Hybrid FSO/RF symbol mappings: Merging high speed FSO with low speed RF through BICM-ID

In this paper, we investigate designing of hybrid free space optical (FSO)/ radio frequency (RF) symbol mappers using bit-interleaved coded modulation with iterative decoding (BICM-ID) with asymmetric data rates between the FSO and the RF links. Data bits are first encoded with a channel code and the coded bits after an interleaver are mapped to hybrid FSO/RF symbols instead of separate conventional RF symbols and on-off keyed FSO transmissions. Iterative decoding is used at the receiver. An orthogonal evolutionary (OE) algorithm to obtain optimal symbol mappings is used. Two cost functions to be used in the OE algorithm are proposed. For high signal-to-noise ratio (SNR) regime, analytical error bound expressions are used and at the low SNR regime, extrinsic information transfer (EXIT) chart is used for the cost function. The proposed mappings achieve significant gains over conventional mappings. For a hybrid FSO/RF link with an FSO and RF channel use ratio of 10:1, the proposed mapping achieves an optical SNR gain of more than 7 dB over conventional mapping. A good agreement between simulation and analysis results is obtained for the proposed scheme at high SNR.