Journal ArticleDOI

# A Cost-Effective Wideband Switched Beam Antenna System for a Small Cell Base Station

Petros Bantavis
08 Oct 2018-IEEE Transactions on Antennas and Propagation (Institute of Electrical and Electronics Engineers (IEEE))-Vol. 66, Iss: 12, pp 6851-6861

TL;DR: The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

AbstractA wideband switched beam antenna array system operating from 2 to 5 GHz is presented. It is comprised of a $4\times 1$ Vivaldi antenna elements and a $4\times 4$ Butler matrix beamformer driven by a digitally controlled double-pole four-throw RF switch. The Butler matrix is implemented on a multilayer structure, using 90° hybrid couplers and 45° phase shifters. For the design of the coupler and phase shifter, we propose a unified methodology applied, but not limited, to elliptically shaped geometries. The multilayer realization enables us to avoid microstrip crossing and supports wideband operation of the beamforming network. To realize the Butler matrix, we introduce a step-by-step and stage-by-stage design methodology that enables accurate balance of the output weights at the antenna ports to achieve a stable beamforming performance. In this paper, we use a Vivaldi antenna element in a linear four-element array, since such element supports wideband and wide-scan angle operation. A soft condition in the form of corrugations is implemented around the periphery of the array, in order to reduce the edge effects. This technique improved the gain, the sidelobes, and helped to obtain back radiation suppression. Finally, impedance loading was also utilized in the two edge elements of the array to improve the active impedance. The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

Topics: Vivaldi antenna (63%), Antenna (radio) (61%), Wideband (59%), RF front end (56%), Directional antenna (56%)

### Introduction

• The system is operating from 2 to 5 GHz providing more than one octave of usable bandwidth.
• A hexagonal shaped approach was adopted in [18] and a wideband Butler matrix was designed, implemented and measured.
• Finally, the conclusions of this work are summarized in section V.

### II. DESIGN METHODOLOGY

• A. 4× 4 Butler matrix design Butler matrix is one of the most widespread analogue beamforming networks.
• Personal use is permitted, but republication/redistribution requires IEEE permission.
• An indicative layout geometry of the technology is illustrated in Fig. 3(b) where the continuous and dashed lines represent different layers.
• The first step is to design the individual subnetworks, which are the hybrid coupler and the phase shifter of the Butler matrix.

### C. Butler Matrix Network Implementation and Measurements

• After the implementation and verification of the two key components of the Butler matrix, the procedure continues with the implemented network verification.
• Furthermore, the distance between the output ports is set to be same as for the designed linear Vivaldi array that will be described in section III.
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• To fully characterize the performance of the Butler matrix the phase difference between the output ports needs to be verified.
• Similar results for the magnitude and phase are verified for the remaining ports.

### III. LINEAR VIVALDI ARRAY DESIGN

• As an array element, an exponential tapered slot antenna (TSA) or Vivaldi [24] has been selected for its wideband and wide-scan performance.
• In this application only E-plane scan is utilized hence the motivation of the Vivaldi as a radiating element in this work.
• Personal use is permitted, but republication/redistribution requires IEEE permission.
• The edge elements were in turn matched to the new acquired impedance and tested to the Butler matrix output vectors.
• The simulated and measured active reflection coefficient of the array is depicted in Fig. 15.

### IV. SYSTEM ASSEMBLY AND PERFORMANCE

• At this point the two components have been successfully designed.
• In effect these cascaded S-parameters contain the active array impedance and the performance of the Butler matrix.
• Personal use is permitted, but republication/redistribution requires IEEE permission.
• The simulated and post processed measured results are in very good agreement.
• It is worth to note that the system is symmetric as was indicated from the measured and simulated beam patterns, see Fig. 17, and similar behavior is shown for the beams 1L and 2L.

### V. CONCLUSIONS

• The system is able to achieve four directional beams at all frequencies.
• The linear Vivaldi array offers improved wideband performance up to 6 GHz and can be utilized with a feed network in parallel with the proposed Butler matrix.
• Personal use is permitted, but republication/redistribution requires IEEE permission.
• This avoids any undesired microstrip cross sections and also improves the iterations of prototyping respectively.
• The authors have introduced a unified design methodology for such multilayer structures that can be expanded to any geometrical shape.

