TalkBack Margin
Q: What is the “TalkBack Margin” used in TAP TalkBack calculations?
A: TAP uses the calculations of the TalkOut coverage (base station to remote radios) to compute the area of TalkBack (remote radios to the base station) using the differences between the two systems, or the “TalkBack Margin.”
Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.
For example, consider a very unusual communication system, consisting of a fixed location base station, such as a repeater, and one or more mobile units:
This is an unusual mobile system because the mobile radios are identical to the fixed base unit. The transmitting specifications (100Watt transmitter, 9dB antenna gain, 1dB line loss) and the receiving specifications (3dB antenna, 0.5dB line loss, -110dBm input threshold) are identical at both ends of the communication circuit.
The TalkOut coverage (from the base station to the area where the mobiles will be operating) can be computed with HDCoverage:
Since the transmit and receive facilities are identical (base to mobile or mobile to base) and assuming the free-space loss and path loss are both reciprocal along each path computed (the losses along each path are independent of direction), then the area where TalkBack service is possible would be the same area as shown for the TalkOut service. (Note that this assumes that the base station transmit and receive antennas are at the same height above ground, and likewise the mobile transmit and receive antennas are at the same height.)
Now consider another communications system, slightly modified:
In this case, all the base and mobile parameters are identical, except that now the mobile transmitter power output (TPO) is reduced from 100Watts (50dBm) to 1Watt (30dBm).
Now the TalkBack communications system is 20dB “less efficient” than the TalkOut system. Therefore, we can represent the TalkBack system by de-rating the TalkOut coverage area by 20dB.
For example, suppose the computed Required Field for the TalkOut system is 6.35dBu, then the TalkBack area can be represented by the area with TalkOut service at least 20dB above that threshold, or 26.35dBu:
This is the approach used in the TAP TalkBack calculations, comparing all aspects of the base-to-mobile and mobile-to-base systems (gains, losses, power levels, receiver thresholds, etc.) The net difference between the two systems is the “TalkBack Margin” as computed from the TAP “Utilities” menu:
The TalkBack System Margin is computed when the Base and Mobile parameters are entered (or when the “…” lookup buttons are used to read values for particular Fixed or Mobile Facility records).
In this simplified example, the TalkBack Margin is an even 20dB. Normally, when all parameters are considered (differences in antenna gains, line losses, power levels, receiver thresholds) the actual value will be the sum of the combined differences, not necessarily a round number as above.
The TalkBack Margin is computed automatically when the TalkBack Calculation function is used from the TAP “Area” menu. The TalkBack System Margin utility is provided to enable TAP users to investigate and better understand the TalkBack calculations.
The TalkBack System Margin calculator can also be used to enable you to show TalkBack coverage as a contour. For example, if HDCoverage is used to compute the 6.35dBu, 95% contour for the TalkOut coverage, then (based on the above example) the 26.35, 95% contour could be computed to represent the TalkBack service area:
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Copyright 2011 by SoftWright LLC
