Incremental redundancy hybrid ARQ protocol design for FSO links

Scintillation mitigation is an important aspect of free-space optical (FSO) communications link design. While a number of hardware-based mitigation techniques have been proposed in the literature and deployed in experimental systems (e.g., aperture averaging, wavelength diversity, etc.), novel baseband signal processing approaches have the potential to complement - or even supplant - these traditional methods at a much lower cost and with minimal size, weight, and power impact. This paper presents a digital baseband approach to scintillation mitigation that employs multiple stop-and-wait incremental redundancy (IR) hybrid automatic repeat request (HARQ) processes. The proposed IR retransmission scheme provides a bandwidth efficient means of capturing the time diversity that is required to combat millisecond long fades at baseband. Additional gains of improved link margin and link adaptability are afforded by the use of TrellisWare´s Flexible Low-Density Parity-Check (F-LDPC) code family. The efficacy of the proposed coded protocol is demonstrated via simulation in a number of representative scenarios.