Atmospheric turbulence causes severe performance degradation in free-space optical communication links. However, by using a reliable low-bandwidth wireline or RF link in parallel, the performance of such links can be improved significantly. This is achieved by using error correction codes at the transmitter and decoding the received data jointly over the two links at the receiver. Our work describes the feasibility study of such a hybrid link. It is observed that a 10% use of the wireline link can result in more than 8-dB performance gain in the optical link.

This paper investigates the behavior of optical and hybrid optical/RF links over varying link distances. We first describe inclusion of various factors affecting the optical link, such as the atmospheric absorption, beam divergence and atmospheric turbulence. A technique to analyze outage probability of the hybrid link for low density parity check (LDPC) codes is presented. Our results show that the hybrid link can improve outage probability by orders of magnitude over an optical only link. In haze conditions, the hybrid link can extend the coverage distance of an optical only link by hundreds of meters by diverting a small fraction of the data through the RF link.

We design irregular Low-Density Parity-Check (LDPC) codes for free-space optical (FSO) channels for different transmitter-receiver link distances and analyze the error performance for different atmospheric conditions. The design considers atmospheric absorption, laser beam divergence, and random intensity fluctuations due to atmospheric turbulence. It is found that, for the same transmit power, a system using the designed codes works over much longer link distances than a system that employs regular LDPC codes. Our analysis is particularly useful for portable optical transceivers and mobile links.