Prediction Methods

Propagation Methodology

  FCC Propagation Curves

The FCC curves were created through a combination of the free-space equations and actual measurements, which augmented the equations with real world experience. Initially, the curves were available only as a set of graphs. However, with the advent of computers, the U.S. Federal Communications Commission employed its staff to translate the curves to a set of digitally stored tables, which could be interpolated by machine. With the input of desired signal level, radiated power, and effective antenna height the curves will give the user an accurate estimate of the distance from the antenna where the signal will exist. The curves can also be used to determine signal level at a distance with the input of power, antenna height and distance from the antenna. Proper use of the curves requires that the input variable "antenna height" be calculated to represent the antenna's height above "average terrain". The FCC specifies certain methods for determining this value. When topographic maps are employed, the Commission requires that at least 50 points be taken from 3.16 to 16 kilometers (FM) and then averaged to produce the height above average terrain. The computer implementation of the curves will generally take terrain samples at one/tenth kilometer intervals. The FCC's method is excellent at representing coverage over somewhat smooth or rolling terrain, however the methods tend to break down in places where the terrain is rugged. Since the method simply averages the terrain elevations, inaccuracies are introduced when the terrain varies widely or when it varies significantly at points beyond the method's 16-kilometer cutoff.

  Longley-Rice Model

In the mid-sixties, the National Bureau of Standards published Technical Note 101. P. L. Rice, A. G. Longley, A. Norton and A. P. Barsis authored this two-volume propagation treatise in the course of their work at the Institute for telecommunications Sciences and Aeronomy at Boulder, Colorado. The concepts expressed in these documents were incorporated into a series of computer routines that came to be known as the "Longley-Rice Model". This model has recently been employed by the Commission to determine the new DTV allocation scheme. It has now become the standard alternative prediction method. Going well beyond the FCC curves, the Longley-Rice method considers atmospheric absorption including absorption by water vapor and Oxygen, loss due to sky-noise temperature and attenuation caused by rain and clouds. It considers terrain roughness, knife-edge, (with and without ground-reflections), loss due to isolated obstacles, diffraction, forward scatter and long-term power fading. The model and our V-Soft Communications implementation require the following inputs for analysis based on multiple point-to-point paths:

Frequency (20 - 20,000 MHz)

Transmitter antenna parameters:

Transmitter antenna height (above mean sea level - meters.) Transmitter antenna height (above ground - meters.) Transmitter power. Transmitter antenna pattern.

Receiver antenna height (above ground - meters)

System antenna polarization (vertical or horizontal)

System Ground Conductivity (mhoS/m)

As opposed to Longley-Rice, TIREM has built-in routines for evaluating radio paths over sea water. TIREM is used in numerous modeling and simulation (M&S) tools at the Department of Defense.

Since the TIREM is a proprietary model it is not possible to tell exactly what its code is doing, which makes the model less attractive to the FCC and other users.

 P.T.P.

The point-to-point  (PTP) propagation method.  In the 1998 Biennial Regulatory Review  (Streamlining of Radio Technical Rules in MM Docket No. 98-93, 98-117) the U.S.A. Federal Communications Commission proposed the PTP method. Authored by Harry Wong of the FCC’s Office and Engineering Technology, this method provided for an analysis of the entire path between the transmitter and receiver. It based its process on radio diffraction and attenuation to the free space path caused by irregular terrain entering the Fresnel zone. According to Wong, major determinants of this method include:

(1.) the amount by which the direct ray clears terrain prominences or is blocked by them, 

(2) the position of terrain prominences along the path,

(3) the strong influence of the degree of roundness of these terrain features, and

(4) the apparent earth flattening due to atmospheric refraction.”

The original code for the PTP method used the 30 arc-second terrain elevation database and applied a static 5 dB of attenuation at points along the path to represent urban clutter. The Commission chose not to adopt this method but reported that it planned to do more work on the model, modifying it to use 3 arc-second terrain and to provide for more flexible clutter calculations.[1]   Mr. Wong updated his method in an abstract available from the FCC, dated November 1, 2002.[2.]  Here he reports that “Comparison with actual propagation measurements, and with the results of other prediction procedures, demonstrates that path loss values calculated by the PTP model are relative accurate; and moreover the accuracy of the PTP model is as good or better than that achieved by alternative prediction procedures.” 

Probe    -  Terrain 3-D