NXDN
Encyclopedia
NXDN is a Common Air Interface (CAI) technical protocol for mobile communications. It was developed jointly by Icom Incorporated and Kenwood Corporation. The NXDN Forum was formed in order to promote the NXDN protocol in North and South America. The forum's members are:
NXDN is implemented by Icom in their IDAS system and by Kenwood as NEXEDGE.
(FCC) 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.”
(TDMA) systems 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 allow many users to share a common spectrum allocation by using spread-spectrum techniques.
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 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 for use on VHF (136-174 MHz) and UHF (400-520 MHz) bands.
Data is transmitted using 4-level frequency-shift keying
(FSK) modulation. NXDN uses the AMBE+2 vocoder (codec
) 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 at the 12 dB SINAD
threshold. The transmission bit rate is 4800 bps.
The following FCC emission designators 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
Encryption
– Spectrum-inversion digital voice scrambling
with a 15-bit key for 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
Kenwood NEXEDGE
for more information on the AMBE vocoder
family and discussions of speech reproduction quality.
The audio reproduction quality of IDAS and NEXEDGE communications systems is dependent on the performance of the AMBE+2 vocoder
used by NXDN. The AMBE family of vocoders has been subjected to comparative testing and found to be adequate for its intended uses, primarily mobile and aeronautical radio. The AMBE+2 vocoder has also been selected for use in the Motorola MOTOTRBO radio family and the Project 25 (P25) mobile radio system. The following reports and papers are descriptions of laboratory-environment evaluations of AMBE+2 and other speech vocoders.
Compromises in audio quality are inherent in the use of any codebook
-based speech coder
, particularly when used in conditions of high background noise. Incremental improvements are being made in the algorithms, which may lead to differences in performance even while the basic method remains unchanged. In the US, the Department of Commerce Public Safety Communications Research laboratory http://www.pscr.gov/projects/audio_quality/mrt/mrt1.php regularly reports on progress in this field. While their work specifically pertains to Project 25 radios, it is directly applicable to any system using similar multi-band excitation coders.
- Aeroflex Wichita, Inc.
- Anritsu Corporation
- CML Micro
- Daniels Electronics, Ltd.
- Icom Incorporated
- Kenwood Corporation
- Ritron, Inc.
- Trident Micro Systems
NXDN is implemented by Icom in their IDAS system and by Kenwood as NEXEDGE.
Applications
The NXDN protocol and the communications products in which it is used are intended for commercial Private Land Mobile Radio (PLMR) and low-end public safety communications systems. The technology satisfies the U.S. Federal Communications CommissionFederal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) 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.”
Technical characteristics
NXDN uses Frequency-Division, Multiple-Access (FDMA) technology in which different communication streams are separated by frequency and run concurrently. Time-Division, Multiple-AccessTime division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
(TDMA) systems 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
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(CDMA) systems allow many users to share a common spectrum allocation by using spread-spectrum techniques.
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
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
system 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 for use on VHF (136-174 MHz) and UHF (400-520 MHz) bands.
Data is transmitted using 4-level frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
(FSK) modulation. NXDN uses the AMBE+2 vocoder (codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
) 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 at the 12 dB SINAD
SINAD
SINAD stands for Signal-to-noise and distortion ratio. It is a measure of the quality of a signal from a communications device, often defined as:...
threshold. The transmission bit rate is 4800 bps.
The following FCC emission designators 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
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
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
– Spectrum-inversion digital voice scrambling
Voice inversion
Voice Inversion scrambling is an analog method of obscuring the content of a transmission. It sometimes used in public service radio, automobile racing, cordless telephones and the Family Radio Service. Without a descrambler, the transmission makes the speaker sound like "Donald Duck"...
with a 15-bit key for 32,768 different codes
Paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
& 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
Icom IDASKenwood NEXEDGE
Audio quality
In all lossy compression schemes, trade-offs are made in voice reproduction quality in return for minimizing the raw bit rate of the transmission. This leads to artifacts and compromises of frequency response in reproduced speech. Encoders and other compression schemes that are highly-optimized for speech are often unsuitable for non-speech audio, such as music or frequency-shift keyed data. Using an inappropriate encoder usually results in the creation of distortion and artifacts in the reproduced audio. See the AMBE entry Advanced Multi-Band ExcitationAdvanced Multi-Band Excitation
Advanced Multi-Band Excitation is a proprietary speech coding standard developed by Digital Voice Systems, Inc.- Overview :AMBE is a codebook-based vocoder that operates at bitrates of between 2 and 9.6 kbit/s, and at a sampling rate of 8 kHz in 20-ms frames...
for more information on the AMBE vocoder
Vocoder
A vocoder is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder...
family and discussions of speech reproduction quality.
The audio reproduction quality of IDAS and NEXEDGE communications systems is dependent on the performance of the AMBE+2 vocoder
Vocoder
A vocoder is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder...
used by NXDN. The AMBE family of vocoders has been subjected to comparative testing and found to be adequate for its intended uses, primarily mobile and aeronautical radio. The AMBE+2 vocoder has also been selected for use in the Motorola MOTOTRBO radio family and the Project 25 (P25) mobile radio system. The following reports and papers are descriptions of laboratory-environment evaluations of AMBE+2 and other speech vocoders.
- NTIA Report 01-386, "Voice Quality Assessment of Vocoders in Tandem Configuration" http://www.its.bldrdoc.gov/pub/ntia-rpt/01-386/01-386_ocr.pdf
- "Performance assessment of 4.8 kbit/s AMBE coding under aeronautical environmental conditions", 1996 IEEE International Conference on Acoustics, Speech, and Signal Processing, 1996. ICASSP-96 Vol 1. Conference Proceedings. http://www.computer.org/portal/web/csdl/doi/10.1109/ICASSP.1996.541142
- "Project 25 Vocoder Evaluation Mean Opinion Score Test", TIA TSB-102.BABE http://standardsdocuments.tiaonline.org/tia-tsb-102-babe.htm
Compromises in audio quality are inherent in the use of any codebook
Codebook
A codebook is a type of document used for gathering and storing codes. Originally codebooks were often literally books, but today codebook is a byword for the complete record of a series of codes, regardless of physical format.-Cryptography:...
-based speech coder
Speech coding
Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
, particularly when used in conditions of high background noise. Incremental improvements are being made in the algorithms, which may lead to differences in performance even while the basic method remains unchanged. In the US, the Department of Commerce Public Safety Communications Research laboratory http://www.pscr.gov/projects/audio_quality/mrt/mrt1.php regularly reports on progress in this field. While their work specifically pertains to Project 25 radios, it is directly applicable to any system using similar multi-band excitation coders.