Sporadic DMR activity.

[KAF6045]

I had to take .575 off my scan list as it seems the City of Grand Rapids is trying to implement DMR on their repeater (while allowing regular FM also)

 

[Lscott]

9 hours ago, KAF6045 said:

I had to take .575 off my scan list as it seems the City of Grand Rapids is trying to implement DMR on their repeater (while allowing regular FM also)

 

You sure about that? To date there are no digital voice modes authorized by the FCC for GMRS. If it’s being done by the city are they operating under a grand fathered license? Checking the FCC database should show what frequencies are licensed and emission mode. The later will be the clue if they have authority to run digital voice.

[KAF6045]

6 minutes ago, Lscott said:

You sure about that? To date there are no digital voice modes authorized by the FCC for GMRS. If it’s being done by the city are they operating under a grand fathered license? Checking the FCC database should show what frequencies are licensed and emission mode. The later will be the clue if they have authority to run digital voice.

It's an old grand-fathered license -- shows up in the 1997 GMRS repeater guidebook I have. The license is likely much older than that, and predates any digital voice format. Seems to mostly be used by parking lot control -- lots of reports of gates not going up or down, payment machines refusing either credit or cash, at least one mention of the Museum...

And I overheard one city user (on analog) explicitly state that the DMR wasn't quite working -- but that didn't stop an afternoon of dit-dit-dit-dit noise (I'm presuming some of that is timeslot breaks; if trying to run DMR over an analog repeater I suspect they'll need to revert to single/no time-slot [Tier 1?] mode, otherwise there's no master timing control).

[Lscott]

1 minute ago, KAF6045 said:

It's an old grand-fathered license -- shows up in the 1997 GMRS repeater guidebook I have. The license is likely much older than that, and predates any digital voice format. Seems to mostly be used by parking lot control -- lots of reports of gates not going up or down, payment machines refusing either credit or cash, at least one mention of the Museum...

And I overheard one city user (on analog) explicitly state that the DMR wasn't quite working -- but that didn't stop an afternoon of dit-dit-dit-dit noise (I'm presuming some of that is timeslot breaks; if trying to run DMR over an analog repeater I suspect they'll need to revert to single/no time-slot [Tier 1?] mode, otherwise there's no master timing control).

The various digital voice modes use variations of FDMA modulation. In the case of DMR it’s also done in 30 milliseconds bursts per time slot. Since the frequency shift is done directly on the carrier, it’s not audio frequency shift keyed, and the bit stream there will likely be very low and high frequency components. The radio’s Discriminator will try to decode it as audio. The high and low frequencies will get filtered out by the audio circuits so when applied to the transmitter there will be high data loss, missing data bits. The loss likely will be too high for the RX radio to perform adequate error recovery so the communications fails.

[WQBI410]

I'd like to see NXDN or P25 Phase 1 allowed. NXDN is 6.25kHz (which seems to be the way the FCC is heading anyway), and P25 sounds good. I don't see the attraction for DMR, so that, to me, is out.

[Lscott]

44 minutes ago, WQBI410 said:

I'd like to see NXDN or P25 Phase 1 allowed. NXDN is 6.25kHz (which seems to be the way the FCC is heading anyway), and P25 sounds good. I don't see the attraction for DMR, so that, to me, is out.

The FCC wants either 6.25KHz bandwidth or the equivalent at some future point in time. When that might happen is anyone's guess. DMR can do the "equivalent" part since it can run two voice streams in a 12.5KHz channel bandwidth.

The one attractive part of DMR is the ability to build a repeater without the necessity of using expensive and bulky cavity filters. This is the so-called SFR, single frequency repeater. The hand units would TX, for example, on slot 1 and RX on slot 2. The repeater would RX on slot 1 and TX on slot 2. All of this transpires on the same frequency. To change the repeater frequency simply requires switching to another frequency by all involved users. No cavity filter re-tuning required.

I've read many comments about how one mode sounds better that another. Consider that DMR, P25 and NXDN all use the same exact vocoder, the AMBE2+, which can run at two different data rates. When running the same data rate, NXDN has two by the way typically offered by Kenwood radios, any differences likely has more to do with other factors with the radio and not the mode as such. Otherwise they should all sound about the same based on the vocoder itself.

