Multi-beam modulated metasurface antenna for 5G backhaul applications at K-band

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.


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 low prole, low cost and reduced power consumption of this kind of structures, along with the adaptability of the design process to dierent 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 elds [5,6,7]. 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 performance. The system is based on a pillbox quasi-optical beamformer [9], which essentially transforms 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 1a), 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 material, 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 f o = 20.7 GHz.

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 1a. The IBC used to model our MTS consists in a sheet transition [11,12], penetrable [13] or transparent impedance [14,15] Z s = jX s 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 eect, the sheet transition impedance is modulated along (1), we use square metallic patches whose size changes according to the spatial variation of X s (x). To that end, we rst 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 Z s . Figure 1b 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 = λ 0 /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 nal structure depicted in

Simulation Results
For the design at hand, we implement (1) to obtain a beam at θ 0 = 15 o for normal incidence (port 1, φ 0 = 0 o ). The employed modulation parameters are p = 11.3 mm, X av = −0.24η 0 (where η 0 is the free-space impedance), and M varying with x to optimize the attenuation of the aperture elds and, therefore, enhance the aperture eciency of the antenna [16]. Switching between the N = 7 ports implies the modication of both θ 0 and φ 0 . The simulated S-parameters of the horns are shown in Figure 2b, showing a bandwidth of 20% and a very good isolation between ports. Figure 3 presents the radiated beams for each source at f o . The obtained angular coverage is displayed in Figure 3a, where the direction of the beam is given in (θ, φ) coordinates. Next, we represent in Figure 3b 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 = φ 0 , where φ 0 is the azimuth angle of maximum gain for each source. The maximum realized gain at f o is G = 27.1 dB for port 1 and the beam-switching losses are up to 2.5dB (realized gain dierence between port 1 and ports 4, 7). We note that the generated beams own the property of frequency scanning in θ. Thus, it would be possible to operate at other frequencies around f o within the bandwidth shown in Figure 2b, modifying then the individual beam directions and the angular coverage.

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 modied 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 dierent ports in the pillbox focal plane. The proposed antenna topology is able to generate high-gain beams at dierent pointing angles, while providing a good scanning range in θ and a wide angular coverage in φ. The extremely thin prole of the structure (h total = 1.28mm), the high-gain behavior, and the multi-beam capability make this antenna a suitable candidate for backhaul applications. Last but not least, this at aperture antenna can be easily integrated with a low visual impact on smart urban furniture, buildings, homes, and oces.

Acknowledgments
This publication has been supported by the European Union through the European Regional Development Fund