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TDMA timing question, near far issues

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Oct 12, 2009
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bridlington, uk
#1
Hello guys,

Does any one on here have first hand experience on tdma near far timing, i am looking to the future we have 5 vhf sites mpt1327 and at some point we will look at moving customers that want to go to digital, i am not looking at the audio quality side of things in this question just distance as i have sites that can easily cover 80 miles. I am just trying to look how tdma/fdma from that point of view would be a issue, we have no issues with timing on mpt1327 at any distance as the site can wait extra slots.

phill, uk.
 
Joined
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bridlington, uk
#3
Yes i know what but other radios dealers in the area have had mototrbo and then gone to fdma simply becuase of the timing issue, if it wasnt a issue then why do gsm base stations have distance limits
 

N4GIX

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#4
I can only offer some anecdotal information, but the W9CTO 70cm DMR repeater (Gary, Indiana) has an effective footprint of around 90 miles radius. We have experienced zero timing issues with mobiles at the extreme edges of coverage. The receive antenna is at 520' and the transmit antenna is at 480'.
 
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#5
Hi N4GIX that what i want to here, many thanks i am going to be buying a mototrbo repeater soon and just wanted some assurances, i understand clearly that the big M and Tait would not do this is there where fundamental issues but just wanted to hear it from the man in the street as we say over here, thanks again
 

N4GIX

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#6
No problem. In fact, W9CTO has replaced his 2m analog with a shiny new SLR5700. I'm ordering a UHF SLR5700 on Monday just so we can have the Brandmeister network available here in NW Indiana as well as our Hoosier DMR and Chicagoland DMR networks.
 
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Toronto, Ontario
#8
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.
 
Joined
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PA
#10
But TDMA is distance limited. The issue is that if the roundtrip time is too long, your transmission is no longer matched up with the expected slot.
the 3ms buffer between timslots translates to about 900 kilometers of round-trip delay (450 kilometers one way) before you'd have timeslot overlap. Relative clock drift is a more significant issue, but only if you're engaging in an extended monologue during a rag chew. For typical radio use where TX length is more like 10-20 seconds, neither is an issue. The signal will fade out long before clock drift or propagation delay has any effect whatsoever.
 
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Messages
788
Location
Springfield, MO
#11
While I know that dual slot TDMA has a theoretical distance limitation, and while that may be the case with some TDMA systems, I've never heard of such a problem with any MOTOTRBO system. And I've spoken with a LOT of dealers across the U.S., and many hams around the world, who are using MOTOTRBO. If this was a -real- problem, I would think that I would have heard of such a problem by now.

John Rayfield, Jr.
 

DaveNF2G

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Rensselaer, NY
#12
Based on comments already posted, I would conclude that nobody is building TRBO systems wherein signals must travel far enough for it to be an issue.
 
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Jul 18, 2014
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Location
PA
#15
Do cheap chinese radios have high quality reference oscillators that are stable enough to not drift and cause problems?
They use the same oscillators needed to stay on frequency. Not necessarily lab-grade calibration reference, but good enough that clock drift is not an issue in normal circumstances.
 
Joined
Oct 12, 2009
Messages
207
Location
bridlington, uk
#16
Tait says;

In practice, if this situation occurs, the DMR range is around 75kms and TETRA around 30kms. This is due to the gap time being longer in DMR. This is unlikely to be a limiting factor in an urban environment but in a flat rural setting, it could be significant.

DMR or TETRA: A Technical Comparison of Coverage | Tait Communications Blog

I have one site that covers 100km without effort so i will try it out but think i refer nxdn anyway.

phillmobile
 
Joined
Jun 16, 2013
Messages
3,241
Location
Texas
#18
I bet Simoco would cut you a deal on repeaters (best price for Tier 3 right now). They are also built for a very, UK frame of mind. 10m sites, 25 sites without any controllers/servers, higher redundancy and failover compared to others.

Also, their SDM730 mobiles handle Analog, MTP1327, Tier 2 and Tier 3.




Sent from my iPhone using Tapatalk
 
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