ARLA/CLUSTER: DMR vs. Tetra (and conclussions for D-STAR)

[João Gonçalves Costa]

Hi Darren,


(Changed the subject line to match content better)


>as usual, you raise a very interesting point: Range (reference to OM access to a repeater 100 km away).


> Tetra is not exactly ideal for "typical" amateur usage because in TMO (repeater) mode, there is a 58 km absolute limit to
> transmission range (in common with the TDM range limits in GSM). This range limit can be slightly extended (83 km) for
> special services such as Tetra communication from helicopters (AGA mode) but the limits are hard coded within the
> specification.


> Given that /\/\otorola digital is TDM based technology (2 time slots per carrier), does anyone know if there is a hard coded
> limit on (base to mobile) transmission range?


> During enhanced conditions I have worked over 320 km (200 miles) with D-STAR on UHF from my car, it would be a
> shame to lose the ability to work DX by moving to other digital technologies that weren't designed for ham radio use.




After doing some searching on what you mentioned, I found two interesting document both compairing DMR (ETSI TS 102 361) with TETRA; which do have some nice conclussion which also apply to D-STAR.

- http://www.rrmediagroup.com/onlyonline.cfm?OnlyOnlineID=174
- http://www.zs6ro.co.za/files/documents/DMR/DMR_vs_TETRA_comparison.pdf
(especially the part "Coverage vs. Traffic")



Some conclussions which are also interesting for D-STAR seams to be:

- D-STAR (like DMR) has a relative simple modulation sceme (DMR uses 4FSK, D-STAR uses GMSK). This is less demanding on the linearity of the amplifier then TETRA which then has a number of concequences:
-> simpeler tranceiver design and the ability to use analog FM-transceiver (as used by all D-STAR homebrew projects)
-> because of that, easier to design high power transceivers (which is good for range).

It should be noted that this easier modulation sceme is a trade-off which results in a lower "bit/Hz" ratio and therefor less capacity per spectrum.
TETRA is designed to handle high demands of capactity in a small concentrated area (say a large fire in an urban area). D-STAR would simply not be able to handle this kind of situation.


- The importance for VHF for long distance. TETRA is UHF only (althou the specification do support 100 Mhz to 999 Mhz). The ability to use 2 meter for D-STAR is a good point for range.

As with the different homebrew projects for D-STAR, we are no longer dependent on i-com for radio and is now possible to move D-STAR down to 70, 50 or even 29 Mhz. It might be interesting to see how far D-STAR range can be extended this way, especially for "mobile" use in rural areas.

- Also, as you mention. D-STAR (unlike both DMR and TETRA) is a single-user-per-channel system. Having multiple users on the same channel (TDMA) imposses constrains on timing, and therefor distance. D-STAR does not have this problem.

It would indeed be sad to lose the ability to catch a rare "DX-contact" on digital voice too!




However, before we start getting to victorious saying "D-STAR is perfect for us", there DO still are other technologies out there that might be interesting to have a look at.

On of them is dMPR (ETSI TS 102 490 / TS 102 658) that Trevor already mentioned in this list. It is also a FDMA system (like D-STAR), but with slightly different modulation parameters:
- C4FM (4FSK) modulation: 2 bits / symbol
- channel width of 6.25 Khz (actually, more around 4 to 5 Khz), vs. about 7 to 8 in reality for D-STAR.

It would be interesting to do some experiment with this, e.g. run D-STAR on top of C4FM in a 6.25 channel.
- What is the impact of range? (smaller channel width would reduce noise, 2 bits / symbol modulation would need higher SNR ratio to demodulate).
- Can it run on 29 Mhz (where the smaller channel-width with would be a big advantage)?
- Can it be implemented and still maintain backwards compatibity for the existing infrastructure (e.g. use it only on 1.5, 4, 6 or 10 meter where all equipement is homebrew anyway, use it only on repeaters for repeater input, ...)



So, sofar my conclussion is, at first sight, D-STAR is actually doing a pretty good job as a ham system and does have a number of big advantages over TETRA; simply because the latter has been designed for a completely different enviroment.
TETRA does not work in the more exotic ham enviroments (like long distance communication. D-STAR would not work well in the high-capacity high-usage enviroments which are more typical for emergency situations.

But, this does not mean that it is not interesting to look what else is out there; learn from it, experiment with it; especially in the systems that do have open specifications and are possible to do in homebrew projects.




73 de Darren
G7LWT
73
Kristoff - ON1ARF

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