Mutual coupling reduction of two PIFAs with a T-shape slot impedance transformer for MIMO mobile terminals

Mutual coupling reduction of two PIFAs with a T-shape slot impedance transformer for MIMO mobile terminals
S. Zhang, B. K. Lau, Y. Tan, Z. Ying, and S. He, “Mutual coupling reduction of two PIFAs with a T‐shape slot impedance transformer for MIMO mobile terminals,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1521‐1531, Mar. 2012. This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder. ©2012 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
1521
Mutual Coupling Reduction of Two PIFAs With a
T-Shape Slot Impedance Transformer for MIMO
Mobile Terminals
Shuai Zhang, Buon Kiong Lau, Senior Member, IEEE, Yi Tan, Zhinong Ying, Senior Member, IEEE, and
Sailing He, Senior Member, IEEE
Abstract—An efficient technique is introduced to reduce mutual
coupling between two closely spaced PIFAs for MIMO mobile
terminals. The proposed mutual coupling reduction method is
based on a T-shape slot impedance transformer and can be
applied to both single-band and dual-band PIFAs. For the proposed single-band dual PIFAs, the 10 dB impedance bandwidth
covers the 2.4 GHz WLAN band (2.4–2.48 GHz), and within the
WLAN band an isolation of over 20 dB is achieved. Moreover, the
dual-band version covers both the WLAN band and the WiMAX
band of 3.4–3.6 GHz, with isolations of over 19.2 dB and 22.8 dB,
respectively. The efficiency, gain and radiation patterns of the
two-PIFA prototypes are verified in measurements. Due to very
low pattern correlation and very good matching and isolation
characteristics, the capacity performances are mainly limited by
radiation efficiency. The single-band and dual-band PIFAs are
also studied with respect to their locations on the ground plane.
An eight-fold increase in the bandwidth of one PIFA is achieved,
when the single-band PIFAs are positioned at one corner of the
ground plane, with the bandwidth of the other PIFA and the good
isolation unchanged.
Index Terms—Antenna array mutual coupling, MIMO systems,
parasitic antennas.
I. INTRODUCTION
M
ULTIPLE-INPUT and multiple-output (MIMO) technology has become an important feature in all future
generation wireless communication systems. This is primarily
because it can linearly increase channel capacity with an increase in the number of antennas, without needing additional
frequency spectrum or power. Moreover, popular wireless communication systems typically operate in rich scattering environManuscript received February 28, 2011; revised July 14, 2011; accepted
October 17, 2011. Date of publication December 19, 2011; date of current
version March 02, 2012. The work was supported in part by VINNOVA
under Grants 2008-00970 and 2009-04047, in part by a scholarship within
the EU Erasmus Mundus External Cooperation Window TANDEM, in part
by Swedish VR Grant (2011-4620), and in part by Chinese 863 project Grant
(2012AA030702).
S. Zhang and S. He are with the Department of Electromagnetic Engineering, School of Electrical Engineering, Royal Institute of Technology,
S-100 44 Stockholm, Sweden, and also with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310058, China (e-mail:
[email protected]).
B. K. Lau and Y. Tan are with the Department of Electrical and Information
Technology, Lund University, SE-221 00 Lund, Sweden.
Z. Ying is with Research and Technology, Corporate Technology Office, Sony
Ericsson Mobile Communications AB, SE-221 88 Lund, Sweden.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2011.2180329
ments, which MIMO exploits to achieve the aforesaid large performance gain. However, the requirement for compactness of
MIMO-enabled user terminals in mobile communications can
potentially induce very high mutual coupling among the antenna
elements, which can dramatically degrade the MIMO systems’
performance. This motivates the need for efficient isolation enhancement techniques for portable MIMO terminals [1]–[6].
Planar inverted-F antennas (PIFAs) have been widely utilized
in mobile terminals due to the advantages of low profile, low
cost and ease of design. However, in order to obtain an isolation
of 20 dB or more between two PIFAs with air substrate on a
common ground plane, their separation distance should exceed
one half of the free space wavelength [7]. Even a 10 dB isolation, which is usually sufficient to achieve low enough correlation for good MIMO performance, will still require an antenna
separation of a quarter of the wavelength. Moreover, this conclusion applies to three different joint orientations (co-linear,
orthogonal and parallel) of the two PIFAs, which indicates limited flexibility in using different relative antenna orientations to
reduce coupling. In recent years, many studies have been performed to find more efficient ways to enhance the isolation between closely-positioned PIFAs, e.g., [8]–[17]. In [8], good isolation is achieved by equipping each of the two PIFAs with a
small (local) ground plane, which is physically separated from
the common ground plane beneath. However, the achieved separation between the two modified PIFAs is still relatively large.
Defected ground structures (DGS) can provide a band stop property and have been studied in [9]–[11]. This method can reduce
the mutual coupling between antenna elements, but at the price
of sacrificing impedance bandwidth and occupying too much
space. A neutralization line is inserted in-between two PIFAs in
[12], [13]. This method can introduce some current on the neutralization lines and create an additional electromagnetic field
to cancel the mutual coupling. Another technique, based on the
former study in [12], utilizes instead a parasitic element that has
no direct connection to the antenna elements to provide the decoupling field [14], [15]. Unfortunately, no matter how the neutralization line or parasitic element is positioned, they will still
occupy some areas between the antennas. In [16], the quarter
wavelength slot formed by the edges of two very closely positioned PIFAs is utilized to induce a resonant mode, which effectively traps the displacement (or over-the-air) coupling current
between the antenna elements and enhances isolation. Hence,
no additional physical structure is required by this approach. In
order to further reduce mutual coupling, a new resonant mode is
0018-926X/$26.00 © 2011 IEEE
1522
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
excited in [17], which utilizes the edges of two PIFAs as well as
another slot cut on the ground plane to form a half-wavelength
U-shape slot in-between. Apart from trapping the displacement
coupling current between the antennas, the new mode also traps
the coupling current on the ground plane in the U-shape slot.
Thus, the resonant mechanism of the slot effectively cuts off
all coupling currents and allows an isolation of above 40 dB to
be realized in simulation for an inter-PIFA spacing of 0.0016
wavelength. However, the methods in [16] and [17] cannot be
utilized for a ground plane with arbitrary size and shape, (e.g.,
that of a mobile phone), because it may not be possible to match
the decoupling slots well enough in these cases in order to excite
them successfully. Moreover, these methods cannot be used for
dual-band applications, which further limit their versatility.
In this paper, a T-shape slot is inserted at the end of a quarterwavelength decoupling slot formed by the edges of two PIFAs,
as an impedance transformer for the decoupling slot. By appropriately adjusting the length of the T shape, the formed decoupling slot can be excited in a ground plane of arbitrary size and
shape. In addition, with the help of the T-shape slot impedance
transformer, a dual-band MIMO antenna with good isolation in
both bands can be achieved.
The paper is organized as follows: In Section II, an impedance
matching T-shape slot is introduced in a single-band two-PIFA
array with a decoupling slot for mobile phone applications.
Section III extends the proposed method to the dual-band
case, where a dual-band isolation property can be achieved.
The diversity and MIMO capacity performances of proposed
single-band and dual-band antennas are evaluated in Section IV.
Finally, our conclusions are presented in Section V.
Fig. 1. Geometry of the proposed single-band PIFA array: (a) top view,
(b) ground plane, (c) back view, (d) side view, (e) front view, (f) and (g) 3D
view. All dimensions are given in mm.
II. CLOSELY SPACED SINGLE-BAND PIFAS WITH A T-SHAPE
SLOT IMPEDANCE TRANSFORMER FOR MOBILE TERMINALS
A. Antenna Configuration and Physical Mechanism
Fig. 1 illustrates the configuration of our proposed singleband PIFAs for mobile terminals. The ground plane is a PCB
with a surface area of 40 mm 100 mm, and it consists of a
0.03 mm thick top copper sheet (i.e., the yellow region in Fig. 1)
and a 1.55 mm thick FR4 substrate (i.e., the orange region in
Fig. 1). The FR4 substrate has a dielectric permittivity of 4.7
and a loss tangent of 0.015. In order to simplify fabrication
(i.e., a pre-requisite for mass production), the proposed PIFAs
are mounted on a 1 mm thick hollow carrier (a dielectric supporting structure indicated in blue in Fig. 1) commonly found
in today’s terminal antenna implementations. The carrier has a
dielectric permittivity and a loss tangent of 2.5 and 0.007, respectively. The two PIFAs have an edge-to-edge separation distance of 0.0088 wavelength. Each PIFA is fed by a feeding pin
provided by the inner conductor of a 50 coaxial cable (see
Figs. 1(d), (f) and (g)). In order to increase the impedance bandwidth at the end of each probe, an
copper sheet is added
to form a capacitively load feed [18]. In this paper, the copper
is also an important parameters for enhancing
sheet width
the isolation. Since the spacing between the copper sheet and the
top part of PIFA will highly affect the performance of the PIFAs,
and in order to further simplify precise antenna fabrication, an
dielectric block is added to this in-between region. The
dielectric block is of the same material as the hollow carrier and
has a thickness of 1 mm. A T-shape slot is etched on the ground
plane and it performs as an impedance transformer of the slot
formed by the edges of two PIFAs. The detailed dimensions of
the designed dual-PIFA structure are provided in Fig. 1.
The physical decoupling mechanism for the proposed MIMO
antennas is as follows: when two PIFAs with shorting walls
are positioned side-by-side, the neighboring edges of the PIFAs
form a quarter-wavelength slot. For some specific sizes of the
ground plane, the formed slot can be excited to reduce mutual coupling [16]. However, in general (e.g., in mobile terminals), the slot may not be able to effectively decouple the
antennas. This is because different sizes of ground plane will
strongly affect the impedance of the formed quarter-wavelength
slot, as well as the current distributions on the ground plane,
such that good matching cannot be guaranteed for the decoupling slot. To address this problem, a T-shape slot is etched on
the ground plane and can be view as an impedance transformer
of the formed slot. In our design, route (1) (see Figs. 1(f) and
(g)) and route (2) (see Fig. 1(f)) are the formed decoupling slot
) and the T-shape impedance
(of length
) of the formed slot, retransformer (of length
and
spectively. By proper optimization of the parameters
ZHANG et al.: MUTUAL COUPLING REDUCTION OF TWO PIFAS WITH A T-SHAPE SLOT IMPEDANCE TRANSFORMER FOR MIMO MOBILE TERMINALS
Fig. 2. Simulated S parameters with and without the T-shape slot impedance
transformer, and measured S parameters of the proposed single-band PIFAs for
MIMO terminals.
