DESIGN OF AN ULTRA-WIDEBAND MIMOANTENNA FOR PDA APPLICATIONS

Jaewon Lee, Seokjin Hong, and Jaehoon ChoiDepartment of Electronics Computer Engineering, HanyangUniversity, 17, Haengdang-Dong, Seongdong-Gu, Seoul 139-791,Korea; Corresponding author: choijh@hanyang.ac.kr

Received 15 December 2009

ABSTRACT: A low-profile, ultra-wideband (UWB), multi-inputmulti-output (MIMO) antenna for a personal digital assistantapplication is proposed. To improve the impedance bandwidth, a 2 mm

� 1 mm connecting strip is used on each antenna element. The isolationcharacteristic between the two antenna elements is improved by

inserting two T-shaped stubs. The optimized design parameters wereobtained through parametric analysis. The designed antenna has a10-dB return loss bandwidth of 9 GHz (2.2–11.2 GHz) covering the

WiBro, Bluetooth, WiMax, S-DMB, and UWB frequency bands, with anisolation characteristic below �20 dB over the operating frequency.

Additionally, the envelope correlation coefficient is less than 0.12.VC 2010 Wiley Periodicals, Inc. Microwave Opt Technol

Lett 52:2165–2170, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.25478

Key words: antenna; MIMO; ultra-wideband

1. INTRODUCTION

Large channel capacity and high spectral efficiency are essential

factors in providing various multimedia services for current mo-

bile communication systems. A wireless communication system

in which multiple antennas are used within a small device is

one solution to satisfy these demands [1, 2]. However, it is very

difficult to reduce the mutual coupling between antenna ele-

ments in a multi-input multi-output (MIMO) system as antennas

must be installed within the limited space inside a handheld de-

vice. Recently, various research efforts have been attempted to

solve this problem by using a ground wall and connecting line

[3], a suspended line [4], or a single unit of negative metamate-

rial [5].

In this article, a low-profile MIMO antenna with good isola-

tion, intended for a wireless communication system, is proposed.

To reduce the size of the antenna and to widen the impedance

bandwidth, a connecting strip is used on each antenna element.

The 10-dB return loss bandwidth for the proposed antenna

ranges from 2.2 to 11.2 GHz. Two T-shaped stubs are symmetri-

cally introduced in the proposed antenna design to attain the

appropriate isolation characteristic between the two antennas.

The details of the antenna design and the measured results of

the proposed antenna are presented and discussed in this article.

2. ANTENNA DESIGN

The configuration of the proposed ultra-wideband (UWB)

MIMO antenna is shown in Figure 1. The antenna consists of

two identical antenna elements and two T-shaped stubs located

symmetrically with respect to the axis. The two antenna ele-

ments are mounted near the two top corners of the 82 mm �80

mm ground plane. The volume of each antenna element is 10

mm � 12 mm � 2 mm, and the antennas have a folded struc-

ture, as shown in Figure 1(b). Figure 2 shows the impedance

bandwidths of the proposed antenna with and without a connect-

ing strip. It is observed that the impedance bandwidth of the

antenna, from 5.5 to 7.5 GHz, is improved by adding the

connecting strip. Based on the parametric analysis shown in

Figure 3, a strip width of 2 mm was selected. The overall

antenna size, including the ground plane, is suitable for practical

personal digital assistant (PDA) applications.

To improve the isolation characteristic, two T-shaped stubs

are adopted between the two antennas on the ground plane. As

the geometrical parameters of the stubs affect the impedance

characteristic, it is very important to identify the proper position

and dimensions of the T-shaped stubs to minimize the effect on

the impedance matching over the operating frequency. The two

T-shaped stubs are placed 19 mm from each feeding port. The

top portion, ST, of the T-shaped stub is asymmetrical and has a

total length of 19 mm. To obtain the proper dimensions of the

T-shaped stub, S-parameters were calculated for different values

of LS1 and LS2. A suitable impedance bandwidth and isolation

characteristics were obtained when LS1 ¼ 15 mm and LS2 ¼ 9

mm, as shown in Figures 4 and 5. The proposed antenna for

Figure 1 Configuration of the proposed MIMO antenna: (a) top view

and (b) detailed dimensions of the antenna

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 2165

MIMO systems was designed and analyzed using a high-fre-

quency structure simulator [6].

