Erasure correcting codes for high-throughput optical ground-to-satellite links

Abstract

Optical satellite links provide increased throughput at a more competitive SWaP compared to radio frequency links. Due to the nature of the light interaction with atmosphere and the limited pointing accuracy, the propagated optical signal suffers from significant variation of the SNR. This causes error bursts in the received bit stream but can also lead to a temporary loss of synchronization causing further loss of data. Since bit rates in optical communications can reach multiple Gbps, millions of bits are affected by these error bursts and dropouts. Therefore, we propose an additional layer of protection, termed erasure coding, which is applied on top of the PHY layer of the communication system. This approach provides time diversity by long erasure code words, so that PHY interleaving can be abstained from. Therefore, optical transceiver and terminal architectures that were not specifically tailored to optical ground-to-satellite links can be reused with this layered approach. The erasure coding scheme corrects packet losses/erasures due to channel impairments, where the packet size and consequently the length of the code consisting of packets as codeword-symbols can be picked in a flexible way. Since only packet erasures need to be corrected, erasure codes show advantages compared to PHY coding schemes in terms of memory utilization and throughput. In this paper, the proposed erasure correction scheme is explained in detail and a performance analysis for typical scintillation channel models of earth-satellite laser communication links is presented. Furthermore, implementation aspects, as well as the encoding and decoding speed are discussed. In perspective, we aim at reaching a throughput in the order of 100 Gbps.