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Received December 22, 2020, accepted January 7, 2021, date of publication January 12, 2021, date of current version January 21, 2021.
Digital Object Identifier 10.1109/ACCESS.2021.3051066
Integration of Sub-6-GHz and mm-Wave Bands
With a Large Frequency Ratio for Future
5G MIMO Applications
MUHAMMAD ZADA 1, IZAZ ALI SHAH 1, AND HYOUNGSUK YOO 1,2, (Senior Member, IEEE)
1Department of Electronic Engineering, Hanyang University, Seoul 04763, Republic of Korea
2Department of Biomedical Engineering, Hanyang University, Seoul 04763, Republic of Korea
Corresponding author: Hyoungsuk Yoo (hsyoo@hanyang.ac.kr)
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology under Grant 2019R1A2C2004774.
ABSTRACT The integration of sub-6-GHz and millimeter-wave (mm-wave) bands has become an important
issue for future fifth generation (5G) wireless communications owing to their large frequency ratios. This
paper proposes a compact-size dual-function antenna operating at 3.5 GHz and the mm-wave band (28 GHz)
for 5G mobile applications using a frequency reconfigurability technique. The proposed antenna comprises a
microstrip patch linked with a meandered radiating structure through a radio frequency PIN diode to achieve
frequency reconfigurability between the two bands. A significant size reduction up to 15.3 mm ×7.2 mm ×
0.508 mm for the proposed antenna was achieved using a meandered line structure and truncated ground
plane. To enhance the functionality, 8 ×8 multiple-input multiple-output (MIMO) with possible long- and
short-edge antenna placement configurations were demonstrated. The system exhibited satisfactory MIMO
characteristics with wide decoupling −10 dB bandwidths of 7.4% and 4.8% at the low- and high-frequency
bands, respectively, without utilizing any external decoupling structure. The simulated results were validated
using fabricated prototypes, and good agreement was observed. Additionally, a safety analysis based on the
specific absorption rate and power density at the prescribed frequency bands was conducted using a realistic
human model, and the results were found to be in accordance with the safety guidelines. Owing to the
integration of sub-6-GHz and mm-wave bands in a single compact structure with a large frequency ratio and
good MIMO performance, the proposed antenna system is suitable for future 5G mobile handheld devices.
INDEX TERMS Frequency reconfigurability, MIMO, mm-wave, PIN diodes, power density, smartphones,
specific absorption rate, sub-6-GHz, truncated ground structure.
I. INTRODUCTION
Mobile communication is commonly used in daily life and is
one of the most active areas of social development. Owing
to the rapid growth of data and information, modern wireless
networks need a high-speed data rate with low latency. Fifth
generation (5G) for cellular and local area networks is fore-
seen to be a promising solution to overcome the limitations of
current communication technologies [1]. The Federal Com-
munications Commission has proposed the millimeter-wave
(mm-wave) spectrum as the operating frequency for 5G com-
munication, including 24, 28, 37–39, and 60 GHz [2]–[4].
The associate editor coordinating the review of this manuscript and
approving it for publication was Hassan Tariq Chattha .
However, the proposed spectrum involves challenges and
shortcomings regarding propagation, which could also affect
network deployment. Therefore, the prime 5G mid-band
(3.4–3.6 GHz) has been assigned by the ITU WRC-15 and
has already been deployed for the broadband cellular com-
munication system; this band can offer wider area coverage
with fewer propagation losses, while the mm-wave bands for
short-range indoor links are still in developing stages [5].
The prime 5G band with upcoming mm-wave bands (e.g.,
28 GHz) will be suitable for dense 5G small-cell networks
in urban areas where additional capacity is vital. These fre-
quency bands also suit macro-cells for wider area cover-
age. To support a large amount of traffic with a high data
rate, the fastest 5G services require approximately 100-MHz
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M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
bandwidth in the 5G mid-band (3.5 GHz), and 1-GHz band-
width at the mm-wave band (28 GHz). To meet these tar-
gets, several countries (including South Korea) have awarded
bandwidths of 80 and 800 MHz per operator in the 3.5- and
28-GHz bands, respectively [6].
To cover the sub-6-GHz and mm-wave bands, antennas
with multiband capability and a large frequency ratio are
of great importance for future 5G applications [7]. Signif-
icant research efforts have been made to design antennas
for sub-6-GHz 5G communication [8]–[16]; further, several
compact-sized mm-wave antennas have been proposed
[17]–[19]. Most reported studies used separate antennas for
the corresponding frequency bands. However, space limita-
tion in the handsets for several single-band antennas is a very
challenging task. Therefore, the integration of the sub-6-GHz
and mm-wave bands has attracted researchers’ interest in this
area. Future 5G technology demands compact-sized antennas
with the capability of frequency reconfiguration to cover
the allocated wireless frequency bands, e.g., sub-6-GHz and
mm-wave bands. Reconfigurable antennas are advantageous
over wideband and multiband antennas owing to their band
notching capability to avoid interference [20].
Recently, several frequency-reconfigurable antennas for
5G applications have been proposed [21]–[27]. In [21],
the author designed a pair of frequency-reconfigurable dipole
antennas backed by an artificial magnetic conductor sur-
face, covering the 3.3–3.6 GHz and 4.8–5.0 GHz bands
for possible 5G applications. However, the overall size of
the proposed structure was very large (72 mm ×72 mm
×3.5 mm) and was unable to achieve the 5G mm-wave bands.
A microfluidic-based frequency-reconfigurable antenna cov-
ering the 5G band (3.5 GHz) was presented in [22], where
piezoelectric pumps were used to realize wideband fre-
quency tuning. The integration of the applied techniques in
portable devices is significantly complex. Similarly, in [23],
a differentially fed antenna with frequency-reconfigurable
characteristics was developed for WLAN and sub-6-GHz
5G applications. The large size (100 mm ×100mm ×
2.5 mm), two-layered structure, and the number of switches
made the design inappropriate for smart handheld devices.
In [24], the authors proposed a low-profile slotted T-shaped
frequency-reconfigurable antenna intended for 5G wireless
networks. The slots were designed with two pairs of switches
to reconfigure the radiating structure at the desired fre-
quencies of 26.5 and 40 GHz. The reconfigurable antenna
designed in [25] exhibited four operating states (0.7, 2.4, 3.5,
and 5.5 GHz) using varactor diodes. However, the antenna
only covered the sub-6-GHz bands and had low gain val-
ues that were insufficient for satisfactory 5G communication
services. The reconfigurable antennas presented in the lit-
erature are single-antenna elements with the corresponding
limitations, and they are unable to cover both sub-6-GHz and
mmm-wave bands. However, in many countries including the
USA and South Korea, the prime 5G mid-band (3.5 GHz) and
mm-wave band (28 GHz) have been assigned for future 5G
communication.
The tremendous growth in 5G communication requires a
system with high throughput capability and efficient spec-
trum utilization [28]. Moreover, new services and standards
are continuously being added to wireless devices. It is chal-
lenging to design an antenna system that can support mul-
tiple wireless standards with the capability of high data
rate and efficient spectrum utilization. The utilization of
a multiple-input and multiple-output (MIMO) system with
frequency-agile capability is the best candidate to address
the aforementioned issues. Frequency-reconfigurable MIMO
antenna systems combine the advantages of high throughput
capability and several band coverages. To the best of the
authors’ knowledge regarding the MIMO antenna system
with frequency-reconfigurable features, the relevant studies
have not yet addressed both the 5G mid-band (3.5 GHz) and
mm-wave band (28 GHz) owing to the large frequency ratio.
Thus, a MIMO antenna system in the lower and higher bands
of the 5G spectrum is required for handheld devices to cover
the allocated bands by altering the switching state. Further,
the majority of the previous studies did not consider user
safety at 5G bands, e.g., by analyzing the specific absorption
rate (SAR) and power density (PD).
In this paper, a compact dual-band frequency-
reconfigurable antenna is proposed for future 5G hand-
held devices. A radio frequency (RF) PIN diode switch
is used to provide two working states; thus, the proposed
antenna can maximally cover 3.39–3.66 GHz (ON state) and
27.40–28.60 GHz (OFF state) for 5G applications. A sig-
nificant size reduction and bandwidth enhancement were
achieved using a meandered radiating patch and a truncated
ground structure. Moreover, to enhance channel capacity and
spectral efficiency for future 5G communication, 8×8 MIMO
with possible short- and long-edge antenna placement con-
figurations were formed. Owing to the meandered radiating
structure and defected ground plane, satisfactory decoupling
bandwidths were achieved for both configurations without
utilizing external decoupling structures. Prototypes of the
proposed proof-of-concept system were built, and the perfor-
mance was validated through measurements. The measured
results reasonably correlate with the simulations. For safety
concerns, SAR and PD analyses were conducted at low and
high frequencies, respectively. Both the SARs and PDs were
found to comply with the prescribed safety limits of the
Institute of Electrical and Electronics Engineering (IEEE)
and the International Commission on Non-Ionizing Radiation
Protection (ICNIRP). The proposed MIMO system offers
the advantages of a simple structure, reasonable gain, high
radiation efficiency, and frequency reconfiguration, making
it well-suited for low- and high-frequency 5G applications,
in contrast to the existing state-of-the-art MIMO antenna
system.
