This paper investigates the usage of hybrid automatic repeat request (HARQ) protocols for power-efficient and reliable communications over free space optical (FSO) links. By exploiting the large coherence time of the FSO channel, the proposed transmission schemes combat turbulence-induced fading by retransmitting the failed packets in the same coherence interval. To assess the performance of the presented HARQ technique, we extract a theoretical framework for the outage performance. In more detail, a closed-form expression for the outage probability (OP) is reported and an approximation for the high signal-to-noise ratio (SNR) region is extracted. Building upon the theoretical framework, we formulate a transmission power allocation problem throughout the retransmission rounds. This optimization problem is solved numerically through the use of an iterative algorithm. In addition, the average throughput of the HARQ schemes under consideration is examined. Simulation results validate the theoretical analysis under different turbulence conditions and demonstrate the performance improvement, in terms of both OP and throughput, of the proposed HARQ schemes compared to fixed transmit power HARQ benchmarks.

翻译：本文研究了混合自动重传请求(HARQ)协议在自由空间光(FSO)链路上实现高能效可靠通信的应用。通过利用FSO信道的长相干时间，所提出的传输方案在同一相干间隔内重传失败的数据包，以对抗湍流引起的衰落。为评估所提HARQ技术的性能，我们建立了中断性能的理论框架。具体而言，给出了中断概率(OP)的闭式表达式，并推导了高信噪比(SNR)区域的近似表达式。基于该理论框架，我们构建了跨重传回合的发射功率分配问题，并通过迭代算法对该优化问题进行数值求解。此外，还分析了所考虑HARQ方案的平均吞吐量。仿真结果验证了不同湍流条件下理论分析的正确性，并表明与固定发射功率HARQ基准方案相比，所提HARQ方案在中断概率和吞吐量方面均实现了性能提升。