Mutual Coupling Reduction Techniques between MIMO Antennas for UWB Applications

International Journal on Recent and Innovation Trends in Computing and Communication ISSN: 2321-8169
Volume: 5 Issue: 9 18 – 22
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IJRITCC | September 2017, Available @ http://www.ijritcc.org
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Mutual Coupling Reduction Techniques between MIMO Antennas for UWB
Applications
K. Jayanthi
AP/ECE
Government College of Engineering
Salem, Tamilnadu
jayanthikathir@yahoo.co.in
Dr. A. M. Kalpana
HOD/CSE
Government College of Engineering
Salem, Tamilnadu
kalpana.gce@gmail.com
Abstract— The recent research has proved that the Multiple-input-multiple-output (MIMO) systems can substantially increase the channel
capacity by employing multiple antennas at both the transmitter and receiver, without increasing either transmitter power or bandwidth. Hence it
is very much essential to know all the aspects of MIMO system. Usually, in any MIMO system the antenna design plays a major role in
improving the system performance and channel capacity. The antenna bandwidth must support the wireless system for transmitting larger data
rates. Also, the mutual coupling effect between the antennas must be taken into consideration, while designing an efficient MIMO system. The
objective of this paper is to discuss various techniques to reduce mutual coupling of MIMO antennas for UWB application.
Keywords: MIMO, UWB.
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I. INTRODUCTION
In the present scenario of wireless communication, people
demand high data rate and reliable communication. UWB has
become the solution for future short-range high-data wireless
communication applications. Federal Communication
Commission (FCC) has allocated the unlicensed frequency
band of 3.1 GHz to 10.6 GHz with an EIRP of−41.3
dBm/MHz to avoid interference with narrowband
communication systems. After the announcement of
unlicensed 7.5 GHz of spectrum, there has been a wide variety
of applications developed for indoor/outdoor communications,
high accuracy radars and imaging systems, etc. In UWB
Communication systems, an antenna is one of the most critical
components to be realized to have a good system performance.
Antenna design for portable devices with compact size is the
one of the main challenge especially for UWB wireless
communication.
The challenges of feasible UWB antenna design also include
wide impedance matching, radiation stability, low profile and
low cost. Moreover, UWB systems also suffer from multipath
fading like other wireless systems. The Multiple-input-
multiple-output (MIMO) technology usually provides
multiplexing gain and diversity gain and further improves the
channel capacity and link quality. So to solve the problem
faced by UWB systems, the MIMO technology is introduced.
The major challenges in the design process of MIMO antennas
for UWB systems are minimize the size of antenna elements
for the MIMO and reduce the mutual coupling between the
antenna elements.
The various methods employed to reduce mutual coupling in
MIMO have little effect on the wideband impedance matching
for the UWB applications. The half wave dipole antenna,
monopole antenna and Microstrip patch antennas are the most
widely used antenna for MIMO systems.
II. MIMO ANTEENA DESIGN
A conventional RF communication system has one antenna in
the transmitter and another antenna in the receiver, commonly
designated as a single-input–single-output (SISO) system. In
the last decade, significant advances in the multiple-input–
multiple-output (MIMO) technology have been achieved by
wireless communication engineers. This is due to the
development of spatial multiplexing and diversity coding
techniques to increase the channel capacity of MIMO systems.
Advanced diversity antennas are required to support this
exciting new development.
A schematic representation of MIMO system with N number
of transmitting and receiving antennas is shown in Fig. 2.1.