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A Cost-Eective Wideband Switched Beam Antenna
System for a Small Cell Base Station
Petros Bantavis, Christos I Kolitsidas, Tzihat Empliouk, Marc Le Roy, B.
Lars G. Jonsson, George. A Kyriacou
To cite this version:
Petros Bantavis, Christos I Kolitsidas, Tzihat Empliouk, Marc Le Roy, B. Lars G. Jonsson, et al..
A Cost-Eective Wideband Switched Beam Antenna System for a Small Cell Base Station. IEEE
Transactions on Antennas and Propagation, Institute of Electrical and Electronics Engineers, 2018,
66 (12), pp.1-4. �10.1109/SaPIW.2018.8401663�. �hal-02087695�

0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2874494, IEEE
Transactions on Antennas and Propagation
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. , NO. , 2018 1
A Cost-effective Wideband Switched Beam Antenna
System for a Small Cell Base Station
Petros I. Bantavis, Christos I. Kolitsidas, Member, IEEE, Tzihat Empliouk, Marc Le Roy, Member, IEEE,
B. L. G. Jonsson and George A. Kyriacou Senior Member, IEEE
Abstract—A wideband switched beam antenna array system
operating from 2 to 5 GHz is presented. It is comprised of
a 4 × 1 Vivaldi antenna elements and a 4 × 4 Butler matrix
beamformer driven by a digitally controlled DP4T RF switch. The
Butler matrix is implemented on a multilayer structure, using
90
hybrid couplers and 45
phase shifters. For the design of
the coupler and phase shifter we propose a uniﬁed methodology
applied, but not limited, to elliptically shaped geometries. The
multilayer realization enables us to avoid microstrip crossing
and supports wideband operation of the beamforming network.
To realize the Butler matrix we introduce a step by step
and stage by stage design methodology that enables accurate
balance of the output weights at the antenna ports to achieve
stable beamforming performance. In this work we use a Vivaldi
antenna element in a linear four element array, since such
element supports wideband and wide-scan angle operation. A
soft condition in the form of corrugations is implemented around
the periphery of the array, in order to reduce the edge effects.
This technique improved the gain, the side lobes and helped
was also utilized in the two edge elements of the array to improve
the active impedance. The proposed system of the Butler matrix
in conjunction with the constructed array can be utilized as a
common RF front end in a wideband air interface for a small
cell 5G application and beyond as it is capable to simultaneously
cover all the commercial bands from 2 to 5 GHz.
Index Terms—Wideband, Butler matrix, RF front end, 5G,
I. INTRODUCTION
M
ODERN wireless communications are driven by high
quality end user experience providing high data rates,
low latency and extended coverage. It is predicted that with the
massive deployment of the Internet of things (IoT), the number
of connected devices will increase exponentially. Network
densiﬁcation, [1], conjointly with heterogeneous networks
(HetNets) will be in the heart of future wireless networks offer-
ing increased network capacity and spectral aggregation, [2].
Additionally, exploiting spatial multiplexing we can further
increase the network capacity and offer higher signal-to-noise
Petros. I. Bantavis was with the Lab-STICC at the university ENSTA-
Bretagne in Brest, France. Now he is with institut d’ electronique
et de Telecommunications de Rennes, Universite de Rennes 1, e-mail
(petros.bantavis@univ-rennes1.fr).
Christos I. Kolitsidas and B. L. G. Jonsson are with with the Department of
Electromagnetic Engineering, School of Electrical Engineering, KTH Royal
Institute of Technology, SE-100 44, Sweden. e-mail: (chko@kth.se).
Tzihat Empliouk and Georgios. A. Kyriacou are with the Microwave Lab-
oratory, School of Electrical Engineering, Democritus University of Thrace,
Xanthi, Greece. e-mail: (gkyriac@ee.duth.gr).
Marc Le Roy is with Lab-STICC at Universit
´
e de Bretagne Occidentale
(UBO) in Brest, France. e-mail: (marc.leroy@univ-brest.fr)
controller
NETWORK 4x4
ARRAY 4x1
Fig. 1: Illustration of dividing data trafﬁc into macro and small
cells and the block diagram of the proposed system.
ratio (SNR) by focusing the RF power to the desired direction.
Small base stations, [3], are in the center of the emerging next
generation wireless networks playing an instrumental role to
the aforementioned demands. Small cell base stations due to
their foreseen massive deployment should also be very cost
effective and budget friendly devices.
Recent works, [4], [5], have shown the potential of switched
beam systems based on a Butler matrix, [6]. These works
have been developed for the 60 GHz ISM band that can
accommodate high data rates but with increased path loss. The
resulted beamforming in the 60 GHz band will be utilized to
electrically optimize the line of sight link. In [7], a 2 × 2
circularly polarized array with a Butler matrix capable to
operate in about 48% bandwidth was presented. Slomian et.al.
in [8] provided an array conﬁguration with a Butler matrix
capable for dual linear and dual circular polarization operating
in 5.2 - 5.4 GHz. In both approaches the bandwidth was
limited in less than 50% with severe impact on the second
approach. A narrow band Butler matrix integrated with a patch
antenna array is presented in [9]. In [10], wider bandwidth was
achieved, however only simulated results are presented.
A wideband switched beam antenna system that will act as
an RF air interface and will be software deﬁned is proposed.
Either a universal software radio peripheral (USRP) or a
micro-controller (µC) can be used to control electronically
the beamforming network. The USRP is proposed to evaluate
the whole system through lab experiments, however a micro-
controller is the best candidate for a massive implementation.
Integrating an analogue RF switched beam system with a
micro-controller offers the advantage of low cost and com-
plexity with spatial multiplexing capabilities. The proposed

