenna has satisfied the safety limit in terms of SAR; low SAR value of 0.687
W/kg confirms the tissue safety. Robust on-body performance of proposed structure makes
it a potential candidate to integrate with on-body medical sensor nodes.
Disclosure statement
No potential conflict of interest was reported by the author(s).
ORCID
Vipan Kumar http://orcid.org/0000-0003-0326-8865

900 A. GUPTA AND V. KUMAR
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