Dependence of uplink beam wander and scintillation on the transmitter beam waist: analytical model and numerical results for Earth-to-LEO links

Scintillation and beam wander in uplink are severe issues in space optical communications, especially to LEO satellites, so that a significant margin is added to account for the statistics of the received intensity. The transmitter beam size (w0) determines these effects; first, a simplified analytical model shows the role of w0, compared to the spatial frequencies of the atmospheric turbulence. Hence, beam wander (scintillation) is principally ascribed to the frequencies of the von Kármán spectrum lower (greater) than 1/w0. This is confirmed by Monte Carlo numerical simulations of vertical beam propagation from Earth to a LEO, using wave-optics and the phase screen method; they confirm that the impairments statistics are largely affected by the w0 value. Changing w0 changes the whole probability density function (pdf) of the received intensity, depending on the turbulence level. Either tracked or untracked, these beams have very different pdf’s (not only in mean value/variance), thus require fairly different margins, to achieve the required outage probability (e.g., 10-2). The obtained dependence of loss in different turbulence regimes is very different, which is especially relevant for tracked beams.