0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2874494, IEEE
Transactions on Antennas and Propagation
2 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. , NO. , 2018
system presented in Fig. 2 is comprised of a 4 × 1 Vivaldi
linear array fed by 4 × 4 Butler matrix connected to Double
Pole Four Throw (DP4T) switch. The system will be controlled
by a µC and have a single transmit/receive chain. This set up
can offer full duplex capabilities. In this way the analogue
beamforming of the Butler matrix and the digital capabilities
of the µC are exploited resulting in a hybrid system. Since
only one µC is required for the proposed system with only
one full duplex channel, the cost is signiﬁcantly reduced. The
system is operating from 2 to 5 GHz providing more than one
octave of usable bandwidth.
Wideband beam forming networks have already been pro-
posed in literature as a base of switched beam systems and
they have been implemented as single layer or multilayer
structures. Single layer wideband Butler matrix structures have
been introduced in [11], [12] where tapered line directional
couplers and Schiffman phase shifters were used to implement
the Butler matrix. Wide bandwidth was achieved but with
the penalty of larger physical size. Several implementations
of Butler matrices have adopted the multilayer slot coupled
technology which was originally proposed in [13], [14] for the
implementation of the required 90
hybrids and phase shifters.
In [15], [16] this concept was extended into elliptically shaped
geometrical structure, whereas in [17] a variation of the
rectangular aperture slot coupler was presented that eliminated
the magnitude rippling on the transmission coefﬁcient. A
hexagonal shaped approach was adopted in [18] and a wide-
band Butler matrix was designed, implemented and measured.
In [19], a trapezoidal shaped geometry was introduced for
the implementation of a wideband Butler matrix and only
simulated results were presented for the corresponding beam
performance. The last two approaches used more a brute
force technique to select the corresponding geometries and no
theoretical motivation or explanation was adequately provided
for the design procedure of the different geometrical shapes.
Fig. 2: The hybrid switched beam system block diagram.
In our previous works, [20], [21], we have focused on
developing switched beam system for Ultra Wide Band (UWB)
applications. In [22] we have presented an early version of the
proposed system. Lower frequency bands and especially the
sub 6 GHz communication bands have received little to no
attention for small base stations as this requires challenging
wideband or multiband RF circuitry development and large
dimensions due to the corresponding wavelength. Due to the
required backwards compatibility with the current wireless
communication systems increased attention is expected to the
sub 6 GHz frequencies for small cell base stations. Next
generation wireless communication systems are expected to
extend the capabilities but continue the support of the current
TABLE I: Ideal complex excitation currents at the antenna
array elements.
BEAMS ANT1 ANT2 ANT3 ANT4
1L 10
1135
1 90
145
2L 10
145
190
1135
2R 10
1 45
1 90
1 135
1R 10
1 135
190
1 45
systems. An illustration of the application of small cell base
stations can be depicted as in Fig. 1, where small cells can be
deployed on buildings or high trafﬁc areas.
The originality of this work refers to the design, fabrication
and testing of an integrated wideband switched beam phased
array system. We present a system in the sub 6 GHz band with
a 4×4 Butler matrix and a 4×1 electrically connected Vivaldi
array. In section II, we introduce the design methodology and
we propose the uniﬁed theoretical approach on the design
of multilayer 90
wideband hybrids and phase shifters. This
approach was applied in the elliptically shaped geometry but
it can easily be extended in any other geometrical shape.
The overall Butler matrix design is also presented in this
section. The proposed Butler matrix is connected with a
linear 4 × 1 linear Vivaldi array that is introduced in section
condition at the array’s periphery are utilized to achieve the
wideband characteristics of such a small array. The overall
system performance is presented in section IV. Finally, the
conclusions of this work are summarized in section V.
II. DESIGN METHODOLOGY
A. 4 × 4 Butler matrix design
Butler matrix is one of the most widespread analogue
beamforming networks. It is composed of N inputs (excitation
ports) and N outputs (antennas ports) where N = 4. The
number of couplers is equal to (N/2) log
2
(N) and the number
of phase shifters to (N/2)[log
2
(N) 1], [6]. Exciting one of
its N inputs produces uniform amplitudes at the output ports
with a phase difference φ. This phase difference between
the output ports is different for every input port excitation and
at the same time it is the factor that steers the beam in the
desired direction in space. As a result of the uniform excitation
currents the radiation ﬁeld is of the form sin x/x. Exciting
the input ports of a N = 4 matrix separately, four orthogonal
beams are produced, so there is no signiﬁcant interference
between the consecutive beams. The operation of the Butler
matrix and the resulting ideal excitation power vectors can be
summarized in TABLE I.
In order to induce wideband characteristics the present But-
ler matrix consists of multilayer couplers and phase shifters.
Another advantage of the multilayer technology is that it
avoids undesired line crossings. Thus, a compact and wideband
beamforming network in the sub 6 GHz communication band
from 2 to 5 GHz as depicted in Fig. 2 is designed, fabricated
and tested. In Fig. 3(a) the schematic of a 4 × 4 Butler
matrix is depicted and it consists of four 90
hybrid couplers
and two 45
phase shifters. This approach is extended for
both the hybrid couplers and phase shifters, especially when

