# Ultra-Wideband Antenna with MIMO Diversity for 5G Wireless Communication

M. Saeed Khan, Adnan Iftikhar, Raed M. Shubair, Antonio-D. Capobianco,
Benjamin D. Braaten, and Dimitris E. Anagnostou A. Iftikhar is with the
Electrical Engineering Department, COMSATS University, Islamabad, Islamabad,
45550, Pakistan e-mail<EMAIL_ADDRESS>M. Shubair is with
Massachusetts Institute of Technology (MIT), Cambridge, MA, USA and New York
University, Abu Dhabi.A. -D. Capobianco is with the Department of Information
Engineering, University of Padova, Via Gradenigo 6/b, 35131, Padova, Italy.B.
D. Braaten is with the Department of Electrical and Computer Engineering,
North Dakota State University, Fargo, ND 58102, USA.D. E. Anagnostou is with
the Department of Electrical Engineering, Heriot-Watt University, Edinburgh,
U.K.

###### Abstract

An eight element, compact Ultra Wideband$-$ Multiple Input Multiple Output
(UWB-MIMO) antenna capable of providing high data rates for future Fifth
Generation (5G) terminal equipments along with the provision of necessary
bandwidth for Third Generation (3G) and Fourth Generation (4G) communications
that accomplishes band rejection from 4.85 to 6.35 GHz by deploying a Inductor
Capacitor (LC) stub on the ground plane is presented. The incorporated stub
also provides flexibility to reject any selected band as well as bandwidth
control. The orthogonal placement of the printed monopoles permits
polarization diversity and provides high isolation. In the proposed eight
element UWB-MIMO/diversity antenna, monopole pair 3$-$4 are 180∘ mirrored
transform of monopole pair 1$-$2 which lie on the opposite corners of a planar
50 × 50 mm2 substrate. Four additional monopoles are then placed
perpendicularly to the same board leading to a total size of 50 × 50 × 25 mm3
only. The simulated results are validated by comparing the measurements of a
fabricated prototype. It was concluded that the design meets the target
specifications over the entire bandwidth of 2 to 12 GHz with a reflection
coefficient better than $-$10 dB (except the rejected band), isolation more
than 17 dB, low envelope correlation, low gain variation, stable radiation
pattern, and strong rejection of the signals in the Wireless Local Area
Network (WLAN) band. Overall, compact and reduced complexity of the proposed
eight element architecture, strengthens its practical viability for the
diversity applications in future 5G terminal equipments amongst other MIMO
antennas designs present in the literature.

###### Index Terms:

Band rejected, compact, diversity, envelope correlation co-efficient, multiple
input multiple output, ultrawide band, 5G communication, 5G terminal devices.

##  I Introduction

Wireless broadband communication system such as Worldwide Interoperability for
Microwave Access (WiMAX (3.4 to 3.6 GHz)), large capacity Microwave Relay
Trunk Network (4.4 to 4.99 GHz), and Wireless Local Area Network (WLAN signals
in 5.15 to 5.35 and 5.75 to 5.8225 GHz bands), impose a limited power spectral
density of the low power and a high data rate UWB signal, even though, the
bandwidth of UWB is wide (3.1 GHz – 10.6 GHz) [1]. The amalgamation of UWB
technology with MIMO permits such wireless systems to achieve high data rates
by transmitting wireless signals over multiple channels without increasing the
input power. For instance, high speed and data rates in Wireless Personal Area
Networks (WPAN) is only possible with UWB-MIMO technology [2]. One key
specification of such MIMO antennas is that isolation between its elements
should be more than 15 dB to ensure mitigation of inter-element
Electromagnetic Interference (EMI) [3, 4, 5, 6, 7]. The inter-element EMI can
be mitigated by ensuring $\lambda$/2 spacing between the MIMO antenna
elements. However, the spacing significantly effects the compactness of the
MIMO antennas which ultimately employ practical restrictions on the MIMO
antenna to be integrated in indoor and outdoor portable devices. Besides,
researchers have proposed various techniques to mitigate inter-element EMI
without effecting the compactness of the MIMO antenna [3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16, 17]. In addition, other wireless communication
standards such as WiMAX, WLAN, and X-band downlink frequencies may
electromagnetically interfere with the UWB spectrum and may affect systems’
performance. Therefore, several techniques have been reported in literature to
mitigate the EMI from these wireless communication standards that are
allocated in the UWB spectrum.

A high gain two-element spanner shaped UWB-MIMO with edge truncation (for
bandwidth enhancement) is proposed for UWB applications [3]. The two element
UWB-MIMO antenna provides an isolation higher than 15 dB by placing elements
0.25 $\lambda$ apart, but, expansion of the proposed antenna to eight element
MIMO antenna significantly enlarges overall size. Stubs have been employed on
the ground plane between two orthogonally placed elements to improve the
isolation [4]. Another design in [5], also exploits the polarization diversity
for UWB-MIMO applications. A four element design with modified slotted ground
plane is presented in [6]. The proposed solution reaches an isolation of 14 dB
with small (45 × 45 mm2) size over the bandwidth of 2 to 6 GHz. Another design
in [7], exhibits an isolation of more than 20 dB with discontinuities between
the antenna elements and ground plane, however the size of the antenna becomes
too large (110 × 114 mm2) for 2 to 6 GHz band. Compact designs (with size 39.8
× 50 mm2) have been proposed for four element UWB-MIMO application in [8, 9].
The elements exploit the polarization diversity to obtain isolation of more
than 17 dB with an addition of complex band stop design on the back side of
the radiators to reject the WLAN band in [9]. With a total size of 60 × 60
mm2, an electromagnetic Bandgap (EBG) structure is employed in [10, 11] to
reject the WLAN for four element UWB-MIMO antenna. Mushroom like stub
structure is used in [10] for obtaining an isolation higher than 15 dB, while
polarization diversity is exploited in [11] for an isolation of more than 17.5
dB, also large number of vias are used for band rejection. In [13], an eight
element array with complementary split$-$ring resonators (CSRR) is detailed to
achieve 20 dB isolation in MIMO system. In another design, array elements are
placed at large distance in eight element MIMO array to obtain an isolation of
10 dB [14]. In [15], an eight element planar antenna is proposed for UWB-MIMO
applications with a complex structure on the bottom side of the elements and
large dimensions of the board. Narrow band polarization diversity antenna with
eight elements is proposed for fifth$-$generation (5G) application in [16].
Identical elements over isolated ground plane are used to obtain high
isolation in an eight port UWB-MIMO design [17], while polarization and
pattern diversity is investigated in [18] by deploying two different slots.
Open slot metal frame is used to design eight port UWB-MIMO antenna for 5G
communications [19]. Dual notch band, sharp rejection of narrow band, and
reconfigurability of the notch band using RF$-$MEMS has been explored on a
single UWB radiator and it has been proven that rejection quality is inversely
proportional to the rejection band [20, 21, 22].

