Optical feeder links (OFLs) benefit from the vast amount of bandwidth available in the THz-regime of the electromagnetic spectrum, and can be considered as enablers for future terabit-per-second satellite systems. A particular challenge for OFLs is to mitigate the effects of fading, caused by a combination of turbulence-induced scintillation, beam wander and pointing errors. The conventional solution is to exploit temporal diversity by a combination of interleaving and forward error correction (FEC). In this study we present an overview of fading mitigation techniques for latency-constrained coherent ground-to-satellite OFL and contribute a generic model which combines various diversity schemes including temporal, spatial, frequency and site diversity. To unlock spatial diversity, multi-beam space-time block coding and multi-beam, multi-λ are proposed and simulated. Though space-time block coding (STBC) provides more diversity gain, it requires accurate timing synchronization at the transmitter and channel state information at the receiver. Temporal, frequency and site diversity all rely on some form of interleaving and the potential diversity, pros and cons of each of these diversity techniques are covered in the presented study. In general, with a strict latency constraint and a tight link budget, frequency diversity, spatial diversity – either by STBC or multi-beam multi-λ – and site diversity can be effective methods to mitigate the effects of fading and close the link budget.
Optical satellite communications is a maturing technology to enable word-wide access to high throughput internet. In the past years a lot of effort has been taken to increase the applicability and the TRL of this technology. In collaboration with industry, TNO initiated several developments for space and ground technologies. Many of these technologies have already passed critical design review (CDR) and are in an advanced state. A missing piece of the puzzle is an in orbit demonstration (IOD), which proves the technologies to be working. This paper presents the plans for an IOD with CubeCAT on the NorSat-TD. As ground segment the TNO optical communications lab is equipped with an 80 cm diameter telescope. By an successful IOD, worldwide available internet at high throughput is yet one step closer.
Optical satellite communication is growing fast and among various applications it requires higher throughput optical feeder links. Optical feeder links for satellite communication necessitate very high data throughput, up to 1 Terabit/s and beyond. Amongst several multiplexing strategies, dense wavelength division holds a key position to enable overall throughput rates above 1 Terabit/s. As a consequence, hardware architectures capable of handling high throughput links must be devised. Complementary to the high throughput requirement, the devices should also cope with the high optical power levels needed in optical ground stations. Combination of spatial aperture multiplexing and free space bulk optics configurations of multiplexers with transmission diffraction gratings are presented as possible concepts. Besides wavelength multiplexing, it is essential to include the beam propagation effects in the performance analysis, since this may affect the overall feeder link properties. A modelling framework is presented that covers the multiplexing behavior as well as the beam propagation of the transmission gratings based concept. The modelling framework based on first principles of optical diffraction is general, and independent of the grating choice. The results suggest that the design of a free space bulk multiplexer for optical feeder link must be approached already at system level. Decisions about telescope sizing, channels distribution and modulation formats may affect the performance of the multiplexer, resulting in severe effects on the link performance. The work discusses the effect of each design parameter and proposes design guidelines for high power satellite communication beam multiplexing.
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