sameh ali
2012-09-11, 07:00 AM
results difference between diff propagation model calculations for planning is mainly due to fact that the mobile networks (or most ground-based radio systems with low antennas) cannot be 100% accurate depending on both model and measurement errors due to missing some parameters
1- Currently, most commercial software releases of the Longley-Rice methodology are based on the NTIA's Irregular Terrain Model (ITM) version 1.2.2; a few use or have as an option, version 7.0; both essentially perform exactly the same algorithm. There should be NO difference in results.
2-The U. S. Federal Communications Commission (FCC) uses an open source FORTRAN version based on version 1.2.2 that calculates in Field Strength, instead of power flux density loss like the unmodified Longley-Rice methodology; a simple conversion formula published by the FCC can be used to compare the results.
Version 7 is essentially a maintenance release, adding a few new input and output options, removing an obsolete alternative calculation, and consolidating a 10-line section of code into 8 lines. It does nothing to fix the dozen or so problems that I have identified in my series of articles in the IEEE-BTS Newsletters on the Longley-Rice algorithm. Most of these problems have been addressed in Version 3 of the ITWOM, my replacement methodology currently available as an open source option with the SPLAT! open source Longley-Rice platform. See: http://www.qsl.net/kd2bd/splat.html (http://www.qsl.net/kd2bd/splat.html)
it became necessary to study, and then develop a better alternative to, the existing Longley-Rice methodology as found in the Longley-Rice Irregular Terrain Model (ITM version 1.22 (and version 7.0)).
For example, one of the major problems with the ITM 1.2.2 is that it is specifically designed to work best with a 30-arc-second GLOBE terrain database. Its results actually get worse when used with a more detailed (3-arc second or 1-arc-second) terrain database. My ITWOM is currently under consideration by the Federal Communications Commission, which reluctantly recognizes the use of the Longley-Rice version 1.2.2 . The ITWOM is under consideration as a future replacement or alternative to the L-R version 1.2.2; for more detail, see www.fcc.gov (http://www.fcc.gov/) website, ECFS search page: http://fjallfoss.fcc.gov/ecfs/comment_search/input?z=x3a9w (http://fjallfoss.fcc.gov/ecfs/comment_search/input?z=x3a9w) , enter proceeding 10-152 for the publicly released documents including c++ source code and preliminary code documentation (over 300 pages), and test results versus the FCC testbed
This new version is far more deterministic (scientifically based), and is much closer to the original vision of the original Longley-Rice documentation (Tech Note 101) than the NTIA's software versions.
,
There is an alternative known as "TIREM" that started out as a fraternal twin of the Longley-Rice version 1.2.2 (a.k.a. Irregular Terrain Model, or ITM v.1.2.2), as both are based on a previous Longley Rice algorithm (ESSA Irregular Terrain Model, a.k.a. ITS-67). It has been continuously developed by a private company for U.S. Government military and intelligence use, and requires U.S. Goverment clearance for access. By comparison, all significant algorithm development for the Longley-Rice ITM version 1.2.2 ended as of the stable 1982
.
Most, if not all of the ITU-R P-series models are empirical.
empirical are the Okumura-Hata and Egli Urban models.I am not familiar with the CEPT, or HCM models.
The Flat Earth model I am familiar with is a simplified version of Longley-Rice for short distances.
Empirical models are valid only for the specific circumstances in which the field data was gathered; extending the model beyond these specific conditions is at the users risk.
most of Empirical models based on a terrain average; the newest ITU-R series have specific terrain database links to consider the exact terrain over which the signal is being transmitted, instead of a general average but still empirical
scientific-based (deterministic) model can be extended beyond the original test conditions.
The Longley-Rice methodology was intended to be as deterministic as possible,
the ITM 1.2.2 software algorithm implementation is only about half deterministic, and half empirical.
One of the major problems with the ITM 1.2.2 is that it is specifically designed to work best with a 30-arc-second database. Its results actually get worse when used with a more detailed (3-arc second or 1-arc-second) database.