, the decoupling slot formed by the edges of the PIFAs
can be placed at an arbitrary location on the ground plane and
efficiently excited, irrespective of the ground plane’s size and
shape. The simulated scattering (or S) parameters of two PIFAs
with and without the T-shape impedance transformer are presented in Fig. 2. The full-wave antenna simulations are carried
out in the frequency domain using the CST Microwave Studio
software. It can be seen that, because of the proposed T-shape
dB
slot, the coupling between the PIFAs is reduced from
dB at the center frequency.
to less than
B. Measurement Results
The fabricated prototype of the proposed single-band PIFAs
with a T-shape slot impedance transformer is presented in
Fig. 3. In the mockup, two 50 coaxial cables are utilized, and
the outer conductors of the cables are connected to the ground
plane and the inner conductors are used to feed the PIFAs at
the feeding pin positions shown in Fig. 1(d). The simulated and
measured S parameters for the proposed PIFAs are given in
Fig. 2. As can be observed, the measured results agree well with
the simulated ones. The measured 10 dB impedance bandwidth
is 100 MHz (2.4–2.5 GHz), which covers the WLAN band
(2.4–2.48 GHz). The isolation within the 100 MHz band is over
20 dB, and up to a maximum of 44 dB.
Fig. 4 shows the measured normalized radiation gain pattern
for PIFA1 (labeled in Fig. 3), which is obtained in an anechoic
chamber. The measured pattern for PIFA2 is omitted here, since
it is in mirror symmetry to the pattern of PIFA1. During the measurement, when PIFA1 (see Fig. 1(a) or 2) is measured, PIFA2 is
terminated with a 50 load, and vice-versa. The measured peak
gain and efficiency are 1.8 dBi and 70%, respectively. The measured efficiency is obtained with an error tolerance of about 13%
(0.6 dB). In addition, the loss in antenna efficiency is mainly attributed to the lack of high precision in the fabrication process
(e.g., the copper tape on the carrier is cut by hand). Based on
the simulation results, it can be estimated that the efficiency can
be increased by 10% or more if a more professional fabrication
process is applied.
1523
Fig. 3. Prototype of the proposed single-band PIFAs for MIMO terminals.
Fig. 4. Measured normalized radiation gain pattern of PIFA1 at 2.45 GHz:
(a) x-z plane (H-plane), (b) y-z plane (E-plane).
C. Parametric Study
To analyze the influence of different parameters and further
understand the decoupling mechanism, a parametric study is
performed in CST. To reduce computation time, the mesh lines
per wavelength and lower mesh line limit are both set to 25,
which are less than that used to obtain Fig. 2 (i.e., 45). However,
this reduction is found to have only a small impact on accuracy.
The results from the parametric study are summarized below:
1) Impedance Matching: Fig. 5 shows the influence of the
capacitively loaded copper sheet of dimensions
. It can
be deduced that
and
mainly determine the impedance
matching of the PIFAs. Furthermore, when
increases, the
mutual coupling will also increase (see Fig. 5(b), where at
2.35 GHz,
mm has a worse impedance matching and a
stronger coupling than
mm). Moreover, we found that
the effects of changing
are similar to those of changing
,
though not as significant.
2) Matching of Decoupling Slot: In order to enhance the isolation between the PIFAs, the formed decoupling slot should
be well matched. As shown in Fig. 6, the matching characteristic is mainly determined by the size of the T-shape slot
on the ground plane. It can be observed that mutual coupling can be effectively reduced by a properly dimensioned T-shape slot. Fig. 6 also reveals that the T-shape slot has
little impact on the operating frequency and the coupling null,
1524
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
Fig. 5. Impact of the capacitively loaded copper sheet parameters: (a) length
, (b) width
.
Fig. 6. Impact of the T-shape slot parameters: (a) length
.
and this indicates that it is not a part of the PIFAs’ radiating
structure.
3) Location of Coupling Null: In Fig. 7, the influence of the
parameters
and
are presented. When
is progressively increased, the coupling null moves to a higher frequency
at a faster rate than the operating frequency. On the other hand,
an increase in
will result in the coupling null moving to
a lower frequency at a faster rate than the operating frequency.
By optimizing
or
, the operating frequency and coupling null can be made to coincide, whereas the isolation level
can be tuned by adjusting the dimensions of the T-shape slot.
contrast, the current along route (2) of length
(see Fig. 1(g)) is significantly weaker, because it only works
as a T-shape impedance transformer of the decoupling slot and
contributes very little to the antenna radiation.
D. Current Distributions
The current distribution of the proposed single-band PIFAs
is shown in Fig. 8. This distribution is obtained when PIFA1
is excited. It can be observed that: First, most of the coupling
current is trapped in the formed slot and cannot flow to PIFA2,
and thus good isolation is achieved by the two PIFAs. Second,
the current distribution along route (1) (see Figs. 1(g) and (f)) is
similar to that of a conventional quarter-wavelength slot, where
the current is very strong at the feeding end (i.e., observe the
current distribution at the
part in Figs. 1(a) and (b)). In
, (b) width
E. Study of the Dual-PIFA Location
To demonstrate the versatility of the proposed dual-PIFA
structure in terms of its location on the ground plane, it is
moved from the upper center position (see Fig. 1) to the upper
left corner of the ground plane, as illustrated in Fig. 9. The detailed parameters of the PIFAs at this new location are given in
Table I, though only the parameters that are changed, relative to
those of the previous location, are listed. This new case reveals
that our proposed method can also work for an asymmetrical
structure. The simulated S parameters are presented in Fig. 10.
