Miniaturized Antenna Array with Low Correlation for Telemedicine

and Body Area Networks Applications

Haider Khaleel

, Chitranjan Singh

, Hussain Al-Rizzo

and Seshadri Mohan

Department of Engineering Science, Sonoma State University, Rohnert Park, California, U.S.A.

Department of Systems Engineering, University of Arkansas at Little Rock, Little Rock, Arkansas, U.S.A.

Keywords: Telemedicine, Antenna Array, Metamaterial, Multiple Input Multiple Output, Body Area Networks.

Abstract: The gain from Multiple-Input Multiple-Output (MIMO) techniques in wireless sensor networks equipped

with miniaturized sensor nodes cannot be fully exploited due to the difficulty encountered when placing

traditional multiple antennas with sub-wavelength physical separation. This paper presents a novel antenna

array design using µ-negative metamaterial (MNG) structures that lead to low correlation between antennas

when placed closely on a user’s body utilizing a telemedicine or a Body Area Network based system. The

obtained correlation coefficient of 0.04 is low enough for realizing full diversity gain from using such

antenna array on sensor nodes in a telemedicine WLAN environment. Furthermore, Bit Error Rate (BER)

simulation result for a 2×2 Alamouti diversity scheme in an IEEE 802.11a system is also presented. Design

and simulations are conducted using CST Microwave Studio which is based on the Finite Integration

Technique. Results suggest that the proposed design would be a suitable candidate for telemedicine and

BAN applications that are constrained by limited space.

1 INTRODUCTION

Telemedicine has evolved rapidly over the past

decade due to the increasing demand for remote

health monitoring of post-surgery patients, recovery

tracking, seniors, athletes, fire fighters and

astronauts (Fong et al., 2012). In telemedicine

systems, important health parameters such as body

temperature, blood pressure, and heart rate are

transmitted wirelessly to remote monitoring stations

(clinics, hospitals, etc…) (Algaet et al., 2013).

Obviously, a reliable wireless scheme which is

enabled by a use of antenna(s) is required for

optimal performance of such systems. In this

specific case, antennas are required to be small in

size, lightweight, robust with desired radiation

characteristics. They also need to be comfortable

and conformal to the shape of the body. Microstrip

antennas offer a favorable advantage in terms of a

close to hemi-spherical radiation pattern, i.e. radiates

away from the user’s body, thus, minimizes the

exposure to electromagnetic radiation. Furthermore,

they offer a low profile solution, low cost, and ease

of fabrication (Khaleel et al., 2010). However, this

class of antennas suffers from a very narrow

bandwidth, hence, a low profile antenna with a

directional radiation pattern and a wide bandwidth is

essential in telemedicine applications. Several

techniques have been proposed to achieve

directional radiation pattern by adding a cavity or a

shielding plane underneath the antenna, or using

absorbers (Haga et al., 2009). However these

techniques lead to either an unacceptable increase in

the antenna's height, or a more complicated

manufacturing process. In (Rowe et al., 2003), a

PEC reflector was inserted between a human head

and a folded loop antenna. This approach increases

the return loss and decreases the antenna’s

efficiency. In (Islam et al., 2009), a Single Negative

Metamaterial (SNG) is utilized to reduce the

electromagnetic exposure, though efficient, it leads

to a high profile system. Flexible Artificial Magnetic

Conductor (AMC) based antenna is proposed in

(Raad et al., 2013) for telemedicine application. A

polyimide based printed antenna is integrated with

an AMC ground plane which is utilized to minimize

the specific Absorption Rate (SAR) and the

impedance mismatch caused by the human tissues

proximity. Although the proposed design offers a

low profile solution, it is relatively large in some

applications.

On the other hand, MIMO techniques employing

Khaleel H., Singh C., Al-Rizzo H. and Mohan S..

Miniaturized Antenna Array with Low Correlation for Telemedicine and Body Area Networks Applications.

DOI: 10.5220/0004799000390044

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2014), pages 39-44

ISBN: 978-989-758-013-0

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

multiple antennas at one or both ends of a wireless

communication link have shown the potential to

reach higher spectral efficiencies (Chou et al., 2008),

(Abouda et al., 2006). By combining signals at

transmit antennas and receive antennas, MIMO

substantially improves either data rate using Spatial

Multiplexing (SM) or reliability using diversity

techniques. Space-Time Block Code (STBC), known

as Alamouti scheme (Alamouti, 1998) for two

transmit antennas, is the simplest technique with

linear receiver complexity to improve the reliability

of a wireless communication system. The capacity

and reliability of MIMO systems depend on the

Signal-to-Noise Ratio (SNR) and the correlation

properties among the channel transfer functions of

different pairs of transmit and receive antennas (Shiu

et al., 2000). One of the basic requirements to realize

the gains from MIMO systems is low correlation in

the effective channel. The correlation comes from

three sources, namely, the correlated fading channel,

correlation among the transmit antennas, and

correlation among the receive antennas. The

correlation between two antennas depends on the

coupling and isolation between them (Kyritsi et al.,

2003). The coupling in turn is dependent on the

physical separation between antennas and the

matching network.

