Hybrid symbology

A hybrid channel is created when two or more different communication technologies are used to transmit data from a transmitter to a receiver. Such systems can involve many different modes of communication, but a common combination is RF with FSO communication. The main advantage of hybrid channels is increased resistance to failure: when one of the modes of communication is severely degraded by unfavourable conditions the other mode or modes can sustain communication, and when both modes are more moderately affected the quality of communication can be maintained at a higher level than either mode alone would allow. There has been interest in hybrid FSO/RF links for some time. It is clear that an RF link can provide a more reliable but relatively low-speed backup for an FSO link, and so the RF link has naturally been seen as a means to improve the reliability of an FSO link. For the most part the RF path has been treated as simply a parallel, independent, often more dependable link, with the encoded data for transmission being separately converted into symbols for RF and FSO transmission. More recently, several groups have shown that significant performance improvements can be realised by introducing error correction codes and splitting the data between the two links; however, the processes of constructing the RF and FSO symbols for transmission is still performed separately.