It is observed that the impedance bandwidth for PIFA1 has been
significantly enlarged. PIFA1 and PIFA2 can cover the bands
of 2.3–3.1 GHz and 2.4–2.5 GHz (WLAN band), respectively
(for the
dB specification). Within the WLAN band where
MIMO can be applied (since it is covered by both PIFAs), the
isolation between PIFA1 and PIFA2 is at 15 dB. Moreover,
the isolation is above 11 dB over the full bandwidth of PIFA1,
ZHANG et al.: MUTUAL COUPLING REDUCTION OF TWO PIFAS WITH A T-SHAPE SLOT IMPEDANCE TRANSFORMER FOR MIMO MOBILE TERMINALS
1525
Fig. 10. S parameters of the single-band PIFAs at the corner location.
TABLE I
CHANGED PARAMETERS OF THE PIFAS AT
GROUND PLANE
Fig. 7. Impact of the decoupling slot parameters: (a)
, (b)
.
Fig. 8. Current distribution of the proposed single-band PIFAs for MIMO terminals at 2.45 GHz.
Fig. 9. Schematic drawing of the single-band PIFAs at the corner location.
THE
CORNER
OF THE
suggesting that the additional bandwidth (outside the WLAN
band) may be good enough for other wireless applications.
The mechanisms for the much larger bandwidth of PIFA1 are
explained as follows: First, when the PIFA array moves from the
upper center location to the upper left corner location, another
resonant mode is introduced by the T-shape slot. In this case,
the T-shape slot works not only as an impedance transformer
for the decoupling slot (at 2.45 GHz), but also as one part of
PIFA1 to provide another resonance (at around 2.9 GHz). This
is the main reason for the larger bandwidth of PIFA1. Second,
it is much easier for a PIFA (PIFA1 in this case) to excite the
so-called chassis mode, when its longer side is placed along the
edge of the ground plane [19]–[22], and this mode can further
increase the bandwidth. Based on these two mechanisms, the
impedance bandwidth of PIFA1 can be efficiently enlarged by
eight times.
Moreover, the 15 dB isolation within the WLAN band can
be further enhanced, if the size of the T-shape slot increases.
However, an improved isolation can only be achieved at the
cost of a smaller PIFA1 bandwidth. This is because the T-shape
slot provides the higher resonant frequency for the bandwidth
of PIFA1, and when it is enlarged, the resonance will move to a
low frequency.
Furthermore, when the PIFA array is rotated by 90 and place
on one corner of the ground plane, the results are very similar
to the case studied before. The PIFA close to the shorter edge of
the ground plane will have a much larger bandwidth and for the
other one the bandwidth remains almost the same. The trade-off
between the isolation of the two PIFAs and the bandwidth of the
PIFA closer to the short edge also exists.
1526
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
Fig. 12. Simulated S parameters with and without the T-shape slot impedance
transformer, and measured S parameters of the proposed dual-band PIFAs for
MIMO terminals.
Fig. 11. Geometry of the proposed dual-band PIFA array: (a) top view, (b)
ground plane, (c) back view, (d) side view, (e) front view, (f) and (g) 3D view.
All dimensions are given in mm.
III. CLOSELY SPACED DUAL-BAND PIFAS WITH A T-SHAPE
SLOT IMPEDANCE TRANSFORMER FOR MIMO TERMINALS
A. Antenna Geometry and Physical Mechanism
The proposed decoupling method can also be extended to
dual-band MIMO applications. The configurations for the designed dual-band PIFAs are shown Fig. 11. The materials used
in this designed are the same as those of the single-band case.
Each PIFA is fed by a feeding pin provided by the inner conductor of a 50 coaxial cable (see Figs. 11(d), (f) and (g)). At
the end of each probe, an
copper sheet is added and
directly pasted onto the inner surface of the hollow carrier. Different from the single-band case, a folded L-shape slot (see route
(3) in Fig. 11(a)) is etched onto each PIFA and some metal parts
are removed in Fig. 11(e) (as compared to Fig. 1(e)) to change
the length of the L slot. By optimizing all parameters properly,
a dual-band dual-isolation property can be achieved, as shown
in Fig. 12.
The physical mechanism for the dual-band dual-isolation
property of the proposed MIMO antennas is described as
follows: For the WLAN band (2.4–2.48 GHz), the decoupling
mechanism is the same as the single-band case and has been
explained in Section II-A. Regarding the 3.5 GHz band, the
operating bandwidth can be determined by part “A” (see
Fig. 11(g)) and the decoupling property is achieved by the
folded L slot (see route (3) in Fig. 11(f)) formed by parts
“A” and “B” in Fig. 11(g). Similar to the case of the WLAN
band, the formed decoupling L slot also needs to be matched
well to work correctly, and the T-shape slot can also tune the
impedance matching of the L slot. In the dual-band case, the
T-shape slot is mainly responsible for the matching of the decoupling L slot. However, this will result in a slight mismatch
of the decoupling slot formed by the two PIFAs in the WLAN
band. In order to solve this problem, another parameter
is
utilized together with the T-shape slot to adjust the matching
of the formed decoupling slot in the WLAN band. Therefore,
a dual-band dual-isolation property can be achieved. The
simulated S parameters between two PIFAs with and without
T-shape impedance transformer are show in Fig. 12. It can
be seen that the isolation is significantly enhanced with the
T-shape slot.
B. Measurement Results
The fabricated prototype of the proposed dual-band PIFAs for
MIMO terminals is shown in Fig. 13. In the fabricated mockup,
the feeding method is the same as that of the single-band case.
