TDMA Repeater?

[slicerwizard]

No, I'd say he's right and you're wrong. Your explanation as to why it can't be done doesn't pass the sniff test.

 

[Raccon]

TIn Europe where it has become popular TETRA operates in a 30 kHz wide slot, [...]

Can you elaborate on that?

 

[jonwienke]

Correction acknowledged. P25 has the continuous control channel with sync for Phase 2 TDMA compatibility. Physics still gets in the way of USRobotics trying to use DMR TS1 for transmit and TS2 for receive on a mobile relay system occupying one single [simplex] RF channel.. You can't turn the mobile relay or field subscriber station's transmitter off and unmute the receiver fast enough to interleave both sides of a conversation, especially if the units are in close physical proximity.

If this was true, you couldn't do TDMA at all. For TDMA to work, the transmitter and receiver (especially the transmitter) have to be capable of turning on and off within the 3ms gap between timeslots. Given that, any TDMA-capable transceiver should be capable of listening on timeslot 1, then rebroadcasting what it hears on timeslot 2 without the crosstalk or RF bleedover you would get trying such a thing with conventional analog FM. The firmware may not support such a thing, but the hardware is capable of doing it.

Multipath interference caused by timeslot 1 echoing into timeslot 2 (or vice versa) is a non-issue. Delaying a signal by 3ms would require it to travel nearly 900km, which is far greater than the range of any transmitter within the scope of this discussion. Multipath interference would be the exact same problem as with any digital transmission--bits from different locations within a frame being received at the same time.

Syncing would not be that difficult--no more so than TDMA simplex. The listening radio hears an incoming transmission, and syncs itself to the incoming signal to decode the digital data and play the audio. In the same way, the repeater would listen to the input frequency, and when it hears an incoming transmission in timeslot 1, it begins rebroadcasting the transmission on timeslot 2, syncing the outgoing transmission with the incoming signal to avoid transmitting at the same time as the incoming signal. This would get screwed up if two or more incoming transmissions hit the repeater simultaneously, but that is an issue with any repeater, analog or digital.

Even if the repeater broadcast on a different frequency than it received, as long as it kept the transmitter in sync with its receiver (so that TX and RX never happened simultaneously), you wouldn't need any special filtering to isolate the transmitter from the receiver.

 

[K5MPH]

Jonathan Naylor (G4KLX) is doing something like this now with an MMDVM Digital Board by Tx and Rx on one single Freq on DMR .....

 

[Project25_MASTR]

If this was true, you couldn't do TDMA at all. For TDMA to work, the transmitter and receiver (especially the transmitter) have to be capable of turning on and off within the 3ms gap between timeslots. Given that, any TDMA-capable transceiver should be capable of listening on timeslot 1, then rebroadcasting what it hears on timeslot 2 without the crosstalk or RF bleedover you would get trying such a thing with conventional analog FM. The firmware may not support such a thing, but the hardware is capable of doing it.

Multipath interference caused by timeslot 1 echoing into timeslot 2 (or vice versa) is a non-issue. Delaying a signal by 3ms would require it to travel nearly 900km, which is far greater than the range of any transmitter within the scope of this discussion. Multipath interference would be the exact same problem as with any digital transmission--bits from different locations within a frame being received at the same time.

Syncing would not be that difficult--no more so than TDMA simplex. The listening radio hears an incoming transmission, and syncs itself to the incoming signal to decode the digital data and play the audio. In the same way, the repeater would listen to the input frequency, and when it hears an incoming transmission in timeslot 1, it begins rebroadcasting the transmission on timeslot 2, syncing the outgoing transmission with the incoming signal to avoid transmitting at the same time as the incoming signal. This would get screwed up if two or more incoming transmissions hit the repeater simultaneously, but that is an issue with any repeater, analog or digital.

Even if the repeater broadcast on a different frequency than it received, as long as it kept the transmitter in sync with its receiver (so that TX and RX never happened simultaneously), you wouldn't need any special filtering to isolate the transmitter from the receiver.

This is why you have to have a PEP capable wattmeter to measure power from SU's transmitting DMR and TETRA. Other proof comes from the fact if it wasn't for the transmitter of the SU turning off during off timeslots, anyone closer to the repeater would bleed over into other timeslots causing interference to the system. (Which is what some Tytera models were actually known for).

