Design of an ultra-wideband MIMO antenna for PDA applications
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Transcript of Design of an ultra-wideband MIMO antenna for PDA applications
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: [email protected]
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:[email protected]
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