Ross,
I really resonate with your desire to just jump right into the protocol its
self, I've played around with many such ideas - it really is fun to dream
about what could be possible!
But I had to realize I was getting ahead of my self though, for something
to really gain traction like you are talking about you need to have a chunk
of hardware that is cheap, looks nice and works really well and is likely
largely software defined. Once people's immigration can be lit up with what
is possible it can attract some good talent to really flesh the whole thing
out. I believe this has been one of the large aims of the SM2000.
For my part I have a few limeSDR's that I've been playing with using Pothos
and GNU Radio to try and learn how everything works well enough that I can
actually contribute to something like this.
Have a great day!
Daniel Mundall
On Sun, Nov 5, 2017 at 10:39 PM, Ross Whenmouth <[email protected]> wrote:
> Regarding TDMA (for Codec2 +), would it be best to spin off a new forum
> for this topic?
>
> I think that it would be sensible to have both half-duplex TDMA (single RF
> frequency) and full-duplex TDMA (split frequency repeater) modes. This is
> because whilst half-duplex TDMA has the advantage of allowing a simple
> "user" radio to work as a repeater, because only a single RF frequency is
> used and cavity filters are not required (excepting shared sites where
> cavity filters are required), it suffers from issues with the speed of
> light, range to users and the length of guard times between slots (shorter
> guard times = better channel efficiency but shorter range limit before slot
> collisions occur). Unfortunately, "timing advance" won't always work
> properly with a half-duplex system if some users are very close to the
> repeater and others are far away from it (slot collisions between uplink
> and downlink bursts - all on the same RF frequency).
>
> Full-duplex TDMA requires cavity filters at the repeater site and two RF
> frequencies, but "timing advance" can be made to work properly as uplink
> bursts sent to the repeater can never collide with downlink bursts sent
> from the repeater as they are on different frequencies. "Timing advance" is
> where the repeater and the user radio measure the RF round trip time
> between themselves, and the user radio then advances its slot timing
> (starts transmitting earlier to compensate for the RF propagation delay) so
> that its burst arrives in the correct time slot at the repeater. GSM is a
> good example: https://www.slideshare.net/singheranil/timing-advances
>
>
> I think that the "default" and "supported by all stations" modulation used
> for default Codec2 voice and control/beaconing in such a TDMA system should
> be constant envelope (MSK, 4FSK, etc) to allow the use of power-efficient
> non-linear transmit chains, but with the option to use more complex
> modulations (8PSK, nQAM, etc) for traffic, if supported by both ends of the
> link and channel conditions (think high-definition digital voice, "picture
> messages", data transfer, etc).
>
>
> Considering that the performance of 1200bps AFSK over FM is at least 7dB
> worse than what can be achieved: http://www.rowetel.com/?p=3799
> I think that it would be a good idea for a ham TDMA system to support data
> as well as voice so that a TDMA machine can be used for APRS/packet BBS/etc
> type use as well as for digital voice. Buy-in from APRS & packet users, etc
> (better coverage & faster data transfer) should increase support for the
> deployment of TDMA repeaters?
>
>
> A hypothetical full-duplex system might have say 4x slots in an 80ms long
> frame, with a frame rate of say 12.5 Hz (2x 40ms Codec2 frames per
> slot/traffic burst) and a slot time of 20 ms less inter-slot guard time.
> Assuming that the interslot guard time is negligible, and that a slot
> request "access burst" is only half the length of the traffic burst which
> normally fills an occupied slot (ala GSM), then the maximum range to a user
> before an access burst could collide with the subsequent slot would be
> about 3*10^8 x (10ms /2) or ~ 1500 km, which is probably good enough for
> any VHF or UHF terrestrial repeater?. Like GSM, the repeater would respond
> to an access burst with a timing advance value, so that the remote user
> radio can ensure its traffic bursts arrive at the repeater in the correct
> time slot. 4x (or more) time slots per frame permits staggering of uplink
> and downlink slots in time by half a frame duration, so that a user radio
> at the say 1500 km limit would still have ~ 10ms between the end of its RX
> slot and the start of its TX slot (time for a modern PLL to QSY and settle).
>
>
>
> Albert Cahalan mentioned "DoubleTalk Carrier in Carrier", which appears to
> be patented (2025 expiry?): https://www.google.com/patents/US6859641
> It does NOT allow a co-located TX & RX to operate full-duplex on the same
> frequency at the same time, what it does do is allow two ground stations to
> simultaneously use the same channel on the "bent-pipe" transponder of the
> satellite. The transponder of the satellite still receives on one frequency
> and re-transmits on another (eg uplink on 6 GHz, downlink on 4 GHz) - this
> technology would not permit the elimination of cavity filters from
> full-duplex machines such as ham repeaters.
>
> 73 de ZL2WRW
>
>
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