User's Manual

UtiliNet® Endpoint User Guide 1-5-2007

Page 24 of 30

For most receive signal measurements, slight changes of antenna location (on the order of 1 wavelength),

or measurements at different times, will show a short-term variance of received signal level. This is

generally due to multipath. An allowance of approximately 8 dB should be made on most radio links at 915

MHz. This is because the difference between a median received signal level and a multipath null does not

exceed 8 dB for about 80% of the measurements you could make. This factor can be reduced by one or

more diversity schemes that are available. All Cellnet spread-spectrum radios inherently employ frequency

diversity; however, it doesn’t hurt to be conservative and still include this factor. Tabulated below are 915

MHz multipath factors for other values of percent coverage. This factor varies slightly (1 to 3 dB) for

various types of environments. Values shown are for 915 MHz urban area, small city.

Table 3.2 Multipath vs. Percent Coverage

Margin of Safety

Radio systems are usually not designed to operate precisely at threshold, or even at threshold plus the

multipath factor. Usually they are designed with some extra margin. This is called a margin of safety, and is

used to account for long-term variance in the received signal level. A value of 0 to 35 dB is suggested—the

actual number depends on how conservative you are. You will use 5 dB for this example. If the actual path

could be held constant at the receiver threshold, and never varied, you would see data success rates of at

least 90%. This is because at receiver threshold, the receiver provides no more than 10% packet error rate.

Since the data link layer of the radio software only passes good packets and blocks bad packets, the data

success rate would be 100% minus 10%, or 90%. In practice, however, received signal levels vary

substantially. Rx signal levels cannot be precisely predicted. In fact, Rx signal levels vary due to such

factors as vehicular air traffic, precipitation and air temp changes, wind movement of trees and antennas,

tree growth, city growth, and population expansion. Systems are usually designed with an additional

margin of safety to overcome these and other variances for the next several years of operation.

Summary

For this example, the link budget is calculated as shown. Note that losses and margin of safety are shown as

negative numbers, since they subtract from link budget.

Table 3.3 Example Link Budget Calculation