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Some digital voice protocols are time slotted (TDMA), i.e. the transmitter of a station is not on all the time, and multiple stations may transmit digital voice on the same frequency at the same time by transmitting small digital bursts, one by one.

For example, DMR (MotoTRBO), which is already somewhat popular in amateur radio circles, has a duty cycle of slightly less than 50%, and two voice transmissions can be retransmitted by the same repeater at the same time.

Would it be possible to make a digital "simplex" voice repeater which would listen on one time slot and immediately repeat the received voice frame on the next time slot, on the same frequency? Do such repeaters exist for some protocol?

It'd work fine with a single antenna, and wouldn't require duplex filters. Much easier to set up than traditional voice repeaters.

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There are currently products from multiple major and minor DMR equipment manufacturers to support single frequency repeater (SFR) function. They may have different names but essentially do the same thing: Repeater input on time slot 1 and output on time slot 2.

What is not certain is whether SFR is interoperable between different manufacturer equipment. From the description by each manufacturer it appears to be interoperable.

Motorola:

Extended Range Direct Mode enables repeater operation on a simplex radio channel. Radios on an ERDM channel transmit on one slot and receive on another slot - the frequency is the same.

With this you get:

  • Repeater-like coverage
  • No need for a 2nd frequency - your simplex channel can be reused.

In this configuration, dual slot operation is not possible and you need to be using a SLR series repeater. IP based dispatch solutions, like SmartPTT Plus and TRBONET Plus, are supported. It also only supports single site conventional operation so no trunking or IPSC.

This feature requires R2.7.0 firmware and is supported on all current generation MOTOTRBO radios. My understanding is that this feature will also work on other makes of DMR radios.

Hytera:

Full Duplex and Single Frequency Repeater

The UHF model variants of the MD785i are also available with Full-Duplex function and can thus also be used as Single Frequency Repeaters (SFR). In Repeater Mode the mobile radio boosts the range of a direct connection between conventional DMR devices without having to use a base station or repeater. In very concrete terms for users this means they now have more freedom of movement and considerably more flexibility.

Belfone: Repeater and Base Station

BF-SFR600 is our new single frequency repeater solution which is also able to function as a base station within a system. As a single frequency repeater, BF-SFR600 allocates one timeslot to receive a signal and the other to transmit it at the same frequency, using DMO mode to extend radio coverage.

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    $\begingroup$ Hello, and welcome to this site! I took the liberty of formatting the quote from the site you linked to, to make it more obvious that it was a quote. You can see how and why I did that on this section of the help section. We look forward to seeing more good answers from you. :-) $\endgroup$
    – Mike Waters
    Nov 6, 2019 at 22:05
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    $\begingroup$ You are correct, I am only citing these sources. Thank you. $\endgroup$
    – BM2NHC
    Nov 6, 2019 at 22:16
  • $\begingroup$ Splendid! Great that the vendors have started to support this protocol feature. It would be so much easier to set up repeaters without the need for duplexers. No second timeslot though. $\endgroup$
    – oh7lzb
    Nov 22, 2019 at 19:28
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Would it be possible to make a digital "simplex" voice repeater which would listen on one time slot and immediately repeat the received voice frame on the next time slot, on the same frequency?

Yes, the protocol could be used this way. It wouldn't be simple to setup without the radios specifically supporting such a mode of operation, though. You could use two radios for the repeater - each set the the opposing timeslot, feeding into each other, with VOX enabled, but it would be, at best, a hack and probably wouldn't be very robust without additional control equipment. It would eliminate the need for resonant filters, though.

Do such repeaters exist for some protocol?

I understand that some Hytera radios support this mode. Mototrbo radios support TDMA operation, but when in TDMA mode you select a single time slot for both transmitting and receiving. The repeater would have to be set up to to always receive on one time slot and transmit on the other, and the users would have to set their radio to receive on the correct timeslot, except when they want to transmit. At that point they'd have to manually switch the radio to the other timeslot, then when done immediately switch it back to the timeslot the repeater transmits on. This is not automated in the Mototrbo radios, so they couldn't easily be used for this.

But it is possible, and it may be interesting to do, merely from the viewpoint of eating up only one 12.5kHz channel and removing the need for expensive, large filters.

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  • $\begingroup$ I'm not sure I understand what do you mean by the reference to full duplex (audio going from user 1 to user 2 and user 2 to user 1 at the same time). The question is about half-duplex voice communications using TDMA radios, using a repeater that does not run it's receiver and transmitter at the same time. $\endgroup$
    – oh7lzb
    Nov 13, 2013 at 12:01
  • $\begingroup$ @oh7lzb I've changed my answer to better respond to your question. $\endgroup$
    – Adam Davis
    Nov 13, 2013 at 12:45
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What you describe is exactly how TETRA DMO repeaters work. They use 4 time slots on a single frequency, using 25MHz bandwidth. The repeater receives on time slot #1 and transmits on slot #4.

