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Annex A

Coverage Determination Methods

A-1. General

The parameters used to calculate the propagation studies are based upon common engineering assumptions as specified in this annex. A change in any of these factors will affect system propagation.

A-2. Methods and Data

Algorithm

Attached are propagation coverage maps of the subject radio system. The tile matrix technique rather than the older and less accurate radial line technique, is used by the by the most TIA (Telecommunications Industry Association) compliant software package available to determine accurate coverage.

The Longley-Rice algorithm (also called NBS 101) is used for mobile and portable talk back coverage. . Longley-Rice is a detailed method of calculation which takes into account factors such as refraction, reflection, the effect of the conductivity of the earth and other factors. Longley-Rice is also a statistical measure of coverage. All Longley-Rice studies in this report are configured at a 90% probability of having the signal strength portrayed on the maps.

Terrain Database

The USGS 3' terrain database is used to determine height and location data for the software model. The 3 arc second database is the one most commonly used for propagation studies. Its point spacing of about 90 meters north-south by an average point 70 meters east-west seems appropriate for many planning purposes, especially when wide-area systems with service radii of 50 km or more are being considered.

The main drawback to the 3 arc second database is its vertical accuracy. For the most part it was derived from the 1:250,000 series of maps covering the US. Most of these maps have contour intervals of 200 feet. The result is that many ridges and hills with peak elevations that lie between 200 foot contour intervals are not properly represented. Even some peaks where USGS benchmarks are shown on the maps were not properly digitized. Occasionally, elevation errors occur, some as great as 200 meters.

Land Use, Land Cover Database

The USGS Land Use, Land Cover (LULC) database is used to present a more accurate portrait of the performance of the radio systems. The LULC database assigns an attenuation value to radio waves based upon the type of terrain (Buildings, Cropland, Forest, etc.) and incorporates this into the field strength and talk back studies.

A-3. Units of Measure

Each study has a legend giving the scale and other important factors used in the production of the coverage map. Generally in a talk-back study the map coloring represents the probability of a mobile or portable unit, with the specified power level and antenna type, in pushing its signal back to the repeater's receiver.

The unit of measurement used to show signal strength is the decibel-microvolt (dBm). In reference to the ability of a radio to receive an radio signal, most manufactures use uV (micro volts), while most engineers use dBm as an equivalent measure of field strength. The conversion that is useful to know is that in a static situation, most modern radio equipment will receive a useable signal at appropriately .3 or .25 uV. In converting, .3 uV is equal to -117 dBm.

As movement and statistical fading is introduced, there is generally about a 10 dB degradation in system sensitivity. This reduces system sensitivity to about -107 dBm and is the basis of the minimum acceptable service level shown on the propagation studies. Signal levels between -107 and -117 dB are shown as light gray because mobile and portable units may be able to talk out of those areas, but at a reduced level of reliability.

A-4. Building Penetration

Generally, buildings have an 6 to 14 dB loss at the ground floor, with 6 dB less loss if windows are present. This loss decreases by about 2 dB for each floor to somewhere between the 9th to 14th floor for highrise buildings. After that height, the loss starts to increase by about the same amount.

Part of the penetration loss is accounted for in the propagation studies by de-rating the receive system somewhat and partly taken into account by the conservative nature of the propagation study itself.

A-5. Other Factors

There are other factors, which can not be taken into account by any software program, except in general terms, which will effect a radio's ability to work on the system. Some of them are: incorrect polarization of portable antennas, incorrect placement or damage to the mobile antennas, radio equipment de-tuned, low battery power, obstructions with high reflectivity to radio waves, sunspot activity, atmosphere inversions, foliage not accounted for in the LULC database, and others. Therefore any computer simulation will always be less than perfect. What it is a a guide to the expected general performance of a system.

A-6. Transmit Calculations

For mobile talk back studies, UHF mobiles of 25 watts, with a unity mobile antenna was used in all calculations.

A-7. Receive Calculations

A large variable in talk back performance is the overall system gain (in dB) of the receive side of the radio systems.

System gain is the total of all positive gain in the system (antenna gain at the repeater and at the mobile, multicoupler amplification) minus all loss due to hard-line, connectors, lightning protection, filters, and jumpers.

An example of system gain calculation:

Antenna gain: +10.0 dB

Line loss: -2.0 dB

Connector loss: -1.0 dB

Amplification: +5.0 dB

Mobile antenna: +3.0 dB

Total System Gain: 15 dB

Amplification - at either the tower top or at the receiver multicoupler - can vary widely, based on frequency spacing, site frequency usage and other factors. While the example above shows 5 dB amplification, amplifiers capable of in excess of 20 dB gain are available.

For these studies, an overall system gain of 10 dB was used, with a -117 dB receive sensitivity of the repeaters.

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