Channel-Prediction-Driven Rate Control for LDPC Coding in a Fading FSO Channel with Delayed Feedback

Free-space optical (FSO) links are sensitive to channel fading caused by atmospheric turbulence, varying weather conditions, and changes in the distance between the transmitter and receiver. To mitigate FSO fading, this paper applies linear and quadratic prediction to estimate fading channel conditions and dynamically select the appropriate low-density parity check (LDPC) code rate. This adaptivity achieves reliable communication while efficiently utilizing the available channel mutual information. Protograph-based Raptor-like (PBRL) LDPC codes supporting a wide range of rates are designed, facilitating convenient rate switching. When channel state information (CSI) is known without delay, dynamically selecting LDPC code rate appropriately maximizes throughput. This work explores how such prediction behaves as the feedback delay is increased from no delay to a delay equal to the coherence time of a fading channel. For analysis purposes, optical channels with coherence times of 5 ms and 10 ms are explored. The optical channel with a coherence time of 10 ms is meant to model the optical channel of a Low Earth Orbit (LEO) satellite. Some of the most common LEO systems typically operate at altitudes ranging from around 160 kilometers to 1,400 kilometers which corresponds to round-trip delay of 1.06 ms to 9.3 ms.