All the aforementioned UWB-MIMO designs have tradeoffs between design
complexity, size, number of ports and bandwidth. The design reported in [3, 4,
5, 6, 7, 8, 9, 10] are suitable for two ports or four ports in planar
configuration and no guidelines are presented to extend the design. Also, if
the design is extended for a large number of ports, it becomes too large to be
deployed in practical scenarios. In most of the eight elements cases, the
design compactness exceeds 0.25 $\lambda$ [15, 17, 18], and has a narrow
bandwidth [13, 14, 16, 19]. Last but not the least, all the designs proposed
with the eight elements have not band rejection capabilities. Whereas, the
proposed design, when compared to current designs, have a clear advantage of
compactness and tunable capability to reject various bands with in the UWB
spectrum.

Moreover, conventional 2 × 2 or 4 × 4 UWB-MIMO antenna for future 5G Customer
Premises Equipment (CPE) may not meet high data rates as required by the 5G
technology. The current deployment of 5G technology in developed countries
(USA, Japan, and China) utilizes sub$-$6 GHz band i.e. LTE band 42 (3.4
GHz$-$3.6 GHz) and LTE band 43 (3.6 GHz$-$3.8 GHz). Besides, non-standalone
solutions with 4G as an anchor, deployed in Gulf Corporation Council (GCC)
countries for 5G communication operates on sub$-$6 GHz (3.5 GHz) band having
operational bandwidth of 50 MHz$-$100 MHz. To achieve higher data rates in the
5G technology, some 8 $\times$ 8 MIMO [23, 24, 25, 26], 8 × 8 UWB-MIMO [19],
and 10 × 10 MIMO [27] antennas have been proposed for the future 5G devices
and those can be utilized for the 5G CPE such as routers etc. However, the
planar configuration of the reported design along with the incapability to
provide necessary bandwidth for 4G and 3G communications having band rejection
feature may not be an appropriate solution for the forthcoming 5G technology
because of low footprint requirements. Likewise, four element reconfigurable
band reject UWB-MIMO antenna, having two elements in planar configuration and
two elements fixed at $\pm$ 45∘ is proposed in [28]. The proposed
configuration offers excellent band rejection, however, hardware complexity
and fixation of angularly placed elements are major hindrances of this design
for modern 4G/5G communication devices. In addition, a 3-D eight element UWB-
MIMO array is presented in [29], but without any band rejection capabilities.
The addition of band rejection structures in the existing 3D UWB-MIMO may
affect impedance, isolation, and radiation characteristics and result in
complex geometrical configurations [29].

Therefore, the objective and novelty of this work is; (a) utilize the space
provided for the antenna more efficiently by incorporating as many elements as
possible for future 5G technology, without increasing the size of the board,
by placing additional elements perpendicular to the board, (b) introduce the
WLAN band rejection capability in all elements without affecting the
performance of the other elements. Keeping in mind all of the above aspects,
an eight element UWB-MIMO/diversity antenna with WLAN band rejection
capabilities is proposed here. The band rejection capabilities obtained by
quarter wavelength stub on the ground plane can also be used to reject other
bands by modifying the length of the stub. Four monopoles exploiting the
polarization diversity amongst them are placed in a planar configuration on a
50 × 50 mm2 board and four additional monopoles are then adjusted
perpendicularly to the same board in order to obtain a compact size. The
orthogonal polarization from the closely spaced monopoles guarantees high
inter-element isolation.

The work of this project adds to the previous contributions [30, 31, 32, 33,
34, 30, 35, 36, 37, 9, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70,
71, 72, 73, 74, 75, 76, 77, 78] reported in the literature in new applications
and evolving technologies.

##  II Design Procedure

Initially, a wideband rectangular monopole with arc$-$shaped feeding section
and a partial ground plane was designed in 3D EM software, ANSYS HFSS. The
ground width ‘w’ is so chosen to obtain good impedance matching throughout the
entire frequency range. For further optimization, corners were truncated for
impedance enhancement at higher frequencies and impedance transformer was
attached to the tapered arc section for impedance enhancement at lower
frequencies. After achieving the impedance matching over the wide bandwidth (2
to 12 GHz), a stub was connected to the ground on the backside of the monopole
to reject the WLAN band. The length of the stub was calculated using
$L_{f}=\dfrac{\lambda_{g}}{4}=\dfrac{c}{4f_{\circ}{\sqrt{\epsilon_{r}}}}$
where $f_{\circ}~{}\approx$ 5 GHz, and $\epsilon_{r}$ is the relative
permittivity of the substrate. The length can also be varied to reject other
bands by changing $f_{\circ}$. The position of the stub to connect the ground
plane was chosen after carefully observations of surface currents on the
ground plane at 5.8 GHz and it was found that centre of the ground plane
beneath the transmission line is the most effective place for the stub
connection to draw the current on the stub.

