Terrain Analysis Package (TAP) Propagation Comparison

Several propagation models are available in SoftWright's Terrain Analysis Package (TAPTM) software. These include the following:

Each of these models represents an attempt to predict radio propagation as it is affected by real-world conditions. A useful comparison of these and other models is found in an article entitled "Coverage Prediction for Mobile Radio Systems Operating in the 800/900 MHz Frequency Range," by IEEE Vehicular Society Committee on Radio Propagation, IEEE Transactions on Vehicular Technology, Vol. VT-37, No. 1, February 1988.

The purpose of this paper is to summarize some of the features of the models as implemented in the TAP software.

Broadcast and Carey

The Broadcast (Part 73) and Carey (Part 22) propagation models are based on the pertinent sections of the U.S. FCC Rules and Regulations. Both of these methods are essentially simplified statistical methods of estimating field strength and coverage based only on a station's effective radiated power (ERP) and height above average terrain (HAAT). Since the terrain information is averaged, neither model takes into account specific individual localized obstructions or shadowing. Also, since the average used for these models only includes the terrain between three (3) and sixteen (16) kilometers from the transmitter site, terrain obstructions outside of this range are ignored. This means that identical results will be calculated whether or not a transmitting antenna has clear line of sight or complete blockage by an obstruction in the first 3 kilometer portion of a path. Likewise, any terrain obstructions beyond 16 kilometers that block the line of sight to a more distant receiving antenna are ignored. The main use for either of these models is for license applications or other submissions to the FCC which specifically require the use of the methods described in Part 73 or Part 22 of the FCC Rules, or other administrative requirements, such as certain frequency coordination procedures..

On the other hand, Bullington, Okumura, and Longley-Rice are more analytical models that consider a number of other factors, such as individual obstructions (either terrain or manmade), terrain roughness, etc. Okumura is often used in urban environments and includes correction factors for various area types, such as urban, suburban, etc. Bullington considers individual obstructions and computes losses, for example, for terrain obstructions, ridges, etc. Longley-Rice is a general model that considers radio horizons and various environmental conditions.

Please note that this discussion applies only to the SoftWright implementation of each of the models. Implementation of these models in software other than TAP may be considerably different, at times even reducing field strength calculations to over-simplified equations that do not afford the level of accuracy actually provided within the particular model. This is often accomplished by setting certain parameters as fixed values in the software. The SoftWright implementation of the models gives the user a high degree of versatility and the ability to independently select values for various parameters in each model based on the particular circumstances and good engineering judgment.

Some of the original engineering papers describing the models include various factors and considerations that are not included in the TAP programs. For example, the SoftWright Bullington model considers only knife-edge diffraction as discussed in the Bullington paper. The Okumura program does not currently implement the "Mixed Land-Sea Path" correction described in the original Okumura paper. The details of each specific SoftWright implementation can be found in the documentation for each of the programs.

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The SoftWright implementation of the Bullington model uses terrain elevation information along radials, and added obstruction file information if available for man-made obstructions, vegetation, etc.) to compute knife-edge diffraction losses based on Figure 10 of the Bullington paper. The obstruction penetration into the first Fresnel zone of the path is computed, and the dB loss corresponding to the penetration is computed. Numerous losses can be computed for each path, and the net received field predicted is equal to the free-space value reduced by the sum of all the losses. Bullington is the model that is best used for air to ground communications, since other models assume a receive antenna at a relatively low height above ground (e.g., up to about 10 meters for Okumura). Bullington knife-edge diffraction loss calculations have for years been the method used by the National Oceanic and Atmospheric Administration for the protection of the Quiet Zone at Table Mountain in Boulder, Colorado. New or modified radio facilities in the area are required to coordinate with NOAA to ensure that the RF noise floor at the Quiet Zone will not be raised. The recommended method of prediction is Bullington, and measurements are taken after a facility is built to ensure compliance. Years of documented field strength measurements by NOAA at the Quiet Zone attest to the accuracy of the Bullington model.

Bullington calculations yield good results from about 80MHz and above, over rough paths when the obstructions are primarily due to actual variations in the topography rather that earth curvature alone. In other words, on long paths that are relatively smooth, the path line of sight may be blocked more by the "hump" of the curvature of the earth that by actual hills or ridges that more closely approximate the model of a knife-edge obstruction. In such cases, the loss predictions by the Bullington model are generally not realistic. For example, for a 200 mile path over water (completely smooth), Bullington may not be the best model to use. Several users have also reported unrealistic results using Bullington at lower frequencies (40-70MHz), although the model has a published minimum frequency of 30MHz.

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The Okumura model is based on a great number of measurements at various frequencies in primarily urban areas in Japan. The measured values and statistical methods were used to determine basic median field strength, and numerous correction factors. The correction factors include adjustments for the degree of urbanization, terrain slope and roughness, receiver location relative to nearby hills and valleys, general street orientation in the service area, localized obstructions, etc. While some of the factors, such as terrain roughness, are computed automatically by the TAP software based on the available terrain data, other parameters must be selected by the user. The Okumura model is not recommended for use below 150MHz, but is frequently used for cellular design, 800MHz systems, etc.