The idea behind the system is to transmit various data streams
using different antennas at the same carrier frequency and
without additional power. When a data stream is transmitted
from pth antenna, it is received at the qth antenna after
travelling in different paths as shown in the Fig. 2.1. The
reflection of signal from different objects in the path produces
the multipath propagation. The signal received at the receiver
is represented as
Xq(t) = 𝒉𝒒𝒑 𝒕 𝑺𝒑 𝒕𝑵
𝒑=𝟏 (1)

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Fig.2.1 A schematic representation of MIMO antenna
Where, xq(t) represents the qth
received data, sp(t) represents
the pth
transmitted data and hqp(t) represents the path gain
between pth
transmitter and qth
receiver. The important
technique for modern wireless communication system is
antenna diversity. Section 2.1 gives an overview of several
diversity techniques and their influence on MIMO
performance. .By employing multiple-antennas in MIMO, the
effects such as multi-path, co-channel interference and small-
scale fading can be reduced. The mutual coupling between
antenna elements may become severe for compact devices due
to their small distance. In order to decrease the mutual
coupling between antenna elements, several techniques have
been proposed and they are explained in section 2.2.
2.1 Diversity Techniques for MIMO Systems
1.1.1 Spatial Diversity
Spatial diversity is achieved by using more than one
antenna element at transmitter and/or receiver to increase the
number of channel paths between TX-RX. With sufficient
element spacing, correct number of elements, and appropriate
array geometry or topology, signal quality over the wireless
channel can be improved. Adopting this scheme for MIMO,
channel capacity can also be optimized. As a rule, antenna
element spacing should be a multiple of its frequency
wavelength, to ensure independent fading on each element.
Insufficient spacing between antenna elements will cause
mutual coupling between the elements, which will cause input
impedance modification and pattern distortion to occur.
2.1.2 Polarization Diversity
Mutual coupling can be mitigated with the use of
polarization diversity, which can be viewed as an extension of
the space diversity scheme. In a typical case of linear
polarization diversity, signals are transmitted and received via
horizontally polarized as well as vertically polarized antennas
of two distinct polarizations constructs independent and
Pairing vertically polarized TX and RX antennas is usually
desirable for optimum performance since vertically polarized
signals normally propagate slightly better than horizontally
polarized signals. Nevertheless, a MIMO system for indoor
applications will suffer from cross-polarization induced by the
highly reflective interior structures. This cross-polarization
factor is a phenomenon where a signal is received with
polarization that is orthogonal to the transmitted polarization.
2.1.3 Pattern Diversity
Designing antennas with distinct radiation pattern, for
the case of array with two elements, constitutes the scheme of
pattern diversity. High correlation effect in a MIMO channel is
minimized by taking advantage of the angle spacing of the TX
and RX signals. With angle diversity, angle of departures of
the TX signals or angle of arrivals of the RX signals are
discriminated with the use of directional antennas pointing
specifically to each angle direction. This ensures isolation
between each TX or RX signals, and thus produces low
correlation effect on the signals. Angle diversity is highly
dominant when each of the antennas is able to receive the
multipath signals from many different directions. This causes
the angle spacing to be narrowly spaced and thus generates
high directivity due to the non-overlapping patterns. The
highly orthogonal antenna patterns produce the desirable low
correlated or uncorrelated MIMO signals. Small antennas, in
comparison, tend to have overlapping omnidirectional patterns
due to larger angle spacing causing high correlation impact on
the MIMO channel. Other method to guarantee high capacity
performance in a MIMO system is to employ the exceptional
technique of multimode diversity. This method exploits the
characteristic of multimode antennas that exhibits orthogonal
radiation patterns for different excitation modes. Moreover, a
single multimode antenna which is excited with fundamental
TEM modes, can offer the advantage of having multiple
orthogonal patterns for low correlation effect while
eliminating the need for multiple antenna elements as opposed
to other diversity schemes.
2.2. Technologies to reduce mutual coupling between
antennas in MIMO system
One of the main challenges to employ MIMO
technology in portable devices is the design of small MIMO
antennas with low mutual coupling. The mutual coupling or
correlation between antenna elements in MIMO through
surface wave propagation, because the antennas share the
common surface currents should be minimized either by
antenna design or by the introduction of features on the ground
plane to inhibit the current flow. Various methods have been
introduced to improve the isolation characteristics of MIMO
antenna for UWB application are
(i) Decoupling structures
(ii) Antenna elements of different types
(iii) Meta material structures

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(iv) Electromagnetic Band Gap (EBG) or
neutralization structures
2.2.1 Decoupling structures
Normally the antenna elements were set orthogonally
with respect to each other to enhance the isolation and pattern
diversity. Various decoupling structures have been inserted
between the antenna elements to enhance the wide band
isolation. Few of them are,
In [1] to enhance isolation and increase impedance
bandwidth, two long protruding ground stubs are added to the
ground plane and a short ground strip is used to connect the
ground planes of the two planer-monopole (PM) together to
form a common ground. To further enhance isolation and
increase impedance bandwidth, two long ground stubs, Stub 1
is placed in parallel with PM 1 and is bent to reduce the
overall antenna area, while stub 2 is a simple straight stub
placed in parallel with PM 2.
