Rate-adaptive Techniques for Free-space Optical Channels

Free-space optical (FSO) communication has witnessed rapid development recently in response to ever-increasing demands for greater bandwidth. FSO links provide fiberspeed with the flexibility of wireless. Commercially available systems offer transmission speeds up to 2.5 Gbps, 5 Gbps and 10 Gbps, and demonstration systems report data rates as high as 160 Gbps. This is usually not possible with any fixed wireless or RF technology. Its advantages also include license free operation, high immunity to interference, low bit-error rate (BER) and ease of deployment. However, FSO systems are sensitive to adverse weather conditions such as fog, rain and snow. In order to improve the availability of FSO channels degraded by atmospheric turbulence and varying weather conditions, the effects of channel gain variations must be compensated. To deal with these signal fluctuations, many approaches have been investigated including spatial diversity, adaptive coded modulation, and rateless coding. In this thesis, two rate-adaptive techniques, punctured low-density paritycheck (LDPC) codes and Raptor codes, are studied using experimental data measured over a 1.87 km terrestrial FSO link under different weather conditions. Rate-adaptive performances with punctured LDPC codes and Raptor codes are evaluated in terms of outage probability and throughput. In comparison to uncoded