II. METHODOLOGY
A. DESIGN OF 8×8MIMO ANTENNA CONFIGU RATIONS
The geometry of the proposed 8 ×8 MIMO antenna sys-
tem with configurations A and B is illustrated in Fig. 1.
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M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
FIGURE 1. Geometry and dimensions of the proposed frequency-
reconfigurable 8 ×8 MIMO antenna system with short and long-edge
antenna configurations (Units: mm).
In configuration A, the eight-antenna elements (Ant-A1–Ant-
A8) are symmetrically arranged along the two short edges
of the substrate, while in configuration B, the antenna ele-
ments (Ant-B1–Ant-B8) are disposed along the long edges
of the system substrate. To obtain the minimum isolation,
13.2 and 41.7 mm separations were set for the two closest
antenna elements in configurations A and B, respectively. The
designed eight-antenna elements are printed on the Rogers
RT/duroid 5880 substrate with εrof 2.2, loss tangent (tanδ)
of 0.0009, and thickness of 0.508 mm. The type and thickness
of the substrate were selected to compensate for the losses
in the mm-wave band and fabrication capabilities. The stan-
dard dimensions of 162.6 mm ×77.7 mm, equivalent to the
Samsung Galaxy S10 5G were selected for the system sub-
strate. The radiating patches with the corresponding switch-
ing technique and ground planes are printed on the front and
back surfaces of the substrate, respectively. The antenna sys-
tem design and optimization were conducted using the finite
element method-based simulator ANSYS HFSS. Safety eval-
uations, such as SAR and PD analysis, were carried out using
a realistic human body model in the finite difference time
domain (FDTD)-based solvers Sim4LifeTM (Zurich MedTech
AG, Switzerland). The head effects on the radiation patterns
were evaluated using FDTD based Remcom.
B. DESIGN OF THE SINGLE-ANTENNA E LEMENT
The geometry of the proposed dual-band frequency-
reconfigurable single-antenna element, including its detailed
FIGURE 2. Detailed geometry and dimensions of the single antenna
element (Units: mm): (a) Top view with PIN diode boundaries. (b) Rear
view.
dimensions, is illustrated in Fig. 2. The antenna radiator
comprises two parts: a rectangular patch and a meandered
radiating structure connected through a PIN diode switch,
as shown in Fig. 2(a). The proposed single-antenna element
has a compact size of 15.3 mm ×7.2mm ×0.508 mm.
A truncated ground structure, as shown in Fig. 2(b), is used to
achieve miniaturization and wide bandwidth. The antenna is
fed through a 50-matched microstrip feed line with a width
of 1 mm.
By utilizing the reconfigurability property, the microstrip
antenna part is combined with the meandered part to form
a single structure to be operated at a sub-6-GHz (3.5 GHz).
The PIN diode is placed between the two parts to resonate
the antenna at 3.5 and 28 GHz by altering the switching
state. The DC blocking capacitors are placed between the two
parts to block the entry of DC biasing voltage into the RF
source. During the ON state of the PIN diode, the antenna
operates at 3.5 GHz; whereas, in the OFF state, it operates in
the mm-wave band at 28 GHz. The proposed eight-element
frequency-reconfigurable MIMO antenna was fabricated in
both configurations. The silicon PIN diode (BER90-22EL)
from Infineon is used in the proposed design owing to its
low insertion loss and fast switching time. In each con-
figuration, eight PIN diodes are soldered, accompanied by
biasing circuitries on the top surface of the substrate. The
biasing voltage to the PIN diode is provided through the
jumper wires. The equivalent circuits used in the simula-
tions for the corresponding ON and OFF states are shown
in Fig. 3. A very low resistance of 2.7 and an inductance
of 0.4 nH were considered for the PIN diode in the ON state.
