 theoretical maximum for the cell size is impossible to achieve in practice. Slow fading causes variations of the received signal level, due to obstacles in the signal propagation path. Therefore the term location probability is introduced, which describes the probability of the receiver being able to capture the signal; i.e. the signal level is higher than the receiver sensitivity. In reality there can never be 100 % location probability because it is impractical with only a reasonable amount of resources. To determine the location probability a distribution for the received signal has to be defined. The slow fading variations in the average received signal level are normally distributed, which is presented in Figure 2.14. The distribution function for slow fading is (2.40) where r is the random variable and I'm the mean value for it, and o is the standard deviation, which is measured in dB. The standard deviation o depends on the area type and is normally 5-10 dB. The value generally rises in dense areas. The slow fading is described by the normal random variable r. The location probability can be expressed by an equation, which is upper tail probability of Equation (2.40). The probability Pxo gives the location probability at a certain point when the random variable exceeds some threshold x0: (2.41) The location probability can be expressed as well as the lower tail in Equation (2.41), and therefore the probability can be calculated below a certian margin. A practical example of the lower tail probability would be to calculate certain location probabilities at the cell edge; e.g. a 70 % % location probability at the cell edge can be calculated by finding the X0 value where below it the signal can be received with a 30% probability. The planning target for the location probability is normally 90-95 % over the whole cell area. The location probability, slow fading margin (xo - I'm), maximum path loss and cell range are all connected. If the location probability is, for example, 80% on the cell edge with a certai