 are less. The slow Prediction of received signal strengths, on a statistical basis, has fading is not strongly dependent on the radio frequency. The rapid been made using a parameter Nh, which characterises terrain rough- fading has a frequency of a few fades per minute at lower frequen- ness (see CCIR Recommendation 370 and Reports 239 and 567). cies and a few hertz at u.h.f. The superposition of a number of However, there is considerable path-to-path variability, even for variable incoherent components would give a signal whose ampli- similar Ah values. Especially in urban areas, screening and multi- tude was Rayleigh-distributed. This is found to be the case when the path propagation due to buildings are important. Moreover, in such distribution is analysed over periods of up to five minutes. If other conditions, and especially for low heights of receiving antenna, types of signal form a significant part of that received, there is a depolarisation effects can impair performance of orthogonally po- modification of this distribution. Sudden, deep and rapid fading has larised systems sharing a common frequency. At the higher u.f.h. been noted when a frontal disturbance passes over a link. In addi- frequencies, attenuation due to vegetation (e.g. thick belts of trees) tion, reflections from aircraft can give pronounced rapid fading. is beginning to be significant. The long-term median transmission loss relative to the free-space The problem of field-strength prediction becomes especially dif- value increases approximately as the first power of the frequency up ficult at the frequencies specified for a range of future public and to about 3GHz. Also, for most temperate climates, monthly median private mobile systems. For example, existing 'cellphone' services transmission losses tend to be higher in winter than in summer, but operating at about 950MHz will be supplemented by 'personal the difference diminishes as the distance increases. In equatorial communication networks' (PCNs) at around 1.5 to 2.5GHz. At climat