Hybrid optical/RF channels: characterization and performance study using low density parity check codes

A hybrid channel, consisting of a free space optical (FSO) link and a parallel radio frequency (RF) link, is considered. The FSO link carries most of the transmitted data while leaving only a small fraction of the data to be carried by the RF link. It is first shown that the capacity of the proposed hybrid structure is much higher than the capacity of a single FSO link. Next the performance of the hybrid channel is studied using a low density parity check code. A density evolution strategy for the hybrid channel is derived and a Gaussian approximation technique is presented. Using these techniques, the conditions for the convergence of the message passing algorithm in terms of minimum data carrying rate through the RF link are derived. Finally, numerical results showing the benefits of the hybrid channel under various channel conditions are presented.