Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)
Printed MIMO Antenna Systems: Performance
Metrics, Implementations and Challenges
Mohammad S. Sharawi
Department of Electrical Engineering,
King Fahd University of Petroleum and Minerals (KFUPM)
Dhahran, 31261 Saudi Arabia,
e-mail: [email protected]
In this review paper, we start with the definition of the
new MIMO antenna performance metrics that are added to the
conventional ones. Following that we present three example on
printed multiband MIMO antenna systems that have recently
appeared in literature covering three major applications; large
wireless devices (i.e. laptops, tablets or wireless access points),
medium size devices (i.e. mobile and smart phones) and small
size devices (i.e. as USB dongles). Four major challenges that
are associated with printed multiband MIMO antenna systems
are then identified with some possible solutions suggested.
Finally, future directions based on industry forecasts are highlighted.
Abstract—Multiple-input-multiple-output (MIMO) technology
has become an integral part of wireless systems nowadays. This
technology depends on the use of multiple antenna elements at
the mobile terminal as well as the base station. The design of
compact printed MIMO antenna systems is a challenging task
specially when it is made for small factor mobile terminals.
The introduction of MIMO brought with it several performance
metrics and measurement methods to allow the designer gauge
the performance of his antenna system in real environments.
This paper will give an overview of such new metrics, it will give
several recent example of printed MIMO antenna systems and
will shine some light of some current challenges that designers
face when dealing with such multi-antenna systems. Some future
perspectives are also presented.
Index Terms- Printed Antennas, Multiple-input-multiple-output
(MIMO), Multi-band, Isolation, Correlation, TARC, Capacity.
II. P ERFORMANCE M ETRICS FOR MIMO A NTENNA
S YSTEMS
I. I NTRODUCTION
Additional performance metrics to those of regular single
antenna systems are required to characterize printed MIMO
antennas. While single antenna element metrics such as efficiency, radiation patterns, and operating bandwidths are also
required for MIMO antennas, metrics such as the total active
reflection coefficient (TARC), the correlation coefficient and
the channel capacity are required for multi-antenna system
characterization. These metrics takes into account the effect
of adjacent placement of antennas on the overall system
performance. In this section, we will present the definitions
of these MIMO antenna metrics in details.
Wireless standards are always evolving to be able to provide
higher data rates to mobile users to enhance their multimedia
experience. According to the famous channel capacity formula,
this can be achieved either by increasing the transmitted power
to enhance the signal to noise ratio (SNR) level or by providing
wider bandwidths. Both of these metrics are usually difficult to
change because of the regulations on power transmission levels
from various wireless terminals to avoid interference with
other equipment, and because bandwidth is very expensive to
buy in a crowded spectrum allocation. One way to overcome
this limit is to use multiple antenna systems at the mobile
terminal as well as the base station.
Multiple-input-multiple-output (MIMO) technology is one
of the major advances in fourth generation wireless standards
that led to the large increase in the data rates and throughput
via the use of multiple antennas on the receiver and transmitter
sides. This means that multiple antennas should be integrated
within the user terminals in very close proximity and these
antennas should be operating simultaneously in several modes
based on the channel characteristics. The design of MIMO
antennas is a challenging area are the designer needs to take
care of several issues that were not there when he/she were
designing single or multiple antenna systems that are covering
different bands. Such issues deal with the mutual coupling, the
field correlation among others. In addition, designing multiple
antenna systems requires introducing more metrics than those
of a single antenna system.
A. Total Active Reflection Coefficient (TARC)
TARC is defined as the square root of the ratio of the sum
of the power available at all the ports minus the radiated power
to the total available power [1],
Γta
=
s
available power − radiated power
available power
(1)
TARC is a real number between 0 and 1. When the TARC
value is zero, this means that all the available power is
radiated. The available power is the sum of powers available
on all the ports of the antenna system. TARC curves are used to
obtain the effective operating bandwidth of the antenna system.
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The general formula for finding TARC for antennas with
high efficiencies from the measured S-parameters is given by
qP
N
2
i=1 |bi |
t
(2)
Γ a = qP
N
2
|a
|
i
i=1
Following the condition
0 ≤ |ρrec | ≤ 1
where ρrec is the correlation coefficient and ρrec,0 is the
correlation coefficient given in Equation (6). The factor
r
1
1
−
1
−
1
is added to the formula by considering
η1
η2
where
b = Sa
(8)
the loss correlation that is normally ignored. This factor
determines the degree of uncertainty in the calculation of the
correlation coefficient using the S-parameters. This factor is 1
for antenna systems having radiation efficiencies of 50% for
both radiators. The unity factor of uncertainty is very high for
any calculation. This factor grows rapidly as the radiation efficiency reduces below 50%. So for antenna systems with low
radiation efficiencies, correlation coefficient values calculated
using the S-parameters do not provide meaningful information.