The ITWOM model version 3.0 is far more deterministic and sophisticated, and the results improve as the database detail improves. It is still under development; a major rework of the diffraction algorithm (based on a new deterministic model of diffraction in the shadow area beyond an obstruction, replacing the ancient Kirchoff-Fresnel approximation) has been sumitted as a paper for presentation before the IEEE-BTS Symposium in late October, 2011 in Washington, D.C., and subsequent release as ITWOM v. 4.0.
deterministic model has the potential to be a far more accurate model than any empirical model, for a specific location and set of circumstances. The Longley Rice ITM, the TIREM, and the Givens & Bell ITWOM computer modes, and other models privately derived from these models and associated predecessors, are the only deterministic models I am aware of.
empirical model, today, will give the best average results for an average cellular-type mobile network in average terrain, especially if the terrain is simple (such as a flat savannah or plain), in that it is based on the average of a large set of actual measurements. This is how it is often used by cellular planning software such as the LS Telecom package which allows use and comparison of many models.
However, for a specific path profile graph of reception between a specific cell tower and a cell phone at a specific location, over irregular or rough terrain, the deterministic model has the potential to provide a far more accurate answer, which is why the FCC uses the ITM 1.2.2 for predicting reception at a specific house location to determine whether a home viewer can receive a local network TV station.
2. The description you sent of the Longley Rice is highly simplified, deals primarily with the deterministic math sections, and leaves out the numerous empirical adjustments added to make it work. For example, the ITM 1.2.2 software, despite what it reports, does not consider the effect of diffraction obstacles until there are two in the path, often far beyond the horizon or first major obstacle, and then it considers only the highest obstacle observable from the transmitter and the highest obstacle observable from the receiver, ignoring what is between them on long paths.
Longley Rice has MANY disadvantages for short-range communications use; for example, many models that are originally intended for broadcast use, are limited for cellular prediction because they are not designed or intended for use at distances less than one kilometer from the transmitter site; this includes (Longley-Rice ITM 1.2.2 and ITU-R P.1546) not use for cellular prediction.
(http://www.ofcom.org.uk/static/archive/ra/topics/research/topics/propagation/vegetation/vegetation-finalreportv1_0.pdf) (http://www.its.bldrdoc.gov/pub/ntia-rpt/08-452/) (http://www.its.bldrdoc.gov/isart/art08/slides08/)
The simple fact is that most commercial propagation prediction software programs today use algorithms that have been around for 15 to 30 years;
1- Currently, most commercial software releases of the Longley-Rice methodology are based on the NTIA's Irregular Terrain Model (ITM) version 1.2.2; a few use or have as an option, version 7.0; both essentially perform exactly the same algorithm. There should be NO difference in results.
2-The U. S. Federal Communications Commission (FCC) uses an open source FORTRAN version based on version 1.2.2 that calculates in Field Strength, instead of power flux density loss like the unmodified Longley-Rice methodology; a simple conversion formula published by the FCC can be used to compare the results.
Version 7 is essentially a maintenance release, adding a few new input and output options, removing an obsolete alternative calculation, and consolidating a 10-line section of code into 8 lines. It does nothing to fix the dozen or so problems that I have identified in my series of articles in the IEEE-BTS Newsletters on the Longley-Rice algorithm. Most of these problems have been addressed in Version 3 of the ITWOM, my replacement methodology currently available as an open source option with the SPLAT! open source Longley-Rice platform. See: http://www.qsl.net/kd2bd/splat.html (http://www.qsl.net/kd2bd/splat.html)
it became necessary to study, and then develop a better alternative to, the existing Longley-Rice methodology as found in the Longley-Rice Irregular Terrain Model (ITM version 1.22 (and version 7.0)).
For example, one of the major problems with the ITM 1.2.2 is that it is specifically designed to work best with a 30-arc-second GLOBE terrain database. Its results actually get worse when used with a more detailed (3-arc second or 1-arc-second) terrain database. My ITWOM is currently under consideration by the Federal Communications Commission, which reluctantly recognizes the use of the Longley-Rice version 1.2.2 . The ITWOM is under consideration as a future replacement or alternative to the L-R version 1.2.2; for more detail, see www.fcc.gov (http://www.fcc.gov/) website, ECFS search page: http://fjallfoss.fcc.gov/ecfs/comment_search/input?z=x3a9w (http://fjallfoss.fcc.gov/ecfs/comment_search/input?z=x3a9w) , enter proceeding 10-152 for the publicly released documents including c++ source code and preliminary code documentation (over 300 pages), and test results versus the FCC testbed
This new version is far more deterministic (scientifically based), and is much closer to the original vision of the original Longley-Rice documentation (Tech Note 101) than the NTIA's software versions.