Two open L-shaped slot (LS) antenna elements and a
narrow slot on the ground plane. The antenna should be placed
normal to each other to obtain high isolation, and the narrow
slot is added to reduce the mutual coupling of antenna
elements in the (3-4.5 GHz) low frequency band.[2]
In [8] two square monopole-antenna elements, a T-
shaped ground stub, a vertical slot cut on the T-shaped ground
stub to reduce mutual coupling, and two strips on the ground
plane to create a notched frequency band.
The MIMO antenna in [13] employed two-folded
monopole elements, each coupled with a parasitic inverted-L
element, to achieve UWB operation. Two meander lines, a
connection line and a short parasitic line, were used to enhance
isolation between the two input ports.
In [10] two antennas share a single radiator, which
reduce the overall size of the MIMO system. T-shaped slot in
the radiator and a stub on the ground achieves high isolation
and the pentagonal radiator with perpendicular feeding
structure produce dual polarization. Decoupling is achieved by
extending a branch in the symmetry axis of two antenna
elements and etching a T-shaped slot in the radiator.
2.2.2 Antenna elements of different types
The technique of orthogonal placement of antenna
feeds/elements provides good isolation among the antenna
elements. However, it will result in a dual-polarized system or
polarization diversity. However, the true challenge will be the
placement of the antenna elements in the
same polarization and to obtain high isolation without any
decoupling structures while maintaining compact dimensions.
In [3] high isolation between slot antennas can be
easily achieved with the help of the directional radiation
properties of a slot antenna. The microstrip-fed stepped-slot
antenna achieves high isolation without any decoupling
network is due to inherent directional radiation properties of
slot antennas (SAs) and their asymmetrical placements.
Two symmetrical half-slot antenna elements with
coplanar waveguide-fed structures and a Y-shaped slot that is
cut at the bottom center of the common ground plane. Y-
shaped slot is employed to improve the isolation performance
at low UWB frequency band. [4]
Antennas in [14] enhanced isolation by notching
rectangular or T-shaped slot on the ground, which suppressed
the surface currents flowing between adjacent ports.
In [7] a printed circular disc compact planar antenna
reducing the mutual coupling and the correlation between the
elements.
An array of 4 monopole radiators and a u-shaped slot
was inserted in radiator 2 to rectify the mismatch and improve
the wideband matching characteristics. A separate partial
ground plane was chosen for each radiator because it plays an
important role in matching and also provides better
isolation.[9]
In [11] A radiation patch connected through a via
with the strip placed beneath the patch. The strip not only
provides another coupling path, but also serves as the
impedance transformer, resulting in good isolation ( dB) and
dual band rejection at WiMAX (3.4–3.7 GHz) and WLAN
(5.15–5.35 and 5.725–5.825 GHz) overthe UWB system
operation (3.1–10.6 GHz).
2.2.3. Metamaterial structures
To concentrate electromagnetic fields and current
near the antenna structure instead of spreading them along the
antenna ground metamaterial structures are used. Because
spreading of fields and currents results high mutual coupling
between the antenna elements. The circuit size is reduced by
using metamaterial technology also this structure produces
better performance in both antenna and passive circuit
applications.
In [19] A microstrip antenna array with a novel Ring
Resonator structure that is included between the antenna
elements for mutual coupling reduction. The effects of the
inclusion of this structure increase the antenna performance.
