Background and Motivation

Today, flying is one of the safest ways to travel or transport goods worldwide. However, the demand for air transportation is continuously growing. Forecasts indicate that in 2035 there will be an increase in the number of flights of approximately 50% over 2012 based on the most-likely growth scenario for Europe.

The increasing number of flights is a big challenge and it is expected that the current Air Traffic Management (ATM) system reaches its capacity limits within the next years in Europe and the US, the world’s regions with the highest aircraft densities. To ensure sustainable air-traffic growth worldwide, a modernization of ATM is required which aims at increasing air-traffic efficiency while simultaneously increasing safety of flight. Therefore, two major projects have been initiated with the final goal to modernize ATM: SESAR (Single European Sky ATM Research) in Europe and NextGen (Next Generation National Airspace System) in the US. Both projects are globally harmonized under the framework of the International Civil Aviation Organization (ICAO).

For ATM modernization new operational procedures have to be devised. This in turn requires improved Communications, Navigation, and Surveillance (CNS) technologies which enable the implementation of these operational procedures. In communications, ATM modernization requests for a paradigm shift away from voice towards digital data communications. Increased and more complex information exchange between controllers and pilots demands the use of modern communications technologies. Voice is not capable to efficiently convey the information required for future operational procedures.

State of the art in aeronautical communications is still to apply analog voice communications between controllers on ground and pilots onboard aircraft. The technology used is DSB-AM (Double-Sideband Amplitude Modulation) and is deployed in the VHF band from 118 to 137 MHz. DSB-AM has been introduced in the 1940s. Although being quite old, the DSB-AM technology ensures sufficiently reliable communication between aircraft and ground stations. However, the DSB-AM technology is very spectrum inefficient and voice communications is not capable to serve the needs for future information exchange between controllers and pilots. In the 1990s, ICAO already standardized the first digital data links for use in the VHF-band, the VHF Data Link (VDL) standards. The 25 kHz bandwidth of the VDL standards fits the VHF channel grid of the DSB-AM system. Out of the three available standards VDL Mode 2/3/4, only VDL Mode 2 is foreseen for ATM communications and currently implemented. Developed more than 20 years ago, VDL Mode 2 provides only limited data capacity. Using a modulation rate of 31.5 kbps in conjunction with carrier-sense multiple-access limits the actual net data rate to a few kbps. This is well below what is expected to be needed to enable full ATM modernization with SESAR/NextGen, according to the COCR document. As a consequence, further modernization of the communications technology is required.

Within ICAO, a common understanding has been reached that a single data link technology is not capable of covering the communications needs for all phases of flight. Therefore, the Future Communications Infrastructure (FCI) has been developed which serves as basis for the development of future aeronautical communications within SESAR and NextGen. The FCI comprises a set of data link technologies: an air/ground communications component, a dedicated data link to be used at large airports, a satellite component, and a direct air/air data link. These FCI data links are integrated into a common communications network, thus, realizing the concept of "networking the sky". The network is based on commercial IP technology and makes available seamless handovers between the different data links.

Figure Networking Sky

For the implementation of the future air/ground communications component of the FCI, ICAO recommended the use of the L-band between 960-1164 MHz if coexistence with legacy systems is ensured. During the World Radio Conference in 2007, this part of the L-band has already been assigned to aeronautical communications on a secondary basis and, thus, utilization is permitted as long as existing services are not interfered. The future air/ground communications systems is termed L-band Digital Aeronautical Communications Systems (LDACS) and is the most important data link within the FCI, since it is foreseen to cover the main part of air/ground communications.

ICAO has recommended further developing and evaluating two LDACS technology candidates, namely LDACS1 and LDACS2. LDACS1 employs a broadband transmission using Orthogonal Frequency-Division Multiplexing (OFDM) together with adaptive coding and modulation. With that, LDACS1 follows modern and highly efficient transmission concepts as also applied, e.g. in the fourth generation of mobile radio systems. Since no large contiguous blocks of free spectrum are available in L-band, the transmission bandwidth is limited to roughly 500 kHz. To avoid splitting up this limited bandwidth between forward and reverse link, frequency-division duplex is applied. As a result, this LDACS1 design approach achieves net data rates ranging from 561 kbps (strongest coding, robust modulation) to 2.6 Mbps (weak coding and higher order modulation) for a pair of forward and reverse link channels. In contrast, LDACS2 follows a more traditional approach. It is based on GSM (Global System for Mobile Communications), i.e. on second generation mobile radio technology. It is a narrowband single-carrier system with 200 kHz transmission bandwidth and time-division duplex. With a modulation rate of 271 kbps the achievable net data rate is around 70-115 kbps.