Differene between DPMR and DMR

[paulears]

The trouble is, Tait give a great technical point of view for promoting time sharing systems, citing the additional requirement to manage two channels as an infrastructure problem, while Icom promote the benefits of frequency sharing for range and infrastructure reasons.

So two big manufacturers spinning the technology to suit their chosen systems. Tait are firmly saying TDMA is best, and Icom, FDMA. As I've already said, it's Betamax and VHS all over again.

There have been comments on the timing issues the cheaper DMR radios have, which suggests that the jury may well still be out - despite me thinking DMR had it?

Am I correct in thinking that TDMA radio to radio, without the repeater then does use up a full 12.5KHz channel, whereas with a repeater, two separate conversations can take place independently? This is the one thing I'm unclear on? If a DMR repeater is not 'intelligent', not doing the retiming functions, then this too removes the two simultaneous audio channel capability?

 

[Voyager]

Just not sure how Voyager can argue with the graphic in post #24.

I'm not arguing with it. It is graphically correct. It's what some are saying that is not correct. FDMA is a single transmitter with 12.5 kHz bandwidth that is divided into two 6.25 kHz talkpaths - just what the graphic is showing.

Just the same, TDMA is a 12.5 kHz bandwidth that is divided in time to provide two talkpaths.

One is divided in Time and the other is divided in Frequency, hence the T and F in TDMA and FDMA. Yes, it really IS that simple. You can't have Multiple Access when only one person can access a transmitter at a time. "Single" is not "Multiple".

If you want to make up a term for it, single-user repeaters are NDSA (No Division Single Access). But, FDMA means something completely different even though many people want to use it to describe such a transmitter.

No matter how much you call your 45 records CDs, that does not make them CDs.

 

[Raccon]

[...]

So two big manufacturers spinning the technology to suit their chosen systems.

Well, that shouldn't surprise anyone, it's called marketing.

Tait are firmly saying TDMA is best, and Icom, FDMA. As I've already said, it's Betamax and VHS all over again.

No it ain't. Both TDMA and FDMA are firmly established regardless of what two of many vendors claim about their systems. Both solutions have their pros and cons, and both have areas of application in which they are more suitable, so to argue which access method is better is pointless.

Am I correct in thinking that TDMA radio to radio, without the repeater then does use up a full 12.5KHz channel, whereas with a repeater, two separate conversations can take place independently? This is the one thing I'm unclear on?

Assuming you mean DMR when you say TDMA then the 2nd assumption is correct. The first one may also be correct but it is also possible to carry two conversations within a 12.5kHz channel, subject to the DMR radios being used.

I'm not arguing with it. It is graphically correct. It's what some are saying that is not correct. FDMA is a single transmitter with 12.5 kHz bandwidth that is divided into two 6.25 kHz talkpaths - just what the graphic is showing.

That has been explained to you already but let's repeat it once more: it shows two single transmitters with a bandwidth of 6.25kHz each ("two channels"), thus utilizing 12.5kHz in total so it can be compared to a single 2-slot TDMA transmitter with a bandwidth of 12.5kHz.

You can't have Multiple Access when only one person can access a transmitter at a time. "Single" is not "Multiple".
If you want to make up a term for it, single-user repeaters are NDSA (No Division Single Access). But, FDMA means something completely different even though many people want to use it to describe such a transmitter.

By that logic FDMA would not exist in radio systems. As said before, it's about sharing a frequency spectrum, regardless if that is multiple repeaters (that may be combined into the same antenna) or multiple sites with single repeaters.

All voyager would have to do is "google it" and lots of examples come up.
here is one from ICOM.
Key Differences between FDMA vs. TDMA Technology

Not a good example because it states that TDMA has a 12.5kHz bandwidth, which isn't true since - there is no direct relation between TDMA and bandwidth. Same goes for their statement about FDMA being 6.25kHz.

 

[Voyager]

That has been explained to you already but let's repeat it once more: it shows two single transmitters with a bandwidth of 6.25kHz each ("two channels"), thus utilizing 12.5kHz in total so it can be compared to a single 2-slot TDMA transmitter with a bandwidth of 12.5kHz.

If you add one word you would be correct: Two adjacent channels each 6.25 kHz wide. (which is the same thing as one 12.5 kHz channel split in the frequency domain into two voice slots)

Without them being adjacent channels, the transmitter cannot be Divided (the D in FDMA).

If they are not adjacent, it's just two conventional transmitters and is not Multiple Access (the MA in FDMA).

Those initials aren't meaningless terms. They describe what is happening. Both FDMA and TDMA Divide the channel into two (or more) paths providing Mulriple Access. They do so in the Time or Frequency domain.

Not a good example because it states that TDMA has a 12.5kHz bandwidth, which isn't true since - there is no direct relation between TDMA and bandwidth. Same goes for their statement about FDMA being 6.25kHz.

That part I agree with. There is no bandwidth limit to TDMA or FDMA. It can be 6.25, 12.5, 100, or any other bandwidth. There is also no limit to how many times it is divided. It can be divided into two (as most Land Mobile is), or it can be divided more times to provide more talk paths (or more video paths in some cases). They defining key is that it is a single transmitter that is divided to allow multiple paths (technically called slots) for multiple access by more than one user.