In these cases, the correlation coefficient values should be
calculated using the 3D radiation pattern using the formula
given in Equation (5). However in spite of this ambiguity in
the calculation of the correlation coefficient values using Sparameters, many researchers are still using this method for
electrically small antenna based MIMO antenna systems.
It is important to mention that the isolation and correlation
coefficient are two different things. High isolation does not
guarantee low correlation coefficient and vice versa. High
isolation and low correlation coefficient are required for good
performance out of the MIMO antenna system.
(3)
where vector a is the excitation applied to the antenna structure. The expression of TARC, can be evaluated by following
formula for two port networks [2],
r
((|S11 + S12 ejθ |2 ) + (|S21 + S22 ejθ |2 ))
t
(4)
Γa =
2
where θ is the input feeding phase, Sxx and Sxy are the sparameters associated with the antenna structure.
B. Correlation Coefficient
The correlation coefficient (ρ) is a measure that describes
how the communication channels are isolated from each other.
This metric deals with the radiation pattern of the antenna
system. The square of the correlation coefficient is known as
the envelop correlation coefficient. In an isotropic communication channel, the envelop correlation coefficient(ρe ) can be
calculated using [3],
2
RR
[F~1 (θ, φ) ∗ F~2 (θ, φ)]dΩ
(5)
ρe = RR4π
|F~1 (θ, φ)|2 dΩ|F~2 (θ, φ)|2 dΩ
C. Channel Capacity
Channel capacity is a measure of how many bits can be sent
per 1 Hz of bandwidth. It is used to compare the performance
improvement of a MIMO system relative to a single antenna
system. It is also a convenient measure to determine the
performance of the MIMO system relative to the case of
ideal all independent channels system. The channel capacity
is usually measured in the form of a cumulative distribution
function(CDF) or relative to the SNR. The first step to find
the channel capacity is to determine the channel matrix. The
channel matrix is determined by the radiation patterns of the
antenna system and the channel between them. The channel
matrix can be determined as [5]
4π
where Fi (θ, φ) is the field radiation pattern of the antenna
when the ith port is excited and all other ports are terminated
to matched load. This is a complicated expression that requires three dimensional radiation pattern measurements and
numerical integration to get the envelop correlation coefficient.
Simple derivation in [3] proves that the correlation coefficient
can be calculated by using the s-parameters and the radiation
efficiency. The general expression is given by,
2
∗
∗
Sjj |
|Sii
Sij + Sji
|ρ(ij) |2 = 2
2
2
2
(1/2)
|(1 − |Sii | − |Sji | )(1 − |Sjj | − |Sij | )|
(6)
where ρij is the correlation coefficient, ρeij = ρ2ij is the
envelop correlation coefficient, Sij is the coupling factor
between the i and j elements and ηradi is radiation efficiency
for ith element. In this formula we need to know only
the S-parameters and the radiation efficiencies that can be
evaluated easily as compared to 3D radiation patterns required
by Equation (5).
However [4] questioned the accuracy of the correlation
coefficient calculated using the S-parameters for an antenna
system having radiation efficiencies less than 50%. The author
provided the upper and lower bounds on the correlation
coefficient that is given as
s
1
1
|ρrec |max,min = |ρrec,0 | ±
−1
−1
ηrad1
ηrad2
(7)
1/2
1/2
H = ψR GψT
(9)
where ψRM ×M and ψT N ×N are the receive and transmit
coefficient matrices respectively and GM ×N is a matrix containing complex Gaussian random numbers representing the
randomness of the channel.
