A Novel Cooperative HARQ Protocol for Free-Space Optical Broadcasting Systems

In this paper, we propose a new hybrid automatic repeat request (HARQ) transmission protocol for broadcasting systems over free space optical (FSO) channels, which is applicable to both incremental redundancy (IR) and chase combining (CC) schemes. In traditional optical HARQ systems, upon receiving a non-acknowledgment (NACK) signal from a given user, the corresponding data packet is retransmitted from the central node to that user. However, in the proposed transmission protocol, called cooperative interuser retransmission protocol (CIRP), the optical users cooperate in the ARQ process. Specifically, instead of relying on the central node, a user who successfully decoded the original data packet and whose distance from the NACK issuing user is smaller than that of the central node, is invited to retransmit this packet to the latter user. This interuser cooperation, in fact, leverages the distance-dependent scintillation effect and path loss, which critically impact FSO channels and degrade the receiver's signal-to-noise ratio (SNR) as distance increases. We provide a detailed performance analysis of both the proposed and traditional protocols in terms of the achievable average sum rate and the next round probability of success. Our numerical results demonstrate that the former outperforms the latter for both IR and CC schemes under various turbulence conditions as well as in the presence of pointing errors.