Design and validation of a CCSDS O3K synchronization front-end, tailor made for a non-coherent 10 Gbps GEO optical downlink
Résumé
Radiofrequency spectrum congestion, SWaP constraints, link security requirements, and OISL maturity advocate for direct-to-Earth optical links, despite the challenges of trans-atmospheric optical communications. Several standards have been released, including CCSDS for noncoherent optical communications [1], SDA [2], and ESTOL [3], while others are under development [4]. For GEO direct-to-Earth optical communication links, preamplifier-based detection schemes provide high sensitivity but require adaptive optics for single-mode fiber coupling. Strong turbulence can generate deep signal fading, potentially causing loss of clock data recovery (CDR) lock, frame synchronization loss, or misdetection of in-band signaling. Rather than increasing onboard power or OGS aperture, robust solutions require appropriate framing structures and receiver algorithms resilient to signal fading. In the CCSDS non-coherent optical communications standard [1], the O3K (Optical On-Off Keying) framing scheme is well suited to this scenario and applicable to higher-performance modulation schemes such as DPSK. However, performance critically depends on CDR sensitivity, reacquisition time, and frame synchronizer strategy. This paper evaluates optical link performance, focusing on synchronization aspects from three perspectives: simulation, laboratory measurements, and real satellite-to-ground link measurements. We first present the simulation model for evaluating the communication chain between a GEO satellite and an optical ground station under strong turbulence, then provide results for different code rates and interleavings defined in the CCSDS non-coherent in-band signaling standard.
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