The correlation coefficient entries can be written as
µij
(10)
ψ i,j = √
µii µjj
where
µij =
Z
E{[Ai (Ω).h(Ω)][A∗j (Ω).h∗ (Ω)]}dΩ
(11)
Ai (Ω) represents the field pattern of the ith element and
h(Ω) represents the incoming waves. This integral is very
complicated and requires complete three dimensional radiation
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patterns. However certain assumptions, under certain practical
conditions, allow us to simplify this integral. So if we assume
a Rayleigh fading channel, only horizontal incoming waves,
uncorrelated orthogonal polarization of incoming waves and
constant time average power density per steradian, then the
correlation coefficient expression becomes [5],
Z 2π
[ΓAiθ (π/2, φ)A∗jθ (π/2, φ)+Aiφ (π/2, φ)A∗jφ (π/2, φ)]dφ
µij =
0
(12)
where Γ is the cross-polarization discrimination and
Aθ (π/2, φ) and Aφ (π/2, φ) are the theta and phi E-field
patterns at theta=90o .
Once the H is known, the capacity can be found using,
ρ
T
(13)
HH
C = log2 det IR +
NT
where IR is NR × NR identity matrix, NR and NT are the
number receive and transmit antennas and H T is conjugate
transpose of the H matrix. It is clear from the formula that
if H is an identity matrix then the capacity is the number of
antennas within the system (rank of H) times the capacity of
one antenna.
To get the CDF, a sequence of the capacity values is
generated by calculating the capacity several times. This is
a random sequence due to the dependence of the capacity on
the channel modeled. This random sequence is used to get the
CDF for the capacity. For calculating the capacity relative to
the SNR, the SNR is increased in regular intervals and the
capacity is calculated with new value of SNR.
Fig. 1. Penta-band MIMO antenna for tablet PC applications [7].
III. R ECENT M ULTI - BAND P RINTED MIMO A NTENNA
S YSTEM E XAMPLES
The number of multi-band printed MIMO antenna system
designs has increased dramatically in the past two years [6]
due to the importance and adaptation of MIMO technology in
4G systems and beyond. The applications cover a wide range
of consumer electronics and wireless standards. In this section,
three examples of printed MIMO antenna systems along with
their performance metrics are presented and discussed. They
cover various application sizes, from large ground plane sizes
(i.e. Laptop and Tablet PC size as well as wireless access point
sizes), to small ones (i.e. USB dongle size).
Fig. 2. Gain and efficiency curves of antenna 1 and 2 of the Penta-band
antenna [7].
A penta-band, four-element printed MIMO antenna for
wireless routers was proposed in [7]. The fabricated geometry
of this antenna is shown in Figure 1. Antenna elements 1 and
2 are printed meandered loop antennas covering the bands
699-798 MHz (LTE) and 2.2-2.4 GHz (WLAN/WiMax), and
elements 3 and 4 which are also printed meandered loop
antennas cover the bands 1.7-2.0 GHz (UMTS), 3.5-4 GHz
(WiMax) and 5-5.8 GHz (WLAN). The multiband printed
MIMO antenna occupied a 150 × 150 mm2 FR-4 board
area with 0.8 mm thickness. Two shorting strips were added
to elements 1 and 2 to improve matching performance and
suppress unwanted resonances.
The total efficiency and peak gain values in dBi are shown
in Figure 2 for elements 1 and 2. The minimum efficiency
of antennas 1 and 2 in the lower band was 70% while in the
higher band, it was 58%. The maximum gain in the lower LTE
band was 2.35 dBi while in the higher WLAN/WiMax it was
3.25 dBi. For elements 3 and 4, the minimum efficiency in
the UMTS band was 74% with a maximum gain of 4.9 dBi.
The minimum efficiency in the WiMax band was 61% with a
maximum gain of 3.9 dBi. Finally, the minimum efficiency
A. Example 1: Printed MIMO antenna systems for Laptop/Tablet Applications
Several printed multiband antenna structures have appeared
in literature for devices with large ground planes such as
laptop and tablet PCs as well as wireless access points as
in [7] - [11]. Printed monopole antennas along with PIFA and
loop ones dominated the antenna structures of such systems.
We will present an example here that demonstrated several
MIMO antenna performance metrics for the proposed design
(as several designs that appeared in literature do not fully list
the MIMO metrics of their antennas). The example presents
a penta-band MIMO antenna system for wireless routers (as
well as large ground plane based devices).
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Fig. 3. Measured and simulated envelope correlation coefficient for antennas
1 and 2 [7].
(a)
(b)
Fig. 4. A dual-band four-shaped dual element MIMO antenna system with
CLL isolation structures [15].
in the 5 GHz WLAN was 71% with a maximum gain of
5.3 dBi. The maximum envelope correlation coefficient (ECC)
was less than 0.1 in the bands of interest and was calculated
based on the s-parameters without including the efficiency. The
measured and simulated values of ECC are shown in Figure 2
between elements 1 and 2.