It is important to select the type of area (urban, suburban, open, etc.) and other factors carefully to accurately apply the model. The Okumura model is especially applicable in urban areas for general coverage calculations where numerous discrete obstructions, such as buildings, exist. It is important to note that the definitions of "Urban", "Suburban", etc., described in the Okumura paper are in the context of Japanese cities, especially Tokyo. The terms must be applied with caution to areas in other countries to ensure similar building density, type of construction, etc. The IEEE article referenced at the beginning of this paper (comparing models in the 800/900 MHz range) suggests of the Okumura model, "The most typical situation in the United States is far from the urban situation.... Experience with comparable measurements in the United States has shown that the 'typical' United States suburban situation is often somewhere between Okumura's suburban and open areas. Okumura's suburban definition is more representative of residential metropolitan areas with large groups of 'row' houses" (p. 41).

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The Longley-Rice model is essentially a computer implementation of many of the techniques described in Tech Note 101 (National Bureau of Standards Technical Note 101, Transmission Loss Predictions for Tropospheric Communication Circuits, by P. L. Rice, A. G. Longley, K. A. Norton and A. P. Barsis, U. S. Department of Commerce, Revised January 1, 1967). Longley-Rice uses terrain information to compute terrain roughness and radio horizons automatically. Other environmental variables must be supplied by the user. These include average climate conditions, soil conductivity, etc. While these factors can be set to custom values, the program includes typical or average values that are applicable in most cases. Longley-Rice is a well established, general purpose propagation model. TAP users who expressed concern about Bullington results at lower frequencies (30-40 MHz as discussed above) have reported significantly better results for field strength calculations compared to field measurements when using Longley-Rice at those frequencies. Longley-Rice is also recommended where the path length and terrain smoothness (as discussed above under Bullington) may be inappropriate for the Bullington knife-edge model.

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The Egli model is a greatly simplified model that assumes "gently rolling terrain with average hill heights of approximately 50 feet" (Land Mobile Radio Systems, Edward N. Singer, PTR Prentice Hall, 1994, p. 196). Because of this assumption, no terrain elevation data between the transmit and receive facilities is needed. Instead, the free-space propagation loss is adjusted for the height of the transmit and receive antennas above ground. As with many other propagation models, Egli is based on measured propagation paths and then reduced to mathematical model. In the case of Egli, the model consists of a single equation for the propagation loss.

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The Hata/Davidson model is derived from the Hata model, which was in turn based on the Okumura model (described above).

Hata ("Empirical formula for propagation loss in Land Mobile radio services," IEEE Transactions on Vehicular Technology, Vol. 29, No. 3, Aug 80) reduced many of Okumura’s adjustment to equations, but the equations were limited to paths of less than 20km, as well as other limits on its application.

Hata/Davidson ("A Report on Technology Independent Methodology for the Modeling, Simulation and Empirical Verification of Wireless Communications System Performance in Noise and Interference Limited Systems Operating on Frequencies between 30 and 1500MHz", TIA TR8 Working Group, IEEE Vehicular Technology Society Propagation Committee, May 1997.

Hata and Hata/Davidson ignore some of the adjustment factors included in Okumura, such as the slope of the terrain, street orientation, and correction for location on hills. The main factors included in Hata/Davidson are the area type (Urban, Suburban, Quasi-open, Open) as well as corrections for the receiver antenna height. Hata/Davidson also includes frequency and distance corrections to extend the limitations on Hata, particularly the distance range to 300km.

It should also be noted that Hata/Davidson computes the basic median field strength (to which the various adjustments are applied) on the Height Above Average Terrain (HAAT) of the transmit antenna (typically based on the 3-16km segment of a path), rather than the topography of the entire path.

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Rounded Obstacle

The Rounded Obstacle transmission loss calculations are based essentially on two sections of Tech Note 101 ("Transmission Loss Predictions for Tropospheric Communication Circuits", 1967, NTIS). Section 7 of Tech Note 101 involves the calculation of diffraction loss over peak obstructions along the path. Some of the source code for the model was adapted from the program QZGBT used for protecting the rf noise floor at the National Radio Astronomy Observatory (Green Bank WV) radio astronomy site.

As to the determination of what constitutes an obstacle, TAP utilizes its Obstacle Peaks method to programmatically assess which points along the path represent critical obstructions in a manner similar to how a person may intuitively "eyeball" peaks along a path. Section 9 of Tech Note 101 involves the computation of tropospheric scatter losses due to these obstructions. Hence, the SoftWright Rounded Obstacle model identifies and "rounds" the obstacles and then calculates associated diffraction and scatter losses for your study.

Using arrays of elevation values and a corresponding array of distances, the TAP Obstacle Peaks method creates hypothetical "contours" or elevation bands beginning at the highest elevation along the path. TAP then analyzes each point along the path and isolates a series of points that enter and exit a band but do not penetrate the top of the band. The "peak" of this series of points is assigned to the center of the range of points in the series and a radius of the peak is calculated from the height and width of the series of points. TAP then determines the peak-to-peak path horizons and applies a specific algorithm to discard intermediate peaks that are deemed irrelevant based due to the geometry of the path. Diffraction loss is calculated for the remaining path segments based on Section 7 of Tech Note 101 and scatter loss is computed based on Section 9 of Tech Note 101. As described in more detail in the Rounded Obstacle article of your Technical Reference Manual, TAP users have several options available related to the use of diffraction loss, scatter loss or a combination of the two.

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