High-data-rate space-to-ground optical links are anticipated to be an alternative to radio-frequency spectrum congestion for the next generation of highthroughput geostationary satellites. An affordable site diversity system based on several optical ground stations (OGSs) and an optical fiber network is necessary to overcome cloud obstruction of the feeder link. In this paper, we report three promising sets of OGSs in the vicinity of existing ingress and egress points of a selected pan-European optical fiber network. These OGS networks are optimized for reaching at least 99.9% feeder link availability while minimizing the overall cost of the system. For the first time, the optical fiber network between OGSs is optimized using existing high-data-rate fiber links to limit its expense. The resulting cost estimates for each OGS network highlight the need to define a new cost model considering optical feeder link specificity. In addition, the link availability is simulated using a 2 year cloud mask data bank, taking into consideration practical cloud blockage forecasting duration and assuming optical transmission through thin ice clouds.
Optical feeders for geostationary High Throughput Satellites (HTS) systems based on 1.55µm wavelength technology are expected to enable to transmit up to several terabits over one active link. A desirable option of transmission architecture is an optical feeder link transparent with respect to the user air interface. This can be implemented using either a digital or an analog modulation of the optical carrier. The digital option increases the optical bandwidth to be transmitted, however it benefits from error correcting codes, interleaving and framing which are efficient against atmospheric turbulence impairments. The analog option is more efficient concerning the optical bandwidth; however the atmospheric turbulence impairments can only be mitigated by a more complex optical ground terminal. Both analog and digital options could be feasible in the 2025-2030 time-frames but the digital option is more mature with respect to the atmosphere impairments mitigation techniques.
A simulation framework based on a physical-layer based abstraction to predict physical layer performances and to compare different forward error correcting (FEC) codes is presented. This framework is used to jointly design interleaving and FEC schemes for free space optical link. A sub-class of regular Low-Density Parity-Check codes is shown to be an interesting alternative to current space communication standard for optical links that require low error floor and high decoder throughput. End-to-end simulations show the feasibility of error free link from a LEO satellite to a high complexity ground station at 25Gbits/s and from a LEO satellite to low complexity optical ground station at 10 Gbits/s. The proposed protection scheme is composed of FG LDPC code and a bit interleaver to span the burst of errors.
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