Is It Time to Replace Longley–Rice?
April 18, 2008
The limitations of the Longley–Rice propagation model used in FCC coverage and interference studies have been known for some time. On Sunday afternoon at the Broadcast Engineering Conference at the NAB Show, Sid Shumate, senior appraiser and director of engineering for the BIA Financial Network (BIAfn) Consulting Division, documented the problems with the Longley–Rice model and offered solutions in his paper “From ITM to ITWOM: Correcting Completing and Updating the Longley–Rice Irregular Terrain Model.”
Accurate propagation studies are essential when designing single frequency networks and plotting mobile coverage. The average in ITM tends to smear results, while the Irregular Terrain with Obstructions Model (ITWOM), provides more accurate results on a cell-by-cell basis. I’ll discuss ITWOM in more detail in an upcoming “RF Technology” column. Sid Shumate’s improvements have not been released and it isn’t clear if the improvements and modified code will available for free. If they are, and the modified code is made available for use in free propagation software such as SPLAT (see my Feb. 6 “RF Technology” column), this could eventually displace Longley–Rice as the standard for propagation modeling. Some of the plots Sid used in his presentation were based on SPLAT, so it looks like this may be possible. If distribution is restricted, his work will likely join TIREM and CRC-Predict as excellent software that is not widely used due to high purchase and/or licensing costs. Sid is looking for sponsors for his work and it would be nice to see broadcast organizations or companies support his work with the goal of making it an open-source standard. While it may seem odd, in the software business large companies like Sun, Novell, IBM and Red Hat are supporting open source software. If you can afford it, more accurate propagation models are available. RadioSoft was showing its new Radio Compass software, which includes an updated version of TIREM (Terrain Integrated Rough Earth Model), and the ability to work with a wide range of high-resolution geographic databases. CRC Canada’s coverage software was featured at its booth in the ATSC Hot Spot; it includes the continuously updated CRC-predict coverage software. People creating accurate propagation models deserve to be compensated and I was pleased to see the products at the NAB Show. However, the industry would benefit if broadcasters and related companies provide the financial support needed to generate a more accurate open-source software model that could be incorporated in all propagation software (free, like SPLAT, or in more full featured commercial software) and by the FCC in its coverage and interference studies. By the way, if you have been wondering what value to use for building attenuation, David Plets’ paper “Evaluation of Buildings Penetration Loss for 100 Buildings in Belgium” presented on Wednesday at the Broadcast Engineering Conference, provided some answers, at least for buildings in Europe. Based on his averages, if you allow 10 dB you should be safe, although in offices with metallic-coated windows the penetration loss could be much higher, greater than 20 dB.
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