Figure 1: (a) Perspective view of the proposed UWB-MIMO antenna with dimension
in mm. $L$ = 50, $W$ = 50, $w_{1}$ = 10, $l_{1}$ = 15, $l_{2}$ = 2.25, $l_{3}$
= 2, $l_{4}$ = 5, $l_{5}$ = 10, $g$ = 5, $w_{2}$ = 1.5, $l_{6}$ = 6, $l_{7}$ =
3.82, $c$ = 3.1, $l$ = 13.5, $w$ = 7, $L_{f}$ = 7.25, and $S_{g}$ = 0.5, (b)
perspective view of fabricated prototype with dimensions shown in Fig. 1, and
(c) photo during pattern measurement in fully calibrated anechoic chamber.

Figure 2: Simulated and measured reflection coefficient below $-$10 dB except
the rejected band, where values reach $-$1 dB almost at all input ports, (a)
simulated port 1 to 4, (b) simulated port 5 to 8, (c) measured port 1 to 4,
and (d) measured port 5 to 8.

After obtaining the band rejection capabilities for a single monopole, a
second one was placed orthogonally at 6.15 mm edge distance to exploit the
polarization diversity. The distance was chosen after parametric study and to
have space for the inclusion of more elements later on. The placement of
second monopole induced surface currents and affected the impedance match of
the second monopole at lower frequencies, which was improved by inserting a
U$-$shaped slot [3]. The polarization purity of element 1 is high while the
polarization of element 2 is low. This is because of the U-shaped slot which
decreased the power level of antenna element 2. The parameters of the slots
were optimized using
$L_{5}=\dfrac{\lambda_{g}}{4}=\dfrac{c}{4f_{\circ}{\sqrt{\epsilon_{r}}}}$ and
$g=\dfrac{\lambda_{g}}{8}=\dfrac{c}{8f_{\circ}{\sqrt{\epsilon_{r}}}}$ where
$f_{\circ}~{}\approx$ 3.60 GHz, and $\epsilon_{r}$ is the relative
permittivity of the substrate. Two more elements were then placed in planar
configuration on a 50 $\times$ 50 mm2 area. The placement of the monopoles was
numerically adjusted to obtain low mutual coupling and impedance matching over
the entire bandwidth. The edge distance between element 1 and 4 was kept 12 mm
so that the design can be placed at the edge of the PCB board and deliver high
isolation. After obtaining the desired performance of four monopoles in the
planar configuration, four more monopoles (labelled as 5, 6, 7 and 8 in Fig. 1
(a)) were employed perpendicularly, preserving the compactness of the design.
The polarization diversity was also achieved in these monopoles along with the
wideband impedance matching, low mutual coupling, and band rejection
capabilities. The dimensions of the proposed 3$-$D eight elements UWB-MIMO
array (total volume 50 × 50 × 25 mm3) are shown in Fig. 1 (a). In the proposed
design configuration, the separated ground plane is used to suppress the
surface current and mitigate the near-field coupling. Since antenna elements
are placed too closely it is necessary to disconnect the ground plane or use
any complex decoupling structure. In the proposed configuration, disconnected
ground plane was preferred, because four elements (elements 5 to 8) were
placed in vertical configuration.

Figure 3: Simulated and measured mutual couplings less than $-$17 dB amongst
all ports, (a) simulated port 1, (b) measured port 6, (c) measured port 1, and
(d) measured port 6.

Figure 4: (A) Simulated and measured radiation patterns for port 1 in the
principle planes, (a) x$-$z plane at 3.5 GHz, (b) y$-$z plane at 3.5 GHz, (c)
x$-$z plane at 5.8 GHz, (d) y$-$z plane at 5.8 GHz, (e) x$-$z plane at 9.8
GHz, and (f) y$-$z plane at 9.8 GHz. The patterns are nearly
omni$-$directional in the x$-$z plane, suitable for UWB-MIMO systems and (B)
simulated and measured radiation patterns for port 6 in the principle planes,
(a) z$-$y plane at 3.5 GHz, (b) x$-$y plane at 3.5 GHz, (c) z$-$y plane at 5.8
GHz, (d) x$-$y plane at 5.8 GHz, (e) z$-$y plane at 9.8 GHz, and (f) x$-$y
plane at 9.8 GHz. The patterns are nearly omni$-$directional in the z$-$y
plane, suitable for UWB-MIMO systems.

##  III Results and Discussion

###  III-A S$-$parameters

The layout shown in Fig. 1 (a) was printed on a FR4 board (thickness = 1.6 mm,
$\epsilon_{r}$ = 4.5 and tan$\delta$ = 0.02), as shown in Fig. 1 (b). The
prototype was then characterized using an Agilent N5242A PNA$-$X network
analyser. Fig. 2 (a) to (d) shows the simulated and measured reflection
coefficients at all input ports. The measured reflection coefficients were
lower than $-$10 dB at all ports except in the rejected band (4.85 to 6.35
GHz). Good agreement between simulated and measured results was found, with a
slight variation ($\pm$ 1.1 dB in most of the band) due to manufacturing
imperfections. The simulated and measured mutual couplings of the ports 1 and
6 are also plotted in Fig. 3. For measurement purposes, the ports 1 and 6 are
selected to realize the effect on the monopole in planar configuration and
perpendicular placement of the monopoles. It can be seen in Fig. 3 (a) to (d)
that both simulated and measured mutual couplings are not exceeding $-$17 dB
level, which is a significant achievement for having such a large number of
antennas in such a compact volume.