0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2874494, IEEE
Transactions on Antennas and Propagation
BANTAVIS et al.: A WIDEBAND SWITCHED BEAM SYSTEM FOR SMALL CELL APPS. 3
implemented with elliptical patches and slots. This multilayer
technology besides the wideband operation overcomes the
usual problem of interconnecting line crossings encountered
in printed microstrip Butler matrices. An indicative layout
geometry of the technology is illustrated in Fig. 3(b) where
the continuous and dashed lines represent different layers. It is
clear that no line crossing occurs in the same layer. In Fig. 3(b)
it is shown the corresponding physical implementation. In Fig.
3(c) the multilayer conﬁguration of the coupler is depicted,
where two elliptical patches are printed on the top and bottom
surfaces of two grounded substrates pressed back-to-back at
their common ground plane. An elliptical slot is etched on
their common ground plane which is oriented orthogonally
to the elliptical patches. To build the respective prototype the
top and bottom substrates are fabricated separately and then
pressed and glued together.
(a)
1L
2R 2L 1R
ANT2
ANT1
ANT3
ANT4
dw
ds
dl
dps
dpl
dpw
(b)
ds
dl
dw
(c)
Fig. 3: (a) Butler matrix block diagram with the continuous
and dashed lines deﬁning different layers (b) Top layered view
of the developed Butler matrix where the middle copper layer
is with brown color and the top bottom copper layers with
light dark and light yellow shades. The denoted dimensions in
mm: d
s
= 13.7, d
l
= 14.9, d
w
= 6.1, d
ps
= 13, d
pl
= 13.8,
d
pw
= 5.9. (c) 3D layered illustration of the hybrid coupler
and top perspective same as (b).
The ﬁnal layout implementation is shown in Fig. 3(b) where
the three distinctive layers are visible. The dimensions of the
coupler and phase shifter that are integrated in the Butler
matrix are shown in Fig. 3(b). A step by step and a stage
by stage design procedure is proposed for the Butler matrix
design. The design is divided in two steps and three stages
as indicated in Fig. 3(b). The ﬁrst step is to design the
individual subnetworks, which are the hybrid coupler and the
phase shifter of the Butler matrix. After the successful design
of the two individual components the coupler is connected
with the phase shifter resulting in a two port network. At
this stage the components are numerically optimized again to
achieve optimal cooperative performance. Next, at stage 2 the
Butler matrix is designed. The magnitude and phase behavior
of this network are evaluated and numerically optimized.
The design procedure continues with the second step, (see
Fig. 3(b)), where the equiphased transmission line network is
designed separately. This line network will align and order in
sequence the Butler matrix output ports to correspond to the
linear antenna array ports enabling their integration. Finally,
in the third stage the circuitry from the two previous stages
is integrated and the overall performance of the ﬁnal Butler
matrix is evaluated.
B. Subnetworks
The adopted technology for the hybrid couplers and phase
shifters consists of three conductive layers interleaved with two
dielectric layers as shown in Fig. 3(b). The top and bottom
layers are coupled through an aperture slot that is located
in the middle layer. The coupling phase and bandwidth are
controlled by the shape of the two conductive patches and
the slot. Rogers 4003C with thickness h=0.813 mm is used
for a substrate as a design material that provides a good
compromise between losses and cost for RF circuit. The phase
shifters adopted in this work are based on the same principle as
the hybrid couplers. However, they provide a constant phase-
shift versus frequency with respect to a corresponding uniform
transmission line.
In this work we propose a uniﬁed design methodology for
the 90
hybrid coupler and phase shifter based on [15], [16]
which in turn are based on the original work from [13] since
the designs share a similar geometrical cross section. The
coupling coefﬁcient can be calculated as:
C =
Z
e
Z
o
Z
e
+ Z
o
(1)
where Z
e
and Z
o
denote the even and odd mode character-
istic impedances. The feeding microstrip line characteristic
impedance is calculated using the geometrical mean Z
0
=
Z
e
Z
o
. The impedances Z
e
, Z
o
are derived from [13] using
the image theory as:
Z
e
=
60π
r
K(k
1
)
K
0
(k
1
)
(2)
Z
o
=
60π
r
K
0
(k
2
)
K(k
2
)
(3)
where K(k) is the elliptical integral of ﬁrst kind and K
0
(k)
its complementary given by K
0
(k) = K(
1 k
2
). This ratio
of elliptic integrals can be approximated either numerically or
using the formulas given in [13]. The elliptic modulus k
1
, k
2
are given based on the geometrical characteristics as follows:
k
1
=
s
sinh
2
(π
2
E
s
/(16h))
sinh
2
(π
2
E
s
/(16h)) + cosh
2
(π
2
E
w
/(16h))
(4)
k
2
= tanh(π
2
E
w
/(16h)) (5)
where E
s
{d
s
, d
ps
} and E
w
{d
w
, d
pw
} with respect to
the notation of Fig. 3(b) and Fig. 3(c). A π/4 multiplier has