3. RESULTS AND DISCUSSION

To investigate the effect of the T-shaped stubs, the surface current

distributions on the antenna and ground plane, at 4 and 8 GHz,

are analyzed. This is done both with and without the two T-

shaped stubs, when antenna 1 is excited and when antenna 2 is

limited to a 50-X load, using the antenna configuration shown in

Figure 6. The current distributions without the two T-shaped stubs

show that the strongest current is generated near antenna 2 for

both frequencies. The current distributions near antenna 2 with the

two T-shaped stubs are much weaker than that for the antenna

without the stubs, at both the 4 and 8 GHz frequencies. Similarly,

in Figure 7, the surface current distributions at 4 and 8 GHz are

shown with and without the two T-shaped stubs when antenna 2

is excited and antenna 1 is limited to a 50-X load. A strong cou-

pling can be observed between the two antenna elements when

the stubs are absent. The return losses and isolation characteristics

of the proposed MIMO antenna, both with and without T-shaped

stubs, are illustrated in Figure 8. The isolation characteristic

between the two antennas is shown to improve over the frequency

band of interest; similarly, the impedance bandwidth is widened at

the lower frequency band.

An HP8719ES network analyzer was used to measure the

S-parameter characteristics of the fabricated antenna, which is

shown in Figure 9. The measured results show that the �10-dB

S-parameter requirement is satisfied over the frequency band of

2.2–11.2 GHz, and the isolation characteristic between the two

antennas is less than �20 dB over the entire bandwidth. Figures

10(a) and 10(b) illustrate the radiation patterns for both the x-yand y-z planes, at 4 and 8 GHz, respectively, and indicate that

the radiation patterns are appropriate for PDA application.

Figure 11 shows the measured peak gains of each antenna as a

function of frequency and indicates that the proposed antenna

has good gain flatness.

For a MIMO application, the correlation between the signals

received by the antennas on the same side of a wireless link is

an important figure of merit. To evaluate the MIMO capabilities

of a multiple antenna system, the envelope correlation coeffi-

cient (ECC) is typically evaluated. Under the assumption that

the MIMO system operates in a uniform multipath environment,

the ECC can be calculated using the following equation [7]:

q12 ¼jS�11S12 þ S�12S22j2

ð1� jS11j2 � jS21j2Þð1� jS22j2 � jS12j2Þ(1)Figure 3 Calculated S-parameter characteristics for different values

of CW

Figure 2 Calculated S-parameter characteristics with and without the

connecting strip

Figure 4 Calculated S-parameter characteristics for different values of

LS1: (a) S11 and S22 and (b) S12 and S21

2166 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 DOI 10.1002/mop

Figure 5 Calculated S-parameter characteristics for different values of LS2: (a) S11 and S22 and (b) S12 and S21

Figure 6 Calculated surface current distributions of the proposed antenna (port 1 excited): (a) 4 GHz and (b) 8 GHz

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 2167

Figure 7 Calculated surface current distributions of the proposed antenna (port 2 excited): (a) 4 GHz and (b) 8 GHz

Figure 8 Calculated S-parameter characteristics with and without the

T-shaped stubs Figure 9 Measured S-parameter characteristics

2168 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 DOI 10.1002/mop

Figure 10 Measured radiation patterns of the proposed antenna: (a) 4 GHz and (b) 8 GHz

Figure 11 Measured antenna gain Figure 12 Measured envelope correlation coefficient characteristics

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 2169

where the S-parameter values are measured. The ECCs of the

proposed antenna are lower than 0.12 over the frequency band

of 2.2–11.2 GHz, as illustrated in Figure 12.

4. CONCLUSIONS

In this letter, a MIMO antenna for a PDA application is pro-

posed. The connecting strip on each antenna element is used to

improve the impedance bandwidth, and the two T-shaped stubs

are symmetrically added between the proposed antennas to sup-

press the mutual coupling between the two antenna elements.

The proposed antenna has an impedance bandwidth of �9 GHz,

from 2.2 to 11.2 GHz, for S11 and S22, less than �10 dB. A

good isolation characteristic of less than �20 dB has been

obtained over the operating frequency band. Furthermore, the

ECC of the proposed antenna is less than 0.12. The proposed

antenna is a solid candidate for use as an UWB MIMO system.

ACKNOWLEDGMENTS

This research was supported by the Ministry of Knowledge Econ-

omy, Korea, under the Information Technology Research Center

support program supervised by the Information Technology

Advancement.

REFERENCES

1. G.J. Foschini and M.J. Gans, On limits of wireless communications

in a fading environment when using multiple antennas, Wireless

Personal Commun 6 (1998), 311–335.