Figure 5: (a) Simulated and measured peak gain over the entire band, the gain
varies from 2.65 dBi to 5.8 dBi except the rejected band, where gain drops to
-3.6 dBi. (b) Simulated and measured efficiency over 86 $\%$ in the entire
band except the rejected band.

Figure 6: Simulated 3D radiation patterns showing pattern diversity in y-z
plane for port1 and 6, (a) only port 1 is excited at 4 GHz, (b) only port 6 is
excited at 4 GHz.

###  III-B Radiation Patterns and Peak Gain

The radiation patterns when feeding port 1 and 6 were measured at three
different frequencies (3.5, 5.8 and 9.8 GHz) in an anechoic chamber in their
respective principle planes and compared with the simulated patterns. During
the measurements, port 1 and 6 were excited one by one and all other ports
were terminated with a 50$-\Omega$ matched load. The results for port 1 and 6
are plotted in Fig. 4 (A) and Fig. 4 (B), respectively. At the lower
frequencies, the patterns are fairly dumbbell shaped in the y$-$z plane and
omni directional in the x$-$z plane for port 1, as shown in Fig. 4 (A)
(a)$-$(b). At 3.5 GHz, the agreement between simulated and measured results is
fair. In the rejection band, the antenna radiates with very low intensity and
gain in both planes, which is clearly visible in Fig. 4 (A) (c)$-$(d) at 5.8
GHz. At the higher frequencies, the pattern is slightly deviated from dumbbell
shaped in the y$-$z plane and the discrepancies in both planes are slightly
higher as compared to the lower frequencies due to more losses at higher
frequencies. This is visible in Fig. 4 (A) (e)$-$(f), when the patterns are
plotted at 9.8 GHz. As obvious from Fig. 1, monopole 1 is identical to
monopole 3 and monopole 2 to monopole 4, so a mirror transformation at 180∘ in
the radiation patterns in the respective planes is observed. Therefore, the
patterns of port 2 are also a mirror transform in the perpendicular plane from
port 1. Similarly, port 6 results (shown in Fig. 4 (B) (a)$-$(f)) exhibit
almost the same behaviour in the z$-$y plane (omni directional) and x$-$y
plane (dumbbell shape) as of port 1 in the x$-$z plane (omni directional) and
y$-$z plane (dumbbell shape), respectively.

The simulated and measured gain at port 1 and port 6 are plotted in Fig. 5
(a). There are slight variations in the measured and simulated values and also
from port 1 to port 6. Due to the identical monopoles, same values of gain are
observed at other ports. The peak gain varies from 2.65 dBi to 5.8 dBi over
the entire spectrum and it drops to $-$3.6 dB in the rejected band, showing
that antennas are rejecting the band. The simulated and measured efficiency at
port 1 and 6 is also plotted in Fig. 5 (b). It was observed that the
efficiency was more than 86 $\%$ in the entire band except the rejected band.

Figure 7: Simulated and measured radiation patterns of monopole pair 3$-$4 in
the x-y plane at 4 GHz showing strong un-correlation at some angles, useful
for diversity applications.

###  III-C Diversity Analysis

In MIMO systems, multipath effects can be mitigated, if different monopoles
have patterns diversity in the respective plane. In the proposed design,
monopole 1 has dumbbell shape (E$-$plane) radiation pattern in the y$-$z
plane, while monopole 6 has omni directional (H$-$plane) radiation pattern in
the y$-$z plane, as plotted in Fig 4 (A) and (B). This behavior can also be
seen in Fig. 6 (a) and (b), where simulated 3D radiation patterns are plotted
at 4 GHz for port 1 and 6, respectively. Similarly, to check the diversity
amongst other monopoles, x$-$y plane radiation patterns of the monopole pair
3$-$4 are plotted in the Fig. 7 at 4 GHz. It is shown that monopole 3 has more
radiation at 0∘ and 180∘ while monopole 4 has nulls in those directions,
however monopole 4 has more radiation at 90∘ and 270∘, while monopole 3 has
nulls in those directions. As a conclusion, these patterns are reasonably
uncorrelated which is very well suited for the diversity applications.

Figure 8: Computed envelope correlation coefficients from far-field radiation
pattern for isotropic (uniform), indoor and outdoor environments. The XPR
values used for indoor and outdoor environment are 5 dB and 1 dB,
respectively. The ECC values are less than 0.45 in all cases.

To further investigate, the diversity performance of the antenna is analysed
by computing the envelope correlation coefficient (ECC) from far$-$field
radiation patterns. For indoor and outdoor environments, the parameters
defined in [12] are used in (1) and ECC is numerically calculated. The
computed ECC from the far$-$field radiation patterns is shown in Fig. 8. It
can be observed from Fig. 8 that the computed values are less than 0.45 for
all cases over the entire band. For uniform scattering environments, the
values are less than 0.15.

