# HARQ Performance Limits for Free-Space Optical Communication Systems

## TL;DR

This paper explores how HARQ protocols can improve the reliability and performance of free-space optical communication systems affected by atmospheric turbulence and noise.

## Contribution

The paper provides a novel information-theoretic analysis of HARQ performance limits in FSO systems using On–Off Keying modulation.

## Key findings

- HARQ protocols significantly enhance the robustness of OOK-based FSO systems under turbulence and noise.
- Channel dispersion is critical for understanding rate–reliability tradeoffs in finite-blocklength FSO systems.
- Feedback delay and retransmission strategies impact the design of low-latency optical communication links.

## Abstract

Free-space optical (FSO) communications represent an attractive technology for future high-capacity wireless and satellite networks, offering multi-Gbps data rates, unlicensed spectrum, and built-in physical-layer security. However, their performance is severely affected by atmospheric turbulence, misalignment errors, and noise, which limit reliability and throughput. Hybrid automatic repeat request (HARQ) protocols provide a powerful mechanism to mitigate such impairments by combining forward error correction with retransmissions. In this paper, we investigate the fundamental performance limits of HARQ applied to FSO systems employing On–Off Keying (OOK) modulation. Using information-theoretic tools, we characterize the achievable rate and the finite-blocklength performance by resorting to channel dispersion, which plays a crucial role in quantifying rate–reliability tradeoffs. We further examine the interaction between HARQ retransmissions, turbulence-induced fading, and feedback delay, providing insights into the design of low-latency, high-reliability optical links. This analysis highlights how HARQ improves the robustness of OOK-based FSO systems and provides guidelines for parameter selection in next-generation space and terrestrial optical networks.