2. O.T.R.W.A. Hottinen, Multi-antenna transceiver techniques for 3G

and beyond, Wiley, West Sussex, England, 2003.

3. K. Chung and J.H. Yoon, Integrated MIMO antenna with high isola-

tion characteristic, Electron Lett 43 (2007), 199–200.

4. G. Park, M. Kim, T. Yang, J. Byun, and A.S. Kim, The compact

quad-band mobile handset antenna for the LTE700 MIMO applica-

tion, Presented at the IEEE Transactions on Antennas and Propaga-

tion Society International Symposium, Charleston, SC, 2009.

5. C.C. Hsu, K.H. Lin, H.L. Su, H.H. Lin, and C.Y. Wu, Design of

MIMO antennas with strong isolation for portable applications, Pre-

sented at the IEEE Transactions on Antennas and Propagation Soci-

ety International Symposium, Charleston, SC, 2009.

6. Ansoft Corporation, Ansoft high frequency structure simulation

(HFSS), Ver. 11, Ansoft Corporation, Pittsburgh, PA.

7. J. Thaysen and K.B. Jakobsen, Envelope correlation in (N, N)

MIMO antenna array from scattering parameters, Microwave Opt

Technol Lett 48 (2006), 832–834.

VC 2010 Wiley Periodicals, Inc.

COLPITTS VCO WITH GATE-SERIESHIGH-QUALITY FACTOR LC RESONATOR

Sheng-Lyang Jang, Li-Te Chou, and Chia-Wei ChangDepartment of Electronic Engineering, National Taiwan University ofScience and Technology, 43, Keelung Road, Section 4, Taipei,Taiwan 106, Republic of China; Corresponding author:m9502216@mail.ntust.edu.tw

Received 16 December 2009

ABSTRACT: A new differential voltage-controlled oscillator (VCO) is

designed and implemented in a 0.13-lm CMOS 1P8M process. Thedesigned circuit topology is an n-core LC-tank VCO with an LC

resonator. At the supply voltage of 1.1 V, the output phase noise of theVCO is �113.8 dBc/Hz at 1-MHz offset frequency from the carrierfrequency of 11.73 GHz and the figure of merit is �192.01 dBc/Hz. The

core power consumption is 1.83 mW. Tuning range is 1.47 GHz from

10.66 to 12.13 GHz, while the control voltage was tuned from0 to 1.2 V. VC 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett

52:2170–2173, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.25466

Key words: 0.13-mm CMOS; Colpitts VCO; accumulation-modevaractor; LC resonator

1. INTRODUCTION

CMOS voltage-controlled oscillators (VCOs) are widely used in

low-cost radio-frequency (RF) products because they are com-

mon functional blocks in modern RF communication systems

and are used for generating the intermediate frequency signal

and modulating or demodulating the RF signal. In the past, a lot

of CMOS VCO circuit architectures have been developed; Col-

pitts VCOs [1, 2] are one of the most popular VCOs because

they can be high-performance oscillators. There are two basic

circuit topologies for Colpitts VCOs, the LC resonator in the

first type is connected between two gates (bases) of metal-

oxide-semiconductor field effect transistor (MOSFETs) (bipolar

junction transistor (BJT)s [3, 4]) and it only plays the role of ac

load to the transistor amplifier. The LC resonator in the second

type is connected between two drains (bases) of MOSFETs [1,

5] (BJTs [6]) and it not only serves as the ac load of the transis-

tor amplifier but also supplies the dc bias for the transistor. De-

spite more design limitation on the LC resonator in the second

VCOs, many published high-performance Colpitts VCOs have

been designed with these circuit architectures. An all-nMOS

Colpitts VCO circuit [7] shown in Figure 1 is a Colpitts VCO

of the former type, it is adapted from the BJT counterpart. The

main part of parallel-tuned LC tank consists of varactors and in-

ductor L. Despite its counterpart in bipolar technologies has

been widely studied and successfully implemented in various

BiCMOS technologies, the circuit topology shown in Figure 1

has received less attention [8, 9].

The goal of this article is to design an 11-GHz differential

CMOS VCO with high performance by using a circuit topology

similar to that in Figure 1 with high-Q factor resonator so that a

high-performance Colpitts VCO can be designed. The LC reso-

nator is connected between the gates of MOSFET amplifiers in

Figure 1 A Colpitts VCO with parallel-tuned LC resonator

2170 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 10, October 2010 DOI 10.1002/mop