 

[Raccon]

If you add one word you would be correct: Two adjacent channels each 6.25 kHz wide. (which is the same thing as one 12.5 kHz channel split in the frequency domain into two voice slots)

Agree, thanks for pointing that out.

Without them being adjacent channels, the transmitter cannot be Divided (the D in FDMA).

There is no "the transmitter" and it certainly can't be divided, there is one transmitter with one frequency having a bandwidth of 6.25kHz for each channel.

If they are not adjacent, it's just two conventional transmitters and is not Multiple Access (the MA in FDMA).

Disagree, 'adjacent' is required for the bandwidth comparision (here 12.5kHz) but not for FDMA as such.

Those initials aren't meaningless terms. They describe what is happening. Both FDMA and TDMA Divide the channel into two (or more) paths providing Mulriple Access. They do so in the Time or Frequency domain.

I didn't say they are meaningless, I said they are not accurate terms to use when discussing details of a particular system; too many times TDMA/FDMA gets thrown around when it is clearly not a function of TDMA/FDMA in general but a particular implementation (e.g. DMR, P25, TETRA, GSM ...) that defines bandwidth, number of slots etc.

That part I agree with. There is no bandwidth limit to TDMA or FDMA. It can be 6.25, 12.5, 100, or any other bandwidth. There is also no limit to how many times it is divided. It can be divided into two (as most Land Mobile is), or it can be divided more times to provide more talk paths (or more video paths in some cases). They defining key is that it is a single transmitter that is divided to allow multiple paths (technically called slots) for multiple access by more than one user.

Please explain how a single transmitter in a PMR system is divided to provide multiple slots using FDMA.

 

[rescue161]

Please explain how a single transmitter in a PMR system is divided to provide multiple slots using FDMA.

I am curious about this too.

 

[Voyager]

The best example is a satellite transmitter. It has one wideband transmitter and each user is assigned a slot on it. How do they divide it into two or more slots? They only modulate the segment on the slots with the input.
|Slot1|-|Slot2|-|Slot3|-|Slot4|-|Slot5|-|Slot6|-|Slot7|-|Etc to Slot X|
Lower freq allocation..................................................Upper Freq Allocation

Or, in the form of the original FDMA graphic which slots 2-slot FDMA:
|Slot1|
|Slot2|
|Slot3|
|Slot4|........X kHz allocation bandwidth
|Slot5|
|Slot6|
|Slot7|
|Etc to Slot X|

Linear Translators work the same way, but the slots are variable. Several people can use them at the same time. Linear translators are basically land-based satellites.

Think of it like CTCSS. It only modulates the segment below 250 Hz (or so), so that part of the carrier 250 Hz above and below around the carrier can only be used by the CTCSS (although CTCSS will use up to 750 kHz of deviation, the segment 300-750 Hz is shared with voice). The voice will use the portion 750 Hz to 8000 Hz above and below the carrier, and the CTCSS will not. (this is based on up to 250 Hz CTCSS at 500 Hz deviation, and up to 3000 Hz voice at 5000 Hz deviation)

Graphically: (showing only one half of the bandwidth - the upper half)
Carrier-----Lowest modulation----<CTCSS only--------CTCSS+Voice---------Voice only
0 Hz-----------20-60 Hz-------------------250 Hz-----------------750 Hz-----------------8000 Hz

While they are intermixed in part of the bandwidth, CTCSS vs voice is nearly a FDMA scheme.

Put another way: CTCSS uses 60-750 Hz Above and below the carrier (depending on exact frequency and deviation)
Voice uses 250-8000 Hz (again depending on frequency components and deviation)

The division is made by filters that remove voice components below 300 Hz and filters that remove CTCSS components above 250 Hz.

WiFi is close to FDMA within a single band, but only uses one channel at a time, so it's not FDMA (just channel selection). If it could use more than one channel simultaneously using the same transmitter, it would be Multiple Access divided in the Frequency Domain.

 

[Raccon]

I didn't ask about satellites nor do statements that contain "nearly", close", "if it could" or "would be" convince me.

Fact is some PMR systems use FDMA and it ain't a single transmitter that provides multiple slots.

 

[Voyager]

Satellites are the best example and most prominent user of FDMA. You may not have asked directly, but when you ask about FDMA, you are asking for examples.

If those PMR systems aren't using multiple slots on a transmitter, it isn't FDMA, TDMA, or (anything)DMA. What you cited would be an example of twisting of the term. It sounds like you are talking about a Trunked system.

 

[hitechRadio]

It's pointless, you will never grasp what most every one is saying to you.

 

[Voyager]

Are you grasping what I'm saying?

As far as your point, I grasp it fine. I simply don't agree that a 2M repeater is FDMA simply because there is another repeater on a different frequency at the same site.

To have Multiple Access, more than one person has to be able to access the same repeater at the same time. P25 Phase II is an example of Multiple Access. P25 Phase I is not Multiple Access, as only one user at a time can use it. If Phase I were Multiple Access, there is no point to having P25 Phase II.