Also known as: TDMA
TDMA (time-division multiple access) splits one frequency into rapid, repeating timeslots, so two or more calls share the channel by taking turns.1 Each call is present only during its own slots but recurs often enough — many times per second — that the conversation sounds continuous, so one RF channel does the work of several.
How it works
Transmitters and receivers agree on a repeating frame divided into equal slots. A radio bursts its data in its assigned slot and stays silent in the others, so several radios time share one carrier without colliding. This demands accurate timing: each burst is surrounded by a small guard time to absorb propagation delay, and radios lock onto a periodic frame-synchronisation pattern to know where the slot boundaries fall. In exchange for that complexity, TDMA doubles or quadruples the number of simultaneous calls per frequency and can turn a subscriber’s transmitter off between bursts, extending battery life.
P25 Phase 2 and DMR use two slots, doubling capacity per channel; TETRA uses four. On a trunked TDMA system a grant therefore names both a frequency and a slot, and a receiver must follow the correct slot as well as the frequency.
Variants
- Two-slot TDMA — DMR and P25 Phase 2 (the P25 Phase 2 air interface is called H-DQPSK / TDMA, giving 6.25 kHz-equivalent efficiency in a 12.5 kHz channel).
- Four-slot TDMA — TETRA, packing four calls into a 25 kHz carrier.
- FDMA/TDMA hybrid — most cellular and trunked networks channelise with FDMA and then time-share each channel, combining the two access methods.
In practice
TDMA is the efficiency play for capacity-constrained systems: a two-slot DMR repeater carries two independent conversations on one frequency pair, which is why DMR became popular for commercial and amateur repeaters. The cost is stricter timing and more complex radios and receivers, and the fact that a single carrier now carries multiple unrelated calls that a monitor must separate by slot.
Relevance to SDR
On a TDMA system a single granted voice channel can carry two (or four) simultaneous calls, so GopherTrunk decodes the relevant slot of the assigned channel rather than the whole carrier. Recovering the slot requires frame synchronisation and burst timing, which GopherTrunk performs as part of its P25 Phase 2, DMR, and TETRA decode paths — contrast FDMA, where the whole frequency is one call.
Sources
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Time-division multiple access — Wikipedia, on sharing one frequency via repeating timeslots. ↩