DMR vs P25

[hill]

FDMA is 2 talkpaths, literally right beside each other, with virtually no separation, right?

No that is TDMA

 

[RRR]

Negative..
Frequency Division is dividing the frequency in half, using one half of the frequency for the voice.
TDMA is using the entire width of the frequency, but alternating it back and forth, IE; Time division.
I ask again, is there any real use of dividing a frequency via FDMA, and successfully using both halves at once, for voice, like TDMA does? Or can data be used on the other "half" of the width?
If so, how would you even program that on KPG111 or D1N?

 

[slicerwizard]

While all this technical stuff is interesting to me and others here, an example of real life DMR unacceptable performance in my area is interesting.
The county sheriff agency converted to DMR awhile back and the had nothing but trouble with voice clarity and range. The radio techs worked on it for months but the sheriff had so many complaints from staff and examples of public safety being compromised that he ordered everything back to analog and it remains there today.

Had a local shop manage to put up a DMR system with the RX filters swapped between channels (filter A on channel B, filter B on channel A). RX sensitivity was down by something like 30+ dB. Symptoms exactly as you describe above.

Techs swore for weeks that everything was set up properly and "That's just how DMR works."

Finally took a demo of a portable DMR radio at the base of the tower (which was on decent high ground) working another portable on simplex at at least two to three times the system's pitiful range to light fires under everyone's butts.

Good times.

 

[slicerwizard]

Negative..
Frequency Division is dividing the frequency in half, using one half of the frequency for the voice.
TDMA is using the entire width of the frequency, but alternating it back and forth, IE; Time division.
I ask again, is there any real use of dividing a frequency via FDMA, and successfully using both halves at once, for voice, like TDMA does? Or can data be used on the other "half" of the width?
If so, how would you even program that on KPG111 or D1N?

Negative. FDMA is using separate RF carriers for separate information streams, such as voice calls.

TDMA is using a single RF carrier to transmit two or more information streams.

FDMA has nothing to do with "dividing a frequency".

 

[nd5y]

FDMA is 2 talkpaths, literally right beside each other, with virtually no separation, right?

No. It only means each talk path is on a different RF channel.

 

[nd5y]

No that is TDMA

No. TDMA is mutiple talk paths on the same RF channel on different time slots.

 

[Echo4Thirty]

Negative..
Frequency Division is dividing the frequency in half, using one half of the frequency for the voice.
TDMA is using the entire width of the frequency, but alternating it back and forth, IE; Time division.
I ask again, is there any real use of dividing a frequency via FDMA, and successfully using both halves at once, for voice, like TDMA does? Or can data be used on the other "half" of the width?
If so, how would you even program that on KPG111 or D1N?

I think you are getting confused with the NXDN concept of splitting 12.5 kHz bandwidth into 2 6.25 kHz channels by putting one center frequency in the top "half" and another in the lower "half" of the total 12.5 channel. We did this with several NXDN multi-site systems. We were not able to do this at a single site as there was no filters that could work with the split channels in the same combiner.

That said, regardless of frequency or bandwidth, NXDN is an FDMA emission. DMR is a TDMA emmission as there are 2 streams of information multiplexed into one RF carrier in the allocated bandwidth. LMR Examples Below. This is by no means an exaustive list.

FDMA:
NXDN
P25 Phase 1
D-STAR
Yaesu Fusion
M17

TDMA:
DMR
P25 Phase II
Tetra
OpenSky

 

[bunangst]

The term FDMA may have made sense when 25 kHz licenses (think old wide band analog 800 MHz) were split into two adjacent 12.5 kHz channels.

Does anyone know if this was done on the same Transmitter and Receiver?

I am not a P25 expert, but Phase 1 only allows 1 talk path per 12.5 kHz.

 

[mmckenna]

I ask again, is there any real use of dividing a frequency via FDMA, and successfully using both halves at once, for voice, like TDMA does? Or can data be used on the other "half" of the width?
If so, how would you even program that on KPG111 or D1N?

FDMA doesn't necessarily mean that the frequencies are side by side, or a split 25KHz or 12.5KHz channel slot. It just means that separate conversations happening at the same time are separated into different channels. The channels could be side by side, or 20MHz apart, doesn't matter.

NXDN and P25 Phase 1 trunked systems are FDMA.

NXDN -can- take a 12.5KHz channel and run two separate repeaters side by side, one on the upper 6.25KHz from the center, and another on the lower 6.25KHz from the center. It's been done, but it isn't very common as it requires a lot of filtering between the two transmitters and receivers to prevent desense. Usually easier to just run two different 6.25 (or 12.5KHz) channels.

 

[kayn1n32008]

NXDN -can- take a 12.5KHz channel and run two separate repeaters side by side, one on the upper 6.25KHz from the center, and another on the lower 6.25KHz from the center. It's been done, but it isn't very common as it requires a lot of filtering between the two transmitters and receivers to prevent desense. Usually easier to just run two different 6.25 (or 12.5KHz) channels.

