Radio Propagation Models
Land-mobile communication is burdened with particular propagation complications compared to the channel characteristics in radio systems with fixed and carefully positioned antennas. The antenna height at a mobile terminal is usually very small, typically less than a few meters. Hence, the antenna is expected to have very little 'clearance', so obstacles and reflecting surfaces in the vicinity of the antenna have a substantial influence on the characteristics of the propagation path. Moreover, the propagation characteristics change from place to place and, if the terminal moves, from time to time.
Statistical propagation models
In generic system studies, the mobile radio channel is usually evaluated from 'statistical' propagation models: no specific terrain data is considered, and channel parameters are modelled as stochastic variables.
Three mutually independent, multiplicative propagation phenomena can usually be distinguished: multipath fading, shadowing and 'large-scale' path loss.
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Multipath propagation leads to rapid fluctuations of the phase and amplitude of the signal if the vehicle moves over a distance in the order of a wave length or more. Multipath fading thus has a 'small-scale' effect.
- Shadowing is a 'medium-scale' effect: field strength variations occur if the antenna is displaced over distances larger than a few tens or hundreds of metres.
- The 'large-scale' effects cause the received power to vary gradually due to signal attenuation determined by the geometry of the path profile in its entirety. This is in contrast to the local propagation mechanisms, which are determined by terrain features in the immediate vicinity of the antennas.
The large-scale effects determine a power level averaged over an area of tens or hundreds of metres and therefore called the 'area-mean' power. Shadowing introduces additional fluctuations, so the received local-mean power varies around the area-mean. The term 'local-mean' is used to denote the signal level averaged over a few tens of wave lengths, typically 40 wavelengths. This ensures that the rapid fluctuations of the instantaneous received power due to multipath effects are largely removed.
More on propagation on this CD ROM
Detailed discussion
- Overview of large-scale propagation models (PS; PDF)
- Statistical Propagation models (PS; PDF)
- Rician and Rayleigh fading
- Statistical behavior of Inphase and Quadrature components (PS; PDF)
- Statistical behavior of amplitude (PS; PDF)
Bibliography
The radio wave propagation
slide show leads you through this topic.
Slides