Large-scale variations caused by shadowing of obstacles are shown to follow a log-normal distribution [17][18][19], which means that when measured in dB they follow a Gaussian distribution. Consequently shadowing effects are usually incorporated into path loss estimates by the addition of a zero-mean Gaussian random variable, with standard deviation σ: N(0,σ), were σ is often estimated by empirical measurements. Commonly accepted values for σ are between 6 dB and 12 dB. Measured values of σ itself seem to display Gaussian distribution as well, in their variations from one area to another, and depend on the radio frequency, the type of environment (rural, suburban, or urban), base station and subscriber station height.