See for instance TETRA DMO modes.

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A better answer is as follows (apart for the spelling ) The dmr protocol is specifically designed to allow repeaters with no duplexer and in fact full duplex working The repeater has to be specifcaly designed to do this as the repeater has to get its timing from the incoming signal as you don't want to put this functionality in every mobile This is a very good way of working as it uses no more channel capacity than the simplex arrangement we are using at present as we cannt nip in and use the channel which is not keyed anyway One day all radio will be dmr and all dmr will be duplex In a few years time the transmit key will be just used for convenience to start call and save battery power during calls and to shut down at the end of the call like the rest on a land line phone Some of the comments above refer to store and forward delays. 1 time slot is not noticeable in human speech
The first big advantage would be to allow the halving of repeater costs and in fact probably an even bigger saving than this as a having in actual costs would bring the repeater costs down to level where we all want one allowing the econonys of scale to reduce prices by another 1/2 at least Basically there is no reason why a repeater without a duplexer is not just a mobile with different software The take up of duplex hand portables and mobiles is a little further away as the tx/rx spacing is only 10 MHz as opposed to 50 megahertz for gsm. This makes switching a little more difficult but not much Personally I would like to see us as amiteurs be quick to follow this route as otherwise there may never be a free of charge solution Which would allow us to enjoy the benifits of mobile phones without the monthly fees This could be the greatest stimulates ever to our hobby Ted slevin Retired chef engineer of burndept electrinics

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  • $\begingroup$ Hi Ted, and welcome to ham.stackexchange.com! We recommend all new users take the tour to get the most from the site, and since you volunteered an answer, it couldn't hurt to also read how to answer. $\endgroup$
    – rclocher3
    Jan 5, 2017 at 18:58
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Unication U4 and U3 radios do exactly that. http://www.comart.com.cn/show_pro_en.asp?classid=152

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There are store-and-repeat repeaters available for simplex, FM analog voice use. Such a repeater listens to the entire sent message, then repeats the entire message on the same frequency.

Another repeater-like option that works without a duplexer is a cross-band repeater. Unlike a typical FM repeater which has input and output on the same band, a cross-band repeater typically has one transceiver on a VHF frequency and one transceiver on a UHF frequency. Whatever it hears on one band, it repeats on the other, often (depending on configuration) in both directions.

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  • $\begingroup$ The question is about TDMA digital voice modes, so this answer does not quite answer the question. "Parrot" repeaters are slow to use (have a delay of the length of each transmission, effectively halving the voice communications possible in a given time). Cross-band repeater requires dual-band antennas and radios, and the repeater needs a (cheaper and simpler) diplexer if using a dual-band antenna. $\endgroup$
    – oh7lzb
    Nov 1, 2013 at 4:45
  • $\begingroup$ I misread the original question. On the subject of a cross-band repeater, with a dual-band radio and a dual-band antenna, no diplexer is needed unless the radio has separate antenna connections for the different bands. $\endgroup$
    – James NF8I
    Nov 1, 2013 at 6:29
  • $\begingroup$ Some mobile radios which have do have a built-in diplexer, yes, but it's still required. I added one more instance of the "digital" word in the question to make it clearer. $\endgroup$
    – oh7lzb
    Nov 1, 2013 at 6:45
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It could be done, but as my understanding there is no easy/default way to do so, yet. theoretically if you create a ip network like 802.x with less protocol overhead and more vocoder performance, you could, sustain multiple conversations within connected units pretty much like IM talk group within one wifi AP range, just with much less bandwidth, here vocoder performance come to play vital role. With advance of semiconductor manufacturing and DSP advance, vocoder are no longer power hungry and too hard to implement.

Some of recent Bluetooth stack originally designed for IoT already come with all requirements mentioned above. We, HAM, are supposed to be pioneer on vocoders, indeed.

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Yes it can be done though the DMR standard is not really designed to support this. https://www.trellisware.com/manet-products/tsm/ are radios designed for this type of application. You would need to use more rf bandwidth due to faster modulation rate and longer gaps between tx and rx.

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    $\begingroup$ Greetings!. We recommend that all new users take the tour to get the most from the site, and since you volunteered an answer it can't hurt to read how to answer also. Welcome! $\endgroup$
    – rclocher3
    Nov 22, 2016 at 2:55

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