A comparison between the measured and simulated S parameters can be obtained from Fig. 12. Based on the measured
results, the proposed dual-band MIMO antennas can cover the
2.4–2.48 GHz WLAN band and the 3.4–3.6 GHz WiMAX
band (for the
dB specification). Within the operating
band, the isolations are above 19.2 dB at the lower band and
above 22.8 dB at the higher band. In general, the measured and
simulated results agree well with each other. Although there
is a slight difference between simulation and measurement in
the frequency of the coupling null at the higher band, the worst
isolation (22.8 dB) within the operating band is still above
20 dB.
The measured normalized radiation gain patterns of PIFA1
at 2.44 GHz and 3.5 GHz are presented in Fig. 14. The peak
gain and efficiency are also measured. As before, the error tolerance of the efficiency measurement is about 13% (0.6 dB). In
the WLAN band and the WiMAX band, the peak efficiencies
ZHANG et al.: MUTUAL COUPLING REDUCTION OF TWO PIFAS WITH A T-SHAPE SLOT IMPEDANCE TRANSFORMER FOR MIMO MOBILE TERMINALS
1527
Fig. 13. Prototype of the proposed dual-band PIFAs for MIMO terminals.
Fig. 15. Impact of parameters (a)
Fig. 14. Measured normalized radiation gain patterns of PIFA1: (a) x-z plane
(H-plane) at 2.44 GHz, (b) y-z plane (E-plane) at 2.44 GHz, (c) x-z plane
(H-plane) at 3.5 GHz, (d) y-z plane (E-plane) at 3.5 GHz.
are 70.3% and 77.2%, respectively, whereas the peak gains are
2.4 dB and 3.4 dB, respectively. These efficiencies and gains are
good enough for MIMO terminal applications.
C. Parametric Study
In the CST simulations, to guarantee good accuracy, both the
mesh lines per wavelength and lower mesh limit are set to 45.
1)
and : The influence of the parameter
, which
determines the length of the L decoupling slot on the PIFAs,
is shown in Fig. 15(a). As the length of
decreases, the L
decoupling slot becomes shorter, while part “A” that determines
the operating frequency at the WiMAX band is unchanged (see
Fig. 11(g)). As expected, the shorter decoupling slot leads to
, (b)
.
the WiMAX band coupling null moving higher in frequency.
The effect of the parameter can be seen in Fig. 15(b). Since a
change in will simultaneously increase or decrease the length
of the L slot and part “A”, the relative location of the coupling
null and operating frequency does not change significantly over
the investigated range of .
2)
and
: The influences of
and
have been
studied in Section II-C for the single-band case. However, apart
from the aforementioned effects, they also have some new roles
in the dual-band case. As illustrated in Fig. 16,
and
can
affect the matching of the operating frequencies of both bands.
Moreover,
can also change the location of the coupling null
for the WiMAX band and the matching performance of the decoupling slot formed by the two PIFAs in the WLAN band. This
is because the coupling current distribution can be viewed as the
source or feed of the decoupling element (L slot or slot formed
by two PIFAs). When the feeding condition is changed, the operating property (e.g., location of coupling null and matching
performance) is also affected.
3)
and
: As mentioned in the single-band case,
and
will affect the matching performance of the
decoupling slot formed by the two PIFAs in the WLAN band.
1528
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
Fig. 16. Impact of the capacitively loaded copper sheet dimensions: (a) length
, (b) width
.
Fig. 17. Impact of the T-shape slot parameters: (a) length
.
Nonetheless, the T-shape slot impedance transformer will likewise affect the matching performance of the decoupling L slots.
In the dual-band case, these two parameters are mainly responsible for the matching performance of the decoupling L slot for
the WiMAX band. For the WLAN band, they work together
with
to tune the matching performance of the decoupling
slot formed by the two PIFAs. In this way, a dual-band dual-isolation property is achieved. The influences of the T-shape slot
dimensions
and
are presented in Fig. 17. Similar to
the single-band case,
and
are mainly used to tune the
location of the coupling null for WLAN band.
Figs. 11(g) and 18(b)) which controls the operating frequency
of the higher band. The coupling currents are blocked by the L
slot and thus cannot flow into the adjacent PIFA.
D. Current Distributions
The current distributions of the proposed dual-band PIFAs
are given in Fig. 18 for 2.44 GHz and 3.5 GHz, which are the
center frequencies of the two operating bands. At 2.44 GHz (the
WLAN band), the currents are mainly concentrated on part “B”
(see Figs. 11(g) and 18(a)), which determines the operating frequency of the lower band. In addition, most of the coupling currents are trapped in the slot formed by two PIFAs. At 3.5 GHz
(the WiMAX band), the currents mainly focus on part “A” (see
, (b) width
E. Study of the Dual-PIFA Location
As in the single-band case, the impact of the dual-PIFA location on the ground plane (adjusted by the parameter
in
Fig. 11(a)) is studied for the dual-band case. For conciseness,
we do not show the S parameter results of this case. Unlike the
single-band case, the moving of the MIMO dual-band antennas
from the center location to the corner location does not produce
the wideband behavior for the antenna closer to the longer edge
of the ground plane, or PIFA1 (see Fig. 10). Instead, it only offers slightly bigger bandwidths in both bands, whereas those
for the other antenna (PIFA2) are slightly reduced. In addition,
the coupling behavior is largely unchanged. The relatively small
change in the bandwidths of PIFA1 in the dual-band case is because the parameter
of the T-shape slot is much shorter
than that of the single-band case. Consequently, the T slot mode
is not efficiently excited so as to enlarge the bandwidth. On the
other hand, if desired, the now slightly different bandwidths of
ZHANG et al.: MUTUAL COUPLING REDUCTION OF TWO PIFAS WITH A T-SHAPE SLOT IMPEDANCE TRANSFORMER FOR MIMO MOBILE TERMINALS
1529
dual-band (see Fig. 11) PIFA arrays are calculated with the measured 3D electric field radiation patterns by assuming uniform
3D angular power spectrum (APS). The envelope correlation
coefficient for the single-band PIFA array is less than 0.09 in
the band of 2.4–2.5 GHz, and for the dual-band MIMO array,
the coefficients are 0.04 and 0.093 in the bands of 2.4–2.48 GHz
and 3.4–3.6 GHz, respectively.
