Passive pin reflection frequency selective surface for interference reduction in the built environment

Proposed is a passive periodic pin reflection frequency selective surface (PR‐FSS) suitable for installation in indoor environments to reduce interference caused by multiple cochannel wireless transmitters. The PR‐FSS is developed from a comb reflection FSS and provides comparable performance with the use of less material. © 2014 The Authors. Microwave and Optical Technology Letters Published by Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1424–1427, 2014

Frequency selective surfaces (FSSs) are increasingly being investigated for their use in modifying radio propagation in buildings, especially through interior walls [1][2][3][4]. Despite this, propagation along corridors is rarely considered, even though it is often the main route of propagation of a transmitted signal [5], especially in buildings with metallized walls [6]. This propagation can cause interference between transmitters in adjacent rooms, and it is, therefore, advantageous to redirect the propagating signal back to the source of transmission to reduce this interference, as seen in Figure 1. This letter presents a pin reflection frequency selective surface (PR-FSS), which is capable of reducing unwanted propagation down corridors. It is developed from a similar comb-based structure described in [7] but aims to reduce the total material use and cost. This reduction in material is likely to result in an inferior performance, and the relationship between pin separation (reduction in material) and surface performance are presented in this letter, with an optimal solution provided. This letter presents results on the reduction in forward scatter for horizontal and vertical polarizations, and provides a comparison of the comb reflection frequency selective surface (CR-FSS) in [7] and the PR-FSS described here.
Simulation results on the reduction in forward scatter at offnormal angles of incidence (h i 6 ¼ 0 ) like in [8] are presented, despite normal incidence usually being considered in FSS literature [1,9,10]. This is due to the fact that only angles of incidence between 30 and 70 are likely to cause interference issues [7,11]. Final discussion is on the broad frequency range at which the surface an example surface is active and its frequency dependence on Bragg's Law [12].

DESIGN AND SIMULATION
As described previously in [7], periodic surfaces can be used to reduce the forward scatter of a propagating signal by redirecting it as backscatter. This letter presents a novel pin based FSS for reducing the forward scatter of an incoming signal. The pin period, a, and separation d are chosen prior to construction of the surface to provide a frequency selective response. The angle of maximum direct backscatter h B can be appropriately chosen by selecting these parameters, and by using Bragg's Law [12]. For practicality of comparisons of the presented simulation results with future experimental ones, the surface is designed to operate in the X and Ku frequency bands. For an arbitrary choice of pin period a 5 16 mm and f 5 10.8 GHz, the angle of incidence, h i where maximum backscatter will occur is at h i 5 sin 21 (c/2af) 5 60 , where c 5 speed of light in a vacuum. There is also an operating forward scatter frequency range, which is dependent on angle of this incidence. This letter will address the consequence of converting the previously researched fin structure in [7] into a pin arrangement.
Scattering simulations were performed using the time domain solver in CST microwave synthesizer (MWS) [13], with the PR-FSS in Figure 2 illuminated by a plane wave with an angle of incidence of 60 . All surface parameters used in the simulation are summarized in Table 1. There were five repetitions of the pins in the x-axis and 40 repetitions in the y-axis, with the plane wave parallel to the y-axis. A field monitor at 12 GHz and Efield far-field probes at 660 were used to obtain the simulation results shown in this letter.

Pin Separation
The optimal PR-FSS would have the highest possible pin separation, without negatively effecting the reduction in forward scatter. Figure 3 shows the relationship between the pin separation distance and the reduction in forward scatter. There are clear differences in the performance of the surface when considering the polarization. At d 5 0 mm, the surface is a CR-FSS and exhibits the best performance, where the forward scatter is reduced by approximately 213 dB, independent on polarization. For a vertically polarized signal, the PR-FSS is able to sustain effective reduction for pin separations lower than 20 mm. Higher than this, the PR-FSS becomes less effective, reaching minimal reduction at 26 mm. The surface exhibits some harmonic effects, with a second peak occurring at 36 mm. For a horizontally polarized signal, the performance decreases rapidly, suggesting a PR-FSS is suitable for reducing the forward scatter.
The effects of increasing the pin separation can be summarized by comparison of the normalized radar cross-section (RCS)   Figure 4 compares the differences for a vertically polarized plane wave, where the PR-FSS offers a 9-dB reduction in forward scatter compared to 13 dB for the CR-FSS. Conversely, the PR-FSS offers little reduction when the plane wave is horizontally polarized.

Frequency Selectivity
The forward scatter frequency range Figure 6(a) is comparable to the CR-FSS in [7], where the range of both is 10-18 GHz. The peak reduction occurs at 10.6 GHz, and this slowly decreases as the frequency is increased. This broadband range can be tuned to cut out predefined frequencies by altering the period of the pins. For the angle of incidence of 60 and the pin period of 16 mm, the peak backscatter is at the expected 10.8 GHz, as seen in Figure 6(b). As the angle of incidence changes, the peak of this backscatter will shift according to Bragg's Law.

CONCLUSION
This letter presents a PR-FSS from a similar comb-based structure that would be installed on a corridor wall to prevent interference between adjacent wireless transmitters. Compared to the CR-FSS in [7], the PR-FSS is only effective for a vertically polarized plane wave. The PR-FSS can offer a 9-dB reduction in forward scatter compared to the 13-dB reduction offered by the CR-FSS, making the PR-FSS an attractive alternative for reduction vertically polarized signals. The PR-FSS offers no reduction for the horizontal polarization. The PR-FSS has a similar frequency range to the CR-FSS, thus is able to offer an effective alternative, with the use of much less material.

ACKNOWLEDGMENT
This work was funded by the EPSRC E-Futures Doctoral Training Centre and British Gas.

INTRODUCTION
Bandpass filter (BPF) is a key component in the radio frequency (RF) front end to achieve the desired and high-performance signals.
In recent years, the development of multiservice wireless communication has attracted much interesting. For example, the combination of global position system (GPS) at 1.575 GHz, or global system for mobile communications (GSM) at 0.9/1.8 GHz, or wireless local area networks (WLANs) at 2.4/5.2 GHz or automotive radar system at 8.2 GHz has created more potential [1][2][3][4]. Therefore, the design of the multiband BPFs is becoming more important, especially in many commercial communication products.
In the past, the triple-passband BPFs were reported by combining of two or more single-passband filters. However, this method needs a large circuit size and additional external networks. Recently, a triple-passband BPF designed at 1.8/2.4/3 GHz was proposed using two sets of coupled stub-loaded resonators and half wavelength resonators [5]. However, the design procedure is complex. Another triple-passband filter designed at 2.4/3.5/5.25 GHz was achieved using assembled half-wavelength stepped impedance resonator (SIR) and a common halfwavelength resonator [6]. However, too much design parameters are used and the insertion losses are high for the tri-passbands. Thus, the requirements of the circuit designers to design a triband filter are to achieve a low insertion loss in the tripassbands and a good passband selectivity between each band using a more simple design method.
In this article, we propose a triband BPF located at 1.8/3.5/ 5.2 GHz for applications of GSM, WiMAX, and WLAN. Only two resonators are used to obtain the desired triband of this