0018-926X (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TAP.2018.2874494, IEEE
Transactions on Antennas and Propagation
4 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. , NO. , 2018
been added when compared to the original formulas from [23].
This multiplier indicates the area ratio between a rectangle,
A
rect
, with sides (a, b) and the corresponding inscribed ellipse
with area A
ell
, as A
ell/A
rect
= (πab/4)/(ab) = π/4. This
π/4 multiplier accounts for the ellipticity of the adopted
geometry. An additional step is required for the phase shifter.
Its corresponding phase shift when referenced to a microstrip
line is given in [16]:
φ = π/2arctan
"
sin(β
eff
d
pl
)
1 C
2
cos(β
eff
d
pl
)
#
+β
ms
l
ms
(6)
where β
eff
= β
0
r
, β
ms
is the corresponding microstrip
propagation constant, d
pl
= λ
ms
/4 = d
l
with λ
ms
the
effective microstrip wavelength and l
ms
the microsrip line
reference length. Equation (6) reveals that there are two
degrees of freedom to achieve the desired phase shift, the
coupling coefﬁcient and the l
ms
. Hence in this particular case
the coupling coefﬁcient C of the phase shifter is chosen based
on the corresponding length required from the layout. This
concludes the proposed uniﬁed approach for the design of the
elliptically shaped aperture coupled phased shifter and hybrid
coupler. Our proposed uniﬁed design methodology is not lim-
ited to elliptically shaped coupling geometries and can easily
be extended to other coupling geometries simply by modifying
accordingly the area ratio. Calculating such multiplier for any
other geometry such as diamond shaped or hexagonal can
provide a priori analytical estimates for the corresponding
physical dimensions. The wideband characteristics and the
small physical footprint of the coupler and the phase shifter are
attributed to the folded aperture coupling occurring in different
layers.
1) Prototypes Construction: For the construction process
silver epoxy was used in order to connect the two printed
circuit boards (PCB) layers. The ground metalization is re-
tained in both PCB layers to avoid via padding for the coaxial
connector and manufacturing ease. Only board routing is
required with this methodology. The ﬁrst step was to spread
this epoxy over the whole surface of one layer and then press
together the metalized layers (ground planes) mechanically.
During the pressing, the PCB is heated for 10 minutes at 100
C. This heating helps the epoxy to get similar conductivity
as the copper and the layers adhere effectively without air-
bubbles trapped in between.
2) Hybrid Coupler: The ﬁrst constructed subnetwork is the
90
hybrid coupler obtained from the overall design process
described above. Since the integrated design is evaluated the
deviation from the ideal results will indicate the difference
resulted from the interconnection of individually designed
components. Starting the equations (1)-(5) the initial ellipses
dimensions denoted in Fig. 3(b) (values in mm) d
s
= 8.1,
d
l
= 12.99 and d
w
= 6.1 are obtained. Ideally, exciting
port 1, as designated in the inset of Fig. 4 the coupler
equally divides the power into ports 2 and 3 with a 90
phase difference, while port 4 remains isolated. The simulation
and measurement results for the magnitude of the constructed
coupler are depicted in Fig. 4. An acceptable variation of
±0.75 dB from the ideal value of 3 dB is observed in the
2-5 GHz band of interest, whereas the reﬂection coefﬁcient
and the isolation are kept below -18 dB.
3) Phase Shifter: The other internal component of the
Butler matrix is the phase shifter. A 4 × 4 Butler requires
two phase shifters with φ = 45
as illustrated in Fig. 3(a).
A differential phase shifter is utilized which is based on the
same multilayer technology by open ending and removing the
transmission lines from ports 3 and 4 of the hybrid coupler.
Using equations (1) - (6) we obtain the initial values in mm
d
ps
= 8.92, d
pl
= 12.99 and d
pw
= 6.22. This phase shifter
is able to provide constant phase shift when referenced to a
corresponding transmission line. The topology of the Butler
matrix has this property inherently since a transmission line
is required to connect the two remaining ports (see Fig. 3(a))
of the ﬁrst stage of hybrid couplers. These initial dimensions
are optimized to achieve the required constant phase shift
throughout the bandwidth of the operation. The constructed
phase shifter and the referenced line are depicted in the inset
of of Fig. 5. Excellent agreement between simulation and
measurements of the designed subnetwork is illustrated in Fig.
5. Very low losses (< 1 dB) are also observed in the band of
interest.
In Fig. 6 the phase responses for both the coupler and
the phase shifter are illustrated. For the coupler the phase
difference between the output ports 2, 3 when port 1 is excited
is illustrated (inset of Fig. 4), whereas for the phase shifter
the achieved phase difference with respect of the reference
transmission line φ = S
21
S
34
is depicted in Fig.
5. Very good agreement between the simulation and the
measurements and the ideal value is observed for the sub-
band 2.5-4 GHz. For the low and the high end of the band,
higher but acceptable deviations up to 5
are observed. The
deviations can also be traced in the manufacturing process
of the subnetworks where we utilized a milling process for
this implementation. The milling process will not protect the
copper of the PCB of being contaminated with particles that
could potentially result in higher resistivity of the ground
connection. The milling process can also affect the thickness
of the substrate which can impact the phase. The ﬁnal Butler
matrix is constructed using etching process and a semi-clean
room environment to avoid the aforementioned issues.
C. Butler Matrix Network Implementation and Measurements
After the implementation and veriﬁcation of the two key
components of the Butler matrix, the procedure continues with
the implemented network veriﬁcation. An ideal Butler matrix
will equally divide the power from an input port and depending
on the activated port, four different sets of phase sequences
with φ = ±135
and ±45
are obtained corresponding to
four respective beams.
The ﬁnal constructed Butler matrix is depicted in Fig. 7
and has overall dimensions 150 mm× 110 mm. Emphasis has
been given to the outputs of the Butler matrix to be placed in
sequential order and thus be ready to connect with the antenna
array. Furthermore, the distance between the output ports is set
to be same as for the designed linear Vivaldi array that will be
described in section III. The latter step has been taken so that

