NXDN Land Mobile Radio

- notes and summary or tutorial of the basics of NXDN a digital standard for Land Mobile Radio or PMR - the private / professional, mobile radio.

NXDN radio technology is a form of land mobile radio or business radio that was developed jointly by Icom and Kenwood in response to the need for radio communications systems that provided greater levels of spectrum efficiency over those that were already available while still keeping costs low.

NXDN is a digital technology that in addition to providing spectrum efficiency also provides greater levels of flexibility and more facilities while retaining the simplicity of analogue FM radios. As a result NXDN radio technology is being deployed in many areas.

NXDN rationale

The NXDN radio technology was developed to meet the needs of the radio industry, particularly in North America where pressure was increasing on the available spectrum. While channel bandwidths had fallen from 100 kHz for the very first land mobile radio systems to 50 kHz, 25 kHz to 12.5 kHz, there was a further need to reduce the channel bandwidth to 6.25 kHz.

There was also a need to increase the overall flexibility and available capabilities of any new system by adopting digital technology.

In order to reduce the channel bandwidth to 6.25 kHz, access technologies were considered. TDMA - time division multiple access, used for systems such as DMR and P25 were not deemed suitable for 6.25 kHz channels. Accordingly FDMA - frequency division multiple access was adopted.

For an FDMA system to work within a 6.25 kHz channel, careful consideration of the modulation format was required. Schemes such as CQPSK - compatible quadrature phase shift keying, and ACSB - amplitude- compandored single sideband, were considered, but 4-level FSK was adopted. This provided better performance within the requirements of the system. For example it did not require the use of linear RF power amplifiers to retain the integrity of the transmission.

The first NXDN radios were introduced in 2006, and now a full range of NXDN equipment is available.

NXDN radio specification

The main aims for the NXDN radio system are low cost, good performance and high spectral efficiency within a 6.25 kHz channel spacing. The cardinal points within the technical specification for NXDN radio systems are detailed in the table below:


NXDN parameter NXDN specification details
Access technology FDMA
Transmission rate 4.8 kbps
Modulation 4-level FSK
Vocoder AMBE+2
Codec rate 3.6 kbps
Codec data partitioning Voice - 2.45 kbps
Error correction - 1.15 kbps

NXDN radio FSK modulation

The NXDN radio system uses a form of modulation known as 4 level FSK or frequency shift keying - 4FSK. This form of modulation allows effective transfer of data. A two bit binary number is mapped to a single symbol which is modulated onto the carrier. In this way the data rate is increased.

The number and symbol mapping is shown in the table below:


Information
(Binary data)
Symbol Deviation
01
+3
+1050 Hz
00
+1
+350Hz
10
-1
-350Hz
11
-3
-1050Hz

It can be seen that a single symbol, represented by a single deviation frequency is able to carry two bits of information. This is an effective way of carrying data on a transmission.

When modulating a carrier, the binary signal enters the circuitry, it is then mapped to the symbols. After this it is filtered and this signal is applied to the frequency modulator.

During reception the demodulator process is the reverse of the transmission scenario within NXDN. The 4FSK signal enters the frequency demodulator, after which it is filtered and passed to the symbol de-mapping to produce the original binary signal.

NXDN summary

NXDN is a standard that is mainly deployed within North and South America, although it has been deployed in a number of other countries. There are links with the dPMR developers as the solutions are very similar.

In terms of performance NXDN performs better than analogue FM, while occupying a narrower channel bandwidth. For an equivalent transmitter power, NXDN is generally accepted as having a greater range and slightly better multi-path characteristics than analogue FM in typical RF environments.

By Ian Poole

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