Frequency selective wallpaper for mitigating indoor wireless interference

Abstract

This thesis presents an effective technique for interference control with the use of frequency selective surfaces (FSSs) in indoor wireless environments. An FSS with a bandstop frequency response was applied to an existing wall as a wallpaper to transform the wall into a frequency- selective (FS) wall. The FS-Wall can be made to deliberately filter out undesired interference, and accordingly improve wireless system performance. In-situ measurements have shown that the FS-Wall prototype can attenuate 5.3GHz{5.8GHz (IEEE802.11a) transmissions by 15dB in addition to the unmodified wall attenuation, whereas other radio services, such as cellular telephony at 1.8GHz, experience only little attenuation. The FS-Wall created performed consistently (with acceptable attenuation levels) with varying signal incident angles from 0± to 56±. The 15dB reduction in signal strength is considered sufficient to isolate a system from external interference, and could potentially improve a WLAN system throughput by 2.2 times. The FSS performance has been examined intensely by both equivalent circuit modelling and practical measurements. Factors that influence FSS performance such as the FSS element dimensions, element conductivities, dielectric substrates adjacent to the FSS, and signal incident angles were investigated. By keeping the elements small and densely packed, a largely angle- insensitive FSS was developed as a promising prototype for FS-wallpaper. Such an FS-wallpaper can be applied directly to the wall (with no intervening spacing) with insignificant mutual interaction between the FSS and the wall. Accordingly, the resultant FS-Wall can be modelled by cascading the effects of the FS-wallpaper and the wall. Good agreement between the modelled and the measured results was observed, which suggests that the equivalent circuit model can be used confidently to assist FSS design. Fabrication techniques and practical installation issues associated with the development of FSS for indoor wireless applications were also investigated in this thesis. With the use of an angle-insensitive bandstop FSS, this research has demonstrated a feasible solution for mitigating external interference into indoor wireless systems. This research work has linked the fields of antenna design, communication systems and building architecture.