Propagation Models

A fundamental element of wireless communication system planning is predicting the signal strength at some location resulting from a transmitter at another location. The transmission of electromagnetic waves from one point to another is a complex physical phenomenon, especially when the environment is complicated with features such as mountains, buildings, and/or foliage, in addition to changing atmospheric conditions, rain and other variable elements. Depending on the transmission frequency, location and so forth, all of these features and elements can potentially affect the strength of the signal at some distance from the transmitter. Because RF transmission is a complex phenomenon, it’s not possible to predict the signal strength or transmission path loss exactly. As a result, over the years many propagation models have been developed to estimate, as accurately as possible, the signal loss over the transmission path and its variations.

However, not all propagation models are created equal. Depending on the circumstances, frequency, type of service, as well as the experience of the engineer using the model, some models are better than others. For that reason, EDX software products currently offer over 30 different propagation models spread across its product line. The propagation models and the programs where they are available are shown and described in Appendix A. Propagation Models. At the beginning is a detailed list of models, their applicable frequency ranges and the product in which they are available. A table at the end shows all the models and the various environmental switches implemented in that model. A detailed discussion of the calculation techniques involved in each of these models.

To select a propagation model, click Studies<Propagation Models and a dialog box will display where you can select the propagation model and enter the parameters which control how it will be used by the program.

At the top of the Propagation Models dialog box, you’ll see a list with 5 rows in it. Each row defines a propagation model. You can set up multiple models and select which one to use in the transmitter details (see Transmitter/Base/Hub/Router Sites). You can also associate a propagation model with specific links as well (see Link Systems). You can add more models by clicking in the last (empty) row of the grid.

Name

Use this column to give the model a user-friendly name. It doesn’t affect the actual mode or results, but is the name used by EDX throughout the application to identify which propagation model line is in use.

Category

 Depending on your program or installed modules, the drop-down list in this column provides access to different types of propagation models. All EDX software come with “Standard” and “Specialized” categories available. The Advanced Propagation Module allows access to 2D and 3D ray-tracing models.

Type

This drop-down list column allows you to select the desired propagation model for your system’s design. If you don’t know which one to pick, start with Anderson 2D, and contact EDX Technical Support for guidance on propagation models and their use.

Percent of Location/Percent of Time/Margin (dB)/Additional Data

The % of Location and % of Time columns allow you to add path loss to the propagation model due to either fast (location) and/or slow (time) fading. Time fading is normally used in fixed links like microwave, where signal variation over time is dominant. Location fading is used in mobile environments where the fading is due to the mobile moving through obstructed areas. Both of these fields are statistical controls and the basis for calculating these factors is discussed in Appendix A. Propagation Models These values are given in percent in order to let the user easily add a path loss value that will, for example, ensure that any given location will have an actual received power at or above the predicted received power at whatever % is listed. For example, with % of Location set to 50%, out of all the range of possible range of signals due to fading variations, the predicted signal level is adjusted so that 50% of the possible levels are above and below the displayed signal level. 

The allowed range for the variability percentage selections is determined by the selected path loss calculation method. For example, when using the FCC-EDX or FCC-FCC propagation model, you will usually want to select a location percentage of 50% and a time percentage of either 50% or 10%. These selections correspond to the F(50,50) and F(50,10) propagation curves, respectively. However, you do have the flexibility of choosing F(50,90) or other statistics with FCC methods, if desired. The ITU-R method offers a greater range of selections. The basis for calculating these variability factors is discussed in Appendix A. Propagation Models. Other time and location settings may be specified by your particular industry as well, such as public safety which usually uses 50% time and 85-95% location for mission critical systems.

Since the field strength or received power level calculations are estimates, the Margin field lets you specify a safety margin in dB so that you can be more confident your signal level estimate is indeed above the specified signal level.

If you select the FCC-EDX model type for the “Standard” category, the Supplemental Data button is enabled under the Additional Data column. Clicking this button opens the FCC-EDX dialog box which allows you override the default FCC frequency table used by this model and select a different frequency table. The dialog box also allows configuring additional loss (dB) to account for antenna height correction.

Environment

K factor

The K factor is an indication of the atmospheric refractivity in the area where the system will be used. A typical value is 1.333 (4/3 effective earth radius).

Ground Conductivity/Ground Dielectric Constant.

The ground constants (conductivity and relative dielectric constant) are used in propagation prediction algorithms for calculating the magnitude and phase of the ground reflection. The contribution of the reflection component is also affected by the antenna polarization selection - either horizontal or vertical - which is made in the base station transmitter/receiver antenna parameter section.

Atmospheric Absorption

At frequencies above 20 GHz, signal path loss due to atmospheric absorption from oxygen and water vapor in the atmosphere becomes important. The next entry, Atmospheric Absorption, allows you to choose from several water vapor density types that may apply to your area. Calculating atmospheric absorption loss as a function of frequency and path length is described in Appendix A. Propagation Models.

