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5.7.2 VHF Communications Digital Links
Note that, although the VHF spectrum is already congested with the result that it is
currently difficult to launch new digital services, the aeronautical community has
reasonably unchallenged access to the VHF band and it makes sense to use the band for
digital communication where possible. Hence the introduction of an efficient VHF link,
either narrow band or wideband, together with a concerted effort to clear out analogue
channels is a reasonable strategy, albeit one which will face initial barriers related to
current spectrum shortages.
VHF datalinks are being standardised, and will be fully implemented in the next few years.
VDL Mode 2 is the first candidate and, as discussed above, is undergoing initial
implementation. VDL Mode 3 is being considered mainly in the USA and VDL Mode 4 is
being considered in Europe, Russia and Asia. Implementation would lead to a decrease
in the reliance on analogue techniques.
5.7.2.1 VDL Mode 3
The VHF Digital Link Mode 3 (VDL3) system provides multiple channels to operate on
one 25 kHz frequency assignment. The VDL3 system uses the same physical layer as
VDL2 (D8PSK modulation), provides a data service and also employs a 4.8 kilobits per
second vocoder for voice operation.
VDL3 has been selected by the FAA for the future provision of voice and data
communications, and forms the basis of the FAA’s Next Generation Air/Ground
Communications (NEXCOM) programme. (Note that it is possible that the FAA will
abandon its VDL3 programme in favour of implementation of 8.33 kHz analogue voice
systems – no firm decision has been made although one could be expected in the next
year).
The FAA will deploy VDL3 to support voice in the high-altitude en-route airspace by 2009
and data around 2012. The FAA does not propose to offer AOC services through their
VDL3 network.
VDL3 employs Time Division Multiple Access (TDMA). The system divides a radio
transmission into four 30-millisecond segments which equates to a 120-millisecond frame.
Each of these segments can be assigned to different user groups to achieve a theoretical
four-fold increase in effective use of a channel.
The system software digitises and compresses 120 ms of the sound into 576 bits that are
transmitted in one burst during a 30-millisecond segment. By using the same software to
decompress the burst on the receiving end, high quality reproduction of voice waveforms
is achieved. Because the basic transmission of the radio is a digital signal, data can also
be transmitted on one of the other time division slots.
The VDL3 system provides for a variety of different system configurations. The various
configurations differ in the way that the different time slot resources are allocated to
different user groups. A user group consists of a ground radio and a number of airborne
radios which are all interconnected by voice and/or data communications.
At any given time different user groups can be in different configurations. An airborne
radio does not need to know configuration information prior to net initialisation. This
information is provided by the ground station.
There are 4-slot configurations and 3-slot configurations. The 4-slot configurations
provide guard time sufficient to allow interference-free communication up to a range of
200 nautical miles (nm). For long range scenarios, the 3-slot configurations provide for
600 nm.
The 4-slot configurations include the following:
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