Hybrid optical/RF channels: Characterization and performance study using LDPC and turbo 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. The atmospheric turbulence effect of the optical channel is modeled using log-normal and gamma-gamma distribution functions. The performance of the hybrid channel is studied using low density parity check (LDPC) and turbo codes.;In the case of LDPC codes, we derive a density evolution strategy and a Gaussian approximation technique for the hybrid channel. 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. Numerical results showing the benefits of the hybrid channel under various channel conditions are presented. Both regular and irregular LDPC codes are considered and trade-offs between FSO and RF links are discussed.;An analysis on the bit error rate (BER) performance of turbo codes over the hybrid channels is presented using the transfer function bounding technique. Random and specific codeword bit selections for transmission through the RF link are considered. Our analytical results are found to match well with the simulation results. In the case of block fading channels, the hybrid channel using turbo codes is found to provide tens of dBs of gain in the signal-to-noise ratio (SNR) over an optical only link. A technique to jointly optimize puncturing and RF bit selection patterns for the hybrid channel is also presented. The proposed method relies on a simple approximate error analysis algorithm based on weight two input error sequences. Results demonstrate significant BER performance improvement obtained from the proposed joint optimization approach.