NXDN is a narrowband digital protocol, and is “6.25 kHz and 12.5 kHz FDMA technology”. NXDN is the result of a joint technical alliance between Icom Incorporated and JVC KENWOOD Corporation. It has become the alternative “de-facto” standard to the Land Mobile Radio (LMR) industry that allowed the development of digital radio products without the excessive premium of some competing digital technologies like Motorola MotoTRBO.
Icom calls their version IDAS and Kenwood has chosen NEXEdge.
NXDN uses (unlike DMR Tier 2 that is TDMA or Time Division Multiplexing ) Frequency Division Multiple Access (FDMA), and uses either 6.25khz or 12.5khz of bandwidth. The narrow 6.25KHz mode is the most popular. This also means that only one conversation can be supported unlike DMR that can have two independent conversations occurring on the same channel.
Without getting too technical, the basic difference between FDMA and TDMA is the definition of a channel and how it is used. In FDMA a particular bandwidth (E.g. 6.25 kHz) at a particular frequency (E.g. 443.200 MHz) is used to define a channel. In TDMA, the same principle applies regarding bandwidth and frequency, but a larger bandwidth (E.g. 12.5 kHz) signal is divided into time slots that allow the channel to create a 6.25 kHz equivalency, but can only be achieved by using a repeater to make the time slots occur.This the Motorola MotoTRBO or DMT standard. With FDMA (Fusion or NXDN), the 6.25 kHz usage of the bandwidth is consistent throughout all products including subscriber units.
Benefits of Digital radio include:
- Clearer audio (NXDN uses the AMBE+2 vocoder)
- Increased range
- Ability to link repeaters via the Internet
This FDMA standard is the same as Yaesu’s C4FM Fusion mode but they are not compatible. The Icom and Kenwood repeaters can be used in “mixed-mode” (just like the Yaesu DR-1/x) , meaning that traditional analog FM at 12.5khz can be used as well. The repeater will sense the appropriate mode and repeat as required. Like DMR, Fusion and D-Star the NXDN repeaters can be linked to other locations increasing the coverage area. This is a powerful feature of Digital Radios in the VHF/UHF bands and the VA3XPR DMR repeater has a Golden Horseshoe talk group linking 4 repeaters together.
NXDN is still very much a niche mode with about 50 repeaters across North America but interest is growing. Popular states include Connecticut, Florida, and Massachusetts.
NXDN like DMR uses talk groups and the original one is 65000 Worldwide.
Kenwood and Icom both use the NXDN CAI over the air, but their internet linking is different. Both send data packets in a UDP stream, but the data in the packet and the ports are different for each manufacturer. This has led to regions typically choosing one brand of repeater for each to stay consistent and be linkable. To combat this interopaerability problem Alan, W7QO, maintains a reflector which converts between the Icom and Kenwood protocols, tying the Kenwood and Icom repeaters together, so that every repeater at least has access to talk group 65000.
The most basic technical specifications are as follows or 6.25 kHz NXDN
Access Method FDMA
Transmission Rate 4800 bps (9600bps for 12.5 kHz NXDN)
Modulation 4-level FSK
Codec Rate 3600bps (Voice 2,450 + Error Correction 1,150 bps) (7200bps for 12.5 kHz NXDN)
Analog/Digital mode Yes
Channel Spacing 6.25 kHz/12.5 kHz
Trunking operation Yes Type-C and Type-D (More details below)
Digital Scrambling Yes (15-bit/32,000 keys)
Encryption Yes (AES/DES)
Audio quality over distance is also greatly improved with 6.25 kHz NXDN™ technology. Instead of the early degradation of audio that you see in an analog signal, the 6.25 kHz NXDN™ digital audio quality remains higher over a comparable distance. This has been seconded in real life use from end users.
With my initial experimentation using LMR based NXDN radios with a Kenwood NX series UHF radio and a commercial repeater site was very interesting. It compares directly with the other digital modes and offers an increase of coverage area due to the power output being sent into a smaller bandwidth. Whitepapers suggest an increase in coverage by 30%. I can confirm this to be accurate comparing the Fusion radio on 70cm against its analog counterpart. Users in Toronto continue to be amazed at the footprint of the VA3XPR DMR repeater. Many digital users will find its reception and transmission to be either 100%, a little garbled, or not heard at all. Another benefit of NXDN is that it uses a Forward Error Correction protocol (FEC) is a technique used for controlling errors in data transmission over unreliable or noisy communication channels. This makes the audio sound superior to D-Star that does not have this benefit in its protocol.
These radios program up like DMR radios and use Code plugs. The DMR Color Code is replaced with Random Access Number (RAN) codes. These codes are similar to the CTCSS tones we use in analog FM.
Here is a High-Level Architecture Overview of the linked network
Source Bill Hausmann N2WNS
Your Local NXDN REPEATER
VE3YYZ Toronto 443.0500 (Ran 1) Digital – (ctcss 103.5) Analog Mixed Mode NXDN and Analog
Talk group ID=1 Local Communications
Talk group ID=65000 Entire Network
VE3YYZ NXDN is a PTT Initiated system. The NXDN Group TG needs to be keyed before the Talk group is linked and times out after 3 minutes, which will require another keyup.