Outdoor Propagation Models
Radio transmission.in a mobile communications system often takes place over irregular terrain. The presence of trees, buildings, and other obstacles also must be taken into account. A number of propagation models are available to predict path loss over irregular terrain.
Outdoor propagation models:
1. Longley-Rice Model
2. Durkin's Model
3. Okumura Model
4. Hata Model
5. PCS Extension to Hata Model
6. Walfisch and Bertoni Model
7. Wideband PCS Microcell Model
- The Longley-Rice model is applicable to point-to-point communication systems in the frequency range from 40 MHz to 100 GHz, over different kinds of terrain.
- The Longley-Rice propagation prediction model is also referred to as the ITS irregular terrain model.
- The Longley-Rice model is also available as a computer program to calculate large-scale median transmission loss relative to free space loss over irregular terrain for frequencies between 20 MHz and 10 GHz.
- For a given transmission path, the program takes as its input the transmission frequency, path length, polarization, antenna heights, surface refractivity, effective radius of earth, ground conductivity, ground dielectric constant, and climate
- The program also operates on path-specific parameters such as horizon distance of the antennas, horizon elevation angle, angular trans-horizon distance, terrain irregularity and other specific inputs
The Longley-Rice method operates in two modes
- When a detailed terrain path profile is available, the path-specific parameters can be easily determined and the prediction is called a point-to-point mode prediction.
- If the terrain path profile is not available, the Longley-Rice method provides techniques to estimate the path-specific parameters, and such a prediction is called an area mode prediction.
- One important modification deals with radio propagation in urban areas and this is particularly relevant to mobile radio. This modification introduces an excess term as an allowance for the additional attenuation due to urban clutter near the receiving antenna. This extra term, called the urban factor (UF),
- One shortcoming of the Longley-Rice model is that it does not provide a way of determining corrections due to environmental factors in the immediate vicinity of the mobile receiver
- It provides an perspective into the nature of propagation over irregular terrain and the losses caused by obstacles in a radio path
- The execution of the Durkin path loss simulator consists of two parts
- The first part accesses a topographic data base of a proposed service area and reconstructs the ground profile information along the radial joining the transmitter to the receiver.
- The second part of the simulation algorithm calculates the expected path loss along that radial.After this is done, the simulated receiver location can be iteratively moved to different locations in the service area to deduce the signal strength contour.
Walfisch and Bertoni Model:
This model developed by Walfisch and Bertoni considers the impact of rooftops and building height by using diffraction to predict average signal strength at street level.
The model considers the path loss, S, to be a product of three factors.
Where P0 represents free space path loss between isotropic antennas given by â€• (2)