Technical Overview

LDACS is a secure broadband communications system based on OFDM (Orthogonal Frequency-Division Multiplexing) modulation and shares many technical concepts with 4G wireless communications systems. It 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 LDACS 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 LDACS 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 LDACS to achieve a sufficiently high transmission capacity.
  • Other than passenger communications, ATS and AOC messages are safety and time critical. LDACS is designed to guarantee a secure, reliable, and timely transmission of the messages.

To meet these requirements, LDACS 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. LDACS 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, LDACS 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 the DME (Distance Measuring Equipment) system.

Figure Inlay Approach

To perfectly use the spectral gaps between adjacent DME channels, LDACS 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, LDACS has implemented several measures, like strong forward error correction coding. This forward error correction coding can also be adapted according to the transmission conditions. In case of good channel quality, weaker coding can be applied which increases data rate. The framing structure of LDACS is designed to 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 LDACS

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

This site uses cookies to customize the site to best suit your needs and improve our services. Your continued use of the website is understood as your consent to our cookie policy. Learn more

Die Cookie-Einstellungen auf dieser Website sind auf "Cookies zulassen" eingestellt, um das beste Surferlebnis zu ermöglichen. Wenn du diese Website ohne Änderung der Cookie-Einstellungen verwendest oder auf "Akzeptieren" klickst, erklärst du sich damit einverstanden.