Toward an optimal combined FSO/RF system via an adaptive bit rate control

In hybrid FSO/RF (Free Space Optics/Radio Frequency) systems, the FSO link is the primary link while the RF link is the secondary (backup) link. In current hybrid systems, once the FSO signal to noise ratio (SNR) decreases above a preset threshold, the system switches from FSO to RF to maintain the communication connection. But this scenario does not provide maximum utilization of the available bandwidth in terms of spectrum efficiency. This paper proposes an adaptive FSO bit rate algorithm to maintain communication using the FSO link as long as its bit rate is greater than that can be offered by the RF, thus requiring no switching to the secondary link. The system switches to the secondary link if and only if the FSO bit rate while maintaining the system bit error rate (BER) drops below the bit rate that can be achieved by the RF link. This paper shows the dependence of FSO bit error rate on channel bit rate using analysis and computer simulation. Furthermore, computer simulation is carried out and the results confirm the algorithm's effectiveness on maintaining higher bit rate communication connection using FSO link. Hence, the overall system performance achieves better throughput.