Several papers including (Caban et al., 2007)

have previously presented analysis and simulation of

the effect of correlation between antennas on the

performance of Alamouti STBC scheme. The

Alamouti scheme starts losing performance as the

separation between the two antennas reaches

somewhere between 0.1λ to 0.3λ. The size of

traditional antenna is a major concern in placing

multiple antennas on a miniaturized sensor node.

The free space wavelength for the frequency band at

5.2 GHz, used in WLAN, is 5.77 cm. To obtain low

correlation for WLAN applications with physical

separation between antennas to be ൒ λ/2 = 2.88cm

is not difficult. However, application of MIMO in

wireless sensor networks, especially in medical

applications, requires multiple antennas to be placed

on tiny sensor nodes. To place multiple antennas

with physical separation of ൒ λ/2 on a sensor node is

very challenging using traditional antenna

technologies.

This paper presents a unique antenna design and

antenna array based on metamaterial that leads to

correlation coefficient of 0.42 without isolation

structure and 0.04 with proper isolation between

antennas. We present the design, radiation pattern of

antennas and BER performance of using them in a

WLAN system.

The design of the model of the proposed system

along with antenna correlation is introduced in

Section 2. The antenna design is presented in

Section 3 where both geometries and characteristics

of the antenna and the metamaterial structures are

provided. The error performance results of the

proposed antenna system are introduced in Section

4. Finally, we conclude the paper in Section 5.

2 SYSTEM MODEL WITH

ANTENNA CORRELATION

We considered a MIMO system with 2 transmit and

2 receive antennas having Alamouti STBC (Chou et

al., 2008) encoder that maps the two consecutive

symbols x

and x

to two antennas over two symbol

periods at the transmitter. The correlation coefficient

between the two transmit antennas Tx1 and Tx2 is α.

The transmitted signals pass through a Rayleigh flat-

fading wireless channel with a (2 × 2) channel

matrix H with element h

= α

exp (jθ

) representing

the gain between transmit antenna j and receive

antenna i and additive white Gaussian noise

(AWGN). The correlation coefficient between the

two receive antennas Rx1 and Rx2 is β. The

decoding is based on linear combining of signals

received at the two antennas over two symbol

periods. This scheme performs maximum likelihood

detection of x1 and x2 using simple linear

combining. The discrete channel model for a (2 × 2)

MIMO system is given below in (1)-(3).

(1)

(2)

(3)

The correlation matrices between antennas at the

transmitter and receiver are given by C

and C

respectively in (3). The simple implementation of

linear combiner at the receiver would lead to a

minimal increase in the complexity of receiver at a

sensor node. The performance gain from using

Alamouti scheme has a direct relationship with the

coupling between the two antennas. The coupling

depends on the physical separation between the

antennas. When we consider a two transmit antennas

scenario, the correlation coefficient for a uniform

distribution of sources is given by (4). The

assumption of uniform distribution holds quite well

in an indoor environment. In (4), S

, S

, are

BIODEVICES2014-InternationalConferenceonBiomedicalElectronicsandDevices

scattering S-parameters for the antenna system. The

S-parameters takes into account the relationship

between voltage and current at the input and output

terminals of the antenna, the loading, and matching

network employed. The analytical derivation of S-

parameters is quite involved and requires several

approximations. The terms S

, and S

contain the

effect of mutual coupling between antennas. The

correlation coefficient can be computed either using

(4) after obtaining the S-parameters from full-wave

3-D electromagnetic simulation of antennas or

through measurement of antenna prototypes or using

(5) from the three-dimensional far-field radiation

pattern.

(4)

(5)