The improvement in the isolation was more than 10 dB the
lower band of operation and more than 3 dB improvement in
the higher band. The measured S-parameters for the printed
MIMO antenna system in its lower operating band are shown
in Figure 5. The lower operating band covered 827-853 MHz
and the higher band covered 2.3-2.98 GHz. Note that the interelement spacing between the two antennas is less than λ/15
in the lower band, and thus large coupling between the two
elements is expected without using the isolation enhancement
structure.
The maximum measured gain in the low band (at 840 MHz)
was -2.8dBi while in the high band (at 2.65 GHz) it was
5.5dBi. The efficiency in the low band was 35% due to
the use of larger size CLLs and the GND plane defect,
while it was approximately 67% in the higher band. The
correlation coefficient was evaluated using the S-parameters
formula including the effect of efficiencies. The values were
0.11 and 0.18 for the low and high bands, respectively. The
TARC curves for different input signal angles were evaluated
based on the measured S-parameters. Figure 5(b) shows the
TARC curves for angle steps of 30 degrees for the low band.
Note the stable behavior in the low band due to the very
good isolation obtained (more than 19 dB). The calculated
mean effective gain (MEG) values were within 1 dB from one
another in both bands and for XPD values of 0 and 6dB.
The channel capacity of this 2-element printed dual-band
MIMO antenna system was measured using a software defined
radio (SDR) platform. The entries of the channel matrix (H)
that are required to calculate the channel capacity of the system
are complex numbers that contain the gain and phase information between the propagation paths from the transmitter to
the receiver. The measured H matrix is then substituted in
(13) to find the channel capacity in the environment under
consideration. The H matrix was measured in an indoor line
of sight (LOS) and non-line of sight (NLOS) environments in
the Electrical Engineering hallways, at KFUPM.
The SDR platform had a Xilinx Vertix 4 FPGA pro-
B. Example 2: Mobile Terminal size Printed MIMO antenna
System
The need of Printed MIMO antennas with multiband operation for mobile phone applications is a necessity nowadays
with 4G and the upcoming 5G standards. Current 4G systems
have two antenna elements and the number of antennas might
increase to 4 in the near future. This poses a real challenge in
the design and integration of such antennas within a mobile
phone terminal size while maintaining good MIMO performance metrics. Several designs have appeared in literature
for this application category such as those in [12]-[18]. The
majority of proposed multiband printed MIMO antenna types
for cellular and small handheld devices were either PIFA or
monopole based.
The example on a cellular phone size printed multiband
MIMO antenna presents a 4-shaped printed MIMO antenna
for mobile phone applications. A two element 4-shaped printed
MIMO antenna geometry with enhanced isolation using capacitively loaded loops (CLL) is presented in [15]. The fabricated
model of the printed MIMO antenna is shown in Figure 4.
The antenna was fabricated on a 100 × 50 × 1.56 mm3 FR4 substrate. Microstrip impedance transformers were used for
proper impedance matching at the higher band. CLLs were
used to enhance the isolation performance between the two
adjacent elements at the lower and higher bands. Two CLL
arrays were used for the dual band operation, a top layer for the
higher band (with lower CLL sizes) and a complementary CLL
one (etched out from the GND plane) on the bottom layer of
the PCB, i.e. on the GND layer, that enhances the isolation in
the lower band. The two CLL structures are metamaterial ones,
and their characteristics were modeled before being applied.
The single cell size on the top layer was 5.727 × 5.8 mm2 and
on the bottom layer was 9 × 8.927 mm2 .
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of path loss variations.
Figure 7 shows that in both LOS and NLOS scenarios,
at least 1 bit/s/Hz improvement is achieved when isolation
is improved by 3 dB between the two antenna elements at
2.45 GHz at an SNR level of 20 dB. In addition, the MIMO
advantage is evident if we compare the measured channel
capacities at 20 dB SNR. They show about 8 bit/s/Hz capacity
compared 6.5 bit/s/Hz from a SISO one. The channel capacity
values for the NLOS were 1 bit/s/Hz higher than the LOS ones.
This is due to the higher propagation induced correlation in
the LOS channels compared to their NLOS counterparts. The
reason for the difference between the ideal channel capacity
performance of a 2x2 MIMO antenna system and the one measured can be explained taking several factors in mind. First,
the printed MIMO antennas have limited efficiency which
was approximately 65% compared to 100% in the ideal case.
