Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.
SUMMARYOptical backhaul downlinks from high-altitude platforms (HAPs) are investigated. An experiment demonstrated the advantages of optical links: a small and lightweight terminal with low power consumption was launched to the stratosphere and data transmitted down to a ground station at a rate of 1:25 Gbit=s: Owing to the chosen system parameters and the high budget margin, disturbing turbulence effects did not decrease the link performance.The scientific aspect of the experiment was to study turbulence effects in order to design future systems with higher transmission performance. On the day of the experiment, measured scintillation and wavefront distortions were minimal in the morning. The best atmospheric conditions were observed about 3 h after sunrise with a peak of the atmospheric coherence length r 0 at 16 cm: An r 0 of 4 cm was measured as the worst case before sunrise and later during the day. This trend could also be observed for power-ðs ; a lognormal intensity probability density function was measured.Apart from the robust intensity modulation scheme with direct detection which was used for the trial, future improved systems could benefit from a coherent transmission scheme. According to the r 0 measurements and further simulations on heterodyne efficiency it turned out that the aperture size can be decreased from 40 to 10 cm without any significant change in the link margin.Future stratospheric optical links between HAPs or links from platforms to satellites will not suffer from cloud blockage but it remains an issue for up/downlinks to a ground station. This can be mitigated by ground-station diversity. Four optical ground stations in the southern part of Europe can lead to an availability of over 98%. The separation distance of the ground stations is about 900 km with a negligible correlation of cloud cover. A change of wavelength from the employed 1.55 to a wavelength around 11-mm with minimum cloud attenuation would increase the link availability for thin clouds.
Optical data links through the atmosphere suffer from turbulence-induced signal scintillation. In a coaxially-symmetric bidirectional link scenario, the variations of the axial intensities at both ends are correlated. This relation can be used as an inherent feedback mechanism, with negligible delay, to enhance the capacity of the transmission system. By experiment, we show the correlation coefficient of both received signals can reach values close to one over long atmospheric distances, provided the receiver apertures are smaller than specific intensity speckle structures, while the correlation reduces gradually with larger apertures. This allows transmission capacity to be optimized with adaptive transceiver systems that take into account the degree of correlation.
An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESA's OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. Wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during the entire campaign with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (10-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory.
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