Coherent optical wireless communications over atmospheric turbulence channels

Recent advances in free-space optics have made outdoor optical wireless communication (OWC) an attractive solution to the “last-mile” problem of broadband access networks. Significant chal- lenges can, however, arise for OWC links with increased levels of atmospheric turbulence from time-varying temperatures and pressures. As a promising alternative to the current generation of on-off keying (OOK) direct detection based OWC system, the coherent OWC system is studied in this thesis for a variety of turbulence conditions. Since coherent OWC system performance is found to be impaired severely under strong turbulence conditions, spatial diversity techniques, e.g., maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC), are adopted to overcome turbulence impacts. The results are then generalized to Gamma-Gamma turbulence for MRC and EGC with perfect channel or phase estimation. The impacts of phase noise compensation error on coherent OWC system performance are investigated, and it is found that such impacts can be small when the standard deviation of the phase noise compensation error is kept below twenty degrees. A postdetection EGC scheme using differential phase-shift keying (PSK) is proposed and is shown to be a viable alternative to overcome phase noise impacts. The subcarrier intensity modulation (SIM) based OWC system has been proposed as another alternative to the OOK system. With a unified average signal-to-noise ratio definition, system per- formance is compared for coherent and SIM links over the Gamma-Gamma turbulence channels. Closed-form error rate expressions are derived for coherent and SIM systems using MRC, EGC and SC schemes. It is found that the coherent systems outperform the SIM systems significantly. The benefits of coherent systems come chiefly from the large local oscillator power which eliminates the effects of the thermal and ambient noises that dominate in SIM systems.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat