Overview
NXDN™ is an Common Air Interface (CAI) technical standard for mobile communications. It was developed jointly by Icom, Incorporated and Kenwood Corporation. The NXDN Forum [1] was formed in order to promote the NXDN protocol in North and South America. The forum's members are:
Aeroflex Wichita Inc.[2]
Daniels Electronics Ltd.[3]
Icom Incorporated[4] and Icom America Incorporated [5]
Kenwood Corporation[6] and Kenwood USA Corporation[7]
Ritron Inc.[8]
Trident Datacom Technologies (Trident Micro Systems) [9]
CML Microcircuits Ltd. http:/cmlmicro.com
NXDN is implemented by Icom in their IDAS™ system [10] and by Kenwood as NEXEDGE™ [11].
Applications
The NXDN standard and the communications products in which it is used are intended for commercial Private Land Mobile Radio (PLMR) [12] and low-end public safety communications systems. The technology satisfies the FCC [[13]] mandate requiring all communications systems covered by Part 90 regulations to use narrowband technology by 1 Jan 2013. Part 90 regulations specify a bandwidth of 12.5 kHz, but the FCC “strongly urges licensees to consider migrating directly to 6.25 kHz technology rather than first adopting 12.5 kHz technology and later migrating to 6.25 kHz technology.” The FCC “will expeditiously establish a schedule for transition to 6.25 kHz narrowband technology.” [14]
NXDN Technical Characteristics
The NXDN standard does not comply with the European Telecommunications Standards Institute (ETSI) [15] Technical Standard 102 490, "Peer-to-Peer Digital Private Mobile Radio using FDMA with a channel spacing of 6,25 kHz with e.r.p. of up to 500 mW". NXDN is a similar FDMA protocol but is not an open standard. ETSI TS102 490 is the air interface protocol for digital Private Mobile Radio (dPMR) which is a 6,25 kHz only FDMA technology. http:/dpmr-mou.org
NXDN uses Frequency-Division, Multiple-Access (FDMA) technology [[16]] in which different communication streams are separated by frequency and run concurrently. (Time-Division, Multiple-Access (TDMA) systems [[17]] combine the communications streams into a single stream in which information from the different streams is transmitted in interleaved time allocations or "slots." Code-Division, Multiple-Access (CDMA) systems [[18]] allow many users to share a common spectrum allocation by using spread-spectrum techniques.[[19]]
The basic NXDN channel is digital and can be either 12.5 kHz or 6.25 kHz wide. 6.25 kHz dual-channel systems can be configured to fit within a 12.5 kHz channel. This effectively doubles the spectrum efficiency compared to an analog FM system [[20]] occupying a 12.5 kHz channel. The architecture of NXDN is such that two NXDN channels, within a 12.5 kHz channel for example, can be allocated as voice/voice, voice/data, or data/data. As of yet this capability has not been implemented in commercially available hardware.
Systems that use NXDN also support mixed analog FM and digital NXDN equipment, including direct radio-to-radio communications. This allows system owners to migrate to a narrowband, digital system without replacing the entire system at once. NXDN equipment is currently FCC type-accepted [[21]] for use on VHF (136-174 MHz) and UHF (400-520 MHz) bands.
Data is transmitted using 4-level frequency-shift keying (FSK) modulation. [[22]] NXDN uses the AMBE+2™ vocoder (codec) [23] for digital audio. This combination provides better weak-signal voice quality than for analog FM. For an equivalent transmitter power, NXDN is represented as having a wider range and slightly better multi-path characteristics than analog FM in typical RF environments, specifically the 12 dB SINAD [[24]] threshold. The transmission bit rate is 4800 bps.
Within the NXDN protocol data structure the codec bit rate is 3600 bps. Of the codec bit rate, 2450 bps are allocated to voice data and the remaining 1150 bps consist of error correction data. Digital data can be included in the data stream with a net data rate of 800 to 950 bps. If the equipment is configured such that only digital data is transmitted, the net data rate is 4400 to 4550 bps. The NXDN frame transmission length is 80 msec.
The following FCC emission designators [[25]] apply to NXDN transmissions:
8K30F1E 12.5 kHz single channel digital voice
8K30F1D 12.5 kHz single channel digital data
8K30F1W 12.5 kHz single channel digital voice and data
4K00F1E 6.25 kHz single channel digital voice
4K00F1D 6.25 kHz single channel digital data
4K00F1W 6.25 kHz single channel digital voice and data
4K00F2D 6.25 kHz single channel analog CW ID
Application Functions Supported by NXDN
The NXDN protocol provides support for the following functions. Implementation of the functions and the user-level interfaces by which they are accessed and used may vary by manufacturer.
Encryption – Spectrum-inversion digital voice scrambling, 15-bit key (32,768 different codes)
Paging & Status Reporting – Radio-to-Radio and Dispatch-to-Radio
User Aliases – 65,545 different Group ID's and User ID's
Man-down and Emergency call
Remote radio management functions - Stun/Kill/Revive and Monitor
Current Implementations of NXDN
Kenwood NEXEDGE [28]
See Also:
ETSI Technical Standard 102 490, "Peer-to-Peer Digital Private Mobile Radio using FDMA with a channel spacing of 6,25 kHz with e.r.p. of up to 500 mW".
Comparison of FDMA and TDMA Systems [29]