To obtain a good correlation between the measurements and
simulation results, the PIN diode should provide the same
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M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
resistance and inductance in the measurements; thus, these
values were considered based on the technical datasheet of
BER90-22EL. Additionally, Fig. 3 provides details of the
biasing circuitry used in the measurements, which includes
RF chokes of 68 nH, DC biasing capacitors of 8.2 pF, and
biasing voltage of 3.3 V (applied to the DC bias terminal).
Further, in the simulation for the OFF state of the PIN diode,
the capacitance and reverse resistance of each diode were
assigned as 0.2 pF and 1.6 k, respectively, according to the
datasheet of BER90-22EL when biasing voltage of 0 V was
applied.
FIGURE 3. Equivalent circuits for the radio frequency PIN diode switch
between ON and OFF states: (a) Biasing circuit of the proposed antenna
used in the measurements. (b) Equivalent circuits used in the simulations
for the corresponding ON and OFF states.
The surface current distributions of the proposed antenna at
the corresponding frequencies are shown in Figs. 4(a) and (b).
These intuitively describe the working principle of the
antenna. In the ON state (Fig. 4(a)), the switch allows the cur-
rent to flow from the microstrip patch part to the meandered
structure that is responsible for resonating at 3.5 GHz. In the
OFF state, the switch limits the radiating structure to only the
microstrip patch part; thus, the current distribution is concen-
trated only on the microstrip patch, as shown in Fig. 4(b),
resulting in resonance at 28 GHz.
III. PERFORMANCE ANALYSIS AND DISCUSSIONS
The simulated reflection coefficients of the antenna elements
in configuration A and their corresponding isolations
at 3.5 and 28 GHz are shown in Figs. 5(a) and (b),
respectively. Meanwhile, Figs. 5(c) and (d) show the reflec-
tion coefficients and isolations for configuration B at the cor-
responding lower and higher frequency bands, respectively.
As depicted in Fig. 5, in both configurations, the proposed
MIMO antenna covers the targeted prime 5G 3.5 GHz band
(3.45–3.55 GHz) when the switch is turned ON, while in the
OFF state, the antenna operates at the mm-wave band 28 GHz
with a satisfactory bandwidth of 1060 MHz. All the antenna
elements in both configurations showed good impedance
matching. The reflection coefficients of some antenna ele-
ments deviated slightly at 3.5 GHz in configuration A owing
to their close placement; however, the achieved bandwidths
were still sufficient to cover the targeted band.
The proposed MIMO antenna system was also evaluated
for mutual coupling among various antenna elements in both
FIGURE 4. Current distributions on the radiating patch of a single
antenna element: (a) ON state (3.5 GHz). (b) OFF state (28 GHz).
configurations. Because of the similar structure and symmet-
rical placement of the antenna elements in each configuration,
only transmission coefficients S12–S18 (port-1 with each port)
and S23 (port-2 with port-3) were evaluated. Owing to the
meandered radiating structure and defected ground plane
(reduced ground effects), desirable isolation was achieved
even at the lower frequency band across the operational
bandwidth. As can be seen in Figs. 5(a) and (c), the isola-
tion of 14 and 24.5 dB was achieved at the prime 5G band
(3.5 GHz) between the closest antenna elements in configura-
tions A and B, respectively. Satisfactory isolation of 33.7 and
36.4 dB were acquired between the nearest antenna elements
at the mm-wave band (28 GHz) for configurations A and B,
respectively. The considerably improved isolation at 28 GHz
FIGURE 5. Simulated scattering parameters of the proposed 8 ×8 MIMO
antenna system in various configurations: (a) ON state and (b) OFF state
of PIN diode in configuration A. (c) ON state and (d) OFF state of PIN
diode in configuration B.
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M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
was due to the very short wavelength in the mm-wave band.
Although the isolation value between the closest antenna ele-
ments (Ant-A1 and -A2) at 3.5 GHz was comparatively small
owing to the short distance between the antenna elements,
the mutual coupling of the proposed MIMO antenna was still
better than that of several 5G antennas at sub-6-GHz and
mm-wave bands [9], [12], [15], [29].