B. MIMO Channel Capacity
With the same number of transmitting and receiving antennas
(i.e., 2) and without channel knowledge at the transmitter, the
ergodic capacity of the 2 2 MIMO channel in uniform 3D
APS can be obtained using the measured antenna efficiencies
and the complex correlation coefficient , assuming that the
PIFA array is on the receive side, and there is no correlation or
coupling on the transmit side [24]
(1)
Fig. 18. Current distributions of the proposed dual-band PIFAs for MIMO terminals at (a) 2.44 GHz, (b) 3.5 GHz.
Fig. 19. Ergodic capacity for: singe-band PIFA array, dual-band PIFA array at
the lower band and dual-band PIFA array at the higher band.
PIFA1 and PIFA2 for the corner placement can be equalized by
enlarging the width of PIFA2 while reducing the width of PIFA1
(to maintain the volume of the entire PIFA array). Furthermore,
the same conclusions as above apply to the case where the PIFA
array at the corner location is rotated by 90 .
IV. CORRELATION AND MIMO CHANNEL CAPACITY
A. Correlation
Correlation coefficient is an important MIMO performance
metric, as it quantifies the ability of the MIMO channel to provide parallel sub-channels, which facilitates good capacity performance. The envelope correlation is defined in [23]. The envelope correlation for the proposed single-band (see Fig. 1) and
where
and
are the determinant, conjugatetranspose and expectation operators, respectively.
is the
total power that is equally distributed to each transmitting
antenna,
is the receiver noise power,
is the 2 2 identity matrix. Moreover,
, where
is a 2 2 matrix of independent
and identical distributed (i.i.d.) complex Gaussian variables
with zero mean and unit variance, is a normalized correlation
matrix whose diagonal elements are 1 and the upper and lower
off-diagonal elements are given by and , respectively, and
is the total efficiency of antenna [24], [25].
The ergodic capacities of the single-band and dual-band PIFA
arrays are shown in Fig. 19. The reference i.i.d. Rayleigh case,
i.e., with
and
in (1), is also shown for comparison. The results confirm that the designed PIFA arrays give
good MIMO performance. The differences in SNR requirement
between the i.i.d. Rayleigh case and the proposed antennas in
achieving a given ergodic capacity is mainly due to reduced total
efficiency, since the correlation is very low.
V. CONCLUSION
This paper demonstrates an efficient decoupling technique
for two PIFAs that is enabled by a T-shape slot impedance
transformer. The technique is not only applicable to single-band
PIFAs, but also to dual-band PIFAs through a dual-isolation
property. The impact of the PIFA array location on the antenna
performance is studied for both single-band and dual-band
cases. When the single-band PIFA array is positioned at one
corner of the ground plane, the bandwidth of one PIFA can be
enlarged by eight times, whereas the bandwidth of the other
PIFA and the good isolation remain almost the same. Correlation and channel capacity results confirm that the proposed
PIFA array can give good MIMO performance. Overall, our
results substantiate that the proposed technique is both effective
and versatile for practical implementation in MIMO terminals.
Finally, it is noted that the proposed technique can be applied
to MIMO antennas operating in a lower frequency band, e.g.,
at 1.5 GHz in LTE band 21. In particular, the T-shape slot can
1530
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 3, MARCH 2012
be modified into a meander-shape slot or a slot of some other
shapes to keep the whole antenna structure compact.
ACKNOWLEDGMENT
The authors thank Dr. A. Sunesson of Lite-on Mobile AB,
Sweden, for his help with antenna pattern measurements.
REFERENCES
[1] B. K. Lau, “Multiple antenna terminals,” in MIMO: From Theory to Implementation, C. Oestges, A. Sibille, and A. Zanella, Eds. San Diego,
CA: Academic Press, 2011, pp. 267–298.
[2] S. Zhang, Z. Ying, J. Xiong, and S. He, “Ultrawideband MIMO/diversity antennas with a tree-like structure to enhance wideband isolation,”
IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1279–1282, Dec.
2009.
[3] J. Zhu and G. V. Eleftheriades, “A simple approach for reducing mutual coupling in two closely spaced metamaterial-inspired monopole
antennas,” IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 379–382,
2010.
[4] S. Zhang, P. Zetterberg, and S. He, “Printed MIMO antenna system of
four closely-spaced elements with large bandwidth and high isolation,”
Electron. Lett., vol. 46, no. 15, pp. 1052–1053, 2010.
[5] R. Tian, V. Plicanic, B. K. Lau, and Z. Ying, “A compact six-port
dielectric resonator antenna array: MIMO channel measurements and
performance analysis,” IEEE Trans. Antennas Propag., vol. 58, no. 4,
pp. 1369–1379, Apr. 2010.
