Making better use of the spectrum

A digital radio system that is designed to make more efficient use of the spectrum will use a 6.25 kHz bandwidth and have increased range.

Called NXDN, its common air interface (CAI) was the result of a joint development between Kenwood and Icom. It is aimed at providing a migration path for land mobile radio users and low-end public safety communications systems to move towards multi-vendor standards. It also opens the platform to vendors, allowing them to develop communications products and interfaced OEM equipment.

Bringing all this together was the NXDN Forum that in 2008 came into being to set out and promote the new platform. The 13 forum members include Kenwood, Kenwood USA, Icom, Icom America, Aeroflex Wichita, Daniels Electronics, Ritron, Trident Micro Systems, Anritsu, CML Microsystems, Etherstack, General Dynamics SATCOM Technologies, Meteor Communications.

Developed as a response to the need for greater spectrum efficiency as frequency allocations run out and the US encourages that two-way radio moves to 6.25 kHz, the system maintains the basic architecture of analog FM radios but uses frequency division multiple access (FDMA) and the digital modulation technique of four-level frequency shift keying (FSK).

Under FDMA the spectrum is divided into workable bandwidths for signal transmission. Digital technology allows this bandwidth to be 6.25 kHz, although it will also operate at 12.5 kHz (coverage and IP data rate advantages when using 6.25 kHz).

Being similar to analog digital gives greater flexibility when migrating to a new platform, as some equipment can be reused in the new system and the NXDN equipment can operate in both analog and digital modes and in shared operation.

The modulation technique used by NXDN is four-level FSK where data information is represented by amplitude so that the data can modulate the carrier generated by a voltage-controlled oscillator for transmission.

Four different deviations of a frequency are used to represent the data bit pairs giving higher data rates.

A vocoder (a combination of voice and encoder) converts the analog voice audio into a digital signal ready for modulation. The vocoder samples the waveform and transmits a reference of the waveform shape along with waveform information, at the receiving end the waveform is reconstructed by using the reference in a look-up table of waveforms (simple explanation) and the waveform information.

NXDN uses the AMBE+2 vocoder having:

• 3600 bps enhanced half rate;

• Voice 2450 bps forward error correction (FEC), 1150 bps;

• 7200 bps enhanced full rate.

NXDN is claimed to have improved coverage, good audio quality, good security and calling features, including a large number of operational options. Using forward error correction, the modulated signal can be recovered when corrupted by signal degradation giving improved coverage and penetration.

The processing power of the vocoder can correct errors and maintain high intelligibility, improving voice quality in noisy environments and fading or weak signal areas.

The system operates at 4800 and 9600 bps at 6.25 and 12.5 kHz respectively and it transmits the voice information while simultaneously transmitting control data. There is built-in security encryption with upgrades available to give higher levels of protection.

Other features of the platform include:

• Individual and group calling;
• Emergency functions;
• Radio stun and kill;
• Data messaging;
• GPS;
• Trunking and roaming;
• Priority monitoring and registration.

Both Kenwood and Icom have released equipment using the NXDN protocol. The Kenwood range is called NEXEDGE and lcom is producing IDAS. Both ranges include portables, mobiles and repeater units covering the UHF and VHF commercial communications bands (VHF 136 to 174 MHz and UHF 400 to 512 MHz).