Improvements in Propagation Models

In my August 6, 2008, column, "Propagation Models in Perspective," I described the software errors Sid Shumate found in the current implementation of Longley-Rice used to determine DTV station coverage and interference in the United States. It is possible that the P.1812 model could be a used in place of Longley-Rice for more accurate coverage and interference predictions. P.1812 takes into account the following model elements:



• line-of-sight
• diffraction (smooth-Earth, irregular terrain and sub-path cases)
• tropospheric scatter
• anomalous propagation (ducting and layer reflection/refraction)
• height-gain variation in clutter
• location variability
• building entry losses

Fig. 1 shows a portion of a test plot of coverage from KNBC-DT and a DTS transmitter on Table Mountain in Angeles National Forest.

Diffraction loss is calculated using a method based on the Deygout construction for a maximum of three edges as described in ITU-R P.452. Recommendation P.1812 notes "other diffraction methods are under consideration now that will lead to an update of this section."

Terminal clutter losses are calculated when the receiver or transmitter antenna (the model is symmetrical) is located below the height representative of the ground cover surrounding the transmitter or receiver. P.1812 offers recommendations for receivers with antennas below clutter height in urban or suburban environments, for mobile systems with omnidirectional antennas at car-top height, receivers with rooftop antennas near the clutter height, and for receivers in rural areas.

Building entry loss is specified for three different frequencies along with a standard deviation. At 200 MHz, the loss is 9 dB with a standard deviation of 3 dB. At 600 MHz, loss increases to 11 dB with a standard deviation of 6 dB. The same numbers apply at 1.5 GHz. The recommendation states these values may have to be updated when more experimental data becomes available.

I'll have more on Recommendation ITU-R P.1812 after I've had a chance to work through the calculations on some specific paths where I have measured data. ICS telecom nG is the only propagation software package I've found with P.1812. More information on it is available at www.atdi.com/icstelecom.php. Recommendation ITU-R P.1812 can be obtained at www.itu.int/rec/R-REC-P.1812-0-200711-I/en. A Google search on ITU-R P.1812 will reveal work on updates.