some DMR questions

Hi all,


First, I am new to the radioreference forum. I see there are three forums on digital voice, so I hope I chose the correct one


I have been doing some reading on DMR and dPMR and I have three of questions concerning DMR:

- I know there is DSD that should be able to decoce DMR, but is there an full open-source stack of DMR?
It would be nice to have the basic stack to write our own dPMR repeater for ham-radio use. Are there already projects for this?

- A know that DMR is TDMA-based so requires a SDR radio. However, if I have read correctly, a DMR repeater does not switch on/off the radio every 30 ms. It does have the concept of TDMA, but transmits continues two timeslots. If one timeslot is not used, it is transmitted anyway be it without content.

Is this correct?
Because if this is indeed the case, this would mean that one can build a DMR radio using the simular technologies like D-STAR repeaters: a modem-card (now doing C4FM instead of GMSK) and a FM transceiver with a "9k6" dataport.

- The ETSI specifications of DMR mention an option to run a repeater on a single frequency, using one timeslot to transmit and the other one to receive.
Does anybody know if this is actually used in practice.

For ham-radio use, this would be interesting for -say- 10 meter (to avoid having to use cavities) or the 4 meter band (which is only a couple of hunderd KHz).


Any thoughts?

73
kristoff - ON1ARF

Hi Kristoff
I can answer some of your questions but not all of them.

With DMR repeaters, the repeater itself does not turn the transmitter on and off during transmit. If there is an empty slot it just fills it with idle data. This is also useful because it shows other radios that they can access the repeater on the empty slot.
On the input side is where TDMA is used. The handhelds/mobile users accessing the repeater must use TDMA even if one slot is not being used, since another user may key up the radio and wish to use that other slot.
I think it would be possible to build a DMR repeater without TDMA capability because it only needs to be able to receive TMDA, not transmit it.

It is meant to be possible to use a single frequency repeater and have the input on one slot and the output on the other, but I have never seen any DMR product that can do this so far. DMR was only designed to be used down to I think 80MHz or something around there. A single frequency repeater would only work up to a certain distance, due to the time limitations of TDMA, so it may not be that useful on 4m and 10m as the range of the repeater would be limited by the timing constraints.
However it would be useful on 2m and 70cm where it would make repeaters a lot cheaper and more portable as, as you say, they would not require and cavities. I say this because on 2m and 70cm it is more likely that the limiting factor on range would be due to weak signal and not reaching the maximum time delay distance.

I know DSD can definitely decode DMR, I don't know about the stack. Also you may want to look at another program DMRdecode which doesn't decode voice but is used for data.

Hi Ben,


Thanks for your reply.
I'm sorry for the late reply but I wanted to verify a couple of things before I posted. (just trying to get my facts correct ).

- thanks for your confirmation about the repeater not needing to be TDMA.
I am currently concentrating a bit on dPMR instead of DMR (I've been in contact with a group of hams in PA-country that experiment with dPMR), but I do want to try this out later. Perhaps if we manage to get a proof-of-concept working of a DMR transmittor using a plain FM transceiver, this might get more people interested. (then again, ham-radio operators and coding??? :-( )

Having your own minirepeater would be nice if you are interested in connecting to some exotic reflector, but do not want to force everybody else in your town to have to listen to what you are interested in.


- concerning single-frequency repeaters.
I agree that timing might be an issue. I know that is one of the parameters that affect the range of GSM mobile phone networks, but someone told me that this might be more an issue related to the GSM protocol itself then just TDMA timing.
Perhaps, if we know the location of the repeater and the handset (using GPS), it might be possible to correct for this.

Feel free to correct me or comment if you have more information on this.


But as there are no repeaters that support this, I think we can fear that there will also be a lot of handheld radios that will not be able to cope with it neither!

- Concerning the lowest frequency for DMR, sofar I found in TS 102 361-1 and -2, I do not really see anything in the protocol that really put a hard limit on this (I just did a quick scan on "MHz" in the document). It does mention that it is "designed to be used from 30 Mhz to 1 Ghz", but that is not a technical limit.
There do seams be to a lot more frequency-related parameters for tier-III (trunking) however.

- I did check out DMRdecode, but currently I do not seams to be able to download the java-executable to try it out. (website down?)
Althou I am not a java programmer, the source-code on github does look to be a nice reference for programming low-level DMR.


Again, thanks for your reply.

73
kristoff - ON1ARF

- I did check out DMRdecode, but currently I do not seams to be able to download the java-executable to try it out. (website down?)

Click to expand...

