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Comptes Rendus
Physique
Jorge Ruiz-García, Marco Faenzi, Adham Mahmoud, Mauro Ettorre,
Patrick Potier, Philippe Pouliguen, Ronan Sauleau and David
González-Ovejero
Multi-beam modulated metasurface antenna for 5G backhaul
applications at K-band
Volume 22, issue S1 (2021), p. 47-52
<https://doi.org/10.5802/crphys.62>
Part of the Special Issue:URSI-France 2020 Workshop
Guest editor: Joe Wiart (LTCI, Télécom Paris, Institut Polytechnique de Paris,
Institut Mines-Télécom, France)
© Académie des sciences, Paris and the authors, 2021.
Some rights reserved.
This article is licensed under the
Creative Commons Attribution 4.0 International License.
http://creativecommons.org/licenses/by/4.0/
Les Comptes Rendus. Physique sont membres du
Centre Mersenne pour l’édition scientifique ouverte
www.centre-mersenne.org
Comptes Rendus
Physique
2021, 22, n S1, p. 47-52
https://doi.org/10.5802/crphys.62
URSI-France 2020 Workshop / Journées URSI-France 2020
Multi-beam modulated metasurface antenna
for 5G backhaul applications at K-band
Jorge Ruiz-García ∗, a, Marco Faenzi a, AdhamMahmoud a,
Mauro Ettorre a, Patrick Potierb, Philippe Pouliguen c, Ronan Sauleau a
and David González-Ovejero a
a Univ. Rennes, CNRS, IETR - UMR 6164, F-35000, Rennes, France
b Direction Générale de l’Armement, 35174 Bruz, France
c Direction Générale de l’Armement, Agence de l’innovation de défense, 75509 Paris,
France
E-mails: jorge.ruiz@univ-rennes1.fr (J. Ruiz-García), faenzi@diism.unisi.it
(M. Faenzi), adham.mahmoud@univ-rennes1.fr (A. Mahmoud),
mauro.ettorre@univ-rennes1.fr (M. Ettorre), patrick.potier@dga.defense.gouv.fr
(P. Potier), philippe.pouliguen@dga.defense.gouv.fr (P. Pouliguen),
Ronan.Sauleau@univ-rennes1.fr (R. Sauleau),
david.gonzalez-ovejero@univ-rennes1.fr (D. González-Ovejero)
Abstract. We explore the use of a new modulated metasurface (MTS) antenna topology as solution for
wireless backhaul at K band. The proposed structure is composed of a quasi-optical beamformer, which
feeds the modulated MTS radiating aperture. These two elements are vertically stacked in a two-layer pillbox
architecture to produce a very compact antenna. Furthermore, our design is able to provide several beams
at different pointing angles and, hence, it offers the possibility of discrete beam steering by beam switching.
The employment of a modulated MTS and the compactness given by the pillbox approach lead to a high-gain
and low-profile antenna that could be an appealing solution for mobile backhaul networks.
Keywords. Metasurface antennas, Low profile, High gain, 5G, Backhaul, K-band, Leaky-wave antennas.
Available online 6th May 2021
1. Introduction
Microwaves and millimeter waves frequencies are among the frequency bands allocated for
small-cell backhaul in 5G networks [1]. The use of metasurfaces (MTSs) for 5G systems has been
studied and proved to be extremely useful in the aforementioned frequency ranges. Indeed, some
antenna prototypes based on MTSs have already been developed with the aim of satisfying the
needs of the new generation of mobile networks [2, 3]. Modulated MTS antennas, which were
initially developed for satellite communications, are a special category of MTS antennas [4]. The
∗Corresponding author.
ISSN (electronic) : 1878-1535 https://comptes-rendus.academie-sciences.fr/physique/
48 Jorge Ruiz-García et al.
Figure 1. Side view of the pillbox fed MTS antenna.
Figure 2. Unit-cell with square metallic patch and curve representing Xs (s) for different
patch sizes.
low profile, low cost and reduced power consumption of this kind of structures, along with the
adaptability of the design process to different frequencies, make them a very attractive solution.
