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Narrow Band Digital
Two-Way Radio
by Kevin Poulter*
Like just about everything else in the 21st Century, professional
two-way radio is making the transition from analog to digital.
As well as potentially much clearer signals and more users in
the same amount of spectrum, digital two-way radio offers
many other advantages.
* on behalf of ICOM Australia
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October 2013 53
Fig.2: Next Generation
Digital Narrowband
(the red line) has a
much sharper range
cut-off than analog
audio (the blue line)
but offers significantly better audio
quality over its
operating range.
Fig.1: the older 12.5kHz-wide analog signal (on the
left) occupies the same amount of bandwidth as two
6.25kHz digital signals (right). These can co-exist without
interfering with each other.
T
owards the end of last century, the growing demand
for radio frequency spectrum in the “Land Mobile”
UHF professional two-way radio band forced a
change from the original 100kHz separation between users
down to 25kHz separation. But even this wasn’t enough to
accommodate the ever-increasing demand.
In the USA, the FCC mandated that all professional
telecommunications equipment have a separation of just
12.5kHz by January 1, 2013, and Australia’s Communications and Media Authority (ACMA) followed suit.
However, it’s widely expected that before too long, even
this will be too wide and a separation between channels
of just 6.25kHz will be required.
Modern radio equipment, such as that made by Icom,
already has this capability built in.
Going digital
Narrowbanding, as it is called, was the ideal opportunity
for transceivers to also migrate from analog transmissions
to digital. Not only does this enable many more users in
the available spectrum, but it also enables improved noise
rejection and voice quality over a greater distance.
Much clearer (and therefore intelligible) conversations
are especially evident at the limits of the transmission range,
with none of the analog background noise.
But digital transmission has a number of other benefits,
such as the ability to further clarify speech by looking
for waveform patterns which match an internal library of
speech waverforms. Any other patterns – such as noise
and even wind noise in the microphone - are assumed to
be noise and are nulled out.
Co-existing with analog
With so many millions of dollars invested in analog
radio equipment, many organisations would be most reluctant to replace their current system, digital advantages
notwithstanding.
For this reason, Icom developed a “mixed mode” system
whereby analog and digital can co-exist and be used in
parallel until the user is ready to replace old gear.
For example, a leading Australian hospital currently has
made the switch to digital inside the building but their
security contractor still uses analog mobiles on the same
channel. Both are received simultaneously with the system
automatically adjusting and replying in the transmission
mode in use.
The same hardware – antennas, power supplies, duplexers, isolators and combiners can be used with both
systems, so the only cost is adding digital transceivers as
and when required.
Next Generation Digital Narrowband
A joint technical alliance between Icom and JVC Kenwood developed Next Generation Digital Narrowband
IDAS single-site trunking (left) is for relatively small
area use, with individual sites only able to communicate
with units on their site. By contrast, multi-site trunking
(above) can handle up to 48 sites, each with 30 channels.
Communication is via an IP network and sites can be
virtually anywhere, even on the other side of the world.
54 Silicon Chip
siliconchip.com.au
(NXDN) technology, an open protocol. Two 6.25kHz digital
narrowband signals can be used next to each other within
a 12.5kHz channel without causing interference to each
other or adjacent channels.
Internet connection
Icom’s Digital Advanced System (IDAS) has a network
interface which can be connected to a LAN or the internet.
Communication range is vastly extended by talking through
IP (internet protocol), eliminating the need for costly RF
links or leased lines for “off site” communication. Hence,
“off site” can be anywhere from the next street to the other
side of the world.
Trunking systems
Insufficient spectrum, increased radio usage and budget
limitation often mean there are insufficient channels available for users in large groups to have their own frequency.
“Trunking” is a commonplace solution to this problem,
where many users share relatively few radio channels and
calls are automatically “routed” via a central computer.
They are divided into “talkgroups” and the computer finds
a vacant channel to allow the communication to proceed.
Icom’s digital transceivers are capable of a variety of
modes of trunking, with their “MultiTrunk system allowing
from one to thirty channels per site, with up to 16 sites. All
channels can be used for voice or data and a priority system
allows emergency communication interruption.
Coding, decoding and scrambling
Digital communication allows advanced coding, with
CTCSS (Continuous Tone-Coded Squelch System),
DTCS (Digital Tone Code Squelch) and RAN (Random
Access Number) coding on a per-channel basis. This
allows the channel to be shared with multiple users,
only springing to life when the code appropriate to
the called station is received.
Similarly, scrambling (for secure communication) is
particularly suited to digital mode, without the significant loss of voice quality that analog scrambling can
cause. In IDAS digital mode, a 15-bit encryption key can
also be programmed, for over 32,000 scrambling codes.
Easy programming
Another feature of the digital age is quick and
easy programming on site, from a suitably
equipped PC. Features such as selective
calling, status message, radio stun/kill/
revive (useful when equipment is stolen) and even GPS position reporting
can be programmed into the system.
Professional Telecommunications Narrow Band Digital has a host of features,
far more than can be summarised here.
If you’d like to know more, and see how
the ICOM range fits into digital radio, visit
SC
www.icom.net.au
Icom’s Digital Handheld transceiver has
512 memory channels with 128 zones,
IDAS and NXDN features, GPS receiver,
man down function, waterproof to IP67
standard and 800mW audio output power.
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October 2013 55
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