Pretty sure it requires the use of a common, external 10MHz reference. It would also require external PA's and hybrid combiners that are rated to have 0khz separation. You are looking at a minimum of 7dB of loss just in the combiner alone.

 

[mmckenna]

Pretty sure it requires the use of a common, external 10MHz reference. It would also require external PA's and hybrid combiners that are rated to have 0khz separation. You are looking at a minimum of 7dB of loss just in the combiner alone.

Yep, exactly. I've heard that there are users doing it successfully. I'm happy with my 500KHz separation between repeater pairs.

 

[kayn1n32008]

Yep, exactly. I've heard that there are users doing it successfully. I'm happy with my 500KHz separation between repeater pairs.

Big cost saving factor between 500kHz spacing and less than 300kHz spacing. Also significantly less losses as well.

 

[mmckenna]

Big cost saving factor between 500kHz spacing and less than 300kHz spacing. Also significantly less losses as well.

Not to mention the protected NPSPAC frequencies….

 

[kayn1n32008]

Yep, exactly. I've heard that there are users doing it successfully. I'm happy with my 500KHz separation between repeater pairs.

I don't want to go down the road of less than 350KHz combining. It's expensive and incredibly lossy. We have 6 UHF ham repeaters with all their transmitters combined using 4" ResLok multicouplers.

If we wanted to have DMR, P25, DStar and Fusion, all on our site on VHF, we would have to use hybrid combiners, add external PA's and separate TX and RX antennas. There just isnt enough ham spectrum to use 6" cavity multicouplers, and we don't have the rack space either lmao.

I guess the positive of doing it all on UHF is we are utilizing the spectrum, that is really under utilized where I am.

 

[RRR]

NXDN -can- take a 12.5KHz channel and run two separate repeaters side by side, one on the upper 6.25KHz from the center, and another on the lower 6.25KHz from the center. It's been done, but it isn't very common as it requires a lot of filtering between the two transmitters and receivers to prevent desense. Usually easier to just run two different 6.25 (or 12.5KHz) channels.

This is what I was referring to. I have seen NXDN advertised before as 2 talkpaths, FDMA. Never seen it done. Wouldn't want to do it, separation is a good thing.

Amused at the "dislike" emoji's and so forth. Nothing I stated was wrong. Just because it isn't done often, and for good reason, doesn't mean it isn't what it says. TDMA & FDMA abbreviations are worth exploring. I don't know it all, and I don't act like I do. But I made my point, for what it's worth. Regardless, when you stop breathing, is when you stop learning. I enjoy these "debates" as everybody can learn something from it. I don't care one bit if I am called stupid or something. But don't just assume there is merely a keyboard rambo trying to show off what they think they know. Some statements are made to entice a response, that may otherwise not get a lot of attention if asked in conventional manner. This was the case.

Y'all try to stay warm, and be safe, got some interesting weather headed in. 🫡

 

[mmckenna]

This is what I was referring to. I have seen NXDN advertised before as 2 talkpaths, FDMA. Never seen it done. Wouldn't want to do it, separation is a good thing.

Right, but it's two separate RF channels, two separate repeaters. Not one repeater doing both.

 

[kayn1n32008]

This is what I was referring to. I have seen NXDN advertised before as 2 talkpaths, FDMA. Never seen it done. Wouldn't want to do it, separation is a good thing.

I wouldn't want to do that either, but I can think up some situations where you may have no choice.

This is what I was referring to. I have seen NXDN advertised before as 2 talkpaths, FDMA.

I've never seen NXDN advertised as 2 talk paths. Kenwood offers repeaters that will do both narrow NXDN(12.5kHz channel) and Very Narrow NXDN(6.25kHz channel), but not at the same time abd only one talk path per repeater.

 

[GTR8000]

Some of the posts in this thread, oh my. There is no shame in not posting if you don't know what you're talking about. Really, we won't hold it against you!

Speaking of carriers on immediately adjacent frequencies...how about an ASTRO 25 system that has two pairs of immediately adjacent 12.5 kHz TDMA channels in the same ESS rack??? Yup, 772.83125 and 772.84375, as well as 773.15625 and 773.16875, operating simultaneously day and night for 12+ years. I guess it's a super special 4-slot TDMA system according to some of the loose "definitions" being thrown around in this thread!

 

[mmckenna]

I think his confusion may come from some of the Kenwood and NXDN Forum documents. Looking at the pictures and not understanding the technology might lead one to believe that FDMA allows two simultaneous talk paths in a single RF channel.


No good reason to run a system like that unless your frequency coordinator REALLY hates you.

 

[RRR]

Right, but it's two separate RF channels, two separate repeaters. Not one repeater doing both.

Ok, fair.

You beat me to it, I had that ready to post.

Anyway, good grief, some of you need to lighten up. This is Rad Ref, we aren't in an official industry chatroom. I'm not, not trolling some of you. This is mild entertainment, sheesh. 😆