A true TDMA repeater on the other hand doesn't actually turn it's transmitter on and off as it is the source of the sync and timeslots.

 

[jonwienke]

I would think this would be fairly easy to implement in a DMR mobile radio; it could be a simple repeater on a single frequency or a frequency pair, using a single antenna, as long as the output was synced to the input properly so it wasn't trying to TX and RX at the same time.

Using 2 separate freqs would allow a standard DMR radio to use the repeater--timeslot 1 of the input channel would be synced to timeslot 2 of the output channel, and vice versa. The repeater wouldn't be able to use both timeslots on the input or output simultaneously though.

 

[Project25_MASTR]

Why could it not be the same frequency?


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[USRobotics]

Still not making sense: so now you are saying "two transceivers"? Repeater only needs 1 transmitter and 1 receiver (despite the implementation might use 2 transceivers, but they are used for either as a transmitter or a receiver only at all time). Which kind of repeaters require two transceivers? (not talking trunking here)

As for what you have described, technology has existed in the 90's for a single frequency, full duplex, voice over radio commercially. The only difference is that it is a last mile telephone extension system, and not a repeater. Therefore, it has "one user", in other words (the other part of the pair), so it can't really be a repeater.[/QUOTE

A typical amateur repeater (which Part 90 of the FCC Rules calls a 'mobile relay station') does indeed require only one T and one R. The receiver information is rebroadcast by the associated transmitter with minimal delay, operating on separate RF channels and with adequate isolation to keep the local T energy out of the local R, regardless of the number of physical antennae being used - 1, 2, or more. It's NOT the same as the full duplex link system you described above because the link provides two isolated and independent one-way RF and information paths. It does not 'repeat' or rebroadcast anything being heard on the receiver channel. From a regulatory standpoint the difference is subtle but quite significant.

I actually know what a typical amateur radio repeater works, because I also maintain a few here locally.

I did not say they are the same thing. I was using that example to illustrate that it is physically possible to receive and transmit on the same frequency to achieve a full duplex voice communication. As for your definition of "repeat', one would only need to put the mic close to the speaker than the system I mentioned would be "repeating" then. (Although repeating back to the same user doesn't help for most.).

Some were saying that it is either too difficult or impossible to have a time slot system to both transmit and receive alternatively on a single frequency. I used a counter example that happened in the 90's (Actually one pair was donated to my club to use as a last mile for the Autopatch system for the repeater. It was IRL.) so to make clear that it was / is possible.

 

[USRobotics]

USRobotics missed a key point of DMR that, in order to keep all of the field units clocks synchronized (so they only talk in their own time slot), each repeater will continuously send clock sync as well as the digital system ID. Since that is being sent out all the time, a local receiver listening on the same frequency will be swamped and unable to hear incoming signals from any field units.

I re-read all my posts and did not find what you said I have said. You seem to be putting words in my mouth here. I didn't say anything about any clocks or what.

As another poster has pointed out, DMR repeater nor radio does not send (i.e. transmit) all the time.

 

[USRobotics]

Correction acknowledged. P25 has the continuous control channel with sync for Phase 2 TDMA compatibility. Physics still gets in the way of USRobotics trying to use DMR TS1 for transmit and TS2 for receive on a mobile relay system occupying one single [simplex] RF channel.. You can't turn the mobile relay or field subscriber station's transmitter off and unmute the receiver fast enough to interleave both sides of a conversation, especially if the units are in close physical proximity.

Again, I am not sure why you kept putting words in my mouth, because I did not say "I wanted to use DMR TS1 for TX and TS2 for RX on a mobile relay system occupying one single frequency / channel". The OP did, and I am not the OP.

Plus you aren't even right on the physics. If physics says it can't be done, then how would a Tetra mobile does it on DMO mode?

Not only that, not just Tetra, but now DMR can do it. See:
http://forums.radioreference.com/hytera-forum/340487-pd98x-radio-internal-repeater.html

I find it a little disturbing that you quoted my screen name multiple times, alleging that I said something that I didn't. And then plus backing it up with something that is supposed to be physics, and yet in real life it has already happened to the contrary.

 

[jonwienke]

Why could it not be the same frequency?