Second, the channels are assumed to be totally uncorrelated
in the ideal limit, but in the case of our antennas, they seem
to be more correlated and thus the MIMO performance was
degraded. But even with these two major effects, the system
achieves more than the theoretical SISO limit.
0
−5
S−parameters(dB )
−10
exp−S11
exp−S12
exp−S22
−15
−20
−25
(a)
−30
−35
700
720
740
760
780
800
820
Frequency(MHz)
840
860
880
900
0
−5
0degree
30degree
60degree
90degree
120degree
150degree
180degree
TARC(dB)
−10
−15
(b)
−20
−25
700
720
740
760
780
800
820
Frequency(MHz)
840
860
880
900
C. Example 3: USB Dongle size MIMO Antenna Systems
Fig. 5. Measured (a) S-parameters and (b) TARC curves for the lower
operating band of [15].
USB dongles are widely used for wireless connectivity in
WLAN and WiMAX applications. Such dongles have two
antennas (more in the future) that are very close to one
another, thus suffering from high correlation and coupling.
Novel antennas that are of small size as well as with isolation
enhancement methods have been reported in literature [19][23]. PIFA antenna types dominated the structures used for
USB dongle applications.
The example presented is an UWB monopole based MIMO
antenna system for USB dongles [22]. The two element MIMO
antenna is shown in Figure 8 and represents a very compact
design occupying only 25 × 40 × 1.55 mm3 on an FR-4
substrate. Antenna element 1 is a traditional UWB monopole
that has a crescent shape slot introduced within it to increase
the slot length between it and the GND plane of antenna 2.
It also enhances the isolation between the two antennas and
increases the operating bandwidth of antenna 2.
Antenna element 2 is a half UWB slot; see Figure 8(b).
Antenna elements 1 and 2 form a complementary pair with
reduced coupling. The 90 degree orientation of the elements
with respect to one another along with the crescent slot
reduces the coupling and provides good polarization diversity
performance. Ferrites are used at the input ports to reduce the
currents flowing from the GND planes back to the input feed
cables. The system covers the bands from 3.13-6.2 GHz with
isolation better than 22 dB across the band of operation.
The maximum measured gain was between 2-6 dBi for both
antennas across the operating band. The measured efficiencies
for both antennas were approximately 62 % (-2dB) across the
band of operation. The use of the ferrite reduced the efficiency
by 25%. The envelope correlation was lower than 0.1 over the
entire band for this MIMO antenna system.
cessing unit, 8-channel ADC/DAC, and quad RF modules
that operated at the 2.4 GHz band. To measure the channel
coefficients, a transmission burst was sent from one channel
of the transmitter with all other channels silent. From the data
received at both antenna inputs of the receiver (since it is
a 2-element MIMO system), the complex valued entries of
the channel matrix were calculated. The process was repeated
using the second channel of the transmitter, and a complete
H matrix was calculated. An average H matrix was obtained
after performing 1000 such measurements and averaging them
out. The measurements were carried out in a 10 feet wide and
8 feet high corridor with concrete walls and conducting sheet
ceiling. The measurement setup is shown in Figure 6.
In the LOS scenario, the transmitter and receiver were
separated by an average distance of 20 feet. In the NLOS
scenario, the receiver was moved around the edge of a wall.
The distance of the transmitter from the edge was 20 feet
while for the receiver it was 5 feet. The layout of measurement
scenarios is shown in Figure 6(b). Six sets of measurements
were conducted, three for the LOS case and three for the
NLOS case. For each measurement, the same antenna was
attached to the receiver and the transmitter (two different isolation methods were used, i.e. two different printed 2-element
MIMO antennas, one with CLL based isolation mechanism
and the other with a defected ground structure (DGS)). A total
of 25 realizations of the H matrix were obtained for each case
by moving the transmitter slightly around its position. The H
matrix was averaged out and normalized to remove the affects
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(a)
(a)
(b)
Fig. 7. Channel capacity curves (a)LOS (b) NLOS.
(b)
Fig. 6. System level performance evaluation of a dual-band dual-element
MIMO antenna system (a) SDR platform with antennas mounted on (b)
platform locations.
IV. C URRENT C HALLENGES IN THE D ESIGN OF P RINTED
MIMO A NTENNA S YSTEMS
The design of closely packed printed antennas has several
challenges. In this section, we will go over the major challenges that face the designer of such systems. Aside from the
challenge of coming up with a new antenna structure for a
specific application (a challenge that is not considered here),
four major challenges have been identified when designing
printed multi-band MIMO antenna systems.
