Flexible Aviation Connectivity through Software Defined Radio Unit Convergence with Integrated Modular Avionics (IMA)

Avionics Evolution Context

The avionics suite of modern commercial aircraft (Figure 1) must evolve to sustain high speed communications connectivity between the cockpit and ground systems. Low latency broadband air-ground communications are the basis to new aeronautical flight control automation concepts, such as trajectory-based operations.

IPv6 capable digital aeronautical inter-networking capability is the target as specified in the International Civil Aviation Organization (ICAO) document 9896 describing the Aeronautical Telecommunications Network (ATN)/Internet Protocol Suite (IPS).

The evolution from Federated Line Replaceable Units – LRUs to distributed avionics, including reconfigurable and programmable radio communications, is paramount to meet the air traffic services applicable in each region and phase of flight. The key enabler technology is software defined radio (SDR) digital electronics which render practical the use of one single transceiver equipment to process different radio waveforms.

In 2014, the Single European Sky Air Traffic Management Research (SESAR) Project 9.44 (Flexible Communication Avionics) investigated the use of SDR for aviation purposes. Subsequent standardisation efforts to define a “Distributed Radio Architecture” took place within ARINC industry activities, notably in the Airlines Electronic Engineering Committee (AEEC). 

The deployment of SDR-based distributed radio architectures remains constrained by reduced radio unit reliability along with challenging maintenance accessibility. Way forward entails higher-levels of integration of CNS radios by combining CNS functions.

The benefits of introducing SDR as the basis for distributed avionics, comprise not only the reduction of hardware and the use of digital/optical data bus but also the ability to reconfigure and modify the supported communication functions in accordance with local requirements. New requirements could be implemented as waveforms through software updates instead of equipment retrofits. That may represent a huge advantage for aircraft already equipped with SDR or functional allocation capabilities such as modern 5^th generation fighters.

Repartition of SDR Radio Processing Functions

As described in a previous article in the Aerospace & Defence Review, modern SDR technologies eliminate conventional packaging architectures, organising the integration of the radio functional blocks over two separate pieces of equipment (Figure 2): the Antenna Unit (AU), including mainly RF front end analogue radio functions; and the Radio Unit (RU), located in the avionics bay and comprising a high performance single board computer/computing platform to digitally process the remaining stages among the digital radio functional blocks.

A fundamental trade-off to be addressed, for any aviation radiocommunications solution based on SDR, is the need to define the “cutting point” in the radio functional chain to establish the repartition of the radio digital processing functions of the successive processing stages, between the radio and antenna units (Figure 3).

Integration of the Radio Unit Functions in Integrated Modular Avionics (IMA)

Increasing avionics integration entails the convergence between Integrated Modular Avionics (IMA) and distributed SDR/Radio Units, considering that IMA is widely available and relies on a similar common computing platform.

The IMA Core System (Figure 4) can be defined as a set of standardised modules communicating across a unified digital network. The IMA Core System processes inputs that are received from the platform’s low and high bandwidth sensors. It can be viewed as a single entity comprising many integrated processing resources which can be used to construct any avionics system regardless of size and complexity.

Multiple systems can be architected and overlaid on the partitioned platform resources to form a highly integrated system with full isolation and independence of each individual system. Nevertheless, the digital bus interconnecting AU with RU entails a certain bandwidth and processing power that is usually not available in generic IMA Core Systems. In fact, IMA lacks adequate bandwidth to enable low latency data exchanges with the Antenna Unit. Hence, integration may only happen with an upgrade of IMAs.

A solution to turn IMA suitable for high demanding quality of service and class of service requirements of future radio systems requires to move “down” the RU-AU “cutting point”, mentioned before, which has some drawbacks: the Antenna Unit becomes more complex and has to embed its own dedicated digital signal processing hardware, bigger in size, and dissipating more heat. Conversely, a lower throughput is required at the level of the digital bus interface and the Radio Unit will then have a simplified design.

Proof of Concept

Validation of using IMA to sustain Radio Unit functions could be based on two initial waveforms such as the L-Band Digital Aeronautical Communications System (LDACS) and the SESAR-defined solution for Inmarsat-based SATCOM designated as IRIS. The physical layers of LDACS and IRIS are well defined in ICAO standards and recommended practices (SARPS) as well as in several other standards such as EUROCAE ED-262 (Technical Standard of Aviation Profiles for ATN/IPS) and RTCA DO-379 (Internet Protocol Suite Profiles).

" The IMA Core System Processes Inputs that are Received from the Platform’s Low and High Bandwidth Sensors. It can be Viewed as a Single Entity Comprising Many Integrated Processing Resources Which can be Used to Construct any Avionics System Regardless of Size and Complexity "

For an end-to-end QoS/performance measurement, several levels of interfacing must be considered (Figure 5) but the focus must be placed on those between RF front-ends and the SDR platforms (AU-RU digital RF interface).

The complete AU-RU interface definition must cover layers 1 and 2, including the selection of the physical cabling (copper or optical) and protocols which determine data formats. The selection of AU-RU interfaces relies on trade-offs between the throughput and round-trip-time, characteristic of each waveform, and its channel bandwidth and the availability of low latency interfaces. Protocol options include Avionics Full Duplex Switched Ethernet (AFDX) which is already used in aviation context and supports data throughputs up to 1 Gbps.

The final validation plan must offer a methodology to determine the performance levels that the AU-RU exchanges can reach with the RU integrated in IMA, considering transmission delay, BER/SNR, dynamic range and packet loss on the basis of a set up that emulates processing units, interfaces, cabling and smart antennae.