Field Guide · term

Also known as: FDMA

FDMA (frequency-division multiple access) is a channel-access method in which each call occupies its own frequency channel.1 It is the simplest way to let many users share the spectrum: divide the band into non-overlapping channels and give one conversation to each, separated by small guard bands so adjacent channels do not interfere.

frequency call Acall Bcall Ccall D
FDMA gives each call its own frequency channel — the access method of P25 Phase 1.

How it works

Each active conversation transmits continuously on a dedicated carrier for the duration of the call. Capacity scales directly with the number of frequencies available: adding conversations means adding channels. There is no need to synchronise transmitters in time, which keeps subscriber radios simple and tolerant of propagation delay — one reason FDMA suited early land-mobile systems. The trade-off is spectral: a channel occupied by a quiet talker still ties up its whole frequency, and narrowing channels to fit more of them runs into filter selectivity and adjacent-channel limits.

Narrowbanding has pushed FDMA voice channels progressively tighter — from 25 kHz analog FM down to the 12.5 kHz of P25 Phase 1 C4FM and the 6.25 kHz of NXDN and dPMR — squeezing more channels into the same band without changing the one-call-per-frequency principle.

Variants

  • Analog FDMA — conventional 12.5/25 kHz FM channels; the original land-mobile model.
  • Digital narrowband FDMA — P25 Phase 1 (12.5 kHz), NXDN and dPMR (6.25 kHz), which achieve 6.25 kHz-equivalent efficiency by modulation alone rather than time division.
  • Hybrid access — many cellular and trunked systems combine FDMA (channelising the band) with TDMA (sharing each channel in time).

In practice

FDMA remains the backbone of public-safety P25 Phase 1 across North America and of NXDN in utility and commercial fleets. Its per-frequency simplicity makes conventional narrowbanding straightforward and gives good range, at the cost of the spectral efficiency that TDMA achieves by packing two or more calls onto one carrier.

Relevance to SDR

On an FDMA system each voice channel is a separate frequency the receiver tunes to, and there is no timeslot to track — a granted channel is simply a carrier to demodulate. GopherTrunk handles this by digitally down-converting the granted frequency from a wideband capture, in contrast with TDMA, where it must also select the correct slot.

Sources

  1. Frequency-division multiple access — Wikipedia, on the one-call-per-frequency access method. ↩

See also

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