Cross-Layer Designs and Analysis of Adaptive-Rate Transmission and ARQ for Free-Space Optical Communications

The impact of atmospheric turbulence is one of the most challenging issues for the widespread deployment of free-space optical (FSO) communication systems. To enhance the systems' performance, adaptive-rate (AR) transmission and automatic repeat request (ARQ) have been separately considered at the physical and data link control layers. This paper introduces a framework of cross-layer design, analysis, and optimization for FSO communication systems, in which ARQ and AR transmission are jointly integrated to further improve the overall system performance over atmospheric turbulence channels. Two cross-layer designs are considered: 1) AR and standard ARQ and 2) AR and ARQ with frame combining. In addition, we newly develop a Markov chain model-based cross-layer analysis to evaluate system performance. System performance metrics, including spectral efficiency, maximum expected number of transmissions, and outage probability, are analytically studied under the presence of atmospheric turbulence. In numerical results, how the cross-layer designs significantly outperform conventional ones is quantitatively shown. Furthermore, we discuss a cross-layer optimization of selecting the ARQ persistence level for the tradeoff between the spectral efficiency and the number of transmissions.