To validate the performance of the proposed 5G MIMO
antenna system, the envelop correlation coefficient (ECC)
and diversity gain (DG) were evaluated. Acceptable perfor-
mance can be achieved when ECC <0.5 [28]. ECC can be
calculated via several methods, e.g., using received signal
envelopes, or by estimating the complex cross-correlation,
using S-parameters or radiation patterns [30]. In this work,
the simulated ECC was calculated from the radiation patterns
of the antenna elements based on the following equations:
ρij =|RR 4π
0[E
Fi(θ, φ)×E
Fj(θ, φ)d]|2
RR 4π
0|E
Fi(θ, φ)|2dRR 4π
0|E
Fj(θ, φ)|2d
(1)
DG =10p1−(ECC)2(2)
where ρij represents the ECC; E
Fi(θ,φ) and E
Fj(θ,φ) are the
radiation patterns of the ith and jth antenna elements, where
i, j =1,2,3,...,8. Fig. 6 shows the simulated ECC per-
formance of the proposed MIMO antenna system in both
configurations for the corresponding switching states. The
maximum ECC between the closest antenna elements was
observed at 3.5 GHz with values of 0.11 and 0.02 for con-
figuration A and B respectively, which were much lower
than the acceptable ECC value of 0.5. Another important
parameter for MIMO performance characterization is the DG,
which quantifies the deterioration in the radiation perfor-
mance of an antenna element using signal to noise ratio [31].
FIGURE 6. Simulated ECC performance between antenna elements in
various configurations: (a) ON state and (b) OFF state of PIN diode in
configuration A. (c) ON state and (d) OFF state of PIN diode in
configuration B.
The corresponding DG values were calculated using equa-
tion 2 and are plotted in Fig. 7. As can be seen from
Figs. 5 and 6, a good MIMO diversity performance was
observed based on the computed ECC and DG values for all
frequency bands in both configurations.
FIGURE 7. Simulated DG performance between antenna elements in
various configurations: (a) ON state and (b) OFF state of PIN diode in
configuration A. (c) ON state and (d) OFF state of PIN diode in
configuration B.
Figs. 8(a) and (b) show the three-dimensional radiation
patterns at both frequencies for configurations A and B. These
patterns were evaluated in realistic human head and hand
models in the FDTD-based simulator Remcom. As can be
seen from Fig. 8, the proposed MIMO antenna elements in
both configurations radiate outward. However, some of the
antenna elements are in close contact with fingers, which
slightly disturb the radiation behavior due to power absorp-
tion in hand tissues.
The important safety aspect regarding electromagnetic
(EM) fields from wireless terminal devices is their coupling
with the user’s head and hand. International bodies such as
the IEEE and ICNIRP have established safety guidelines to
limit human exposure to EM waves. These guidelines provide
basic restrictions in terms of (a) SAR to prevent tissue heating
for frequencies less than 10 GHz and (b) PD to prevent tissue
heating near the body surface for frequencies above 10 GHz
[32]–[34]. Thus, to ensure user safety, the peak 10-g averaged
SAR should not exceed 2 W/kg; whereas, the PD should
be less than 10 W/m2. A safety analysis of the proposed
MIMO antenna in close proximity to the hand and head was
conducted using the realistic human model Duke in Sim4Life.
Figs. 9(a) and (b) show the distributions of the 10-g average
SAR at the 3.5-GHz band for configurations A and B, respec-
tively. An input power of 24 dBm was set for each antenna
element because this has been used for the SAR calculation
of long-term evolution smartphones; however, for mm-wave
frequencies, the power standard has not yet been defined [29].
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M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
FIGURE 8. 3D radiation behavior of the proposed 8 ×8 MIMO antenna
elements at both the operating bands: (a) Configuration A.
(b) Configuration B.
FIGURE 9. Peak 10-g average SAR distributions of the proposed
8×8 MIMO antenna system at 3.5 GHz: (a) Configuration A.
(b) Configuration B.
As expected, the maximum peak SAR was observed at the
finger closest to the antenna element in each configuration,
as shown in Fig. 9; however, it was in compliance with
the safety limits in the ICNIRP guidelines. The peak 10-g
average SAR values for each port in both configurations are
summarized in Table 1.
To prevent tissue heating at higher frequencies (>10 GHz),
PD is currently preferred because of the difficulty in deter-
mining a reasonable averaging volume for SAR estimation
at very low penetration depth [35]. Considering a realistic
scenario, the distance between the MIMO antenna system
(ground surface) and the head surface was set to approxi-
mately 15 mm. The PD distributions for both configurations
FIGURE 10. PD distributions of the proposed 8 ×8 MIMO antenna system
at 28 GHz: (a) Configuration A. (b) Configuration B.