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, Hui Liu1, Wang He1
TL;DR: A low-profile dual-band dual-polarized antenna with an artificial magnetic conductor (AMC) reflector that can be used alone for 5G indoor base station, or as the element of an array for5G outdoor base station is proposed.
Abstract: A low-profile dual-band dual-polarized antenna with an artificial magnetic conductor (AMC) reflector is proposed for 5G communications. The antenna consists of a pair of crossed dual-polarized dual-band bowtie dipoles and a dual-band AMC reflector. By introducing trapezoidal slots and U-shaped slots on the bowtie dipoles, miniaturization and dual-band characteristics are achieved. Moreover, T-shaped feeding structures are utilized to broaden the bandwidth of the bowtie dipoles. By adopting a dual-band AMC reflector instead of a conventional perfect electric conductor (PEC) reflector, the distance between the radiator and the reflector can be reduced from $0.25\lambda _{0}$ to $0.08\lambda _{0}$ (where $\lambda _{0}$ is the free-space wavelength at 3.5 GHz). The radiator can maintain the impedance bandwidth and a high gain can also be achieved, even if it is close to the AMC reflector. A geometrical optics model is used to explain the mechanism of the AMC. The optimal parameters of the AMC depend on the antenna operating frequency and the distance between the radiator and the reflector. Measurements show that the proposed dual-band antenna has an impedance bandwidth of 19.8% (3.14-3.83 GHz) and 13.2% (4.40-5.02 GHz), covering the sub-6 GHz frequency spectra of 5G mobile communications. The peak gain is 7.1 dBi in the lower band and 8.2 dBi in the upper band. Port isolation better than 20 dB is achieved. The proposed antenna can be used alone for 5G indoor base station, or as the element of an array for 5G outdoor base station.

