FINAL REPORT - Stakeholders - Ofcom

Parameter Value Notes
Channel access
method
Frequency availability
(allocation status)
TDMA Limited to 4 slots
No operational frequencies
currently assigned in Europe
or North America
Typical Configurations
4 Slot:
4V (4 voice)
3V1D (3 voice / 1 data)
2V2D (2 voice / 2 data)
3T (trunked voice and data)
3 Slot (extended range):
3V (3 voice)
2V1D (2 voice / 1 data)
3S
2V1X
Dependencies Synchronised time source needed to maintain
TDMA timing for multiple ground stations
operating simultaneously on the same frequency
in a given area.
Table 5- 4 Characteristics of VDL Mode 3
5.7.2.2 VDL Mode 4
VDL Mode 4 (VDL4) is a digital data link designed to operate in the VHF frequency band
using one or more standard 25 KHz VHF communications channels. It is capable of
providing both point-to-point and broadcast services between mobile stations, as well as
between mobiles and fixed ground stations.
VDL4 has support in some European states, particularly from Sweden, Russia, Germany
and Italy as well as some Low Cost Carriers and the General Aviation Community, but is
not well supported in the US. Comm4Solutions propose a network of VDL4 ground
stations for AOC. The system has undergone significant development and demonstration
in Europe, with particular emphasis so far on its ability to support ADS-B applications.
VDL4 uses the Gaussian-filtered Frequency Shift Keying (GFSK) modulation scheme
which has a modulation rate of 19,200 bits/s.
Access to the VDL4 medium is time-multiplexed. The system uses the Self-organising
Time Division Multiple Access (TDMA) concept, known as STDMA, which was invented
and developed in Sweden.
The STDMA scheme firstly divides the communication channel into 'time-slots'. Each
time-slot may be used by a radio station (whether mounted on aircraft, ground vehicles or
at fixed ground stations) to transmit a message.
Secondly, access to the time-slots is organised. This means that each station is
responsible for prior selection and reservation of the slots it wishes to use. The use of
organised time-slots reduces the chances of message conflict.
Page 163