Updated: Apr 27, 2020
Even in today’s “performance-based world” the underlying technology of Communication, Navigation and Surveillance (CNS) remains crucial, as no performance-based specification can cover the fact that we still use “steam-powered” systems, especially for communication. If you talk to somebody involved in communication engineering outside aviation, they cannot believe what technologies are being used on aircraft. The bandwidth available today for Controller-Pilot-Data-Link-Communication (CPDLC) is more a reminder of a turtle rather than a Formula One car, not to mention voice communication over VHF/HF. It is very common for passengers of an airliner to have a much faster communication data link at their disposal than the flight crew.
But there is no reason to panic, help is underway!
Figure 1: Ground worker talking on a VHF radio
A glimpse into the future
Tomorrow’s applications for CNS and Air Traffic Management (ATM) require high-speed datalinks, in the air, on the ground and in between . The applications are almost unlimited, high-resolution real-time weather animations, improved flight-data monitoring and automated conflict avoidance to name just a few. To cope with this, ICAO set up a multi-link concept, involving a communication technology roadmap, which basically covers three areas, the airport environment, medium-range and long-range communication . We are going to look at the first two (airport & medium range), as the technology definition for those is relatively mature already.
Around the airport: AeroMACS
The Aeronautical Mobile Airport Communications System (AeroMACS) is based on the commercially available 802.16 (4G) standard  . This well-established technology ensures global interoperability and cost savings. The 5 GHz-band allows for high-speed data transmission including video streaming  . The network provides a common link for airport services, airlines, maintenance and many others . This forms part of the System-Wide-Information-Management (SWIM) approach governed by ICAO .
Figure 2: AeroMACS application in the airport environment 
Testing is organized on a global scale and several tests have been conducted at different airports in China, Europe, the US and Japan . A typical cell-size will measure around 3 km in radius and the users can move with relative velocities up to 50 kts . EUROCAE and RTCA standards are well underway and the launch of this technology is on the doorstep  . This system is very well-suited for the airport environment, but due to its limited adaptability for fast-moving mobile applications, we need something more….
Enroute (medium-range): LDACS
The L-band Digital Aeronautical Communications System (LDACS) is a medium-range, medium-bandwidth communication data-link and thus a key enabler for the ICAO 4D-Trajectory management concept . It can provide air-to-air and air-to-ground connectivity . Originally, there were two versions of LDACS proposed, LDACS1 and LDACS2 . They use different modulation schemes and access control techniques and were both tested for interoperability with existing L-band systems, especially Link16 and DME  . Using part of the L-band (969-1164 MHz) is a compromise between the saturated and bandwidth-limited VHF spectrum and the attenuation-limited C-band (5 GHz) . As the L-band is also heavily congested, an ingenious inlay scenario has been developed, effectively "squeezing" the LDACS signals between the existing DME frequencies .
Figure 3: LDACS1 inlay deployment 
LDACS1 is the system being further developed within the SESAR2020 project and is now just called LDACS . The system is designed to connect up to 512 users per cell (about 200 NM radius), providing a data rate of typically 1 Mbit/s . A draft LDACS specification has been produced and initial ICAO SARP’s are expected to be available by the end of this year, final version around 2026 . Additionally, LDACS can offer more than just communication capability, it has been successfully tested as a candidate for Alternative Positioning, Navigation and Timing (APNT). The testbed aircraft (Falcon 20) flew several enroute and approach scenarios. Multipath and DME interference were investigated, including rectification by Doppler-smoothing, leading to remarkable results (mean accuracy within 2-3 m, SD within 5 m) . Compared to other APNT candidates, such as eLORAN, LDACS is able to provide a 3D-position with a performance that would enable approach operations. LDACS offers a number of promising services and might enable civil aviation to establish a powerful data link, merging communication, navigation and surveillance applications; a concept that has been successfully used on the military side for decades...
 ICAO, Global Air Navigation Plan 2016-2030, 2016
 Ehammer, Pschernig, and Gräupl, AEROMACS - AN AIRPORT COMMUNICATIONS SYSTEM, Digital Avionics Systems Conference October 16-20, 2011
 Eurocontrol, AeroMACS – Factsheet, 2016
 Plass, Future Aeronautical Communications, InTech, 2011
 Rihacek et al., L-BAND DIGITAL AERONAUTICAL COMMUNICATIONS SYSTEM (LDACS) ACTIVITIES IN SESAR2020, 2018 Integrated Communications Navigation and Surveillance (ICNS) Conference April 10-12, 2018
 Shutin et al., LDACS1 RANGING RESULTS WITH DOPPLER SMOOTHING FROM NEW FLIGHT EXPERIMENTS, German Aerospace Center (DLR) / IEEE, 2014
 Gligorevic, Epple and Schnell, The LDACS1 Physical Layer Design, in: Future Aeronautical Communications, InTech, 2011