$\rho_{e}=\frac{\left|\int_{0}^{2\pi}\int_{0}^{\pi}\left(F_{\theta(1,2),~{}\phi(1,2)}~{}\right)d\Omega\right|^{2}}{\int_{0}^{2\pi}\int_{0}^{\pi}\left(F_{\theta(1,2),~{}\phi(1,2)}~{}\right)d\Omega\times\int_{0}^{2\pi}\int_{0}^{\pi}\left(F_{\theta
2,~{}\phi 2}~{}\right)d\Omega}.$ (1)

Where,

$F_{\theta(1,2),~{}\varphi(1,2)}=XPR.E_{\theta 1}.E_{\theta
2}^{\ast}.P_{\theta}+E_{\varphi 1}.E_{\varphi 2}^{\ast}.P_{\varphi}$ (2)

and

$F_{\theta 2,~{}\varphi 2}=XPR.E_{\theta 2}.E_{\theta
2}^{\ast}.P_{\theta}+E_{\varphi 2}.E_{\varphi 2}^{\ast}.P_{\varphi}$ (3)

Figure 9: (a) Simulated and measured TARC computed from S-parameters. (b) CCL
values between element 1, 2 and 2, 7.

Figure 10: Surface current distribution at different frequencies. (a) exciting
port 1 only at 3.5 GHz, and (b) exciting port 6 only at 3.5 GHz, (c) exciting
port 1 only at 5.8 GHz, and (d) exciting port 6 only at 5.8 GHz, (e) exciting
port 1 only at 9.8 GHz, and (f) exciting port 6 only at 9.8 GHz. The plot
illustrates, that stub only draw current at 5.8 GHz to reject the band and
does not affect others band much.

Figure 11: Effects of the stub on the reflection coefficient. (a) Effect of
the gap variation between ground plane, the values are changed from 0.25 mm to
1.50 mm, the rejected band width can be controlled from 1 GHz to 2.6 GHz, (b)
effects of the length variation of the stub, the values are changed from 4.25
mm to 10.25 mm, the rejected band shifted towards higher frequencies and lower
frequencies, when length is decreased and increased, respectively.

Next, Total Active Reflection Coefficient (TARC) and the Channel Capacity Loss
(CCL) are computed using eq. (4) and (8) of [11]. For a desirable performance,
TARC should be less than 0 dB while CCL should be no more than 0.5 bits/s/Hz.
In the proposed design, the value of TARC was found to be less than -8 dB over
the entire band, as shown in Fig. 9 (a) and CCL less than 0.3 bits/s/Hz except
the reject band, as shown in Fig. 9 (b). Another important factor to evaluate
the performance of MIMO antenna is Mean effective gain (MEG). The ratio
obtained after calculation in different scenarios (such as isotropic, indoor
and outdoor) was found close to 1.

Furthermore, to further show the effect of the stub on each monopole, the
surface currents are plotted in Fig. 10 at different frequencies (3.5 GHz, 5.8
GHz, and 9.8 GHz) by exciting port 1 and 6. It can be noticed, when the
surface currents are plotted at 3.5 GHz in Fig. 10 (a) and (b), while exciting
port 1 and 6 individually, very little current is present on the stub, which
does not impact the band at this frequency. Whereas, when surface currents are
plotted at 5.8 GHz as shown in Fig. 10 (c) and (d), the current is present
with high intensity on the stub, which behaves like a LC stop band filter and
rejects the band. Similarly, when currents are plotted at 9.2 GHz for both
ports excitations one by one as shown in Fig. 10 (e) and (f), the stub does
not draw much current.

##  IV Parametric Analysis On Stub Length and Gap Variation

One major benefit of the proposed stub is that the bandwidth of the rejection
can be optimized according to the requirement by changing the gap ‘$S_{g}$’
between the stub and ground plane. The results of gap variations are plotted
in Fig. 11 (a). The gap variation changes the capacitance between stub and
ground plane, resulting in the change of the rejection bandwidth. At the
minimum gap 0.25 mm, the rejection bandwidth is 1 GHz (5.25 to 6.25 GHz),
while at 1.5 mm gap, the bandwidth is around 2.6 GHz (3.7 to 6.3 GHz). Also,
the rejection band can be tuned to higher or lower frequencies by changing the
length ‘lf’ of the stub as shown in Fig. 11 (b). The stub length is varied
from 4.25 mm to 10.25 mm and shift in the rejection band is clearly visible in
Fig. 11 (b).