A MTS is generally formed by sub-wavelength elements arranged on a periodic lattice and
either printed on a grounded dielectric slab or grown on a metallic base-plate. By changing the
geometry of these constitutive elements in the lattice unit-cell, one can exert a high degree of
control on the aperture fields [5–7]. The MTS layer can be modeled as a continuous impedance
boundary condition (IBC) due to the small size of the elements compared to the wavelength.
In modulated MTS antennas, a surface wave (SW) is excited on the aperture and gradually
transformed into a leaky-wave owing to its interaction with the periodically modulated IBC,
which results in a radiated beam [8]. By tuning the properties of the modulation one can control
the attributes of the beam, such as the pointing angle, shape, and polarization.
This paper presents a modulated MTS antenna operating at K-band with multibeam perfor-
mance. The system is based on a pillbox quasi-optical beamformer [9], which essentially trans-
forms the cylindrical wave propagating on the pillbox’s lower layer into a plane wave in the upper
layer. By adding a modulated MTS on the top layer (Figure 1), one obtains a compact antenna. A
pillbox-fed modulated MTS antenna at X-band was described in [10]. As mentioned before, the
entire design process can be adapted for other frequency bands by properly modifying the ma-
terial, the beamformer, and the MTS elements dimensions. Thus, in this work, we use a strategy
similar to that described in [10] to design an antenna operating at fo = 20.7 GHz.
C. R. Physique — 2021, 22, n S1, 47-52
Jorge Ruiz-García et al. 49
Figure 3. Top view of MTS antenna. Disposition of patches and ports in the pillbox beam-
former.
2. Design of a multi-beam modulated metasurface antenna
In the following, we will refer to the Cartesian reference system (x, y , z) shown in Figure 1. The
IBC used to model our MTS consists in a sheet transition [11, 12], penetrable [13] or transparent
impedance [14, 15] Zs = jXs which lies on top of a grounded dielectric slab. This structure
supports the propagation of a TM surface wave. In order to get the desired radiation effect, the
sheet transition impedance is modulated along x-direction as
Xs (x) = Xav(1+M(x)cos(2πx/p)), (1)
where Xav is the average reactance, M(x) is the modulation index (0 ≤ M(x) ≤ 1) and p is
the period. The chosen material is Rogers RO3006 (εr = 6.15, tanδ = 0.002) with thickness
h = 0.64 mm. In order to implement (1), we use square metallic patches whose size changes
according to the spatial variation of Xs (x). To that end, we first build a database that relates the
patch dimensions to the sheet transition IBC values. Taking a unit-cell (a single MTS element)
of side a on a substrate of thickness h, and assuming it inside a regular lattice to preserve the
local periodicity principle, one can vary progressively the metallic patch size s and extract the
equivalent sheet impedance Zs . Figure 2 shows the curve that relates both parameters as well
as an inset with the geometry of the unit-cell. The MTS element has a constant size a = λ0/7,
which makes it small compared to the SW wavelength, as indicated in Section 1. Once we
have characterized the unit-cell, the next design step consists in retrieving the square patch
dimensions that better match the ideal values in (1) to obtain our modulated MTS. The latter
is placed then on the beamfomer top layer to get the final structure depicted in Figure 3.
In addition, it is important to note that the use of a pillbox allows one to place several sources
(here, H-plane horns) in the bottom layer. These horns are arranged at the focal plane of the
pillbox’s reflector (Figure 3) and each one provides a beam pointing at a different direction.
Indeed, when the source position is shifted in the focal plane (along y-direction), the direction of
propagation of the resulting plane-wave changes. As a result, the placement of N ports originates
up to N beams at different pointing angles. This feature will be illustrated in the next section.
Obviously, the number of sources is limited by the pillbox dimension, the horn size, and the
HPBW of the radiated beams. The antenna dimensions are sketched in Figures 1 and 3, from
where we emphasize a very low profile, with a total thickness of 1.28 mm.
The main advantage of operating at a higher frequency with respect to the X-band model
in [10] is the reduction of the antenna dimensions. First, for the require bandwidth, it is possible
C. R. Physique — 2021, 22, n S1, 47-52
50 Jorge Ruiz-García et al.
Figure 4. S-parameters for central and side ports, remaining ports behave as their symmet-
ric ones.
to employ a thinner substrate that keeps providing the needed impedance range (Figure 2), which
enforces the low-profile faculty of the system. The total radiating aperture size is also decreased,
going up to almost a 50% smaller area than that in the X-band design. Another important aspect
is the diminution of the feeding horn width, since the cut-off frequency is now larger. This allows
one to set more sources along the pillbox focal plane and, hence, the number of beams on the
scanning region is increased. The latter point is commented and exploited in Section 3.