,
There is an alternative known as "TIREM" that started out as a fraternal twin of the Longley-Rice version 1.2.2 (a.k.a. Irregular Terrain Model, or ITM v.1.2.2), as both are based on a previous Longley Rice algorithm (ESSA Irregular Terrain Model, a.k.a. ITS-67). It has been continuously developed by a private company for U.S. Government military and intelligence use, and requires U.S. Goverment clearance for access. By comparison, all significant algorithm development for the Longley-Rice ITM version 1.2.2 ended as of the stable 1982
.
Most, if not all of the ITU-R P-series models are empirical.
empirical are the Okumura-Hata and Egli Urban models.I am not familiar with the CEPT, or HCM models.
The Flat Earth model I am familiar with is a simplified version of Longley-Rice for short distances.
Empirical models are valid only for the specific circumstances in which the field data was gathered; extending the model beyond these specific conditions is at the users risk.
most of Empirical models based on a terrain average; the newest ITU-R series have specific terrain database links to consider the exact terrain over which the signal is being transmitted, instead of a general average but still empirical
scientific-based (deterministic) model can be extended beyond the original test conditions.
The Longley-Rice methodology was intended to be as deterministic as possible,
the ITM 1.2.2 software algorithm implementation is only about half deterministic, and half empirical.
One of the major problems with the ITM 1.2.2 is that it is specifically designed to work best with a 30-arc-second database. Its results actually get worse when used with a more detailed (3-arc second or 1-arc-second) database.
The ITWOM model version 3.0 is far more deterministic and sophisticated, and the results improve as the database detail improves. It is still under development; a major rework of the diffraction algorithm (based on a new deterministic model of diffraction in the shadow area beyond an obstruction, replacing the ancient Kirchoff-Fresnel approximation) has been sumitted as a paper for presentation before the IEEE-BTS Symposium in late October, 2011 in Washington, D.C., and subsequent release as ITWOM v. 4.0.
deterministic model has the potential to be a far more accurate model than any empirical model, for a specific location and set of circumstances. The Longley Rice ITM, the TIREM, and the Givens & Bell ITWOM computer modes, and other models privately derived from these models and associated predecessors, are the only deterministic models I am aware of.
empirical model, today, will give the best average results for an average cellular-type mobile network in average terrain, especially if the terrain is simple (such as a flat savannah or plain), in that it is based on the average of a large set of actual measurements. This is how it is often used by cellular planning software such as the LS Telecom package which allows use and comparison of many models.
However, for a specific path profile graph of reception between a specific cell tower and a cell phone at a specific location, over irregular or rough terrain, the deterministic model has the potential to provide a far more accurate answer, which is why the FCC uses the ITM 1.2.2 for predicting reception at a specific house location to determine whether a home viewer can receive a local network TV station.
2. The description you sent of the Longley Rice is highly simplified, deals primarily with the deterministic math sections, and leaves out the numerous empirical adjustments added to make it work. For example, the ITM 1.2.2 software, despite what it reports, does not consider the effect of diffraction obstacles until there are two in the path, often far beyond the horizon or first major obstacle, and then it considers only the highest obstacle observable from the transmitter and the highest obstacle observable from the receiver, ignoring what is between them on long paths.
Longley Rice has MANY disadvantages for short-range communications use; for example, many models that are originally intended for broadcast use, are limited for cellular prediction because they are not designed or intended for use at distances less than one kilometer from the transmitter site; this includes (Longley-Rice ITM 1.2.2 and ITU-R P.1546) not use for cellular prediction.
(http://www.ofcom.org.uk/static/archive/ra/topics/research/topics/propagation/vegetation/vegetation-finalreportv1_0.pdf) (http://www.its.bldrdoc.gov/pub/ntia-rpt/08-452/) (http://www.its.bldrdoc.gov/isart/art08/slides08/)
The simple fact is that most commercial propagation prediction software programs today use algorithms that have been around for 15 to 30 years;