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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