TABLE I: Performance Comparison With Previous Literature

Published literature |  Total PCB size without height / No. of elements |  Bandwidth (GHz) |  Notch Band (GHz) |  S11 at notch (notch quality) |  Bandwidth control / Complexity |  Mutual coupling (dB) |  Gain Var-(dBi) / Notch band gain (dBi) |  ECC using far-field patterns
---|---|---|---|---|---|---|---|---
[6] Anitha et al. |  45 × 45 mm2 / 4 |  2.2$-$6.28 |  No |  N.A |  N.A |  $<-$14 |  4 / N.A |  $<$ 0.25 (uniform)
[7] Lin et al. |  110 × 114 mm2 / 4 |  2$-$6 |  No |  N.A |  N.A |  $<-$20 |  2.7 / N.A |  N.A
[9] Asif et al. |  50 × 39.8 mm2 |  2.7$-$12 |  4.8$-$6.2 |  $-$2 dB |  Not presented / complex design structure used to obtain notch |  $<-17$ |  4 / $-$3.8 |  N. A
[10] Keim et al. |  60 × 60 mm2 / 4 |  2.73$-$10.68 |  5.36$-$6.04 |  $-$1.8 dB |  Not presented /complex EBG structure |  $<-15$ |  5 / $-$3.7 |  N.A
[11] Wenjing et al. |  60 × 60 mm2 / 4 |  3.0$-$16.2 |  4.0$-$5.2 |  $-$1.4 dB |  Not presented /complex EBG structure with vias |  $<-17.5$ |  6 / $-$1 |  $<$0.3 (uniform)
[13] Sharawi et al. |  100 × 50 mm2 / 8 |  4.95$-$5.05 |  No |  N.A |  N.A |  $<-10.5$ |  0.8 / N.A |  N.A
[14] Al-Hadi et al. |  110 × 55 mm2 / 8 |  3.4$-$3.6 |  No |  N.A |  N.A |  $<-10$ |  N.A / N.A |  N.A
[15] Saleem et al. |  60 × 93 mm2 / 8 |  3$-$10.6 |  No |  N.A |  N.A |  $<-15$ |  N.A / N.A |  N.A
[16] Li et al. |  68 × 136 mm2 / 8 |  2.5$-$2.6 |  No |  N.A |  N.A |  $<-15$ |  0.7 / N.A |  0.2 (uniform)
[17] Sipal et al. |  38 × 90 mm2 / 8 |  3$-$15 |  No |  N.A |  N.A |  $<-20$ |  4.5 / N.A |  N.A
[18] Rohit et al. |  85 × 85 mm2 / 8 |  3$-$10.6 |  No |  N.A |  N.A |  $<-15$ |  4.8 / N.A |  0.2 (uniform)
[19] Xugang et al. |  75 × 150 mm2 / 8 |  3$-$3.6 |  No |  N.A |  N.A |  $<-11$ |  N.A / N.A |  0.1 (uniform)
[79] Palaniswamy et al. |  70 × 70 mm2 / 8 |  2.9$-$12 |  No |  N.A |  N.A |  $<-16$ |  N.A / N.A |  0.39
[80] Alsath et al. |  90 × 90 mm2 / 8 |  3.1$-$12 |  No |  N.A |  N.A |  $<-17$ |  3.2 / N.A |  0.16
Proposed antenna |  50 × 50 mm2 / 8 |  2$-$12 |  4.85$-$6.35 |  $-$1 dB |  Yes / simple LC stub |  $<-17$ |  3.15 / $-$3.6 |  $<$0.15 (uniform) $<$0.45 (indoor) $<$0.45 (outdoor)

In Table I, a comparison of the proposed antenna with several four element and
eight element MIMO antenna designs is presented. The list is not complete but
provides a reasonable understanding on the current work. Most of the eight
element MIMO designs are only for narrow band operations, some designs do not
provide band notch characteristics, some available designs provide band notch
characteristics but for four element design. The proposed UWB-MIMO antenna has
improved performance, when compared to all components of other antennas.

##  V Discussion

It can be observed from the literature review and comparison table that though
many MIMO antennas with orthogonal orientation exist with antennas placement
horizontally and vertically, however the proposed design has additional
benefits and novelty in terms of:

  * •

Capability to reject the WLAN band in 3-D configuration of MIMO antenna with
horizontal and vertical placement of antennas having orthogonal orientation.

  * •

The planar configuration of the proposed design i.e. 50 × 50 mm2 which is
considered compact as compared to designs available in literature. For
example, design presented in [79] has board size of 70 × 70 mm2, whereas board
size is 90 × 90 mm2 in [80]. It is worth mentioning here that usually common
ground plane antennas are preferred in MIMO/diversity applications, however in
some application such as biomedical imaging using 5G, the common ground plane
can be avoided to obtain the desired results but that depends only on the
specific applications. Many disconnected ground plane MIMO/diversity antennas
are commercially available [81] for practical application and provide best
possible outcome in certain environments where improved isolation is required
such as LTE/WiMax mobile terminals [82].

The practical application of the proposed UWB-MIMO in a 3D configuration with
disconnected ground plane can be for wireless communication in vehicular
networks and imaging radars. The use of disconnected ground plane have been
investigated for vehicular networks in [79], [83]. In addition, disconnected
ground plane UWB-MIMO antennas have been proposed for radar imaging
applications [84]. Separate ground planes in a MIMO antenna array provide
additional feature of compactness along with the suppression of near-field
coupling for closely placed antenna elements. In addition, separate ground
planes in a design removes the incorporation of complex decoupling structure
that creates fabrication and hardware complexity. Therefore, other than 5G
applications, the proposed design configuration can be used for communication
in vehicular networks and radar imaging where multiple antennas are required
in a limited confined space.

##  VI Conclusion

A compact eight port UWB-MIMO/diversity antenna having perpendicular placement
of antenna element to utilize the height of CPE as compared to the available
eight and ten port MIMO antennas is detailed in this report. Band rejection of
all monopoles is accomplished by trapping current on an LC stub that is
connected to the ground plane. Also, the proposed band-stop stub can be used
to control the bandwidth of the rejected band by changing the gap between the
stub and the ground plane, and can also shift the rejected frequency to upper
band by decreasing the stub length and to lower band by increasing the stub
length. Monopoles 1$-$4 are orthogonally placed on the same board to exploit
the polarization diversity for high isolation. Monopoles 5$-$8 are placed
orthogonally to the planar board (in between the monopoles 1-4), still
exploiting the polarization diversity. The entire antenna measures only 50 ×
50 mm2 with all eight elements in planar board. The results of the fabricated
prototype on FR4 laminate matches well with the simulated results of
reflection coefficient, mutual coupling, peak gain and radiation patterns over
the entire spectrum of 2 to 12 GHz. The simplicity, more elements in compact
size, and good performance of the proposed design makes it a very strong
candidate for small portable devices, vehicular network, vehicle to vehicle
communication, and imaging radar. The work of this project adds to the
previous contributions [30, 31, 32, 33, 34, 30, 35, 36, 37, 9, 38, 39, 40, 41,
42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78]
reported in the literature in new applications and evolving technologies.

## Acknowledgment

This work was supported in part by the EU H2020 Marie Skłodowska-Curie
Individual Fellowship (ViSionRF) under Grant 840854, by the MIUR (Italian
Minister for Education) under initiative “Department of Excellence” (law
223/2016), and by COMSATS University, Islamabad under COMSATS Research Grant
Program (Project $\\#$ 16-63/CGRP/CUI/ISB/18/847).

