A Flexible Aviation Radio to Boost Connectivity, Avionics Optimisation and Civil-Military Interoperability

Aircraft connectivity challenges

Flight efficiency benefits from the introduction of advanced operational concepts and cutting-edge technologies in aviation. The modernisation of aviation infrastructure is underway based on digitalisation, automation and hyperconnectivity.

Higher levels of connectivity entail a new generation of broadband air-ground communication data links which are to be implemented in the framework of future communication infrastructure (FCI) technologies based on ICAO’s Aeronautical Telecommunication Network (ATN) Internet Protocol Suite (IPS).

FCI encompasses legacy (i.e. VHF voice and VHF Data Link Mode 2) as well as new communication systems. The new FCI communication systems include the Aeronautical Mobile Airport Communications System (AeroMACS), satellite-based data link(s) (SATCOM), covering oceanic and continental environments, and the terrestrial L-band Digital Aeronautical Communications System (LDACS) for continental airspace.

The new FCI data links will sustain new advanced operational concepts like trajectory-based operations, relying on near real-time downlink of flight management parameters. Air traffic control will no longer be based on where the aircraft “is” but on where the aircraft “will be”. There will be an optimal synchronisation between the airborne- and ground-based trajectories, the exchange of time constraints.

Huge benefits can be achieved if FCI technologies are implemented based on distributed software-defined radio (SDR) architectures. Such benefits range from upgradability and reusability of the design (since a greater number of common radio functions are implemented through software), weight savings on wiring and equipment, reduction of RF interference and functional flexibility facilitating integration between communication, navigation and surveillance data exchanges.

SDR as an interoperability multiplier and avionics optimiser

SDR technologies are reconfigurable and programmable, with multiple functions integrated in one “box”/form factor, mitigating space constraints, reducing hardware and rationalising equipage. The rapidly evolving SDR digital electronics render practical the use of one single transceiver equipment to receive and transmit different radio waveforms based solely on software.

Today’s avionics architectures remain fragmented and insufficiently integrated. Modern SDR technologies eliminate conventional packaging architectures, organising the integration of the radio functional blocks over two separate pieces of equipment (Figure 1):

• Antenna unit (AU) (with RF front end) – integrated close/next to the antenna and including analogue/superheterodyne radio components (e.g. power amplifier (PA), low noise amplifier (LNA)), AD/DC conversion stage and, sometimes, certain elements of the digital processing stages);

• Radio unit (RU) – located in the avionics bay, comprising a high performance single board computer/computing platform to digitally process (through software) the remaining stages among the digital radio functional blocks. This unit can be multi-instantiated to support, concurrently, multiple aircraft radios.

Figure 1 - (A) Conventional and (B) distributed software-defined radio architectures

The radio unit will be able to host and process simultaneously multiple waveforms and support avionics interfaces (Figure 2). Based on strong partitioning, it is possible to merge radio software onto a common platform supporting the software corresponding to multiple radios. 

The interconnections between the radio units and the antenna units are switchable/selectable and waveforms installed are dynamically reconfigurable.

Figure 2 - Distributed flexible radio architecture concept

A radio resource management function will assign the execution of waveforms and adapt RU-AU digital interface pairing as determined by operational conditions (e.g. the flight phase and the geographical area). Figure 3 depicts a possible topology for FCI aircraft implementation on the basis of distributed SDR architecture.

Figure 3 - Distributed SDR implementing FCI

A distributed SDR approach is particularly important to enable an FCI multilink concept (onboard) to ensure adequate levels of required communication performance when at least two links are in use.

SDR to boost civil-military interoperability

Greater focus on SDR technologies would facilitate military adherence to civil data link requirements, possibly on the basis of waveform accommodation and more integrative and performance-based approaches.

"Air Traffic Control Will No Longer Be Based On Where The Aircraft “Is” But On Where The Aircraft “Will Be”"

The most recent, i.e. 5th, generation of fighters (e.g. F35 Joint Strike Fighter Lightning II) carries communication, navigation and identification (CNI) avionics suites supported by SDR technology, with conformal and adaptive (multi-frequency band) antenna arrays, which use reconfigurable FPGA (field programmable gate arrays)/RF (radio frequency) hardware and computer processors to run software which produces the desired/selected waveform(s).

Distributed SDR could help to overcome military aircraft integration constraints by using a common software processing mechanism, and facilitate the implementation of FCI data link capabilities defined as waveforms. It would also facilitate co-existence with other military capabilities. A flexible radio solution for military aircraft based on SDR would avoid cumbersome military aircraft retrofits, eliminating duplicated equipage and senseless architecture configurations.

State of play and prospects

Today, SDR is an established industry technology. SDR waveforms are highly portable between different hardware platforms, which has resulted in concepts like the Software Communications Architecture (SCA) taking advantage of open market RF integrated circuits (RFICs) and digital signal processing-intensive FPGAs.

In Europe, significant industrial research on the use of SDR for aviation was conducted as part of the Single European Sky Air Traffic Management Research (SESAR), namely in Project 9.44 – Flexible Communication Avionics. Important follow-on technical work is being undertaken in ARINC Industry Activities/the Airlines Electronic Engineering Committee (AEEC).

The final stage of LDACS validation is still ongoing in SESAR where EUROCONTROL, as a founding member of the SESAR Joint Undertaking, contributes in coordination with other industry partners. Nevertheless, distributed SDR solutions for civil aircraft have not yet been fully defined and validated. The following research gaps remain:

• the distribution of the radio digital processing functions of the successive processing stages between the radio and antenna units (trade-off concerning the location of a “cutting point” in the radio functional chain);

• the need to use a specific antenna and a specific analogue RF stage for each aircraft radio. This limits the possibility to update an SDR system to accommodate a new waveform via a software change only. This requires changes in frequency, bandwidth, modulation, and power and antenna polarisation. Future wide-band/multi-band RF front ends and smart/multiband antennae may make this possible in the near future;

• the need to converge distributed SDR solutions with integrated modular avionics (IMA) as the latter is widely used and relies on a similar common computing platform. In the IMA core system, the functional application software is available in mass memory storage devices and is downloadable to the modules upon which they are to execute.

The final stage of research and standardisation (e.g. ICAO Standards and Recommended Practices – SARPS) as well as industrialisation and preparatory deployment activities to bring LDACS, AeroMACS and SATCOM to the adequate level of readiness for implementation are progressing at a remarkable pace. The next implementation steps must consider distributed SDR radios in order to get these FCI technologies deployed with maximum benefits in terms of avionics optimisation and civil-military interoperability.

Beyond FCI, the introduction of software radios may induce a more integrative approach through COM-NAV-SUR synergies, covering various communication exchanges supporting navigation or surveillance applications, i.e. offering backups as needed.

In conclusion, SDR could have a huge transformational effect in aviation technologies by organising infrastructure development and deployment on the basis of a fully integrated and holistic approach, breaking away from the traditional technology life-cycle of aviation.