In order to overcome the losses growth due to the frequency rise, the length of the lower layer
is shortened (Figure 1). This is traduced into a minimization of the pillbox focal distance, while
ensuring the required performance for the side lobe level (SLL ≤ −15 dB) and the beam-width
at −3 dB (HPBW = 5°). Such a modification is realized by adjusting the horn dimensions and the
pillbox reflector diameter.
The proposed design is a good alternative to phased array antennas [16, 17]. In fact, the ab-
sence of phase-shifters makes our MTS antenna simpler, cheaper and less power-consuming.
Furthermore, the structure can be fabricated using PCB technology, which facilitates the manu-
facturing process and reduces the cost.
3. Simulation results
For the design at hand, we implement (1) to obtain a beam at θ0 = 15° for normal incidence (port
1, φ0 = 0°). The employed modulation parameters are p = 11.3 mm, Xav = −0.24η0 (where η0
is the free-space impedance), and M varying with x to optimize the attenuation of the aperture
fields and, therefore, enhance the aperture efficiency of the antenna [18]. Switching between the
N = 7 ports implies the modification of both θ0 and φ0. The simulated S-parameters of the horns
are shown in Figure 4, revealing a bandwidth of 20% and a very good isolation between ports.
Figure 5 presents the radiated beams for each source at fo . The obtained angular coverage
is displayed in Figure 5a, where the direction of the beam is given in (θ,φ) coordinates. Next,
we represent in Figure 5b the radiation pattern in elevation at the E-plane for every port. The
patterns are plotted by cutting every 3D beam at the angle φ=φ0, where φ0 is the azimuth angle
of maximum gain for each source. The maximum realized gain at fo is G = 27.1 dB for port 1 and
the beam-switching losses are up to 2.5 dB (realized gain difference between port 1 and ports 4, 7).
We note that the generated beams own the property of frequency scanning in θ. Thus, it would
C. R. Physique — 2021, 22, n S1, 47-52
Jorge Ruiz-García et al. 51
Figure 5. (a) Radiation performance in (θ,φ) plane, and (b) realized gain at the E-plane cut
for each horn.
be possible to operate at other frequencies around fo within the bandwidth shown in Figure 4,
modifying then the individual beam directions and the angular coverage.
The described model is under fabrication and a measuring experiment is planned. Besides,
the scanning area can be extended to the region θ < 0° (Figure 5a) by arranging an additional
reflector and horns opposed in the x-axis to the existing ones [19].
4. Conclusion
We presented the design and numerical results of a compact modulated MTS antenna at 20.7
GHz. The radiating aperture consists of metallic patches whose size is modified to produce an
equivalent modulated impedance. The MTS antenna is fed by a plane SW, which is obtained
by means of a quasi-optical beamformer in a pillbox architecture. Moreover, one can change
the propagation direction of this SW by exciting different ports in the pillbox focal plane. The
proposed antenna topology is able to generate high-gain beams at different pointing angles,
while providing a good scanning range in θ and a wide angular coverage in φ. The extremely thin
profile of the structure (htotal = 1.28 mm), the high-gain behavior, and the multi-beam capability
make this antenna a suitable candidate for backhaul applications. Last but not least, this flat
C. R. Physique — 2021, 22, n S1, 47-52
52 Jorge Ruiz-García et al.
aperture antenna can be easily integrated with a low visual impact on smart urban furniture,
buildings, homes, and offices.
Acknowledgements
This publication was supported by the European Union through the European Regional Develop-
ment Fund (ERDF), and by the French Region of Brittany, Ministry of Higher Education and Re-
search, Rennes Métropole and Conseil Départemental 35, through the CPER Project STIC & On-
des. It has also been supported by the Direction Générale de l’Armement (DGA) and by Britanny
Region under ARED program.
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Multi-beam modulated metasurface antenna for 5G backhaul applications at K-band

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