Where things get sticky with DMR is when a particular site has no need for the extra voice stream, effectively it goes to waste so you end up with one voice stream occupying a 12.5KHz channel. Unless two separate sites have radios with DCDM, duel capacity direct mode and within range of each other, the radios can't sync to a master time slot signal. You also then have to coordinate between sites who uses which time slot etc. This would just make the job of the frequency coordinator far more difficult. The job of the frequency coordinator is to reduce or eliminate interference between sites by distance and frequency diversity means, just the opposite of what DCDM would require.

A true 6.25KHz bandwidth would allow the frequency coordinator more flexibility to assign channels since there would be more of them, physically, due to the narrower bandwidth possible. Not every site out there needs a repeater, the usual source for the master slot timing signal, simplex communications works fine for them. This point is very often overlooked by proponents of DMR.  

[intermod]

I look forward to trying DMR SFR mode out - my Hytera models are too old to have the option.  SFR should dramatically reduce the cost and complexity of repeater deployment (like 30-40%).   But it is challenging to efficiently fit this into the current frequency allotments as well as higher-elevation shared communications sites using a single antenna. 

 - If the SFR transmits on the 462 side, its receiver is in the same portion of the band used by other co-located transmitters (460-465 MHz; desensitization and noise), and it will hear all the repeater outputs from miles away, as well as all the bubblepack traffic; its receiver will get interfered with.

 - If operated on the 467 MHz side, it will will interfere with other co-located site receivers, and well as other repeaters with receivers on 467 MHz.

So today, this seems to be a great solution when you have limited co-channel signals, a site without many other UHF repeaters, low-elevation applications and temporary emergency use.        

The Hytera approach also seems compatible with some non-Hytera model radios (can I just set them for normal Direct Mode?), as I read here that Motorola's approach (using a repeater instead of mobile or portable radio) requires split Color Codes.  But Hytera's approach may also need Pseudo-trunking capable radios (ability to listen for traffic on either slot).  Motorola may be the only manufacture that refuses to provide this capability.  Hytera, Kenwood, Kirisun and possibly some Anytone radios can do this today. 

[Lscott]

15 hours ago, intermod said:

I look forward to trying DMR SFR mode out - my Hytera models are too old to have the option.  SFR should dramatically reduce the cost and complexity of repeater deployment (like 30-40%).   But it is challenging to efficiently fit this into the current frequency allotments as well as higher-elevation shared communications sites using a single antenna. 

 - If the SFR transmits on the 462 side, its receiver is in the same portion of the band used by other co-located transmitters (460-465 MHz; desensitization and noise), and it will hear all the repeater outputs from miles away, as well as all the bubblepack traffic; its receiver will get interfered with.

 - If operated on the 467 MHz side, it will will interfere with other co-located site receivers, and well as other repeaters with receivers on 467 MHz.

So today, this seems to be a great solution when you have limited co-channel signals, a site without many other UHF repeaters, low-elevation applications and temporary emergency use.        

The Hytera approach also seems compatible with some non-Hytera model radios (can I just set them for normal Direct Mode?), as I read here that Motorola's approach (using a repeater instead of mobile or portable radio) requires split Color Codes.  But Hytera's approach may also need Pseudo-trunking capable radios (ability to listen for traffic on either slot).  Motorola may be the only manufacture that refuses to provide this capability.  Hytera, Kenwood, Kirisun and possibly some Anytone radios can do this today. 

The first "miracle" required is just getting the FCC to allow digital voice modes on GMRS in the first place.

[intermod]

Agreed.   We have several GMRS DMR repeaters now but they are under temporary FCC experimental licenses.  Testing SFR could fall under this license however. 

G 

[tweiss3]

1 hour ago, intermod said:

Agreed.   We have several GMRS DMR repeaters now but they are under temporary FCC experimental licenses.  Testing SFR could fall under this license however. 

G 

I just looked up this license, and saw they got 100W approved for the repeater. Interesting.

[Lscott]

1 hour ago, tweiss3 said:

I just looked up this license, and saw they got 100W approved for the repeater. Interesting.

Being TDMA with about a 50 percent duty cycle that works out to an average power of about 50 watts. Maybe that's how the FCC looked at it for the license grant.

[tweiss3]

7 minutes ago, Lscott said:

Being TDMA with about a 50 percent duty cycle that works out to an average power of about 50 watts. Maybe that's how the FCC looked at it for the license grant.

Thats a thought. Not to derail the the conversation, but DMR would make sense in more than 1 way.