(a)
A. Miniaturization and Integration Issues
When it comes to mobile terminals with small form factors
(i.e. cellular phones, USB dongles, etc.), antenna miniaturization as well as integrating several antennas close to one another
are big design challenges. Antenna miniaturization affects the
radiation efficiency of the antennas as well as its operating
bandwidths. Not to mention that taking the effect of the system
packaging as well as the hand and head effects can severely
degrade the performance of the designed antenna system. All
(b)
Fig. 8. UWB USB dongle MIMO antenna[22].
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these factors needs to be carefully investigated for the design
constraints under consideration and the effect of each needs
to be addressed on the overall system performance. A good
study on the effects of such factors was conducted in [24].
parasitic elements are not physically connected to the
antennas such as those in or connected to the ground
structure to form sort of a resonator. These elements are
designed to control the frequency band of isolation, the
bandwidth as well as the maximum amount of coupling
reduction. The idea of using parasitic elements is to
reduce mutual coupling by creating opposite fields to
the original ones created by the excited antenna.
These parasitic elements can be of simple types, such
as floating or shorted stubs such as those in [33]-[35],
or can be of a resonator type such as those in [36][38]. Both methods have been demonstrated to give good
isolation improvement levels.
4) Defected Ground Plane Structures: The system ground
plane directly affects the behavior of printed antenna
elements as it serves as the path of the return current
and sometimes becomes part of the radiating structure
at lower frequencies of operation as seen in previous
chapters. Since all antennas with a printed MIMO antenna system share the same system ground plane, the
current induced on the ground plane can easily couple to
adjacent antenna elements causing high coupling levels
that degrade the MIMO antenna system isolation and
correlation performance.
The mutual coupling between adjacent MIMO antenna
elements can be reduced by introducing defects within
the ground plane. The defects will act as band reject
filters and will suppress the coupled fields between
adjacent parts when properly designed. This mechanism
of reducing mutual coupling is denoted as defected
ground structures (DGS). There are various types of
defects that can be introduced within the ground plane
to reduce mutual coupling. Some of these are based on
introducing a group of slits such as those in [39]-[41], or
the use of dumble-shaped defects as in [42]-[43]. Other
types of defects also exist and a good survey of such
geometries can be found in [44].
5) Neutralization lines: When one of the two adjacent
MIMO antennas radiates (or gets excited), it induces
some currents on its neighbor, which increases their
mutual coupling and correlation values. A neutralization
line (NL) is an isolation enhancement technique in
which the current at the excited element is taken at a
specific location, then its phase is inverted by selecting
an appropriate length of the NL and then the inverted
current is fed to the nearby antenna to reduce the amount
of the coupled current (similar to the parasitic element
method). The selection of the point is critical in this
method. The point on the radiating antenna should be
with minimum impedance and maximum current. The
effective bandwidth of the NL technique depends on the
variation of the impedance at the selected point. Thus,
a low impedance point on the structure of the radiating
antenna with stable impedance throughout the band of
operation is selected as the starting point of the NL. The
use of this method to reduce mutual coupling between
adjacent MIMO antenna elements can be found in [45][48].
B. Antenna Coupling and Isolation enhancement
Placement of adjacent antennas at distances less than λ/4
causes high coupling levels. In medium to small size device
dimensions such as in handheld mobile phones or PDAs,
and USB dongles, MIMO antennas are very closely placed
and high coupling levels are inevitable. These high coupling
levels will degrade the efficiency and capacity of the MIMO
system. Several methods have been suggested in literature
to minimize or eliminate such coupling effect based on the
antenna structure and its radiation and feeding mechanisms.
These can be summarized into six methods, they are:
1) Antenna placement and orientation: closely positioned
antennas within wireless terminals will have high coupling between them through the system level ground
plane as well as the radiated fields. This will degrade
their efficiencies as well as their correlation coefficients,
thus degrading the channel capacity that can be achieved.
A simple way to reduce these effects is by placing
the antennas far apart within the mobile terminal, i.e.
the two top corners, or one at the top and another at
the bottom edges. A detailed position study should be
conducted to access the MIMO antenna parameters at
various locations.
