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2015

The 8th Annual International Conference ATMS -2015, Bangalore

© ATMS (INDIA)

Internal Compact Quad-Band Diversity Antenna for Next Generation Mobile Handsets Applications

1Hari Shankar Singh, 1Gaurav Kumar Pandey, 1Pradutt K bharti, 2Rajesh Kumar Singh, and 1Manoj Kumar Meshram 1Department of Electronics Engineering

Indian Institute of Technology (Banaras Hindu University) Varanasi, India

2Centre for Airborne System (CABS), DRDO C V Raman Nagar, Bengluru, India

hssingh.rs.ece@iitbhu.ac.in, gkpandey.rs.ece@iitbhu.ac.in, pkbharti.rs.ece@iitbhu.ac.in, rajeshsingh0532@gmail.comand mkmeshram.ece@iitbhu.ac.in

Abstract In this paper, we presented a multiband antenna for next generation of mobile communications.The proposed antenna consistsof one main radiating element and two element elements. The elements are directly connected with the main radiating elements. The designed diversity antenna operates over GPS L1, LTE2500, WiMAX, and HiperLAN bands with below -10 dB isolation over all the operating bands. At the lower resonance frequency (at 1.575 GHz), the antenna is not well matched after rigorous tuning of the antenna parameters. To match the lower resonance, a matching circuit ( -match) is used to obtain the desire impedance matching to cover the GPS L1 band. The -matching circuit consistsof one discrete capacitor and two discrete inductors whichact as highpass matching circuit. The simulated S-parameters, far-field results, and diversity performances are presented.

Keywords

diversity antenna; global positioning system; matching circuit; planar inverted F-antenna.

I. INTRODUCTION

In recent years, wireless communication systems have become part of human life where people can communicate everywhere and anywhere in the world with very high speed. Such demand of high speed with better quality wireless communication can be achieved by promising technology like multiple-input multiple-output (MIMO). The MIMO communication systems have been one of the most significant breakthroughs in modern wireless communication due to overcoming the limited channel capacity of conventional systems [1-2]. MIMO is implemented by utilizing multiple antennas at the input and output sides of the communication system to exploit multipath fading for improved capacity of the channel, data rates, link reliability, and network coverage.

Now-a-days, mobile communication technologies have moved forward from 1G, 2G, and 3G to now 3.5G, which is also known as Long Term Evolution (LTE). LTE standardized by 3rd Generation Partnership Project (3GPP) in order to satisfy the current market demand. Such advancement in the field of mobile communication leads to a great demand for developing slim antennas to support multiple applications that can be easily integrated within the small space available inside

the device. Conventional microstrip antenna designs are based on half-wavelength of operation, whereas the planar inverted-F antenna (PIFA) designs invoke quarter-wavelength operation. The quarter wavelength of PIFA operation is due to the connection of the radiating element to the ground plane through a shorting strip or pin. Thus, PIFA is a promising structure to meet these requirements of portable devices [3-6]. But the size of above reported PIFAis larger compared to the modern day mobile handsets.

In the next generation mobile communication world, the integration of GPS band along with the other wireless communication bands is also desirable to support intelligent transportation systems and to integrate location-finding service with mobile devices to enhance user safety.Various GPS antennas developed are mainly right-handed circularly polarized (RHCP) [7-10]. However, a GPS-enabled device has to be operated in a typical urban environment where the signal from the satellite may undergo reflections from structures like buildings and trees. As a result of this reflection, signal suffers amplitude and phase distortion, and also there is chance that an RHCP signal may become left-handed circularly polarized (LHCP). Moreover, the presence of a human body near the device affects the polarization purity of the signal [11].Use of linearly polarized GPS signal provides the solution to the problem as it is less susceptible to distortion caused by the environment and the presence of the user. A linearly polarized antenna has better coverage than a circularly polarized antenna and hence receives signal from a greater number of satellites of the GPS satellite constellation [11], [12]. It is more convenient to design a simple linearly polarized antenna rather than a more complex and expensive circularly polarized one. A number of PIFA structures have been developed for this purpose [3-6], [11-12]. Unfortunately, footprint of these antenna elements makes them less attractive for mobile phone applications and also operating frequency bands of these antennas are not suitable for future generation mobile applications. From the literature survey, it is found that very few research has been done on the MIMO antenna that

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2015

The 8th Annual International Conference ATMS -2015, Bangalore

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incorporate GPS application band along with other wireless frequency bands [13-16].

In this paper, compact diversity antenna with multi branch for multi band operation including future generation operating bands (LTE2500) is proposed. The lower frequency band is not well matched by tuning the antenna parameters. To match the lower resonance, a simple matching circuit ( -match) is used to obtain the desire frequency bands. The proposed antenna operates over GPS L1 (1561-1586 MHz), LTE2500 (2500-2780 MHz), WiMAX (3378-3434 MHz), and HiperLAN (5000-5366 MHz) based on -6 dB reflection coefficient with better than -10 dB isolation between antenna elements. The proposed PIFA is designed using 0.1 mm thick copper sheet over the small ground plane which acts as printed circuit board (PCB) for mobile handsets. Section II of this paper demonstrates the antenna geometry and design considerations. In Section III, we analyse the mechanism of the proposed antenna. Diversity performances of the proposed antenna are discussed in Section IV. Finally, a conclusion is drawn in Section V.

