Before this:Framing, error correction & interleaving
TDMA vs. FDMA: fitting more calls on a channel
Key takeaways FDMA (frequency-division multiple access) puts one call per frequency — the way P25 Phase 1 and NXDN work. TDMA (time-division multiple access) splits one frequency into repeating time slots so several calls share it: DMR and P25 Phase 2 use 2 slots, TETRA uses 4. Two voice paths in a 12.5 kHz channel works out to roughly 6.25 kHz-equivalent capacity per call. For a decoder, TDMA adds a job: you must track which slot a call is in, because each slot can carry a different conversation and talkgroup.
The last lesson packed bits into frames and bursts. This lesson asks the next question: how do many users share a limited set of frequencies at the same instant? There are two answers, and a system’s choice between them shapes its capacity, its channel plan, and how you decode it.
FDMA: one call per frequency
FDMA is the straightforward approach: every call gets its own frequency for its duration. A trunked FDMA system still shares frequencies over time — the control channel hands a free one to each call and reclaims it afterward — but at any single moment, one frequency carries exactly one conversation. P25 Phase 1 and NXDN are FDMA.
FDMA is simple to reason about and simple to decode: tune the voice frequency and the call is there, the only one on it. The downside is capacity. To carry more simultaneous calls you need more frequencies, and spectrum is the scarce, expensive resource trunking exists to conserve.
TDMA: many calls per frequency
TDMA squeezes more out of each frequency by dividing time. The channel is cut into short, repeating time slots, and each slot is an independent path. Slot 1 and slot 2 alternate continuously, so two calls run on the same frequency without ever colliding — each simply uses its own slots. DMR and P25 Phase 2 use two slots; TETRA uses four.
The slots cycle far faster than a conversation, and the vocoder frames are buffered so neither talker hears a gap — each call sounds continuous even though it’s only on the air half (or a quarter) of the time.
The capacity win
The payoff is spectrum efficiency. A DMR or P25 Phase 2 channel is 12.5 kHz wide but carries two voice paths. Two calls in 12.5 kHz is the same amount of spectrum per call as a 6.25 kHz channel would give — which is why these systems are marketed as “6.25 kHz-equivalent.” TETRA’s four slots in its channel push the sharing further still.
| System | Access | Slots | Calls per voice frequency |
|---|---|---|---|
| P25 Phase 1 | FDMA | 1 | 1 |
| NXDN | FDMA | 1 | 1 |
| DMR | TDMA | 2 | 2 |
| P25 Phase 2 | TDMA | 2 | 2 |
| TETRA | TDMA | 4 | 4 |
This is one of the headline reasons systems migrate from P25 Phase 1 to Phase 2: double the voice capacity on the same frequencies, without buying more spectrum.
What TDMA means for a decoder
FDMA is easy on a decoder — tune the frequency, decode the one call. TDMA adds a crucial extra job: slot tracking. The decoder demodulates the whole channel continuously, but it must sort the incoming bursts by slot, because slot 1 and slot 2 can be entirely different calls, with different talkgroups and radio IDs. Follow the wrong slot and you’d splice two conversations together.
Practically, this means a TDMA channel grant from the control channel specifies not just a frequency but a slot, and the decoder honours both. GopherTrunk handles this internally when it follows a DMR or P25 Phase 2 call — it locks the right frequency and the right slot — but it’s worth knowing it’s happening, because a system that decodes on one slot and not the other is a slot-tracking symptom, not a tuning one.
Quick check: how does TDMA fit more than one call on a single frequency?
Recap
- FDMA gives one call per frequency — used by P25 Phase 1 and NXDN.
- TDMA splits a frequency into repeating time slots — DMR and P25 Phase 2 use 2, TETRA uses 4.
- Two voice paths in a 12.5 kHz channel ≈ 6.25 kHz-equivalent, the capacity win behind Phase 1 → Phase 2 migration.
- A TDMA decoder must track which slot a call is in, because each slot is a separate conversation.
- A channel grant on TDMA names both a frequency and a slot.
Next, we meet the channel that coordinates all of this: the control channel, the system’s heartbeat.
Frequently asked questions
What is the difference between TDMA and FDMA?
FDMA, frequency-division multiple access, gives each call its own frequency, so one channel carries one conversation. TDMA, time-division multiple access, splits a single frequency into repeating time slots, so several calls take turns on the same frequency. TDMA packs more calls into the same spectrum at the cost of more complex timing.
Which trunking systems use TDMA and which use FDMA?
P25 Phase 1 and NXDN are FDMA, one call per frequency. DMR and P25 Phase 2 use two-slot TDMA, so two calls share a channel. TETRA uses four-slot TDMA. Knowing which a system uses tells you how many simultaneous calls a single voice frequency can carry.
How does two-slot TDMA give 6.25 kHz-equivalent capacity?
A DMR or P25 Phase 2 channel is 12.5 kHz wide but carries two independent voice paths in alternating time slots. Two calls in 12.5 kHz works out to the same spectrum per call as a 6.25 kHz channel would provide, which is why these systems are described as 6.25 kHz-equivalent.
What does TDMA mean for a decoder?
With TDMA the decoder must track which time slot it is looking at, because slot 1 and slot 2 can carry entirely different calls and talkgroups. It demodulates the whole channel continuously but separates the bursts by slot, following one call in slot 1 while another runs in slot 2.