FIGURE 11. PD field strength along a 15-mm line: (a) Configuration A.
(b) Configuration B.
at 28 GHz are shown in Fig. 10. Moreover, Figs. 11(a) and (b)
show the spatial PD strengths along the line (15 mm) for con-
figurations A and B, respectively. The corresponding max-
imum peak PDs were 2500 and 2000 W/m2and decreased
with the distance, as depicted in Figs. 11(a) and (b) (zoomed
inset views). The PD exposure into the human head model
was estimated for every port and is listed in Table 1. The
maximum PD values in both configurations complied with
the prescribed limit of 10 W/m2at the head surface for an
input power of 24 dBm.
IV. MEASURED RESULTS
The 8 ×8 MIMO antenna prototypes in both configura-
tions were fabricated and measured to validate the sim-
ulation results. End launch connectors (50 ) from the
southwest microwave were used for measurement purposes.
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TABLE 1. SAR and PD Values from Different Ports at 3.5 and 28 GHz.
The fabricated MIMO antenna system with configurations
A and B are shown in Figs. 12(a) and (b), respectively.
The scattering parameters were measured using the E8364B
Network Analyzer from Agilent Technology. The measured
performances are discussed in the following subsections.
A. SCATTERING PARAMETERS
FIGURE 12. Fabricated prototypes of the proposed MIMO antenna system
in both configurations with an enlarged view of a single antenna element
with biasing circuitry.
The measured reflection coefficient and isolation curves
for both configurations are shown in Figs. 13(a)-(d) at the
corresponding low and high-frequency bands. Owing to the
symmetric structure and placement of the antenna elements
in each configuration, the reflection coefficient and isolation
measurement results are shown only for the five ports. In gen-
eral, the measured scattering performances of the proposed
MIMO antenna were in good agreement with the simulation
results. However, slight discrepancies were observed owing
FIGURE 13. Measured scattering parameters of the proposed 8 ×8 MIMO
antenna system in various configurations: (a) ON state and (b) OFF state
of PIN diode in configuration A. (c) ON state and (d) OFF state of PIN
diode in configuration B.
to fabrication tolerances and losses in the end launch con-
nectors, resistors, and DC bias lines. As shown in Fig. 13,
satisfactorily measured bandwidths for both configurations
are achieved, which are sufficiently wide to support the
desired 3.5- and 28-GHz bands. The measured bandwidths
at 3.5 and 28 GHz for configuration A were 270 MHz
(3.39–3.66 GHz) and 900 MHz (27.6–28.5 GHz), respec-
tively; whereas, configuration B exhibited 140 MHz (3.45–
3.59 GHz) and 1200 MHz (27.4–28.6 GHz) for the ON
and OFF states, respectively. Fig. 13 also demonstrates the
measured isolations between the antenna elements in both
configurations at the corresponding frequencies. Minimum
measured isolations of 16.4 and 36.4 dB were observed
between the closest antenna elements Ant-A1 and Ant-A2
(configuration A) at 3.5 and 28 GHz, which is promising
compared to the simulated isolation of 14 dB. The obtained
measured isolations in both configurations across the targeted
bandwidths are advantageous for attaining good diversity and
multiplexing performances.
B. MEASURED ECC AND DG
As aforementioned, the ECC can be calculated from
the far-field pattern data and S-parameters. Calculating
the ECC from the measured radiation pattern involves a
time-consuming integral calculation. To facilitate the process,
the measured ECC values for the proposed MIMO antenna
system were calculated based on the measured S-parameters
using the following equation [36].
ρij =|S∗
ii Sij +S∗
ji Sjj|2
(1 −(|Sii|2+ |Sji |2))(1 −(|Sjj|2+ |Sij |2)) (3)
The measured ECCs among the five antenna elements in
each configuration are shown in Figs. 14(a)–(d). A maxi-
mum ECC value of 0.02 was observed between the closest
antenna element in configuration A, which was far below the
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FIGURE 14. Calculated ECC from the measured results in various
configurations: (a) ON state and (b) OFF state of PIN diode in
configuration A. (c) ON state and (d) OFF state of PIN diode
in configuration B.
acceptable criterion (ECC <0.5). The measured ECC values
based on the S-parameters were smaller than the simulated
values because the proposed MIMO antenna system exhibited
a very high efficiency (>90%) in both frequency bands.
