A hybrid free space optics/radio frequency antenna - design and evaluation

Mansour Abadi, Mojtaba (2017) A hybrid free space optics/radio frequency antenna - design and evaluation. Doctoral thesis, Northumbria University.

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Abstract

Free space optical (FSO) communication provides high speed data communications with high flexibility and cost-effectiveness. However, FSO links are sensitive to atmospheric effects such as fog, smoke and turbulence. To address the problem, this research is investigating a hybrid FSO and RF technology to ensure link availability under all weather conditions as part of the last mile access networks. The research exclusively investigates design, implementation, and assessment of a novel dual purpose hybrid FSO/RF antenna. The technical issues are interference between FSO and RF parts; compactness of the design; quality of service; and robustness. As part of the design a conventional RF antenna scheme, known as Cassegrain antenna is adopted, and a new design scheme for a hybrid antenna is proposed. For the FSO part an optical transceiver aperture, which is composed of optical lenses and optical fibres, is designed and incorporated in the shadowing region of the sub-reflector of Cassegrain antenna. The use of lens and fibre ensures the isolation between the optical and RF parts. Based on the initial design, modifications are made to enhance the hybrid antenna performance. In this work the focus of the research is on the optical part and how it is incorporated as part of the RF antenna. As part of the optical design, spatial diversity and differential signalling techniques are adopted. Majority-logic combining is adopted from RF technology and the performances of a FSO system implementing combining methods are compared for various turbulent regimes. The concept of differential signalling is investigated in terms of the channels correlation and it is shown that the variation of the detection threshold level reduces in correlated channels. A new design method is given based on spatial diversity and differential signalling techniques. To simulate the RF part, CST STUDIO SUITE® software is used, whereas Monte-Carlo simulation is used for performance estimation of the FSO link. Also provided are the detailed mathematical modelling of the hybrid FSO/RF system. The proposed hybrid antenna is fabricated and evaluated and results are compared with simulation and predicted data. Based on the recorded data of a real hybrid FSO/RF channel the performance of hybrid FSO/RF link employing the hybrid antenna with a switching mechanism is evaluated. Through this thesis, the detailed guidelines on design of the hybrid antenna are outlined and when necessary, the significant issues are discussed and addressed. Since the propose of this PhD is to outline and demonstrate the proof of concept for the proposed hybrid antenna, therefore the focus has been on the design, evaluation and the minimum performance requirements rather than the best possible performance or optimising the communication link quality. The outcome of the thesis will be a prototype antenna with a gain of 29.2 dBi and efficiency of 59 % at the frequency of 10 GHz for use in a hybrid FSO/RF system. It will be shown that the designed antenna is able to provide a hybrid link with 99.8 % availability and 1 Gbits/sec data rate at the recorded fog and rain channel conditions.

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