Measurement and Modelling of a Free-Space Optical Link and In-Field OFDM Experiment

Abstract

<p>Free-space optical (FSO) communication is a potential technology for last-mile applications. Key advantages are the unlicensed spectrum, high transmission rates, and inherent security. Moreover, Radio-over-FSO (RoFSO) allows seamless integration between the incompatible radio frequency (RF) and optical networks. Such advantages qualify FSO systems to take a front seat in next-generation broadband communication networks. However, the main challenge for FSO systems is the performance degradation imposed by the atmospheric attenuation and turbulence. To exploit the advantages of FSO systems, accurate and computationally-efficient channel models are required. This thesis represents in-field experimental work related to FSO channel measurement as well as the transmission of orthogonal frequency division multiplexing (OFDM) over the FSO channel. A 1.87-km FSO link installed at McMaster University is employed. A high-speed field-programmable gate array (FPGA)-based digitizer board is used as the underlying hardware platform for interface with the link. A system-on-three-FPGAs is implemented to act as a universal transceiver for signals composed using MATLAB. A new technique is developed for conducting the FSO channel measurement. An optical signal intensity-modulated by a high frequency sinusoid is transmitted. The received signal undergoes a fast-Fourier transform (FFT) to filter out a large portion of the interfering noise providing more accurate measurements. Fitting with the log-normal distribution is investigated. A finite-state Markov model is also derived and its accuracy is verified by the simulation results. The first realization of an in-field OFDM over FSO transmission system is implemented and tested over the link. The received signal is investigated on the symbol level and constellation diagrams are visualized. Transmission rates up to 300 Mbps are achieved with average symbol-error rate (SER) on the order of 10^-6.</p>