# Dual-Band Single-Layer Fractal Frequency Selective Surface for 5G Applications
## 1 Introduction  
Fifth Generation (5G) cellular communications system is all the rage at the moment; it is planned to be implemented by every country and major telecommunication operator. Frequency allocation to such a system was carried out, grouping all available bands into two groups: FRI involving all sub-6 GHz bands, and FR2 for millimeter-wave bands [1, 2, 3, 4, 5]. The technology used for this deployment is mature, and auctions have been organized all around the world in order to assign spectrum usage to different 5G operators [6, 7]. This expected-to-be-fast development of the new technology and subsequent rollout seem to have slowed down due to the economic effects of the COVID-19 pandemic on both society and businesses. However, the lockdown confinement periods due to this same reason boosted the social valuation of communication networks and increased the importance of accessing fast and trustworthy services. In such dichotomy, 5G was born as a support for the new 'online way of living' but limited by face-to-face restrictions: operators have to install new facilities with equipment that someone had to manufacture, and commercials have limited access to their future clients. As with previous transitions between successive communication standards, the move from 4G (Fourth Generation) to 5G will not be easy. The spectrum requirements of the new technology are large, and the refarming of spectrum (re)allocation had been carried out, removing obsolete analogue systems and liberating the bands occupied to be used by new deployments [8]. This harmonization seems to be a must for the operators, as having access to low frequency bands (i.e., sub-1 GHz) would be used as capital to provide widespread coverage in urban and rural areas. Mid-bands (3.5 GHz, but also refarmed spectra by operators at 1800 MHz or 2 and some GHz) would maintain a good balance between capacity and coverage for many end user applications. Both are within the FR1 spectrum portion. The use of these new bands carries the transmission of some old issues to frequencies not previously considered. Solutions to those bands should be adopted by tuning the forgoing proposals or by designing new ones. This is the case with isolation or site shielding applications: they are needed for several situations, from mitigating the interference between adjacent access points or providing cybersecurity protection, to allowing calm and peaceful spaces (in hospitals, commercial centers or mindfulness places such as churches or meditation areas). For such requirements, frequency selective surfaces (FSS) arise as a possible solution [9; 10]. Such an FSS performs as a distributed electromagnetic filter, providing a frequency response that can be tuned, and then adapted to the frequency allocation of the wireless system wanted to be impacted [9; 10]. It can act as a band-pass or band-stop filter for radio waves propagating across the surface itself. As the FR1 spectrum is the one which will be used by operators in the first 5G deployments, this paper focuses its design efforts on these lower 5G frequency bands, particularly at 700 MHz and 3.5 GHz. The proposal is to design an FSS suitable for mitigating and even blocking radio signals at both bands, taking into account that the bandwidth for both frequencies needs to be at least 400 MHz and of use in real-life applications. Then, this paper aims to provide an appropriate, less blunt, approach towards shielding devices from 5G. An attempt has been made to find a fitting solution using a self-similar design. Earlier research such as [11] showed promising results for a single-layer FSS dual-band design. The self-similar structures and fractal antennas are handled in Section 2, and the unit cell is described in Section 3. Section 4 details a couple of equations to apply this design to other frequencies with simple parameter adjustments, which extend the usability of the proposal, and provides a tool for radio network designers.