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Journal ArticleDOI
Abstract: A wideband and low-profile cross-slot pattern reconfigurable antenna for electromagnetic imaging systems is proposed. The antenna is designed to cover the human chest area with steerable unidirectional radiation patterns at 0.7–0.9 GHz. The antenna is comprised of a corrugated cross-slot as the main radiator, and four parasitic slots with four p–i–n diodes operating as reflectors. An embedded feeding network with six p–i–n diodes is used to feed the main slot and switch the feeding path to modify the direction of the radiated beam (vertical/horizontal). In addition, an inductive mesh-grid surface is used to reduce the back lobe and enhance the operating frequency bandwidth. The antenna dimensions are ${0.8} {\lambda }_{0} {\times 0.} {8} {\lambda }_{0} {\times 0.18} {\lambda }_{0}$ (where ${\lambda }_{0}$ is the wavelength of the lowest frequency operation). By turning the parasitic slot p–i–n diodes to ON/OFF modes, the main beam can be steered along five distinctive positions at 0°, and at ±30° in two perpendicular planes. The antenna achieves a peak gain of 9 dBi at 0.8 GHz with 1 dB gain variations over the band of operation with a peak front-to-back ratio of 20 dB.

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### Cites background from "A Cost-Effective Wideband Switched ..."

• ...See https://www.ieee.org/publications/rights/index.html for more information. techniques include the use of Rotman Lens [27], Parallel plate lens [28], Butler Matrix [29], Luneburg lens [30], Phased array antenna [31], [32], and Parasitic controllable elements [33], [34]....

[...]

• ...plate lens [28], Butler Matrix [29], Luneburg lens [30], Phased array antenna [31], [32], and Parasitic controllable...

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Journal ArticleDOI
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TL;DR: An interfering angle based method for the joint resource (channel and transmit power) allocation problem to the mobile and fixed GAA users is proposed and results show improved capacity from the proposed method while satisfying a predetermined interference constraint.
Abstract: Spectrum access system (SAS) is a spectrum sharing framework proposed to share the spectrum between the incumbent users and the citizen broadband radio service devices, i.e. Priority access users and general authorized access (GAA) users. In this paper, we propose an interfering angle based method for the joint resource (channel and transmit power) allocation problem to the mobile and fixed GAA users. With mobile GAA users, the set of GAA users that can hear each other will change at different time instants making the resource allocation problem more challenging. The resource allocation of fixed and mobile GAA users is done considering coexistence with priority users, as well as coexistence between mobile and fixed GAA users. For the conflict-free resource allocation to fixed and mobile GAA users, we propose to use the maximum allowed transmit power for the beams of fixed GAA users that lie within the interference range of mobile GAA users. The simulation results show improved capacity from our proposed method while satisfying a predetermined interference constraint.