The orientation of the antennas can affect the phase of
the coupling currents as well as the polarization of the
radiated fields. Adjacent antennas can be oriented in
quadrature with each other (i.e. 90o ) to minimize the
ground coupling as well as field coupling. This has to
be also accessed properly though a complete study of the
antenna system proposed as different antenna types have
different coupling mechanisms and thus these methods
can provide varying results. Some work in this area has
been shown in [25]-[28]
2) Decoupling Networks: The mutual coupling between
adjacent antenna elements can be reduced using decoupling networks. Such networks will decouple the input
ports of adjacent antennas thus increasing their radiation
efficiency and lowering their radiation correlation. Using
a decoupling network usually requires the use of a
matching network to provide proper input port matching.
This technique has been applied to several designs such
as those in [29]-[32]. Lumped elements as well as
distributed ones have been effectively used to enhance
the isolation between adjacent antennas.
3) Parasitic elements: Another method of reducing mutual
coupling between two adjacent MIMO antenna elements
to enhance its efficiency, isolation and correlation coefficient is to use parasitic elements between the antennas to
cancel part (or most) of the coupled fields between them.
The parasitic element will create an opposite coupling
field that reduces the original one, thus reducing the
overall coupling on the victim antenna. Usually these
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6) Use of Metamaterials: Metamaterials are man-made
structures that have negative permittivity or permeability
or both. Such materials have very interesting features
that allowed researchers to realize systems that were
only discussed in theory in the past such as cloaks and
perfect lens. Metamaterials can also be used for isolation
enhancement between adjacent elements of a printed
MIMO antenna system due to the presence of a band
gap in their frequency response. If designed properly,
the band gaps can act as band reject filters and suppress
mutual coupling between adjacent antenna elements.
Several examples have appeared in literature that utilized
MTM based structures for isolation enhancement such
as those in [49]-[52]. The two most widely used MTM
basic structures for isolation enhancement between adjacent elements are the use of split-ring resonators (SRR)
[50] or the use of capacitively-loaded-loops (CLL) [51][52].
Fig. 9. Reverberation Chamber diagram.
C. Correlation Coefficient Calculation
The correlation coefficient will directly affect the diversity
performance. Ideally, we want zero correlation between the
antenna radiation patterns. In practice, this is impossible. Thus,
reducing the field interactions is a major design challenge
especially when the antennas are placed within close proximity with inter-element distances of less than 0.2λ. Although
several methods have been proposed to improve the isolation
between adjacent MIMO antenna elements, this does not
guarantee good correlation performance as equation (6) does
not consider field interactions. Thus (5) should always be used
and correlation levels appearing in literature using (6) are
misleading in a sense that they underestimate the correlation
values. The two values that are considered acceptable in the
industrial standard specs (i.e. 3GPP) for the correlation and
envelope correlation coefficients are 0.3 and 0.5, respectively.
Many researchers believe that these values are high and should
be tightened to get better capacity levels.
The correlation coefficient should be evaluated based on
the radiation patterns obtained from the design. Thus it
is important to design MIMO antenna elements with tilted
radiation patterns to minimize their field correlations, and
then characterize their performance through measurements
in a reverberation chamber that can provide values for the
measured correlation coefficient in a specific environment.
The repeatability of the tests is another issue. Several
organizations around the world are trying to come up with
a standardized method for MIMO OTA conformance tests,
but until now, no one standard exists. These organizations are
the third-generation partnership project (3GPP), the European
cooperation in science and technology (COST 2100) and the
International Association for the Wireless Communication Industry (CTIA). The figure of merit that we are interested in for
MIMO OTA testing is the system capacity (data throughput).
The individual metrics such as correlation coefficient, the
radiation patterns, etc., will all impact the final throughput
of the system. In OTA testing, the creation of a repeatable
measurement that represents a real MIMO channel for various
operating modes of the device (different channel conditions) is
the major challenge to characterize its real performance. Thus,
several methods have been devised in Literature and Industry
to evaluate MIMO OTA performance. The three methods
widely used are:
1) The Anechoic chamber multi-probe OTA method [53].
2) The two-stage OTA method [54].
3) The Reverberant chamber OTA method [55]-[56].
The reverberation chamber method is the preferred one these
days. A reverberant chamber is a highly reflective cavity that is
isolated from the outside environment as it has metallic walls.
The waves radiated from the transmit antennas are reflected
within the cavity creating standing waves. Mechanical stirrers
(movable metallic sheets) are used to create a controlled near
ideal isotropic Rayleigh fading channel (the channel model for
urban and indoor environments) representing a MIMO channel
that can accurately characterize the MIMO performance of the
DUT in a statistical fashion over many samples .
