Rateless coding for hybrid free-space optical and radio-frequency communication

Free-space optical (FSO) transmission systems enable high-speed communication with relatively small deployment costs. However, FSO suffers a critical disadvantage, namely susceptibility to fog, smoke, and conditions alike. A possible solution to this dilemma is the use of hybrid systems employing FSO and radio frequency (RF) transmission. In this paper we propose the application of a rateless coded automatic repeatrequest scheme for such hybrid FSO/RF systems. The advantages of our approach are (a) the full utilization of available FSO and RF channel resources at any time, regardless of FSO or RF channel conditions and temporal variations, and (b) no need for a-priori rate selection at the transmitter. In order to substantiate these claims, we establish the pertinent capacity limits for hybrid FSO/RF transmission and present simulation results for transmission with off-the-shelf Raptor codes, which achieve realized rates close to these limits under a wide range of channel conditions. We also show that in conditions of strong atmospheric turbulence, rateless coding is advantageous over fixed-rate coding with rate adaptation at the transmitter.