V-Soft Communications - More on Propagation Prediction Software

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)

.001 = Poor Ground

.005 = Average ground

.020 = Good ground

5.000 = Sea water

.010 = Fresh Water

System dielectric constant (Permitivity)

4.0 = Poor ground

15.0 = Average ground

25.0 = Good ground

81.0 = Sea and fresh water

System minimum monthly mean surface refractivity (Adjusted to sea level.)

200 to 450 (available from map, 301 N-units is default.)

Climate Code:

1 = Equatorial

2 = Continental sub-tropical

3 = Maritime Subtropical

4 = Desert

5 = Continental temperate (default for U.S. continent)

6 = Maritime temperate

7 = Maritime temperate overseas

Probability Factors:

Qt = (Time variability) The percentage of time the actual path loss is equal or less than the predicted path loss (Standard broadcast coverage = 50%)

Ql = (Location Variability) The percentage of paths (all with similar characteristics) whose actual path loss is less than or equal to the predicted path loss. (Used with area mode only.)

Qc = (Prediction Confidence or "Quality") The percentage of the measured data values the model is based on that are within the predicted path loss. (Standard broadcast = 50%, DTV = 90%.)

V-Soft Communication's implementation of Longley-Rice predicts received signal strength level at some 264,000 points. Our programs Probe and Terrain-3D allow instantaneous manipulation of these points to produce numerous graphic representations of the coverage pattern. The user can choose any of the pre-defined signal level representations or input a user-defined signal level. Costal features, cities, political boundaries and streets to the individual road level are available for plotting.

Okumura Propagation Model

The basic Okumura propagation model uses the height above average terrain to calculate path loss and does not consider specific terrain obstacles. The Okumura propagation model that Probe uses is the Okumura/Hata/Davidson implementation. Hata developed a set of equations that provide Okumura model predictions for computer use. The Davidson correction factors extend the frequency and base antenna height range.

COST-231 Propagation Model

Probe implements the COST-231/Hata version of the COST-231 propagation model. This model uses the HAAT along each radial to determine the attenuation based the following equation:

Path Loss (dB) = 46.3 + 33.9*log( F ) - 13.82*log( H ) + [44.9 - 6.55*log( H )]*log( D ) + C
where

F = Frequency (MHz)
D = Distance between base station and receiver (km)
H = HAAT in the direction of the receiver (m)
C = Environmental-correction factor (dB)

The Hata correction for receiver height and frequency is then applied to calculate the final attenuation.

International Telecommunications Union - ITU-R P.1546-1

The ITU-R P. 1546-1 propagation model was developed field strength predictions for terrestrial services in the 30 MHz to 3000 MHz frequency range. It uses a set of propagation curves that are based on measurement data mainly relating mean climatic conditions in temperate climates. The model considers the transmitter height above average terrain, the receive antenna height, and incorporates a correction for terrain clearance angle when making field strength predictions. The ITU model is used widely in Central America and in Eruope.

TIREM

TIREM stands for Terrain Integrated Rough Earth Model. The model is licensed by Alion Science and Technology Corporation, Annapolis , Maryland . This model started with a Tech Note 101 base but has been modified over the years to make up for believed inaccuracies in the Longley-Rice model.

TIREM predicts median propagation loss from 1 MHz to 40 GHz. The techniques used to calculate these losses include:

Free-space spreading
Reflection
Diffraction
Surface-wave
Tropospheric-scatter
Atmospheric absorption

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.”

[1] Second Report and Order : The 1998 Biennial Regulatory Review - Streamlining of Radio Technical Rules in MM Docket No. 98-93, 98-117. Parts 73 and 74 of the Commission’s Rules

[2] Field Prediction in Irregular Terrain – The PTP Model, Harry Wong, FCC OET, November 1, 2002: FCC URL: http://www.fcc.gov/oet/fm/ptp/report.pdf

Probe - Terrain 3-D