Signal distortion caused by tree foliage in a 2.5 GHz channel

Fixed terrestrial broadband wireless systems work very well when a clear line-of-sight is available. In near line-of-sight transmission where a few foliated trees block the line-of-sight, the signal can experience large and rapid fading. This paper investigates the time-variant nature of the frequency response of a 6 MHz wireless channel obstructed by a few fully foliated trees. Measurements were taken on fixed wireless paths with trees in the vicinity of the receive antenna, and under different weather conditions. It was found that signal fading is a function of the foliage density, the amount of wind, the multipath in the channel, and whether the leaves are wet or dry. The fades were largely flat across the band but with some frequency selective fading. Fading rates of 0.5 to 2 fades/s were measured, and occasionally the slope reached 50 dB/s.