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|  M. S. Khan is currently running a small company related to RF projects. He
received his B.Sc. degree in electrical (telecom) engineering from COMSATS
University, Islamabad, Pakistan, in 2011\. Based on his achievement during his
B.Sc. degree, he was awarded EMMA WEST Exchange Scholarship for his B.Sc.
mobility Program. He received his Ph.D. degree in 2016 from University of
Padova, Italy. He was also recipient of a fully funded Ph.D. scholarship from
Cariparo Foundation which provides scholarship to top 15 candidates from all
over the world. During his Ph.D., he also spent 18 months at North Dakota
State University, USA as a visiting scholar. Dr. Saeed was Head of Department
of Electrical Engineering Department of Riphah International University,
Lahore Campus for one year (2015- 2016). From 2016 to 2017, he also worked as
a researcher (Post-doctorate fellowship) with University of Padova. His
current research interests include advanced techniques and technologies for
antenna design for medical applications, phased array for radar systems,
reconfigurable antennas for advance applications, UWB-MIMO antennas, and novel
material based antennas. During this short period of time, he has authored/co-
authored more than 65 peer-reviewed journal or conference papers (h-index 14,
more than 750 citations).
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|  A. Iftikhar (S’12–M’16) received his B.S degree in electrical engineering
from COMSATS University Islamabad (CUI), Pakistan, M.S degree in Personal
Mobile and Satellite Communication from University of Bradford U.K., and Ph.D.
degree in Electrical and Computer Engineering from North Dakota State
University (NDSU), USA in 2008, 2010, and 2016, respectively. He has authored
and co-authored 30 journals and conference publications. His current research
includes applied electromagnetic, reconfigurable antennas, leaky wave
antennas, phased array antennas, UWB-MIMO antennas, and energy harvesting for
low power devices. He is member of IEEE and currently an Assistant Professor
in Department of Electrical and Computer Engineering at COMSATS University
Islamabad (CUI), Pakistan.
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|  Raed M. Shubair (S’85–-M’93–-SM’01) is a Full Professor of Electrical
Engineering and Visiting Scientist affiliated with MIT Research Laboratory of
Electronics (RLE), Massachusetts Institute of Technology (MIT), USA and
Department of Electrical and Computer Engineering, New York University (NYU),
Abu Dhabi, UAE. Prof. Raed Shubair is also affiliated with Center of
Intelligent Antennas and Radio Systems, Univer-sity of Waterloo, Canada and
has been visiting at MIT Department of Brain and Cognitive Sciences and
Harvard Medical School, USA. Beside his academic positions at MIT, NYU, and
Waterloo, he serves since 2017 as a Senior Advisor in the Office of
Undersecretary for Academic Affairs of Higher Education, Ministry of
Education, Abu Dhabi, UAE. He has been a Full Professor of Electrical
Engineering at Khalifa University (formerly Etisalat University College), UAE,
which he joined in 1993 up to 2017. Prof. Raed Shubair received his B.Sc.
degree in Electrical Engineering (with Distinction and Class Honors) from
Kuwait University, Kuwait in June 1989 followed by his Ph.D. degree in
Electrical Engineering (with Distinction) from the University of Waterloo,
Canada in February 1993. His PhD thesis received the University of Waterloo
‘Distinguished Doctorate Dissertation Award’. Prof. Raed Shubair has several
research interests including Terahertz and Wireless Communications, Modern
Antennas and Applied Electromagnetics, Signal Processing and Machine Learning,
IoT and RF Localization, and Nano BioSensing. Prof. Raed Shubair was the first
faculty member in his university to be elevated to IEEE Senior Member grade in
2001 Prof. Raed Shubair has over 300 publications in the form of US patents,
book chapters, and papers in IEEE transactions and IEEE conference
proceedings. Prof. Raed Shubair received, several times since 1993, both the
‘University Teaching Excellence Award’ and ‘University Distinguished Service
Award’. He is also recipient of several international awards including the
‘Distinguished Service Award’ from ACES Society, USA and from MIT
Electromagnetics Academy, USA. Prof. Raed Shubair organized and chaired
numerous technical special sessions and tutorials in IEEE flagship
conferences. He delivered over 60 invited speaker seminars and technical talks
in world-class universities and flagship conferences. Prof. Raed Shubair is a
standing member of the editorial boards of several international journals and
serves regularly on the steering, organizing, and technical committees of IEEE
flagship conferences in Antennas, Communications, and Signal Processing
including several editions of IEEE AP-S/URSI, EuCAP, IEEE GloablSIP, IEEE
WCNC, and IEEE ICASSP. He has served as the TPC Chair of IEEE MMS2016 and TPC
Chair of IEEE GlobalSIP 2018 Symposium on 5G Satellite Networks. Prof. Raed
Shubair holds several leading roles in the international professional
engineering community. He is Board Member of the European School of Antennas,
Regional Director for IEEE Signal Processing Society in IEEE Region 8 Middle
East, and Chair of IEEE Antennas and Propagation Society Educational
Initiatives Program. Prof. Raed Shubair is Fellow of MIT Electromagnetics
Academy and Founding Member of MIT Scholars of the Emirates. Prof. Raed
Shubair is Editor of the IEEE Journal of Electromagnetics, RF, and Microwaves
in Medicine and Biology and Editor of IEEE Open Journal in Antennas and
Propagation. Prof. Raed Shubair is a Founding Member of five IEEE society
chapters in UAE which are IEEE-UAE Communication Society Chapter, IEEE-UAE
Signal Processing Society Chapter, IEEE-UAE Antennas $\&$ Propagation Society
