Link Adaptation for Throughput Optimization of Parallel Channels with Application to Hybrid FSO/RF Systems

Hybrid free space optical (FSO)/radio frequency (RF) communication systems provide an attractive solution for high-throughput wireless connectivity. Such systems promise high data rates (Gbps) due to the FSO channel, weather permitting. The reliability of these systems is enhanced by the more robust RF channel. A critical question is how to efficiently transmit data using hybrid systems with highly variable channel conditions. In this paper, a hybrid FSO/RF communication system is modeled as two independent parallel channels. Under the assumptions of quasi-static channels and perfect channel knowledge at the transceivers, we propose an adaptive design resulting in a capacity-approaching and seamless joint system. In our design, information bits are jointly encoded and decoded. The encoder rate, modem transmission rate, and modulation scheme of each channel are dynamically adjusted according to the channel state. Analytical and simulation results give insight into the relationship between the adaptive parameters and the performance of the hybrid system.