Computing the Threshold Value

Land mobile receivers yield sensitivities of typically -116dBm for 10dB SINAD.  This means that in a steady state (on the bench) a usable signal to noise ratio at the loudspeaker will be generated for quite a low signal level of around 0.3uV p.d..  If we put this sensitivity into the path budget for this Land Mobile Coverage Plot, we will be ignoring the practicalities of the communications.  There are two margins to be added.

Fade Margin

Mobile radio signals vary considerably with location and this yields a 'locations variability' as the mobile moves across each terrain point (equal to the terrain resolution at 200m in this case).  To achieve a usable signal for a high percentage of locations as the mobile moves through fades requires a fade margin.

Without a fade margin the communications would be possible in 50% of locations.  An acceptable locations percentage would be 90% for most commercial applications.  A fade margin of 10dB is needed to raise from 50% to 90% and a further 10dB would be needed to raise from 90% to 99%.

Planning Margin

The propagation prediction models used in this plot are highly accurate when compared to some but nonetheless there are errors.  Without making use of ground clutter, the errors are inevitably higher in urban areas.  Prediction models are described by their average error and standard deviation of error.  Typically the standard deviation of error of the model used here is 8dB. 

Planning margin is considered when the planner wants to be certain that at least the coverage indicated will be achieved.

To be 90% certain that at least the coverage shown will be achieved, one must add 1.25 times the standard deviation as a planning margin.  This would need a further 10dB.

Practical Application

To be 90% certain of achieving a location's probability of at least 90% in all of the areas shown one must add a 20dB margin leading to a threshold of -96dBm.