[6] Y. J. Sung, M. Kim, and Y. S. Kim, “Harmonics reduction with defected ground structure for a microstrip patch antenna,” IEEE Antennas
Wireless Propag. Lett., vol. 2, pp. 111–113, 2003.
[7] H. Carrasco, H. D. Hristov, R. Feick, and D. Cofre, “Mutual coupling
between planar inverted-F antennas,” Microw. Opt. Technol. Lett., vol.
42, no. 3, pp. 224–227, Aug. 2004.
[8] Y. Gao, X. Chen, Z. Ying, and C. Parini, “Design and performance
investigation of a dual-element PIFA array at 2.5 GHz for MIMO terminal,” IEEE Trans. Antennas Propag., vol. 55, no. 12, pp. 3433–3441,
Dec. 2007.
[9] D. Ahn, J. S. Park, C. S. Kim, J. Kim, Y. Qian, and T. Itoh, “A design of the low-pass filter using the novel microstrip defected ground
structure,” IEEE Microw. Theory Tech., vol. 49, no. 1, pp. 86–93, Jan.
2001.
[10] C. Caloz, H. Okabe, T. Iwai, and T. Itoh, “A simple and accurate model
for microstrip structures with slotted ground plane,” IEEE Microwave
Wireless Comp. Lett., vol. 14, no. 4, pp. 133–135, Apr. 2004.
[11] C.-Y. Chiu, C.-H. Cheng, R. D. Murch, and C. R. Rowell, “Reduction
of mutual coupling between closely-packed antenna elements,” IEEE
Trans. Antennas Propag., vol. 55, no. 6, pp. 1732–1738, Jun. 2007.
[12] A. Diallo, C. Luxey, P. L. Thuc, R. Staraj, and G. Kossiavas, “Study
and reduction of the mutual coupling between two mobile phone PIFAs
operating in the DCS1800 and UMTS bands,” IEEE Trans. Antennas
Propag., vol. 54, no. 11, pp. 3063–3074, Nov. 2006.
[13] C. Yang, J. Kim, H. Kim, J. Wee, B. Kim, and C. Jung, “Quad-band
antenna with high isolation MIMO and broadband SCS for broadcasting and telecommunication services,” IEEE Antennas Wireless
Propag. Lett., vol. 9, pp. 584–587, 2010.
[14] A. C. K. Mak, C. R. Rowell, and R. D. Murch, “Isolation enhancement
between two closely packed antennas,” IEEE Trans. Antennas Propag.,
vol. 56, no. 11, pp. 3411–3419, Nov. 2008.
[15] B. K. Lau and J. B. Andersen, “Simple and efficient decoupling of compact arrays with parasitic scatterers,” IEEE Trans. Antennas Propagat.,
vol. 60, no. 2, pp. 464–472, Feb. 2012.
[16] S. Zhang, J. Xiong, and S. He, “MIMO antenna system of two closelypositioned PIFAs with high isolation,” Electron. Lett., vol. 45, no. 15,
pp. 771–773, 2009.
[17] S. Zhang, S. N. Khan, and S. He, “Reducing mutual coupling for an extremely closely-packed tunable dual-element PIFA array through a resonant slot antenna formed in-between,” IEEE Trans. Antenna Propag.,
vol. 58, no. 8, pp. 2771–2776, Aug. 2010.
[18] C. R. Rowell and R. D. Murch, “A capacitively loaded PIFA for compact mobile telephone handsets,” IEEE Trans. Antennas Propag., vol.
45, no. 5, pp. 837–842, May 1997.
[19] J. Villanen, J. Ollikainen, O. Kivekas, and P. Vainikainen, “Coupling
element based mobile terminal antenna structure,” IEEE Trans. Antennas Propagat., vol. 54, no. 7, pp. 2142–2153, Jul. 2006.
[20] W. L. Schroeder, A. A. Vila, and C. Thome, “Extremely small wideband mobile phone antenna by inductive chassis mode coupling,”
in Proc. 36th Eur. Microw. Conf., Manchester, U.K., 2006, pp.
1702–1705.
[21] W. L. Schroeder and C. T. Famdie, “Utilization and tuning of the
chassis modes of a handheld terminal for the design of multiband
radiation characteristics,” in Proc. IEE Wideband Multiband Antennas
and Arrays, Sept. 7, 2005, pp. 117–122.
[22] S. R. Best, “The significance of ground-plane size and antenna
location in establishing the performance of ground-plane-dependent
antennas,” IEEE Antennas Propagat. Mag., vol. 51, no. 6, pp. 29–42,
Dec. 2009.
[23] M. B. Knudsen and G. F. Pedersen, “Spherical outdoor to indoor power
spectrum model at the mobile terminal,” IEEE J. Sel. Areas Commun.,
vol. 20, no. 6, pp. 1156–1168, Aug. 2002.
[24] R. Tian, B. K. Lau, and Z. Ying, “Multiplexing efficiency of MIMO
antennas,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 183–186,
2011.
[25] J. W. Wallace and M. A. Jensen, “Modeling the indoor MIMO wireless
channel,” IEEE Trans. Antennas Propag., vol. 50, no. 5, pp. 591–599,
May 2002.
