80 miles is near the DMR limit.
10.2.3.1.3 Propagation delay and transmission time
A 1 ms propagation delay allowance is built in to the Normal Burst structure for propagation delay and time base clock
drift. This allowance protects against inter-slot interference at the base station receiver. The MS shall time synchronize
with the BS before transmitting. Actual time synchronization at beginning of transmission will deviate from 0 ms by the
propagation delay. As MS transmits, the time base clock drift error may cause further time deviation from "true" time
synchronization.
This 1 ms allowance theoretically enables a mobile to operate up to 150 km from the BS without inter-slot interference.
However, as the MS transmits it may deviate from true synchronization and cause inter-slot interference. Therefore a
system trade off is necessary to account for both propagation delay and clock drift deviations. This trade off is between
the maximum MS operating distance from the site and maximum transmission time of the MS. The following example
details how this trade off is calculated.
The amount of time allocated to propagation delay is determined by the intended maximum coverage distance.
Maximum round trip propagation time is defined by:
Maximum Round Trip Propagation Time = 2 × (Maximum Distance/c)
where:
• c is the speed of light.
NOTE 1: The factor of two is included for round trip propagation delay.
For a 75 km maximum distance between the base station and a mobile station, 0,5 ms of the 1 ms allowance is
dedicated to propagation delay. This leaves 0,5 ms for time base clock drift during transmission. The maximum time
base clock drift error as specified in clause 10.1.4 is ± 2 ppm and worse case occurs when one MS clock drifts fast and
one MS clock drifts slow. Under this situation, the MS maximum transmission time is defined by:
Maximum Transmission Time = 0,5 × ((Clock Drift Error Allowance) / (Drift per Symbol)) × Symbol Time
where:
• clock drift error allowance is 1 ms - Maximum Round Trip Propagation Time; and
• Drift per Symbol = 0,4167 ns for 2 ppm clock stability.
NOTE 2: The factor 0,5 is included to take into account drift from two independent MS drifting in opposite
directions in time.
For the case where Clock Drift Error Allowance = 0,5 ms, the Maximum Transmission Time is 125 s.