Serial, parallel or hybrid: Towards a highly reliable transmission in RF/FSO network systems

Free Space Optics (FSO) is an attractive high-speed communication technique, with advantages of low power consumption, low cost, easy setup and secure data transmission. On the other hand, the FSO channels are sensitive to atmospheric effects, which can decrease the reliability and capacity of the FSO system. This paper proposes a novel method to achieve a reliable FSO transmission, that combines redundancy of parallel transmission supported by additional radio frequency (RF) or FSO links with the redundancy achievable through the adaptive channel coding. We provide a comparative analysis of three scenarios: serial transmission over one FSO path, parallel transmission over multiple FSO paths, and hybrid transmission with one FSO path and one RF path. For each scenario, we derive analytical expressions of expected code rate as well as the upper bound to evaluate the expected information bit rate. Finally, we show that a reliable transmission is possible in each scenario. The hybrid RF/FSO system shows the most promising performance in terms of reliability and system capacity. The best performance can be achieved in systems where the RF link can offer a variable transmission rate, i.e., able to adapting to the atmospheric effects of the FSO channel.