The split ring resonators (SRR) and their variants are
used as a metamaterial structure in[18].
Two single metamaterial antennas which are
constructed based on the modified composite right/left-handed

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(CRLH) model. In order to reduce the mutual coupling of the
antenna, a defected ground structure (DGS) is inserted to
suppress the effect of surface current between elements of the
proposed antenna.
A unit cell structure which has a inductive spiral loop
embedded in a dielectric substrate. A magnetic field normal to
the plane of the spiral induces a current in the loop, a
phenomenon that effectively creates an inductance within the
substrate and creates magnetic energy storage in the unit cell.
This storage enhances the magnetic permeability of the
otherwise non-magnetic substrate material. This”induced”
inductance along with the capacitance in the structure forms a
resonance structure.
In [12] WLAN and Wimax bands are achieved by
using slotted ground structure and metamaterial rectangular
split ring resonator.
2.2.2 Electromagnetic Band Gap (EBG) or neutralization
structures
Electromagnetic Band Gap (EBG) structure was used to
improve the isolation by blocking surface wave propagation.
The EBG structure with no metallic vias or vertical
components was formed by etching two slots and adding two
connecting bridges to a convenient unipolar EBG unit cell.
Two identical monopole antenna elements with a
comb line structure on the ground plane used to improve the
impedance matching and enhance the isolation [6].
2.3 Performance comparison of various MIMO antennas
Ref Size Bandwidth
(Ghz)
Isolatio
n(db)
ECC(ρe
)
Gain(dbi)
1 40X26mm2
2.9-10.6 <-15 <0.2 6.5
2 32X32mm2
3.1-10.6 <-15 <0.02 1.7-4.2
3 42X25mm2
3.1-12 <-22 <0.01 4
4 23X18mm2
3-12.4 <-15 <0.015 4
5 3X19X0.4m
m3
2.3-2.4 <-15 <0.16 2
6 26X31mm2
3.1-10.6 <-25 <0.001 <5
7 59X27mm2
3.1-10.6 <-24 <0.06 4.7
8 22X36mm2
3.1-11
Notched at
5.15-5.85
<-15 <0.06 Decreased
due to
notched fr
9 50X39.8mm2
2.9-10.6 <-17 <0.03 4.2
10 40X40X0.8m
m3
3-11 - <0.02 10.2
11 30X40X0.8m
m3
3.1-10.6
Notched at
WiMax
and WLAN
<-15 <0.05 Decreased
due to
notched fr
15 35× 40× 0.8
mm3
3–11.6 −16 0.01 < 6.5
16 26 × 40 × 0.8
mm3
2.9–10.6 −15 0.2 < 6.5
17 27 × 30 × 0.8 3–11 −20 0.012 < 5.25
2.4 Envelope Correlation Coefficient
The behavior of the MIMO antenna can be analyzed in
terms of two important parameters: ECC and CCL. The
acceptable limits of these parameters are ECC<0.5 and CCL
<0.4 bits/HZ/s. The envelope correlation coefficient (ECC) is
an important parameter to evaluate diversity performance,
which depicts the extent of isolation or correlation of different
communication channels. The ECC can be obtained from the S
parameters and radiation efficiency of UWB MIMO antenna
ρe = 𝜌𝑖𝑗 2
=
𝑆𝑖𝑖∗ 𝑆𝑖𝑗 +𝑆𝑗𝑖 ∗ 𝑆𝑗𝑗 2
1− 𝑆𝑖𝑖 2 – 𝑆𝑗𝑖 2 1− 𝑆𝑗𝑗 2 – 𝑆𝑖𝑗 2 𝜂𝑟𝑎𝑑 ,𝑖𝜂𝑟𝑎𝑑 ,𝑗
(2)
Where ηrad,I is the radiation efficiency
of the ith
antenna element.
III. CONCLUSION
This paper focuses the basic and important concepts of
MIMO antenna. Moreover this paper gives the various
technologies used to improve the isolation between antennas
in MIMO. Also a comparative study of performance of the
above said techniques were discussed.
REFERENCES
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