Based on the attained ECC values, the corresponding DG
values were evaluated and are plotted in Fig. 15. The highest
measured DG values of 9.97 and 9.99 dB were observed
for configuration B at 3.5 and 28 GHz, respectively. The
measured DG values correlate well with the simulated values
and are sufficiently good for MIMO performance.
FIGURE 15. Calculated DG based on the measured ECC: (a) ON state and
(b) OFF state of PIN diode in configuration A. (c) ON state and (d) OFF
state of PIN diode in configuration B.
A comparison of the simulated and measured radiation
patterns (co- and cross-polarization) for Ant 1 (other ele-
ment ports terminated with 50-matched loads) of both
FIGURE 16. Simulated and measured co-and cross-radiation patterns for
configurations A and B: Co- and cross-comparison in (a) azimuthal plane
at 3.5 GHz, (b) elevation plane at 3.5 GHz, (c) azimuthal plane at 28 GHz,
and (d) elevation plane at 28 GHz.
configurations are shown in Figs. 16(a)–(d). The radiation
patterns of the other antenna elements in both configura-
tions are omitted as they have similar patterns. Fig. 16(a)
shows the measured and simulated radiation patterns in the
azimuthal (E-plane) and elevation (H-plane) planes for co-
and cross-polarization at 3.5 GHz. The maximum radiation
occurred at θ=0◦in both configurations, while a difference
of more than 30 dB between co- and cross-components was
observed. A similar behavior was observed in the elevation
plane in Fig. 16(b) for both cases. The co-polarization in
the elevation plane is more than 30 dB over the cross-
polarization, which shows the EMC feature of the proposed
antenna. Figs. 16(c) and (d) show the 2D polar radiation pat-
tern in the E- and H-planes for both co- and cross-polarization
at 28 GHz, respectively. Directional radiation was observed
in both planes with a difference of more than 15 dB between
co- and cross-components. Additionally, in all cases, a rea-
sonable correlation was noted between the simulated and
measured results. A small distortion can be observed in the
radiation patterns at the high-frequency band because of the
current nulls, which increase with frequency. Moreover, these
small discrepancies may be due to the losses in the end
launch connectors and biasing circuity. The peak realized
gain and total efficiency for configurations A and B at the
FIGURE 17. Efficiency and realized gain of the proposed 8 ×8 MIMO
antenna system against the desired frequency bands in various
configurations: (a) ON state (3.5 GHz). (b) OFF state (28 GHz).
11248 VOLUME 9, 2021
M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
TABLE 2. Proposed frequency reconfigurable MIMO antenna comparison with state-of-the-art 5G antennas.
corresponding lower and higher frequency bands are shown
in Figs. 17(a) and (b), respectively. The results are shown
for Ant-A1 and -B1 only owing to the symmetrical place-
ments of antenna elements in each configuration. In the ON
state (3.5 GHz), the simulated and measured peak realized
gain values for Ant-A1 were 1.88 and 1.76 dBi, while for
Ant-B1, the corresponding values were 2.39 and 2.23 dBi,
respectively. In the OFF state (28 GHz), the measured peak
realized gain values for Ant-A1 and -B1 were 7.5 and 7.7 dBi,
respectively. The total simulated radiation efficiencies for
configurations A and B at 3.5 GHz were 92.39% and 93.66%,
while at the 28 GHz band, the corresponding values were
97.14% and 98.6%, respectively. Thus, the simulated and
measured results exhibit reasonable agreement. The small
difference is due to the loss from the PIN diode switch, SMA
connector, and coaxial line in the measurements.
C. COMPARISON WITH PRIOR WORKS
The proposed reconfigurable MIMO antenna system is com-
pared based on some key parameters with recently pub-
lished works in Table 2. It is apparent that by combining
the total volume required for sub-GHz and 5G (mm-
wave) antenna footprints, [16], [17] require a large antenna
profile making them less suitable for handheld devices.
Moreover, the frequency-reconfigurable antennas reported in
[21]–[23], [25] are single-antenna elements with consid-
erably large sizes, covering only sub-6-GHz 5G bands.
Because of the limited space in handheld devices, recon-
figurable antennas with large footprints are not suitable for
MIMO smartphone applications. In this study, the proposed
design successfully realizes the integration of sub-GHz and
mm-wave bands for the first time in one compact structure
with a size of only 15.3 mm ×7.2 mm ×0.508 mm using the
frequency reconfigurability technique. The proposed antenna
system exhibits satisfactory gain and efficiency values with
a considerably smaller size compared to existing antenna
systems. Moreover, the safety aspects of the reported designs
were not considered in previous studies.