3 ANTENNA DESIGN

As mentioned previously, the reduction of mutual

coupling between closely-spaced antenna elements

is essential to the performance of MIMO systems

due to the fact that the mutual coupling affects the

phase and distribution of the current, input

impedance and radiation pattern in each antenna

element which significantly reduces the capacity of

the MIMO systems. Several techniques have been

reported to reduce the mutual coupling between

radiating elements in MIMO systems. Some of these

techniques are based on the use of Electromagnetic

Band Gap (EBG) structures (Ikeuchi et al., 2011),

defected ground plane (Caloz et al., 2004), and the

use of µ-Negative (MNG) structures. In (Bait-

Suwailam et al., 2010), MNG structures have been

used to reduce the mutual coupling between two

high profile monopoles, where the achieved

reduction in mutual coupling was 20 dB. In this

paper, we utilize MNG to reduce the mutual

coupling between two conformal micro strip

radiating elements sharing a common substrate

intended for applications that require compact space,

i.e, wearable medical devices and miniaturized

sensor nodes. When MNG structures are excited

with a specific polarization, an electric current is

induced through the loops of split ring resonators, as

a consequence, the structures act as magnetic dipoles

and an effective medium with a negative

permeability over a certain frequency range is

generated. As a result, the existence of real

propagating modes is prevented within this medium

(Pendry et al., 1999). This behaviour is utilized to

block the mutual coupling between the radiating

elements of the proposed antenna. The proposed

design consists of two elliptical shaped patch

elements with a major axis of 14 mm and a minor

axis of 4 mm placed on a 19 mm × 14.5 mm ×

0.85mm RO3006 substrate with a dielectric constant

of 6.15 backed by a ground plane. Two identical

elliptical radiating elements were designed to

resonate at 5.2 GHz. The inter-element separation

distance is 10.6 mm (0.18 λ), where λ is the free

space wavelength. Parametric study was performed

for the two coaxial feed locations to achieve optimal

impedance matching. The optimized locations are 8

mm along the major axis and 3 mm along the minor

axis for the first element, and 6 mm and 2.5 mm for

the second element. The return loss is -20 dB with a

-10 dB bandwidth of 50 MHz. A unit cell of a square

ring resonator is designed to provide an effective

negative permeability of -8 at 5.2 GHz, which

provides a reasonable isolation between the two

elements. Next, a set of 8 unit cells is arranged

horizontally between the radiating elements,

separated by 1.5 mm from each other. The unit cell

consists of two square metal strip inclusions with

opposite orientation printed on both sides of a 4.4

mm × 4.4 mm × 0.8mm RO3006 substrate. The gap

in each ring is 0.3mm. The height of the antenna

along with the metamaterial structures does not

exceed 6 mm which categorizes the design under

low profile antennas. The front view and side view

of the proposed antenna design along with

corresponding dimensions are depicted in Fig.1 and

Table 1, respectively.

Table 1: Dimensions of the proposed antenna system.

L 14.5 W

4.4

W 19 L

4.4

14 T 0.85

4 L

3.5

S 1.5 W

0.6

0.45 G 0.3

The antenna and MNG unit cell were designed and

simulated using both time-domain and frequency-

domain solvers of CST Microwave Studio which is

based on the Finite Integration Technique (FIT). The

simulated S-parameters for the proposed antenna are

shown in Fig. 2.

From the transmission coefficient S

, we

observe a large reduction in mutual coupling (-26

dB) at 5.2 GHz for the design with MNG structures

MiniaturizedAntennaArraywithLowCorrelationforTelemedicineandBodyAreaNetworksApplications

Figure 1: Two-element antenna array based on elliptical

patch radiating elements with MNG. Top view (top) and

side view (bottom).

Figure 2: Simulated S-Parameter for both cases (with and

without MNG).

compared to -4 dB for the design without MNG. We

also notice a slight shift in the resonance frequency

for the MNG case (around 20 MHz), which can be

compensated for by slightly adjusting the patch

length in order to keep the patch resonance

frequency identical in both cases. Moreover, we

simulated the correlation coefficient for both cases

(with and without MNG structures). The correlation

coefficient for the MNG case is 0.04 versus 0.42 for

the case without MNG. It is also worth mentioning

that the simulated correlation coefficient has been

extracted from the far-field analysis which is more

accurate than the S-parameter method (Blanch et al.,

2003). According to the S-parameters and

correlation coefficient analysis, the design with

MNG provides significant isolation between the

radiating elements compared to the design without

MNG with the same element spacing. The simulated

E-plane and H-plane combined far-field radiation

patterns at 5.2 GHz for the MNG case is presented in

Fig.3.

4 ERROR PERFORMANCE

SIMULATION

The setup for testing the performance of 2 × 2

Alamouti STBC scheme using the antenna array

proposed in Section 3 is based on WLAN IEEE

802.11a system with 5.2 GHz carrier frequency,

QPSK modulation scheme and simulation is

performed under flat-fading channel with Rayleigh

Fading coefficients. We used two transmit antennas

on one substrate at the transmitter and two receive

antennas on another substrate at the receiver. The

BER which is an essential parameter in

telecommunication systems, is the percentage of

transmitted bits that have errors relative to the total

number of bits received in a specific transmission

process.

The BER performance for a) uncorrelated

antennas at the transmitter (TX) as well as at the

receiver (RX), b) antenna array designed using

MNG structure (correlation coefficient at both TX

and RX is 0.04), and c) antenna array designed

without MNG structure (correlation coefficient at

both TX and RX is 0.42), proposed in Section 3, is

shown in Fig. 4.

We notice negligible performance loss for

antenna array from MNG structure as compared to

the performance for uncorrelated antennas.

5 CONCLUSIONS

In this paper, we presented a novel antenna array

designed with MNG structure for exploiting

diversity gain in a system where miniaturized sensor

devices are equipped with multiple antennas. The

negligible performance loss achieved by using the

proposed MNG based antenna array compared to

uncorrelated antennas renders the design suitable for

integration on medical sensor nodes used in

telemedicine systems that require miniaturization.

We presented design, radiation pattern of such

antennas and BER performance for such systems

BIODEVICES2014-InternationalConferenceonBiomedicalElectronicsandDevices

(a)

(b)

(c)

Figure 3: simulated (a) E-plane (YZ), (b) H-plane (XZ) and

at 5.2 GHz.

Figure 4: BER performance of (2 × 2) MIMO with

correlated transmit and correlated receive antennas.

operating in a WLAN environment. Results suggest

that the proposed design would be a reasonable

candidate for telemedicine and BAN applications

that are constrained by limited physical space.

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