How an NXDN Repeater Works

NXDN is the quiet minimalist of the group. Jointly developed by Icom and Kenwood for commercial and light public-safety use, its whole reason for being is fitting more radio into less spectrum — it runs in a channel just 6.25 kHz wide, half the width of everything else here. On the ham bands it’s the least common of the digital voice modes, run on surplus commercial radios in a lean, conventional style. Here is how that narrow little machine works.

It starts as an ordinary repeater

An NXDN repeater listens on an input frequency and retransmits on an output frequency with the usual offset. The modulation is 4-level FSK, the access method is FDMA — one conversation per channel — and the vocoder is AMBE+2. So far, a close relative of C4FM and P25. The twist is how little space it takes up.

The narrowest channel in digital voice

NXDN comes in two channel widths: a 12.5 kHz “wide” mode at 4800 bit/s, and the signature 6.25 kHz narrowband mode at 2400 bit/s. That 6.25 kHz figure is the whole point — where DMR, D-STAR, C4FM, and P25 all live in 12.5 kHz channels, NXDN can fit two conversations’ worth of channels into the same slice of band. For a commercial operator squeezed for frequencies, that efficiency is the entire sales pitch.

Same slice of band, two ways to fill it. Others 12.5 kHz one 12.5 kHz channel — DMR / D-STAR / C4FM / P25 NXDN 6.25 kHz 6.25 kHz channel 6.25 kHz channel Two independent NXDN channels fit where one ordinary digital channel lives.

NXDN’s 6.25 kHz narrowband packs two channels into the space others use for one.

The RAN: NXDN’s access code

Every NXDN transmission carries a Radio Access Number — the RAN, a value from 0 to 63 — and the repeater checks it just as a DMR machine checks its color code, a P25 machine its NAC, or an FM machine a CTCSS tone. Matching RAN, and the repeater responds; wrong RAN, and it ignores you. It’s the gate that keeps a co-channel system from keying your machine, and a required field when you program an NXDN channel.

Two names, one mode

IDAS and NEXEDGE

NXDN was created jointly by Icom and Kenwood, and each sells it under its own brand: Icom calls its NXDN gear IDAS, Kenwood calls its NEXEDGE. They’re the same underlying protocol, so an IDAS radio and a NEXEDGE radio can talk to each other. If you go shopping the surplus market for NXDN equipment, those are the two names to look for — both are NXDN under the hood.

NXDN on the ham bands

Like P25, NXDN is a commercial mode, so hams run it on retired IDAS and NEXEDGE radios from the used market rather than buying new. Commercial NXDN systems can be conventional or trunked, but on the amateur bands it’s run conventional — a single frequency pair with a RAN, no trunking controller. To reach beyond the local machine, an MMDVM controller links the repeater over IP to an NXDN reflector, where talkgroups gather users and bridges can connect NXDN to the other digital modes.

Solid = 4FSK RF over the air   ·   dashed = internet Your HT RAN set NXDN Repeater conventional checks RAN MMDVM IP link NXDN Reflector talkgroups bridges to modes RF Conventional RF on one narrow pair, linked over IP — no trunking on the ham side. From the reflector, your voice reaches every other linked NXDN repeater and hotspot.

A ham NXDN repeater: a conventional narrowband machine plus an MMDVM controller linking to a reflector.

Follow a transmission through

Say you key up on a conventional NXDN repeater linked to a reflector talkgroup:

  1. Your radio — the AMBE+2 vocoder encodes your voice as 4FSK in the 6.25 kHz channel, tagged with the RAN and your talkgroup.
  2. Uplink — the signal reaches the repeater on its input frequency. It checks the RAN; a match, and it accepts you. Being FDMA, it handles one conversation at a time.
  3. To the controller — the MMDVM controller passes your frames over IP to the repeater’s NXDN reflector.
  4. Out to the talkgroup — the reflector copies your frames to every other repeater and hotspot on that talkgroup, near or far, and across any bridges to other modes.
  5. Downlink — each destination repeater transmits your frames on its own output frequency, and every NXDN radio in range decodes them back into your voice.

The one-sentence version

An NXDN repeater is a lean, conventional 4FSK machine — usually surplus Icom IDAS or Kenwood NEXEDGE gear with an MMDVM controller — that fits in a 6.25 kHz channel, gates access with a RAN, and reaches the world through a reflector.

That covers the machine and its linking. For the mode itself — how NXDN encodes and how it compares to the others — see How NXDN Works; and for how it arrived in amateur radio, History of NXDN in Amateur Radio.

A noncommercial hobby reference compiled by N6JET, gathered from public sources and shared freely for anyone interested in amateur digital voice.