II. ANTENNA CONFIGURATION AND DESIGN

The configuration of the proposed MIMO antenna is shown in Fig. 1(a) and detail dimensions of the proposed antenna are shown in Fig. 1(b). It is composed of two symmetrical back-to-back PIFA elements, which are located on top two corners of the mobile circuit of size 100 × 60 mm2

which is made of 0.8 mm thick FR4 (Flame Retardant) substrate of dielectric constant 4.4 and loss tangent 0.018. The total volume of the single antenna element is 12 × 9.85 × 5 mm3. The antenna is designed using 0.1 mm thick copper sheet.

The proposed antenna consists of one main radiating element and two other elements. The additional elements are directly connected with the main radiating elements. Each antenna element is with an air gap of 5 mm above the ground plane and is connected to the ground plane via a shorting plate. Basically, the antenna is designed based on PIFA conceptby using the formula given in [17],

WL

cf

4 (1)

where, C is speed of light, L and W are length and width of the elements, respectively.

Initially, the design startsby keeping the compactness as well as multi-band performance of the antenna in mind. To make antenna structure of compact size, we have chosen spiral like structure as main arm which is resonating at 1.8 GHz.Simultaneously one higher order mode is generated at 4.9 GHz which is 2.7 times of the fundamental mode. To make multi-band performance of the proposed antenna, two elements are directly connected with the main arm. When element 1 is added with main arm, the quarter wavelength of the element 1 helps to resonates at 2.6 GHz with higher order resonance at 5 GHz which cover the LTE2500 and HiperLAN band only. Further, element 2 is added to the main radiating element to cover the WiMAX frequency band. The quarter wavelength of the element 2 helps to resonant at 3.1 GHz in

addition to the 1.7 GHz and 5.1 GHz resonance as shown in Fig. 2. When both the elements are attached with main arm, antenna resonant at 1.58 GHz, 2.63 GHz, 3.3 GHz, and 5 GHz which covers LTE2500, WiMAX, and HiperLAN bands but due to poor matching at 1.58 GHz is unable to cover the GPS L1 band. This is due to the poor impedance matching between the antenna input port and feedline in this band. In view of this, a high pass -matching circuit is designed by taking the antenna as load impedance which helps in impedance matching at 1.575 GHz. The designed matching circuit is shown in Fig. 3. The designed matching circuit consists of two discrete inductors and one capacitor. The high pass matching circuit is designed in CST Design Studio and values of discrete components are tuned to get the desire resonance at 1.575 GHz.

The optimized values of the antenna shape parameters are given in Table I and that of discrete components are given in Table II.

III. RESULTS AND DISCUSSIONS

A. Simulated S-parameters

The proposed antenna is originally designed and optimized in Ansoft s HFSS [18]. Further, to validate the optimized results of HFSS we import the HFSS designe into CST MWS [19]. The results of both the Electromagnetic (EM) simulator are approximately matched and are shown in Fig. 4. The small variation is observed due to the applied numerical techniques in two different softwares.It is also found that the impedance matching at 1.58 GHz are very poor in both the EM softwares. To achieve the good impedance matching at 1.575 GHz, a high pass -maching circuit is designed in CST Design Studio [19]. With the help of this matching circuit, the impedance matching at 1.575 GHz become good. So the proposed MIMO antenna is coveringGPS L1 (1561-1586 MHZ), LTE2500 (2500-2780 MHZ), WiMAX (3378-3434 MHZ), and HiperLAN (5000-5366 MHz) bands based on -6 dB reflection coefficient with better than -10 dB isolation over all the opearting bands between antenna elements.

B. Diverisity Performances

To evaluate the diversity performance of the proposed MIMO antenna, key parameters such as the envelope correlation coefficient (ECC), mean effective gain (MEG), and MEG ratio are analysed. The CST MWS is used to calculate the diversity performances as well as radiation performances.

TABLE I. OPTIMIZED SHAPE PARAMETERS OF PROPOSED PIFA

Parameters Value

(mm)

Parameters Value

(mm)

Parameters Value

(mm)

L1

11 L5

8 L9

8.25

Wl

1.5 W5

1 W9

1.75

L2

8.5 L6

5 L10

12

W2

1 W6

1 W10

0.75

L3

10 L7

4.5 L11

8.25

W3

0.5 W7

1.25 Ls

4.9

L4

6.5 L8

3.25 Ws

1.5

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The 8th Annual International Conference ATMS -2015, Bangalore

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W4

1.75 W8

2.5 H1

2

Xf

4

TABLE II. OPTIMIZED VALUES OF THE DISCRETE COMPONENT

Discrete parameters Values

C 3.8 pF

L1

3 nH

L2

6.4 nH

(a)

(b)

Fig. 1(a) Configuration of the proposed antenna (b) Detail of single antenna element.

Fig. 2 Effect of different configuration on reflection coefficient.

C

L1 L2

Input

Fig. 3 Designed impedance matching circuit.