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It can be on the same frequency, but doing so would require firmware updates on most DMR radios to allow separate time slot settings for transmit and receive. Most of the radios I've looked at only allow the frequency to be different for TX and RX; the time slot, color code, and talkgroup settings are common to both TX and RX.

If you use separate TX and RX frequencies you can work around this firmware limitation, but you can still only use one timeslot at a time on them unless you go to the standard FM repeater's filter setup.

 

[Project25_MASTR]

It can be on the same frequency, but doing so would require firmware updates on most DMR radios to allow separate time slot settings for transmit and receive. Most of the radios I've looked at only allow the frequency to be different for TX and RX; the time slot, color code, and talkgroup settings are common to both TX and RX.



If you use separate TX and RX frequencies you can work around this firmware limitation, but you can still only use one timeslot at a time on them unless you go to the standard FM repeater's filter setup.

You couldn't do it on split frequencies without firmware updates as once the subscribers are put in a split they expect the "handshake" with the repeater to sync.

The repeater does not cyclically key like a subscriber though. It is the source for the sync and transmits for the sync preceding ts1, ts1, sync preceding ts2 and ts2. The carrier is always active if there is activity on the receiver and/or network regardless of activity on only one time slot.


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[jonwienke]

Not if the repeater operated like a simplex radio. The whole point of this is to sync the repeater to the subscriber unit, rather than having the repeater broadcasting all the time.

 

[Project25_MASTR]

Not if the repeater operated like a simplex radio. The whole point of this is to sync the repeater to the subscriber unit, rather than having the repeater broadcasting all the time.

Both of which require firmware modifications. There is no getting around it (for DMR) with any current DMR manufacturer.

Now Simoco is the closest to having a similar setup. Their full duplex calling feature utilizes both slots (one for TX and one for RX) to pass phone patches and could probably be modified to allow a SU to become a simplex repeater and the other SU's to make the proper shifts.


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[jonwienke]

The firmware will require modification, but the hardware is capable. It's just a matter of whether there is enough interest in a "cross-band repeater" type feature that doesn't have to be cross-band or even use multiple frequencies.

 

[n1das]

Hytera seems to have figured it out. Hytera has TDMA single frequency repeater capability in their new PD98X series portables. The portable can function as a TDMA single frequency repeater for other like portables to talk thru it.
Hytera

I will be playing with some of these in a few weeks.

 

[MSS-Dave]

Hytera seems to have figured it out. Hytera has TDMA single frequency repeater capability in their new PD98X series portables. The portable can function as a TDMA single frequency repeater for other like portables to talk thru it.
Hytera

I will be playing with some of these in a few weeks.

They didn't figure it out, they just brought a Tetra feature (ETSI standard) to the US market. Here is a PowerPoint presentation on DMO operation, may be helpful.

http://cloud.tapatalk.com/s/584c97a...n-Selex_Communications_Francesco_Pasquali.ppt


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[Your_account]

they just brought a Tetra feature (ETSI standard) to the US market.

that do you mean with?
Tetra is an Network Based Radio System.
DMR is an Radio 2 Radios Based System.

What interesting to me its possible to operate many of the PD98X like the open Wifi projects who every device share the Data to other Repeater...?

 

[MSS-Dave]

that do you mean with?
Tetra is an Network Based Radio System.
DMR is an Radio 2 Radios Based System.

What interesting to me its possible to operate many of the PD98X like the open Wifi projects who every device share the Data to other Repeater...?

TETRA is an ETSI standard. Normally, trunked radio systems that can be networked. There are also provisions for direct operation.

DMR is an ETSI standard as well that can be stand alone repeat or networked or simplex. Tier II and Tier III are different versions of the DMR standard. Motorola Cap +, Hytera XPT, etc are based on this standard but contain proprietary features to make them not 100% compatible with either the standard or each other.

Your statement that "DMR" is radio to radio based system isnt totally correct. It is a STANDARD, not a brand name or an indicator of simplex or standalone repeater or trunked repeaters on one site or NETWORKED to many or even frequency range. TETRA, NXDN, P25, etc are standards too.

I have no idea what the second part of your post means, sorry.

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[Your_account]

I have no idea what the second part of your post means, sorry.

Why does "traditional" Radio Systems use that: https://en.wikipedia.org/wiki/Optimized_Link_State_Routing_Protocol