Here is a post with the with program:

http://forums.radioreference.com/vo.../303215-dmrdecode-build-74-a.html#post2297844

Regarding single frequency repeaters:
GSM phone networks have a limit of 35km due to the timing. They sync up the transmitting radio (phone) so that it will fit into it's slot, without interfering with other slots. This is done just by the base station telling the phone the transmit so many milliseconds/microseconds earlier. The time offset has to be adjusted as the phone moves about.
Because the base station is only at one location, all incoming signals can be timed to arrive at exact times so they are within their timeslot. There is of course a guard band to allow some variation as the phone moves but it will be corrected again if it gets too close to another timeslot.
GSM can be changed to allow a range greater than 35km by listening in the next timeslot for an incoming transmission. This is not implemented as standard because it means using an extra timeslot for initialisation. After synchronization, the extra timeslot would no longer be required.
With DMR single channel repeaters, the other stations (which are receiving the repeated signal) would be on the same frequency. This would cause problems if the transmitting radio is told to move to a slightly earlier time, because then for some users it may overlap with the repeater output slot, depending on how far it is from the repeater, and how far the receiving radios are.
For this reason I guess that the radio would have to stay within a range that allows it to stay inside it's timeslot without any adjustment. I'm not an expert on this, but this is just my understanding.
I think there would also have to be enough time in the guard band for the repeater to switch between transmit and receive.

DRM maximum range: 150 km

Hi Ben,

Thanks for your reply. Very interesting stuff and it is nice to be able to compair GSM with DMR.

You message got me delving in the specs again, and .. it actually turns out this is all very well documented.
(See section C.1 of TS102 361-1).
This document define a propogation allowance of 1 ms, as part of the inter-timeslot guardband. This means that the theoretical maximum distance for DMR is 150 km.

This is indeed a lot further away then GSM. This is mainly due to the fact the DMR is 2 timeslot TDMA, while GSM has 8 timeslots. This results in longer timeslots and -therefor- much spare space for the inter-timeslot guardband.


Thx for the reply.

(also thanks to nunyax for the link to DMRdecode)

73
kristoff - ON1ARF

I emailing some engineers at Simoco about the possibly of increased sales opportunities for supporting simplex DMR repeaters yesterday. Funny this thread comes alive.

krbonne said:

Hi all,
Is this correct?
Because if this is indeed the case, this would mean that one can build a DMR radio using the simular technologies like D-STAR repeaters: a modem-card (now doing C4FM instead of GMSK) and a FM transceiver with a "9k6" dataport.

Click to expand...

Yes, commercial market have a lot of solutions like this, ex. Fylde Micro - Digital Radio Trunking

TDMA for downstream needed only in 'one frequency' repeater mode.

bben95 said:

Regarding single frequency repeaters:

Click to expand...

krbonne said:

You message got me delving in the specs again, and .. it actually turns out this is all very well documented.
(See section C.1 of TS102 361-1).
This document define a propogation allowance of 1 ms, as part of the inter-timeslot guardband. This means that the theoretical maximum distance for DMR is 150 km.

Click to expand...

You will not get 150 km (or anywhere close to it) out of a single-frequency repeater.

slicerwizard said:

You will not get 150 km (or anywhere close to it) out of a single-frequency repeater.

Click to expand...

Does anyone currently have DMR equipment that operates below 130 MHz? If not, you're primarily relying on LOS to begin with so 30-50 km would be a much more practical limit in most circumstances.


Sent from my iPhone using Tapatalk

LMR users push the limits. A delivery driver in Niagara Falls will contact his dispatcher back in Toronto - he doesn't have much of a problem hitting the repeater antennas that are 330+m AGL. That's a 70 km hop using 420 MHz.

On a few trips to the Parry Sound area, one of our local news choppers had no problems maintaining contact with their news room via their 900 MHz Privacy Plus mobile. 200 km back to the 110+m AGL repeater antennas.

Those aren't the average distances that LMR radios have to transmit to hit their repeaters, but some users do it every day.

Hi,

W5PKY said:

Does anyone currently have DMR equipment that operates below 130 MHz? If not, you're primarily relying on LOS to begin with so 30-50 km would be a much more practical limit in most circumstances.

Click to expand...

But i did have a QSO with somebody the day before yesterday who was 105 km away from the repeater I was on. It's not that this is "normal" repeater-usage but it isn"t that uncommon.


Anycase, I mention the 150 km limit as that is what is in the specification; but that is for direct mode (i.e. simplex) where two stations need to operate on the same frequency and one station needs to syncronise itself with the other station on the frequency.

For me, I am actually interested how well all the different DV modes match up in simplex. (I really would like to try FM-based DV where there is Ee on 10 or 6 meter )


73
kristoff - ON1ARF

Propagation delay is the limiting factor below 140mHz. Even at those frequencies, performance is noticeable worse then at 440mHz.

It just falls flat on its face below that.