Technical Overview

LDACS1 stands for ‘L-band digital aeronautical communication system, type1’. The system provides digital data links between ground stations and aircraft in the continental domain. In the oceanic domain, it is supplemented by satellite links. The scope of LDACS1 is on air traffic services (ATS) and aeronautical operational control (AOC), but not on passenger communication. The system will be operated in the aeronautical part of the L-band, which ranges from 960 to 1164MHz. This frequency band has been assigned to future aeronautical communications at the world radio conference in 2007.

The most important requirements for the design of LDACS1 are

  • Providing a sufficiently high transmission capacity. The expected growth of the air traffic in the future and the shift from voice to data communications places high demands on the design of LDACS1 to achieve a sufficiently high transmission capacity.
  • Other than passenger communications, ATS and AOC messages are safety and time critical. LDACS1 has to be design to guaranteeing a reliable and timely transmission of the messages

To meet these requirements, LDACS1 pursues a cellular point-to-multipoint concept, which means that the airspace is segmented into cells. In each cell, all aircraft are connected to a centralized ground station which controls the entire air/ground communication within the cell.  LDACS1 is designed as a frequency-division duplex system, which enables the ground station to transmit continuously at a certain frequency, while all aircraft within the cell transmit in parallel at a different frequency.

 Figure Cellular Concept

Unfortunately, the L-band is already utilized by different legacy systems and unused spectrum is scarce. To guarantee a high transmission capacity, LDACS1 would be preferably deployed using an inlay approach. In this case the system tries to employ the spectral gaps between channels which are used by DME system.

 Figure Inlay Approach

To perfectly use the spectral gaps between adjacent DME channels, LDACS1 uses OFDM as modulation technique, since it allows a flexible occupation of spectrum. This modulation technique is also employed in system like WiFi or LTE.

To ensure a reliable data transmission, LDACS1 has implemented several measures, like a strong forward error correction. This forward error correction can also be adapted according to the transmission conditions, which increases the data rate. The framing structure of LDACS1 is designed guarantee timely channel access for the aircraft. In addition, the framing sizes are chosen according to the expected message sizes in the aeronautical domain.

 Figure Framing LDACS1

More detailed information about the design of LDACS1 can be found in the specification.