Large vs small scale models; propagation mechanisms

Large-scale vs small-scale propagation models

Large-scale models predict the mean received signal strength at a given T–R distance; small-scale (fading) models describe the rapid fluctuations of signal strength over very short distances or times.

Propagation mechanisms in a mobile environment

1. Reflection

• Occurs when a wave hits an object with dimensions much larger than λ, e.g. the earth's surface, buildings and walls.
• Part of the wave is reflected and part transmitted, depending on the Fresnel reflection coefficient Γ, which depends on material, polarization, angle and frequency.
• Ground reflection gives the two-ray model, in which loss rises at 40 dB/decade. Reflections from many buildings cause multipath.

2. Diffraction

• Occurs when the path is blocked by a surface with sharp edges (hills, rooftops).
• Secondary wavelets (Huygens' principle) bend around the obstacle and reach the shadow region, so a signal is received even without LOS.
• Loss depends on how much of the first Fresnel zone is blocked; modelled by knife-edge diffraction with parameter v.

3. Scattering

• Occurs when the wave meets objects smaller than or comparable to λ, or rough surfaces (foliage, lamp posts, signs).
• Energy is spread in all directions, giving additional received energy not predicted by reflection and diffraction alone.
• Roughness is judged by the critical height hc = λ/(8 sin θi).

           diffraction at roof edge
  BS  ----->  _/|_____         scattering (tree)
   \          |  bldg |  ~~~>   *
    \ reflection      |        / \
     \______ground_____\_____ MS

Other effects: absorption/penetration loss through walls and foliage, and waveguiding along streets. Together these produce path loss, shadowing and multipath fading.