V. CONCLUSION
In this paper, the integration of sub-6-GHz (3.5 GHz) and
mm-wave (28 GHz) bands using a frequency reconfigurabil-
ity technique was discussed for future 5G MIMO smartphone
applications. The proposed antenna comprises a microstrip
patch linked with a meandered line structure through a PIN
diode to achieve reconfigurability between the two frequency
bands by altering the switching state. The compact size for
the single antenna element was achieved using a meandered
line structure and truncated ground plane. The MIMO perfor-
mance of the proposed concept was demonstrated for short-
and long-edge antenna placement configurations. The system
exhibited satisfactory MIMO performance across the entire
bandwidth in both configurations. Moreover, a safety study
was conducted out in accordance with the IEEE and ICNIRP
safety guidelines. Based on the enhanced diversity and mul-
tiplexing performance, the proposed configurations fulfilled
the requirements of the MIMO antenna system at sub-6-GHz
and mm-wave bands; therefore, they are promising
candidates for future 5G wireless applications.
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MUHAMMAD ZADA received the B.Sc. degree
in telecommunication engineering from the Uni-
versity of Engineering and Technology, Peshawar,
Pakistan, in 2015. He is currently pursuing the
M.S./Ph.D. degrees in electronic engineering with
Hanyang University, Seoul, South Korea.
He has published over 14 articles in high-quality
journals and conferences proceedings in the field
of telecommunications and biomedical engineer-
ing. He is serving as a Reviewer for the IEEE
TRANSACTIONS, RFCAD, and Elsevier journals. His current research inter-
ests include implantable antennas and devices, intra-oral tongue drive
systems, wireless power transfer, millimeter-wave antennas, wearable sen-
sors and antennas, MRI and RF coils, microwave breast cancer detection,
frequency-selective surfaces, and EBGs. He was awarded the Best Student
Paper Competition 2018 from the Korean Institute of Electromagnetic
Engineering and Science (KIEES).
11250 VOLUME 9, 2021
M. Zada et al.: Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio
IZAZ ALI SHAH received the B.Sc. degree in
telecommunication engineering from the Univer-
sity of Engineering and Technology, Peshawar,
Pakistan, in 2016. He is currently pursuing the
M.S./Ph.D. degrees in electronic engineering with
Hanyang University, Seoul, South Korea. He has
coauthored seven journal articles and two con-
ference papers. His current research interests
include implantable antennas and devices, wireless
power transfer to electric vehicles and implantable
devices, MRI and RF coils, frequency selective surfaces, and EBGs.
Mr. Shah received fully funded scholarship awarded from the Prime
Minister’s National ICT Research and Development fund throughout the
bachelor’s degree program. He was awarded for the Best Student Paper Com-
petition from the Korean Institute of Electromagnetic Engineering (KIEES),
in 2018 and 2020. He is also serving as a Reviewer for the IEEE TRANSACTIONS
and Elsevier Journals.
HYOUNGSUK YOO (Senior Member, IEEE)
received the B.Sc. degree in electrical engineer-
ing from Kyungpook National University, Daegu,
South Korea, in 2003, and the M.Sc. and Ph.D.
degrees in electrical engineering from the Uni-
versity of Minnesota, Minneapolis, MN, USA,
in 2006 and 2009, respectively.
In 2009, he joined the Center for Magnetic Res-
onance Research, University of Minnesota, as a
Postdoctoral Associate. In 2010, he joined the
Cardiac Rhythm Disease Management, Medtronic, MN, USA, as a Senior
EM/MRI Scientist. From 2011 to 2018, he was an Associate Professor with
the Department of Biomedical Engineering, School of Electrical Engineer-
ing, University of Ulsan, Ulsan, South Korea. Since 2018, he has been an
Associate Professor with the Department of Biomedical Engineering and the
Department of Electronics Engineering, Hanyang University, Seoul, South
Korea. He has been the CEO of E2MR, a startup company, since 2017.
His current research interests include electromagnetic theory, numerical
methods in electromagnetics, metamaterials, antennas, implantable devices,
and magnetic resonance imaging in high-magnetic field systems.
Dr. Yoo was awarded Third Prize for the Best Student Paper from
the 2010 IEEE Microwave Theory and Techniques Society International
Microwave Symposium.
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