Shuai Zhang received the B.E. degree from the
University of Electronic Science and Technology
of China (UESTC), Chengdu, in 2007. In the same
year, he began working toward the Ph.D. degree
at Zhejiang University (ZJU), China. In 2010, he
received an Erasmus Mundus scholarship to continue his studies as a joint Ph.D. student at the Royal
Institute of Technology (KTH), Stockholm, Sweden.
From September 2010 to June 2011, he was a
Guest Researcher at the Department of Electrical
and Information Technology, Lund University,
Sweden. Since June 2011, he has been a Visiting Researcher in the Corporate
Technology Office, Sony Ericsson Mobile Communication AB, Lund, Sweden.
His research interests include ultrawideband (UWB) antennas, MIMO antenna
systems, RFID antennas, SAR, and human-body effects on antennas.
Buon Kiong Lau (S’00–M’03–SM’07) received the
B.E. degree (with honors) from the University of
Western Australia, Perth, Australia, and the Ph.D.
degree from the Curtin University of Technology,
Perth, Australia, in 1998 and 2003, respectively,
both in electrical engineering.
During 2000–2001, he took a year off from his
Ph.D. studies to work as a Research Engineer with
Ericsson Research, Kista, Sweden. From 2003
to 2004, he was a Guest Research Fellow at the
Department of Signal Processing, Blekinge Institute
of Technology, Sweden. In 2004, he was appointed a Research Fellow at the
Department of Electroscience, Lund University, Sweden. In 2007, he became
an Assistant Professor at the Department of Electrical and Information Technology (formerly Department of Electroscience), Lund University, Sweden,
where he is now an Associate Professor. During 2003, 2005 and 2007, he was
also a Visiting Researcher at the Department of Applied Mathematics, Hong
Kong Polytechnic University, China, Laboratory for Information and Decision
Systems, Massachusetts Institute of Technology, and Takada Laboratory, Tokyo
Institute of Technology, Japan, respectively. His primary research interests
are in various aspects of multiple antenna systems, particularly the interplay
between traditionally separate disciplines of antennas, propagation channels
and signal processing.
Dr. Lau is an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS
AND PROPAGATION and a Guest Editor of the 2012 Special Issue on MIMO Technology for the same journal. From 2007 to 2010, he was a Co-chair of Subworking Group 2.2 on Compact Antenna Systems for Terminals (CAST) within
EU COST Action 2100. Since 2011, he chairs Subworking Group 1.1 on Antenna Systems Aspects (ASA) within EU COST Action IC1004.
ZHANG et al.: MUTUAL COUPLING REDUCTION OF TWO PIFAS WITH A T-SHAPE SLOT IMPEDANCE TRANSFORMER FOR MIMO MOBILE TERMINALS
Yi Tan received the Bachelor degree in China in
2003. Currently, he is working toward the Master
degree at Lund University, Lund, Sweden.
During 2004–2007, he was an RF Engineer
in Laird Technologies, Beijing, China, where he
worked on mobile antenna design and production.
In 2008, he joined Lund University to pursue his
Master degree study. In 2010, he took a year off
from his studies to work as a Project Assistant at EIT
in Lund University.
Zhinong Ying (SM’05) is an expert of antenna
technology in Network Research Lab. Technology
office, Sony Ericsson Mobile Communication AB,
Lund, Sweden. He joined Ericsson AB in 1995. He
became Senior Specialist in 1997 and Expert in 2003
in his engineer career at Ericsson. He has been Guest
Professor in Zhejiang University, China, since 2002.
His main research interests are small antennas, broad
and multi-band antenna, multi-channel antenna
(MIMO) system, near-field and human body effects
and measurement techniques. He has authored
and coauthored over 80 papers in various journal, conference and industry
publications. He holds more than 70 patents and pending in the antenna and
mobile terminal areas. He contributed a book chapter to the well known Mobile
Antenna Handbook, 3rd edition. He invented and designed various types of
multi-band antennas and compact MIMO antennas for the mobile industry.
One of his contributions in the 1990s was the development of non-uniform
helical antenna. The innovative designs are widely used in mobile terminal
industry. His patented designs have reached a commercial penetration of more
than several hundreds million products in worldwide.
1531
Mr. Ying received the Best Invention Award at Ericsson Mobile in 1996 and
Key Performer Award at Sony Ericsson in 2002. He was nominated for the President Award at Sony Ericsson in 2004 for his innovative contributions. He served
as TPC Co-Chairman in the International Symposium on Antenna Technology
(iWAT), 2007, and served as session organizer of several international conferences including IEEE APS, and as a reviewer for several academic journals. He
was a member of scientific board of ACE program (Antenna Centre of Excellent
in European 6th frame) from 2004 to 2007.
Sailing He (M’92–SM’98) received the Licentiate
of Technology and the Ph.D. degree in electromagnetic theory from the Royal Institute of Technology
(KTH), Stockholm, Sweden, in 1991 and 1992,
respectively.
Since then he has worked at the same division of
the Royal Institute of Technology as an Assistant Professor, an Associate Professor, and a full Professor.
He is also with Zhejiang University (ZJU, China) as
a Distinguished Professor of a special program organized by the central government of China, as well as
a joint research center between KTH and ZJU. His current research interests
include electromagnetic metamaterials, optoelectronics, microwave photonics
and biomedical applications. He has first-authored one monograph (Oxford University Press) and authored/coauthored about 400 papers in refereed international journals. He has given many invited/plenary talks in international conferences, and has served in the leadership for many international conferences.
Prof. He is a Fellow of the Optical Society of America (OSA) and The International Society for Optical Engineering (SPIE).
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertisement