The probability distribution that signal fading will exceed a given value relative to a specified reference level. Note 1: In the case of phase interference fading, the time distribution of the instantaneous field strength usually approximates a Rayleigh distribution when several signal components of equal amplitude are present. Note 2: The field strength is usually measured in volts per meter. Note 3: The fading distribution may also be measured in terms of power level, where the unit of measure is usually watts per square meter and the expression is in dB.