The future of aviation worldwide is one of significant continuing growth in air travel, air cargo, and private general aviation. In parallel with this growth, a significant increase in the supporting information infrastructure will need to occur. The infrastructure will support two primary functions: aviation operational needs (which includes airline operations, air traffic management/air traffic control, flight information services, and crew communications), and passenger services (internet connectivity, voice/data, travel services, etc.). This infrastructure must also enable radical new air traffic management methods, which require a large flow of information between the aircraft, air traffic service providers, and airline operations centers.
Four categories of communications are defined in annex 10 of the International Civil Aviation Organization (ICAO) regarding their safety level.
– ATSC: Air Traffic Services Communication (critical). This class regroups communication between pilot and ATC to ensure the safety, speed and efficiency of the flight.
– AOC: Aeronautical Operation Control (critical). “Communication required for the exercise of authority over the initiation, continuation, diversion or termination of flight for safety, regularity and efficiency reasons”. Used by airline companies to communicate with aircraft (e.g. maintenance messages, fuel levels, exact departure and arrival time…).
– AAC: Aeronautical Administrative Control (non-critical). “Communications necessary for the exchange of aeronautical administrative messages” AAC are neither linked to the security nor the efficiency of the flight. A few examples of AAC are information regarding passengers (list of passengers, connections), special cleaning requests, hotel booking for flight attendants…
– APC: Aeronautical Passenger Communication (non-critical). Examples of such communication are VoIP, e-mail, web browsing.
It has to be noted that critical communications follow very stringent international rules defined by ICAO (for example only some dedicated frequency band can be used) and are based on dedicated systems. The latter must meet very specific QoS requirements (transaction time, continuity, availability, integrity). These regulatory constraints does not apply to non-critical communications, even if they may have to meet some requirements according to the applications (e.g. delay for passenger VoIP).
Current aeronautical communications between the air and ground are primarily in the VHF band (118-137 MHz) which will be unable to provide the required system capacity and coverage. Hence, hybrid information infrastructure architectures, which include satellite communications links as a key component, are being proposed. The addition of satellite communications links into an information network advantageous because of their inherent properties: economical wide-area broadcast capabilities, large scale geographic coverage including oceanic and remote regions, coverage of all altitudes, lower terrain blockage, and larger available bandwidth at the higher frequencies. Satellite navigation methods (utilizing augmented GPS and other global positioning systems) are a recognized part of future air navigation and air traffic management.
The ever-increasing interest in providing both Internet access and cellular connectivity in the passenger cabin has led to the emergence of in-flight Wireless Fidelity (WiFi) based both on satellite connectivity and on the Gogo Air-to-Ground (A2G) network. However, they suffer from expensive subscription, limited coverage, limited capacity and high end-to-end delay.
As a complement and/or design alternative, the Aeronautical Ad-hoc Network (AANET) concept has been conceived as a large-scale multi-hop wireless network formed by aircraft, which is capable of exchanging information using multi-hop Air-to-Air (A2A) radio communication links as well as integrating both the satellite networks and the ground. Furthermore, AANETs are also beneficial for automatic node and route discovery as well as for route maintenance as aircraft fly within the communications range of each other, hence allowing data to be automatically routed between aircraft and to or from the GS.
Mobile Ad-Hoc Network (MANET) is an infrastructure-less wireless network of autonomous collection of mobile nodes (Smart phones, Laptops, iPads, PDAs etc.) that distribute coordination and control. All the nodes are free to move and organize themselves into a network. These devices collaborate with each other to offer the essential network functions in a distributed manner. In a MANET, a node functions both as a host and as a router to forward the packets in appropriate direction.
As a new breed of networking, AANET aims to establish an ad hoc network amongst aircraft for their direct communication in high-velocity and high-dynamic scenarios, in order to handle the increasing flow of data generated by aircraft and to provide global coverage.
The representative benefits of AANET are
- Extended Coverage: AANETs extend the coverage of A2G networks offshore to oceanic or remote airspace by establishing an ad hoc network among aircraft and GSs. The GSs may also communicate with each other as part of an AANET or they may act as a gateway for
connecting with the Internet via a fixed line. More specifically, AANETs are capable of substantially extending the coverage range in the oceanic and remote airspace, without any additional infrastructure and without relying on satellites.
- Reduced Communication Cost: Avoiding satellite links directly reduces the airlines’ cost of aeronautical communication since the cost of a satellite link is usually significantly higher than that of an A2G link.
- Reduced Latency: Another potential benefit of AANET is its reduced latency compared to geostationary satellite-based access, hence it is capable of supporting more delay-sensitive applications such as interactive voice and video conferencing.
Given their high-velocity node mobility, dynamic topology, decentralised architecture and limited transmission range, legacy Internet routing and transport protocols do not work properly in MANETs, hence the recent research efforts have been focused on these areas.
To manage concurrent accesses to the media three allocation methods have been considered: by frequency (Frequency Division Multiple Access, FDMA), by time (Time Division Multiple Access, TDMA) and by spreading code (Code Division Multiple Access, CDMA). Combinations of these methods are possible. Researchers have found that Frequency division cannot share the available bandwidth in the considered topology without significantly reducing the available bandwidth for every connection. And time division raises an important issue on clock synchronization. Researchers have proposed Direct Sequence CDMA (DS-CDMA) as a solution that has the major advantage that it does not require any coordination between nodes and it allows multiple simultaneous transmissions. The collisions which occur can be resolved in the receiver thank to the low intercorrelation between two different and well chosen spreading codes. It has to be noted that this collision recovery can only be done within the limits of Multiple Access Interferences (MAI). This represents the major limiting factor of the performance of CDMA systems.