28-11-2012, 05:40 PM
Advanced Telecommunication
1Telecommunication.ppt (Size: 3.47 MB / Downloads: 60)
Radio Propagation Mechanisms
Refraction
Conductors & Dielectric materials (refraction)
Propagation wave impinges on an object which is large as compared to wavelength
- e.g., the surface of the Earth, buildings, walls, etc.
Diffraction
Fresnel zones
Radio path between transmitter and receiver obstructed by surface with sharp irregular edges
Waves bend around the obstacle, even when LOS (line of sight) does not exist
Scattering
Objects smaller than the wavelength of the propagation wave
- e.g. foliage, street signs, lamp posts
“Clutter” is small relative to wavelength
Propagation Models
Large scale models predict behavior averaged over distances >>
Function of distance & significant environmental features, roughly frequency independent
Breaks down as distance decreases
Useful for modeling the range of a radio system and rough capacity planning,
Experimental rather than the theoretical for previous three models
Path loss models, Outdoor models, Indoor models
Small scale (fading) models describe signal variability on a scale of
Multipath effects (phase cancellation) dominate, path attenuation considered constant
Frequency and bandwidth dependent
Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time.
Log-Normal Shadowing Model
Shadowing occurs when objects block LOS between transmitter and receiver
A simple statistical model can account for unpredictable “shadowing”
PL(d)(dB)=PL(d)+X0,
Add a 0-mean Gaussian RV to Log-Distance PL
Variance is usually from 3 to 12.
Reason for Gaussian
Longley-Rice Model
Point-to-point from 40MHZ to 100GHz. irregular terrain model (ITS).
Predicts median transmission loss, Takes terrain into account, Uses path geometry, Calculates diffraction losses
Inputs:
Frequency
Path length
Polarization and antenna heights
Surface refractivity
Effective radius of earth
Ground conductivity
Ground dielectric constant
Climate
Disadvantages
Does not take into account details of terrain near the receiver
Does not consider Buildings, Foliage, Multipath
Original model modified by Okamura for urban terrain
Indoor Propagation Models
The distances covered are much smaller
The variability of the environment is much greater
Key variables: layout of the building, construction materials, building type, where the antenna mounted, …etc.
In general, indoor channels may be classified either as LOS or OBS with varying degree of clutter
The losses between floors of a building are determined by the external dimensions and materials of the building, as well as the type of construction used to create the floors and the external surroundings.
Floor attenuation factor (FAF)