It is foreseen that Extremly large telescope (ELT) will include Adaptive Optics Systems to provide diffraction-limited images in the near infrared. The preliminary atmospheric turbulence properties which affects laser guide star (LGS) generation are presented here. The results of modern model developed in optical communication through atmosphere is used to evaluate LGS size for many different atmospheric situation, with the turbulence intensity concentrate to different heights. This introduce the focusing problem and requirements above the observatory where the adaptive optics system with a reference source will be install. In other case, focus a laser in the mesosphere is impossible in a generic place. In high altitude observatories this is possible, like in canarian observatories. We took 6 experimental turbulence profile to evaluate the beam radius with altitude. A mean value of 0.2 m are found for the diameter of mesospheric volume illuminated by the laser. This is small enough to create an useful LGS for adaptive optics proposes.