Fig. 4 Variations of optimized S-parameters with frequency.

1) Envelope Correlation Coefficient (ECC)

The antenna pattern correlation is important diversity parameters for a multi antenna system in a signal fading environment. For Rayleigh fading channel, the correlation of received signal can be characterized by the ECC [20],

ce2

(2)

using the radiation patterns, the simplified complex cross-correlation is given by,

2

0 0

2

0 02211

2

0 012

ddsin,Addsin,A

ddsin,A

c (3)

where, ,E*n,,m,E,E*

n,,m,XPREmnA , in

which E denotes the electric far field of the antenna (m, n = 1, 2 but m n) and XPR represents the cross-polarization ratio. The variation of ECC for proposed antenna is shown in Fig. 5. It is observed that the computed ECC satisfy the criteria of e< 0.5 and P1 P2 (P1 and P2 is the average signal power received at each branch of the antenna), which show good diversity gain can be obtained.

2) Mean Effective Gain (MEG)

In a multipath environment, the MEG is defined as the ratio of the mean received power to the mean incident power [21],

ddsin),(P),(GXPR

),(P),(GXPR

XPRMEG

2

0 0 1

1

1

(4)

where, XPR represents the cross-polarization ratio when the polarization of the transmitted radio waves in horizontal and to the reciprocal of the cross-polarization when the polarization

S11

S21

-30

-24

-18

-12

-6

0

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

Ref

lect

ion

coef

fici

ent (

dB)

Frequency (GHz)

Only main radiating elementWith element 1With element 2Main radiating and additional elementsProposed antena (with matching circuit)

-70

-60

-50

-40

-30

-20

-10

0

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

S-pa

ram

eter

s (d

B)

Frequency (GHz)

HFSSCST MWSWith Matching Circuit

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

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transmitted wave is vertical, G , G are the power gain patterns, P

and P are the - and - components of angular

density functions of the incident power, respectively. The distributions of the angular density functions depend on the surrounding environment. Here,P

andP are assumed to be

Gaussian in elevation and uniform in azimuth [21]. Table III gives the computed MEG for different XPR at different frequency bands by setting the, mv=100, mH=100, v=150, and

H=150. It is clearly seen that ratio of the MEG1/MEG2 is equal to unity which satisfy the equality criteria of the two antennas.

C. Radiation Performances

The 3D far-field radiation patterns of the proposed MIMO antenna are given at 1.575 GHz, 2.65 GHz, 3.4 GHz, and 5.175 GHz) by exciting the single antenna (Ant. 1 or Ant. 2) at a time (Fig. 6). It is clearly observed that two antenna can receive different signals from different direction at the same time, which is the definition of pattern diversity. The simulated peak gain at port 1 is -6.6 dBi, 1.3 dBi, 1.2 dBi, and 2.8 dBi at 1.575 GHz, 2.65 GHz, 3.4 GHz, and 5.175 GHz, respectively and shown in Fig. 7. The total radiation efficiency is computed by considering the mismatch losses and is shown in Fig. 7. It is observed that total radiation efficiency is found to be 15%, 90%, 64.4%, and 93.6% at 1.575 GHz, 2.65 GHz, 3.4 GHz, and 5.175 GHz, respectively.

Fig. 5 Variation of ECC with frequency.

TABLE III. VARIATION OF MEGS FOR PROPOSED MIMO ANTENNA

Freq.

(GHz)

XPR = 5 dB XPR = 1 dB XPR = 0 dB

MEG1 MEG2 MEG1 MEG2 MEG1 MEG2

1.575 -6.5 -6.5 -5.3 -5.3 -5 -5

2.65 -5.5 -5.5 -4.99 -4.99 -4.8 -4.8

3.4 5.67 -5.67 -5.1 -5.1 -5 -5

5.175 -2.39 -2.39 -3.2 -3.2 -3.4 -3.4

IV. CONCLUSION

In this paper, an internal compact quad band diversity antenna has been demonstrated successfully. The spiral shaped main arm supports compact structure and additional two elementsare responsible for multi-band operations. The

operation of proposed antenna over GPS L1, LTE2500, WiMAX, and HiperLAN bands after implementation of the high pass -matching circuit and achieve isolation is well below -10 dB over all the operating bands. The radiation patterns of the proposed antenna in free space show pattern diversity that is suitable for mobile communication by mitigating the multipath fading. The multi-band operations and diversity performances of the proposed antenna can be considered as a strong contender for MIMO applications.

Fig. 6 Simulated 3D far-field radiation patterns at different frequency.

Fig. 7 Variation of simulated peak realized gain and total efficiency with frequency.

REFERENCES

GPS L1

LTE2500

WiMAX

HiperLAN

0

0.1

0.2

0.3

0.4

0.5

0.6

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

EC

C

Frequency (GHz)

0

20

40

60

80

100

120

-15

-10

-5

0

5

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

Total E

fficiency (%)

Pea

k R

eali

zed

Gai

n (d

Bi)

Frequency (GHz)

Peak Realized Gain (dBi) Total Efficiency

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The 8th Annual International Conference ATMS -2015, Bangalore

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