Although the three presented methods of MIMO OTA
testing give close results, the 3GPP, CTIA and COST2100
organizations have not yet set a single standard that all agree
on. This is also due to the fact that MIMO systems are more
complicated and support much more modes compared to their
D. Measurements
The characterization of MIMO terminals is a challenging
task compared to their SISO counterparts. The end to end
performance testing in a real environment is called over
the air testing (OTA). A major challenge in OTA testing
for MIMO devices is to emulate an RF environment that
accurately represents the MIMO device performance in the
real wireless propagation scenario. MIMO OTA testing is a
challenging task because it depends on various factors such as
the receiver baseband processing, mode of operation, antenna
system performance and the channel propagation conditions.
8
Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)
ACKNOWLEDGMENT
SISO counterparts. The reverberant chamber method seems to
be the best candidate for MIMO OTA Testing, but also care
should be taken when characterizing MIMO systems as the
mode of operation should be mentioned when listing the performance metrics. The type of modulation and coding used as
well as the SNR levels are critical. Spatial multiplexing mode
for example does not mean anything if the SNR level is low as
the device will switch to operate in a transmit diversity fashion.
The effect of user hand and head, the effect of interference and
the like are also other factors that can significantly affect the
MIMO system performance. Thus, testing in real environments
as compared to an emulated one such as the reverberation
chamber should be conducted to access the mobile terminal
behaviour. This also required special functions that needs to
be embedded within the mobile terminal electronics to give
such measurements and performance metrics, meaning more
complexity in the handset design. A good review on such
measurement issues can be found in [57], [58].
The author would like to thank the Editor in Chief of
FERMAT Prof. Raj Mittra for his invitation to contribute
to this newly launched journal and the reviewers for their
comments that enhanced the content of the manuscript.
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MIMO antenna systems will also be used for next generation wireless terminals, i.e. 5G and beyond. Printed MIMO
antennas in particular are of great importance for the design
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and channel capacities are growing within every new wireless
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expected from mobile terminals. This means that much larger
bandwidths are required. Such large bandwidths are available
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New potential bands in the 28 GHz, 48 GHz as well as
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VI. C ONCLUSIONS
MIMO technology will be used in all future wireless
terminals and standards. This technology depends on the
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Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)
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AUTHOR B IOS
Mohammad S. Sharawi was born in Wolfsburg,
Germany, in 1977. He obtained his BSc degree
in Electronics Engineering from PSUT, Jordan, in
2000, and his MSc and PhD degrees from Oakland
University, Michigan, USA, in 2002 and 2006, respectively. Currently he is an Associate Professor of
Electrical Engineering at King Fahd University of
Petroleum and Minerals (KFUPM), Dhahran, Saudi
Arabia. Dr. Sharawi is the founder and director
of the Antennas and Microwave Structure Design
Laboratory (AMSDL). He was a Research Scientist
with the Applied Electromagnetics and Wireless Laboratory in the Electrical
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Forum for Electromagnetic Research Methods and Application Technologies (FERMAT)
and Computer Engineering Department, Oakland University, Michigan, USA,
during 2008-2009, and a visiting professor in the summer of 2013. Dr.
Sharawi was a faculty member in the Computer Engineering Department at
Philadelphia University, Amman, Jordan, during 2007-2008. He served as
the Organization Chair of the IEEE Conference on Systems, Signals and
Devices that was held in Jordan in July 2008. During 2002-2003 he was
a hardware design engineer with Silicon Graphics Inc., California, USA. Dr.
Sharawi has more than 90 refereed international journal and conference paper
publications. His research interests include Printed and MIMO Antenna design
and characterization, RF electronics, Applied Electromagnetics, Wireless
Communications and Hardware Integration. He has 2 issued, 3 published and
6 pending patents. Dr. Sharawi is a Senior Member IEEE.
E DITORIAL C OMMENT
It has been quite some time since the original introduction
of the MIMO concept was introduced. It is coming of age
just now by being implemented in a number of practical
applications whose numbers are on the rise. The key feature
of MIMO is the promise to deliver increased channel capacity.
But there are several practical issues that must be dealt with
when implementing a MIMO system in order to realize the
increased capacity. This paper covers these issues for three
different sizes of wireless devices, namely small, medium and
large, and suggests ways for addressing these issues. It also
looks at the crystal ball to predict future trends of MIMO for
similar applications.
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