Link-budget calculations are a common way to assess system parameters, such as the required transmitter power and receiver sensitivity, in free-space optical ͑FSO͒ communication systems. One of the biggest challenges for long-range FSO deployment is its signal propagation under turbulent atmospheric conditions, which produce intensity fluctuations. Methods to estimate atmospheric-fading loss in radio-frequency systems cannot be adapted to the FSO channel. Until now no general closed-form methods have been developed to describe the fading loss in such a channel. A method to calculate the losses due to scintillation fading in the threshold approach, based on lognormal statistics of the received power, is presented.
Channel measurements were performed by the German Aerospace Center in various near ground optical channels including a 1.5 km horizontal path as well as a 61km path. These measurements clearly showed that the atmosphere causes very slow fading (compared to the high data rates usually used in optical communication systems), which significantly degrades the transmission quality. As transmitter power and receiver sensitivity are limited by the transmission technology, fading depicts a severe problem, that can be reduced by the use of forward error correction schemes (FEC) in order to improve system performance. Therefore FEC was subject of investigations by means of simulations. To figure out which FEC methods are useful for applications in the atmospheric optical channels simulations of standard block codes and interleavers have been done. They were based on data sets taken in the various channel measurements. The simulations point out that only very long interleaving can increase performance significantly.
High data-rate atmospheric free-space optical (FSO) lasercom systems typically suffer from relatively long time duration link degradations. These are caused by pointing-and tracking-errors or deep signal-fades produced by the index of refraction fluctuations caused by atmospheric turbulence. Based on measurement results we will present in this paper a channel characterization model for free-space optical links. Further a forward-error-correction (FEC) coding scheme is introduced that is able to overcome link outages. The performance of these codes has been proven by measurements. Code design recommendations and validation test results are discussed in this paper.
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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