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

Rate-adaptive coding enables reliable communication while efficiently utilizing the available channel mutual information in free-space optical (FSO) communication. While adaptive coding has been explored in numerous articles, the effect of feedback delay is often overlooked. To adapt the code rate to current channel conditions, the transmitter must sense the channel or rely on receiver feedback. FSO channel state information (CSI) cannot be reliably estimated at the transmitter because optical scintillation caused by atmospheric turbulence may differ for signals traveling in opposite directions. In real-world FSO systems, feedback from the receiver provides CSI. Even if the receiver sends accurate CSI, the channel conditions might change by the time the feedback reaches the transmitter and the new signal travels to the receiver. To mitigate throughput performance degradation caused by feedback delay, this paper applies linear and quadratic prediction to estimate future CSI and dynamically select the appropriate low-density parity check (LDPC) code rate. Protograph-based Raptor-like (PBRL) LDPC codes supporting a wide range of rates are designed, facilitating convenient rate switching. When 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. Optical channels with coherence times of 5 ms and 10 ms are explored, where 10 ms channel is meant to model the optical channel of a Low Earth Orbit (LEO) satellite.