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TDMA, specifcally DMR, slot timing errors limiting range?



I've read about there being range limitations when using TDMA digital modes, like DMR, due to clock drift in the base and mobile stations. The issue seems to be the slot timing verses distance and length of transmissions. See section on page 112 in the following link to one of the ESTI DMR standards.


Anybody have any experience with this or did the detailed calculations for their installation? I've seen comments where some have gone to FDMA type modes to avoid this issue. 

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Off the cuff (emphasis is mine):


In the case where one MS (MS 1) is transmitting on slot 1 in very close proximity to the BS, the 1 ms propagation delay allowance enables another MS (MS 2) to transmit on slot 2 up to 150 km from the BS without inter-slot interference if there is no additional impact of clock drift error.

Most DMR radios are NOT transmitting through repeaters (the base station providing timing) at distances of 150km (93 miles)! Even a 25 mile radius seems to be doing well for the DMR repeaters in my area (the MI5/CMEN system has THREE repeaters near me: Lowell, Grand Rapids, Byron Center -- Byron Center to Lowell is just 20 miles as the radio waves fly; an HT accessing Byron Center [from Metro Chemo Center] has no reach to GR or Lowell])

HT to HT (or other simplex combinations) are probably synching to each other, and at much shorter distances than most repeaters. Other than repeaters (which have to handle both time slots to justify TDMA) I suspect mobile to mobile rigs only handle one time slot at once and don't have to really synchronize enough to let "many" units into the signal stream (What, your group outing has two radios on time slot 1 and another two on time slot 2, but on the same frequency, and you expect simplex to /not/ interfere when there is a distinct lack of a master time source?)


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21 minutes ago, MichaelLAX said:

Have you ever compared the range of an analog FM broadcast station to its HD-1 digital component?


The issue was looking at the range limit due to problems when the radios can't sync the time slots from signal delays. If the time slots overlap then communications basically fails. That was the focus of the mentioned section in the ESTI standard. I was curious if anyone has experienced this, or at least can be reasonably sure that was the cause.

The second issue was clock drift in the radios. Unless one is using extremely accurate clocks, like synchronized to an atomic standard, they will drift over time. BTW old cell sites used Rubidium atomic clocks but now use GPS disciplined clocks to maintain sync.



The clock would be the internal master oscillator reference used for the frequency reference. After some period of time, likely just seconds, after the initial slot synchronization for a transmission the slot timing may overlap between slots 1 and 2. Remember the frequency drift is for both radios at each end. Again communications fail. That seems to place a piratical limit on the transmission time length too.

If you look closely at the cheaper DMR radios, compared to the more expensive commercial versions, they have sort of crappy oscillator accuracy and drift spec's. For example the few Motorola XPR-6550's I have spec at +/- 0.5PPM, my Kenwood TK-D340's are +/- 1.0 PPM. The popular D878UV is +/-2.5PPM ! Ouch.

Some base radios, and or repeaters, could be using OCXO, oven controlled crystal oscillators, to maintain frequency accuracy and very low drift rates. The best hand held radios only have TCXO, temperature compensated crystal oscillators, because the power drain for the oven heater is far to high for battery pack powered radios.



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4 hours ago, MichaelLAX said:

In my limited experience with United States FM-HD, the analog signal has a longer distance range than the HD digital channels.

In one respect, I think that is true of all digital modes given equal ERP.

Digital tends to stay "pure" until you reach the distance where bit loss starts to occur, and then just drop off.

Analog at distance tends to slowly degrade (more "hiss", less fidelity, but none-the-less legible) until you are basically out of the transmitter area of coverage.


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