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Thread: Mototrbo repeater using only TDMA time slots (no duplexer)

  1. #1
    Alpha2K No Longer Registered

    Default Mototrbo repeater using only TDMA time slots (no duplexer)

    Forgive me for the noob question, but from what I understand, it is possible for there to be two time slots on a single RF frequency, thus providing two voice channels in the bandwidth of a single analog 12.5 Khz allotment. So I could have two conversations happening at once, at the same time, on the same RF frequency allotment. (I know the can be one voice, one data, etc).

    So technically, I see no reason why a repeater could not be built that receives on one timeslot and re-transmits on the other. I know this was done in the 90's for a Glenayre device called the WiLE (or Wireless Line Extender) which emulates a full duplex telephone across a simplex channel on UHF. It does so by encoding and compressing the data to about 40% the time, and sending and receiving rapidly (4 times a second if I recall), and piecing it all together at the ends.

    This simplex trbo repeater setup would require no duplexer, and would be easier to multiplex into existing antenna systems. Less loss, more ERP, win-win.

    What limits this from being a possibility? Is it simply firmware? I think this would be a HUGE selling benefit, and I see no technical reason why it could not be done.

    Dave


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    Quote Originally Posted by Alpha2K View Post
    Forgive me for the noob question, but from what I understand, it is possible for there to be two time slots on a single RF frequency, thus providing two voice channels in the bandwidth of a single analog 12.5 Khz allotment. So I could have two conversations happening at once, at the same time, on the same RF frequency allotment. (I know the can be one voice, one data, etc).

    So technically, I see no reason why a repeater could not be built that receives on one timeslot and re-transmits on the other. I know this was done in the 90's for a Glenayre device called the WiLE (or Wireless Line Extender) which emulates a full duplex telephone across a simplex channel on UHF. It does so by encoding and compressing the data to about 40% the time, and sending and receiving rapidly (4 times a second if I recall), and piecing it all together at the ends.

    This simplex trbo repeater setup would require no duplexer, and would be easier to multiplex into existing antenna systems. Less loss, more ERP, win-win.

    What limits this from being a possibility? Is it simply firmware? I think this would be a HUGE selling benefit, and I see no technical reason why it could not be done.

    Dave
    How would you isolate the transmit power from the receiver?

    *Edit* Oops, never mind, I see what you're saying about using simplex and switching slots...interesting idea that I would have to think on...

  3. #3
    Alpha2K No Longer Registered

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    I guess the question comes down to what happens if you have two trbo handhelds, one in each hand, and a similar setup a few hundred feet away, can you carry on two conversations? Are the TRBO mobiles and handhelds capable of doing TDMA without a base station/repeater to sync them?

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    Quote Originally Posted by Alpha2K View Post
    I guess the question comes down to what happens if you have two trbo handhelds, one in each hand, and a similar setup a few hundred feet away, can you carry on two conversations? Are the TRBO mobiles and handhelds capable of doing TDMA without a base station/repeater to sync them?
    Yes. As of the latest firmware (in my XPR7550 anyway) there's a new mode called 6.25e (equivalent). This allows dual-slot TDMA simplex. Here's what CPS says:

    Dual Capacity Direct Mode (DCDM):
    The Dual Capacity Direct Mode (DCDM) feature supports two simultaneous subscriber transmissions within a 12.5 kHz channel bandwidth for Direct Mode transmissions. The radios within the same group must use the same timeslot so that the group using timeslot 1 does not interfere with the group using timeslot 2. For proper operation, the radios need to identify and track the timeslot structure. A unique radio is elected as a channel timing leader. The other radios adjust their clocks to synchronize as possible with the channel timing leader.
    Notes:
    This feature is supported in Digital mode only.

  5. #5
    mtp850 No Longer Registered

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    Good to see moto now offering what hytera has been for some time....

    Quote Originally Posted by Mars View Post
    Yes. As of the latest firmware (in my XPR7550 anyway) there's a new mode called 6.25e (equivalent). This allows dual-slot TDMA simplex. Here's what CPS says:

    Dual Capacity Direct Mode (DCDM):
    The Dual Capacity Direct Mode (DCDM) feature supports two simultaneous subscriber transmissions within a 12.5 kHz channel bandwidth for Direct Mode transmissions. The radios within the same group must use the same timeslot so that the group using timeslot 1 does not interfere with the group using timeslot 2. For proper operation, the radios need to identify and track the timeslot structure. A unique radio is elected as a channel timing leader. The other radios adjust their clocks to synchronize as possible with the channel timing leader.
    Notes:
    This feature is supported in Digital mode only.

  6. #6
    syntrx No Longer Registered

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    Listening on one slot and transmitting on another is exactly how a TETRA DMO Repeater works.

    Simoco are working on a proprietary extension to support full duplex private calls and phone patch on their upcoming DMR equipment by using two timeslots.

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    Mr Syntrx, are you at the RCC show?

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    Quote Originally Posted by Mars View Post
    Yes. As of the latest firmware (in my XPR7550 anyway) there's a new mode called 6.25e (equivalent). This allows dual-slot TDMA simplex.
    Yes, it's only for the newer 'Gen2' TRBO radios...the 1.5 million or so 'Gen1' units Motorola sold can't do it.

  9. #9
    oh2lak No Longer Registered

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    The DMR air interface specification does includes note related to Time-Domain Duplex (TDD) (ETSI TS 102 361-1; 5.1.4.4), where the two timeslots are utilized in sequence for both uplink and downlink. It would limit the channel capacity to a single communication channel but enable single-frequency duplex operation. The same technology is used in TETRA, as mentioned earlier in the DMO repeater functionality.

    I haven't seen any vendor yet announcing this feature, as it would eventually kill the benefit of having two communication channels in one carrier thus doubling the capacity and splitting the bandwidth requirement.

    ..
    Erik

  10. #10
    syntrx No Longer Registered

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    It'd be fantastic for portable repeaters where you only need one talk path though, especially on VHF.