JPL's Wireless Communication Reference Website

Chapter: Wireless Channels.
Section: Path Loss.

Path Loss

Path loss models describe the signal attenuation between a transmit and a receive antenna as a function of the propagation distance and other parameters. Some models include many details of the terrain profile to estimate the signal attenuation, whereas others just consider carrier frequency and distance. Antenna heights are other critical parameters..

Propagation prediction

Topographical terrain data is used extensively to predict propagation conditions, to optimally select locations for installing cellular base stations.

In an interview, Daniel Davarsilvatham of Bellcore discusses how adequate planning can avoid or reduce costly field tests.


Source: Siemens TORNADO D Cellular Planning Tool
In a rough coverage estimation, pathloss is computed using a basic propagation model, such as Egli's model, considering neither the topography nor terrain features in great detail. If no interference were present and if omni-directional antennas were used, such coverage plots tend to show circular cell layouts. However, real-life cell layouts substantially differ from the theoretical hexagonal or circular lay-out.
Source: Siemens TORNADO D Cellular Planning Tool
In practical cell panning, pathloss is computed from the terrain features and antenna data. Received signal powers and interference power levels, determine to coverage of each base station. Mathematical methods have been developed to calculate the signal outage probability, given all these propagation parameters. The link budget is more of a back-of-the-envelope alternative.
Source: Siemens TORNADO D Cellular Planning Tool

Example

In the above path profile, the most appropriate path loss model depends on the receive location:

Path-loss law

  Figure: Average path loss versus distance in UHF bands, measured in Northern Germany.
(a, green): forestrial terrain
(b, orange): open area
(grey): Average of (a) and (b)
(black): Egli's model

 
  Models such as the Egli Path Loss Law can be used for a rough coverage estimation. However, the prediction error standard deviation is on the order of 12 dB. Compare the predicted smooth cell boundaries by the more accurate estimates in the screen displays above.

On the right: Results generated by the TORNADO D planning tool by Siemens AG for cellular telephone at 900 MHz. (Qatar, Arabian Peninsula )


Source: Siemens TORNADO D Cellular Planning Tool
 

 

 
 
  New York

Received signal power and attenuation

Path loss is one of the mechanisms causing attenuation between the transmitter power amplifier and receiver front end. Some other effects are listed below, with an indication of the order of magnitude in a GSM-like system (see also pdf slides on GSM frequency planning). These parameters all appear on the link budget.

Summary

Several path loss models have been proposed in literature, for instance

SPEC
  See the Special Purpose Embedded Path Loss Predictor, a spread sheet to forecast path loss.  

 

 

    Effect of
antenna height
Effect of
frequency
Effect of
distance
  Free space none 20 log f 20 log d
  Theoretical
plane earth
6 dB/oct none 40 log d
  Egli
plane earth
6 db/oct 20 log f 40 log d
  Measured
urban
6 dB/oct 20 log f 32 log d



JPL's Wireless Communication Reference Website 1993, 1995.