Tutorial NXDN

Tutorial: NXDN

What it is

NXDN is a digital two-way radio system from Icom and Kenwood used by businesses, utilities, transport and public services in many countries. OnAir logs calls with unit IDs and talkgroups, status and short data messages, and shows the site information of a trunked system. Voice needs an mbelib you install yourself.

What you need

• Any SDR that tunes the band: an RTL-SDR covers roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps (see Radio-RTL-SDR), and so do HackRF, Airspy, SDRplay and others (Supported-radios). The narrow signals here need only a small sample rate; the mode asks for 2 Msps.
• Antenna: a VHF or UHF vertical, outdoors.
• Frequencies (database): the PMR446 digital channels (6.25 kHz) starting at 446.10625 MHz carry NXDN-capable equipment; business and public-safety channels use local licensed frequencies. Type the frequency or use the Scan tab. The channel is 12.5 kHz (6.25 kHz NXDN4800 also exists).

Before you start

Set the gain so that the ADC level in the status line stays clear of full scale; use Auto-tune or Optimise gain (Your-first-reception). Too much gain makes strong neighbours (paging, FM broadcast, repeaters) leave spurs in the channel. Make sure the radio is on frequency: a cheap RTL-SDR can be several kHz off at 430 MHz, so enter its correction (ppm) in Radio settings or watch the carrier offset the panel shows. Narrow digital voice channels need an offset under about 1 kHz.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Two-way radio and choose NXDN.
2. Enter the frequency. Press Start; the panel says start the receiver to hear NXDN.
3. When a call arrives, Now shows ON AIR with the system (NXDN4800 or NXDN9600, RAN, joined-late flag), the unit and the target.
4. Open Calls for the log (Carrier, Time UTC, Kind, From, Type, RAN, Details, Length), Messages for short data and Site for the location ID and system / site (trunked control channels send these).
5. Install mbelib yourself if you want to hear voice: Voice-codecs lists the file names and folders. Without it the panel says no mbelib: frames counted only.

What you should see

Calls with a unit ID, a type and the RAN (radio access number). encrypted (cipher, key): no sound appears when a call is encrypted. The bit error counters of the control channels (SACCH, FACCH1, CAC) show the quality; the status shows the symbol SNR, +3 deviation and offset.

Reading the screen

Now: current call. Calls: the log. Messages: short data. Site: no site information on conventional repeaters, location ID and system / site on a control channel. Unit / Heard: the units heard.

Options and settings

• Copy a call log row.
• The panel adapts to NXDN4800 or NXDN9600.

Troubleshooting

• Nothing heard. Many NXDN systems are trunked and use many channels: find the control channel or use the Scan tab.
• Calls with a lot of bit errors. Weak signal or offset.
• Silent calls. mbelib missing or the call is encrypted.
• Nothing on PMR446. Few devices use NXDN there; most use analogue or DMR.
• Inverted spectrum. The panel says inverted when it detects it.

Tips

• Record the channel (Tutorial-Record-IQ-then-the-Signal-workbench) while it is busy and replay it to try settings.
• A scan (Main-window, tab Scan) helps to find an active channel.

Legal and privacy notes

Digital voice and data on land-mobile and amateur channels can be private. Receiving is regulated differently in each country: follow the rules where you live. OnAir decodes only what is sent in the clear: encrypted calls are reported as encrypted and nothing is decrypted, and OnAir offers no help to defeat encryption. The AMBE / IMBE vocoders are patented, so the voice sound comes only from an mbelib you install yourself (see Voice-codecs); calls, IDs and data work without it. Do not publish what you hear. Transmit works in OnAir (used on a real radio by the maintainer and in loop-back tests) but needs the right licence for the service; see Transmit and Legal.

Under the hood

NXDN uses 4-level FSK at 4800 baud in 12.5 kHz (NXDN9600, 9.6 kbit/s) or 2400 baud in 6.25 kHz (NXDN4800), with the channel structure LICH, SACCH, FACCH1 and CAC protected by convolutional codes with CRC, and AMBE+2 voice. The specification is maintained by the NXDN Forum.

Related

Mode-NXDN, Tutorial-DMR, Tutorial-P25, Tutorial-dPMR-PMR446, Tutorial-Digital-voice-DMR-D-STAR-P25, Voice-codecs, Radio-RTL-SDR, FAQ