stem. In [LBLC02], new models which benefit from a statistical character- for example, the authors tried to understand the reason of isation of the urban layout. The overall path-loss model is a coverage gap in a 400 MHz MPT 1327 trunking system then defined as a weighed sum of these components, and by using a 3D RT tool (EPICS). A full 3D representation the model yields estimates of the field fluctuations as well. of the environment, including terrain height, was adopted The obstruction probability, which is defined as the and the study was carried out by comparing measure- probability that a link between a transmitter and a receiver ments and RT simulations. The study confirmed that bad is obstructed, defines the boundary between the LoS and coverage was due to the effect of the buildings and NLoS regions. Then, the contributions to the field suggested possible solutions to the problem. strength propagating in the two planes (lateral and verti- cal) are assumed independent and each of them can be modelled with a Gaussian random variable with known 4.7.3.7 Simplified or hybrid models mean and standard deviation. With this hypothesis the field strength and its standard deviation VS. distance result Simplified or hybrid models may represent a very attrac- from simple formulas involving some environment pa- tive compromise between the complexity of ray models rameters (q, MLAT and MVERT) which depend on synthetic and the oversimplification of Hata-like models. The term topological characteristics of the considered area (e.g. 'simplified' refers to the process of progressive simplifi- mean building height, mean street width, etc.) and, as cation which usually leads to the development of a sim- usual, are expressed in dB. The parameters' values must be plified model starting from a complete ray model or from extracted from the city map and from measurements or a set of measurement data. The required input database is RT simulations. A comparison in terms of path loss VS. often simplified too, with a consequent acquis