Climate type

The Climate Type selection on the next line is used to calculate the extent of path loss variations for time variability. It also has a small effect on the median (50% time) signal level value. For short paths, the degree of variability change resulting from different climate types is very small. More information on the impact of the climate type selection can be found in Appendix A. Propagation Models. Climate selection is irrelevant for path loss methods using the FCC or ITU-R propagation curves.

Use Building Data

If a building database is available, you can check this option to use the height of each building to increase the effective terrain elevation where the building is located so that areas behind the building are shadowed. Note that certain propagation models available in the Advanced Propagation Module make a more nuanced use of building data. Appendix A. Propagation Models explains how each of the models makes use of building data.

Include Ground Reflection

This checkbox toggles the ground reflection calculation component on and off.

Include Fresnel Zone Loss

In most cases you want the program to include path loss for paths that are close enough to being obstructed: where the 0.6 first Fresnel zone is partially obstructed. However, in special circumstances, you may want to exclude this loss factor. Make the appropriate selection choice when indications dictate.

Show on Legend

When the Legend is displayed, you can optionally choose to display the propagation parameters as part of the legend information. This check box controls if this particular propagation model will be displayed.

Include Tropospheric Scatter

Checking this box add calculations for tropospheric scatter using the ITU-R 452 recommendations.

Clutter

Clutter Attenuation File

For a given land use/land clutter (also referred to as: LULC, land use (clutter), LUC, land use, clutter or groundcover) database, there is an attenuation file that goes along with it. This file translates the numbers from the data file into actual clutter types; i.e. clutter code 1 may be water, clutter code 2 may be forest, etc. It also allows you to specify different attenuations for each clutter type at different frequencies. For more details on the creation of this file, (see Land Use (Clutter)). This attenuation file can be specified in two places: here in this field and also under the databases menu, which controls how it gets shown on the main map and the status bar and actually loads it onto the program. This allows for multiple versions of the attenuation file for different attenuations based on location or time of year.

You can edit the chosen file by clicking the Edit button. If you have only one attenuation file and it’s the same one used when you first set up the database under the database menu, you can click the Use file from Land Use database definition checkbox to use that file.

Add Clutter Loss

If a land use (clutter) database is available, you can check this option to apply an additional path loss value to each path, based on the land use category at the mobile/remote unit or receiver location. These path loss values are defined in the Clutter Attenuation File discussed above.

Add Clutter Pass-Through Loss

Rather than assuming clutter is a “hard” obstacle, as in the add clutter height option below, selecting this checkbox allows calculating the loss from clutter as the signal passes through the various clutter types. The clutter height value for each clutter category is used to determine when the signal is passing through the clutter. The loss value is specified in the Clutter attenuation file in dB/km units. These values may need to be quite high (in the hundreds) to model sufficient loss for foliage, buildings, etc. 

Add Clutter Height to Terrain Height

This checkbox can be used along with or in place of Add clutter loss. Checking this box uses the height for each clutter type to increase the effective terrain elevation where that type is located. This is particularly useful in areas where there are buildings that may cause severe signal shadowing but you don’t want to create a building database with each individual building described. The clutter exclusion distance sets the radius around each receive point where the clutter height is not added to the terrain height. This method can produce very conservative results with a standard land use database, but may be appropriate for systems that are very dependent on Line-Of-Sight.

Test Against Measurements

Validating the propagation model results is an important step in designing a real-world system. EDX has tools to allow the validation both of the underlying propagation model and a finished area study grid. This function allows you to make a direct comparison between field measurement data and the predictions made by the software using your current propagation model settings. To utilize this feature, you’ll need to first acquire geographically referenced field measurement data (i.e.: a list of longitude, latitude coordinate pairs with an associated signal level) from a single transmitter (sector) which has been modeled in your EDX project. 

Tune Pass-through Loss

Click the Test Against Measurements button to bring up the following dialog box:

The Test Against Measurements dialog box has options to limit the measurement points that will be included in the comparison. The points may be limited by: distance from server, azimuth from server, a boundary polygon file, and/or received power. You may also spatially average the values. In the bottom section of the dialog, you’ll point to the measurement data file path and select the appropriate file format.

The EDX format is a simple comma-delimited ASCII file described in Appendix I. Miscellaneous Files and Functions. You’ll also need to specify the sector and mobile/remote unit that represent the transmitter and receiver used to gather the data. 

To perform the comparison, click the Run Analysis button. 

The software will then make a received power prediction to every point being considered from the measurement file and produce a scatter-plot of the predicted vs. measured values. Note that blue dots represent measured values; red X’s represent predicted values. If a land use database is in use, this dialog will also display the land use categories from the attenuation file and give the number of study points found within each land use category, along with the loss value associated with that category. You can use this to increase or decrease the amount of loss given for each clutter category to more closely match the measurement results, then save a copy of the modified clutter attenuation file. There is also an auto tune function that will adjust the loss values automatically. In this way, clutter loss values may be ‘tuned’ for a given environment.