3 citations

Journal ArticleDOI
TL;DR: A wideband beam-switching metasurface antenna using programmable unit-cells is proposed for electromagnetic torso scanning and is successfully tested on altering the intensity of the electric field at right, center and left sides of a torso phantom.
Abstract: A wideband beam-switching metasurface antenna using programmable unit-cells is proposed for electromagnetic torso scanning. The design aims at changing the intensity of the electric field inside the torso without any mechanical movements and thus enables fast electronic scanning of the torso. The antenna consists of an H-shape microstrip-fed slot as the radiator and a metasurface layer containing 5 × 5 programmable square ring resonator as the superstrate layer. Four PIN diodes are embedded in each cell to alter the electric field intensity within the metasurface layer and consequently switch the radiation pattern in the azimuth plane, elevation plane, and diagonal axis of the metasurface layer. As a proof of concept, a prototype antenna capable of switching the radiation pattern from -25° to +25° in the azimuth (x-z) plane is fabricated and measured. The antenna, which has the compact size of 0.9λ 0 ×0.9λ 0 ×0.06λ 0 (where λ 0 is the wavelength at the center operation frequency), achieves a wide bandwidth of 30% at 0.9-1.2 GHz. The peak measured gain is 9.5 dBi with maximum front to back ratio of 12 dB. The fabricated antenna is successfully tested on altering the intensity of the electric field at right, center and left sides of a torso phantom.

3 citations

### Cites background from "A Cost-Effective Wideband Switched ..."

• ...Traditional beam switching techniques such as beamforming networks [19], [20], lens structures [21], [22] and phased arrays [23], [24], are effective....

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Journal ArticleDOI
TL;DR: Several techniques for hybrid coupler to achieve the required bandwidth and size reduction are highlighted, such as the T‐shape, meander line, two sections, three‐section, and parallel couple lines.

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TL;DR: This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue.
Abstract: What will 5G be? What it will not be is an incremental advance on 4G. The previous four generations of cellular technology have each been a major paradigm shift that has broken backward compatibility. Indeed, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities, and unprecedented numbers of antennas. However, unlike the previous four generations, it will also be highly integrative: tying any new 5G air interface and spectrum together with LTE and WiFi to provide universal high-rate coverage and a seamless user experience. To support this, the core network will also have to reach unprecedented levels of flexibility and intelligence, spectrum regulation will need to be rethought and improved, and energy and cost efficiencies will become even more critical considerations. This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue.

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### "A Cost-Effective Wideband Switched ..." refers background in this paper

• ...Network densification [1] conjointly with heterogeneous networks will be in the heart of future wireless networks offering increased network capacity and spectral aggregation [2]....

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Journal ArticleDOI
, Junyi Li1
TL;DR: This article explores network densification as the key mechanism for wireless evolution over the next decade if it is complemented by backhaul densification, and advanced receivers capable of interference cancellation.
Abstract: This article explores network densification as the key mechanism for wireless evolution over the next decade. Network densification includes densification over space (e.g, dense deployment of small cells) and frequency (utilizing larger portions of radio spectrum in diverse bands). Large-scale cost-effective spatial densification is facilitated by self-organizing networks and intercell interference management. Full benefits of network densification can be realized only if it is complemented by backhaul densification, and advanced receivers capable of interference cancellation.

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### "A Cost-Effective Wideband Switched ..." refers background in this paper

• ...Network densification [1] conjointly with heterogeneous networks will be in the heart of future wireless networks offering increased network capacity and spectral aggregation [2]....

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714 citations

### "A Cost-Effective Wideband Switched ..." refers background in this paper

• ...Recent works [4], [5] have shown the potential of switched beam systems based on a Butler matrix [6]....

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• ...The number of couplers is equal to (N/2) log2(N) and the number of phase shifters equal to (N/2)[log2(N)−1] [6]....

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Abstract: A transversely corrugated surface as used in corrugated horn antennas represents a soft boundary. A hard boundary is made by using longitudinal corrugations filled with dielectric material. The concept of soft and hard surfaces is treated in detail, considering different geometries. It is shown that both the hard and soft boundaries have the advantage of a polarization-independent reflection coefficient for geometrical optics ray fields, so that a circularly polarized wave is circularly polarized in the same sense after reflection. The hard boundary can be used to obtain strong radiation fields along a surface for any polarization, whereas the soft boundary makes the fields radiated along the surface zero. >

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### "A Cost-Effective Wideband Switched ..." refers methods in this paper

• ...characteristics, we employ a soft condition in the surrounding area of the array [25]....

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Abstract: A technique to compensate for mutual coupling in a small array is developed and experimentally verified. Mathematically, the compensation consists of a matrix multiplication performed on the received-signal vector. This, in effect, restores the signals as received by the isolated elements in the absence of mutual coupling. This technique is most practical for digital beamforming antennas where the matrix operation can be readily implemented. >

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