Adaptive coded modulation for FSO links

The terrestrial free space optical (FSO) link promises a number of advantages in terms of higher data rates and license free bandwidth, but is challenged by atmospheric propagation conditions. Designing a system for the worst channel conditions leads to an inadequate use of resources while opting for a system design based on clear sky conditions lacks reliability. The key idea behind this work is to adapt the system design in terms of modulation order and code rate according to the channel conditions for obtaining high data rates as well as reliability. The adaptation is based on an estimate of received signal strength through a low rate RF feedback path. Modulation schemes and coding techniques are adapted at the transmitter based on the signal estimate. M-PAM and M-PPM have been studied along with LDPC codes with different code rates to select an appropriate combination of modulation and code rate based on channel conditions. 16-PPM with LDPC code rate 2/3 has been selected in this work for slightly adverse channel condition providing an overall gain of 13 dB compared to 2-PPM (the equivalent of IM/DD) and 8-PAM with LDPC code rate 2/3 if channel is in good state.