Chapter, IEEE-UAE Microwave Theory and Techniques Society Chapter, IEEE-UAE
Engineering in Medicine and Biology Society Chapter. Prof. Raed Shubair is the
founder and counselor of the IEEE Student Branch at New York University Abu
Dhabi. Prof. Raed Shubair is a nominee for the Regional Director-at-Large of
the IEEE Signal Processing Society in IEEE Region 8 Europe, Africa, and Middle
East. Prof. Raed Shubair is also a nominee for the IEEE Distinguished Educator
Award of the IEEE Antennas and Propagation Society. Prof. Raed Shubair has
been honored and serves currently as an invited speaker with the prestigious
U.S. National Academies of Sciences, Engineering, and Medicine.
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|  Antonio-D. Capobianco was born in Padova, Italy in 1965. He received the
M.S. degree in electronic engineering from the University of Padova, in 1989
and the Ph.D. degree in electronic and communication engineering from Padova
University, Italy, in 1994. From 1998 to 2017, he was an Assistant Professor
with the Department of Information Engineering of Padova University. Since
2017, he has been an Associate Professor with the Department of Information
Engineering of Padova University. He is the author of more than 160 articles.
His research interests include silicon photonics, non-linear optics, nano
antennas, microwaves antennas, and plasma antennas.
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|  Benjamin D. Braaten (S’02–-M’09-–SM’14) received the B.S. degree in
electrical engineering in 2002, the M.S. degree in electrical engineering in
2005, and the Ph.D. degree in electrical and computer engineering in 2009, all
from the North Dakota State University, Fargo, ND, USA. During the 2009 Fall
semester, he held a Postdoctoral Research Position in the South Dakota School
of Mines and Technology, Rapid City, SD, USA. At the end of the 2009 Fall
Semester, he joined the Faculty of the Electrical and Computer Engineering
Department, North Dakota State University, and was promoted to Associate
Professor with tenure in 2015. He has authored or co-authored more than 100
peer reviewed journal and conference publications, several book chapters on
the design of antennas for radio frequency identification, and holds one U.S.
patent on wireless pacing of the human heart. His research interests include
printed antennas, conformal self-adapting antennas, microwave devices, topics
in EMC, topics in BIO EM, and methods in computational electromagnetics. Dr.
Braaten received the College of Engineering and Architecture Graduate
Researcher of the Year and College of Engineering and Architecture Graduate
Teacher of the Year awards. He also serves as an Associate Editor for the IEEE
ANTENNAS AND WIRELESS PROPAGATION LETTERS and is a member of the National
Honorary Mathematical Society PI MU EPSILON. Currently, Prof. Dr. Braaten is
chairman of ECE department at NDSU, Fargo, ND, USA.
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|  Dimitris E. Anagnostou (S’98–M’05-–SM’10) received the B.S.E.E. degree from
the Democritus University of Thrace, Greece, in 2000, and the M.S.E.E. and
Ph.D. degrees from the University of New Mexico, Albuquerque, NM, in 2002 and
2005, respectively. From 2005 to 2006, he was a Postdoctoral Fellow with the
Georgia Institute of Technology, Atlanta, GA. In 2007, he joined as Assistant
Professor the SD School of Mines $\&$ Technology, SD, USA, where he was
promoted to Associate Professor with tenure. In 2016, he joined the Heriot
Watt University, Institute of Signals, Sensors and Systems (ISSS), Edinburgh,
UK as an Associate Professor. He has also worked at the Kirtland AFB, NM in
2011 as an AFRL Summer Faculty Fellow, and at the Democritus Univ. of Thrace,
Greece as Assistant Professor. Dr. Anagnostou is currently supported by a
European H2020 Marie Skłodowska-Curie Individual Fellowship on wireless
sensing technologies. He has authored or coauthored more than 150 peer-
reviewed journal and conference publications (h-index 24, more than 2200
citations), one book chapter, and holds two U.S. patents on MEMS antennas and
on optically scannable QR-code ‘anti-counterfeiting’ antennas. His research
interests include: antennas (reconfigurable, adaptive, miniaturized and
electrically small, space/satellite, wearable), microwave circuits and
packaging, radar sensing, 5G arrays, functional phase-change materials such as
VO2 for reconfigurable electronics and metasurfaces, direct-write electronics
on organics, RF-MEMS, and applications of artificial neural networks, deep
learning and signal processing in electromagnetics, health care and assisted
living. Dr. Anagnostou serves or has served as Associate Editor for the
_TRANSACTIONS ON ANTENNAS AND PROPAGATION_ (2010-2016) and the _IET
MICROWAVES, ANTENNAS AND PROPAGATION_ (since 2015). He is Guest Editor He is
Guest Editor for _IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS_ (two Special
Clusters), and _MDPI Electronic_. He is a member of the _IEEE AP-S Educational
Committee_ , and of the Technical Program Committee (TPC) of the _IEEE AP-S_
and _EuCAP International Symposia_. He has received the 2010 _IEEE John D.
Kraus Antenna Award_ , the 2011 _DARPA Young Faculty Award_ by the U.S.
Department of Defense, the 2014 _Campus Star Award_ by the American Society
for Engineering Education (ASEE), the 2017 _Young Alumni Award_ by the
University of New Mexico, the _H2020 MSCA Fellowship_ , and four _Honored
Faculty Awards_ by SDSMT. His students have also been recognized with IEEE and
university awards (_Engineering Prize_ at Heriot Watt University, _Best PhD
Thesis_ at South Dakota School of Mines, and others). He is a member of HKN
Honor Society, ASEE, and of the Technical Chamber of Greece as a registered
Professional Engineer (PE).
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