The main objective of WP4 is to look into state-of-the-art wireless networking concepts and their applicability in aeronautical systems. In particular, a multi-hop communication system approach seems interesting to pursue, since it represents an efficient way of extending the range of a communication link. In addition, it can provide diversity through several possible communication links between a transmitter and a receiver.

Numerous systems are involved in aeronautical communications today, and the number will increase in the coming years. Some are complementary as they are used for different services, operating at different frequency bands etc. Others will be in competition, such as e.g. different satellite communication systems. In both cases interoperability and cooperation are of utmost importance to optimise the capacity, ensuring sufficient QoS for each individual service. It should also be noted that the potential of satellite communication links in the context of aeronautical communications is at present far from fully utilized.

Based on this, it is natural to let WP4 contain the following tasks:

• T4.1: Cooperative multi-hop relaying techniques and dynamical routing protocols. Taking a generic approach, an aeronautical communication network may consist of fixed ground nodes, fast moving airborne nodes, and distant fixed or mobile satellite nodes. Given a range of services, all with given safety and QoS requirements, and information about the environment (continental, oceanic, etc.) and radio technologies involved, novel routing algorithms and relaying strategies for such a network model will be developed, for the purpose of ensuring optimal utilisation of resources. An important issue will be to obtain seamless handover, such that the switching from one system to another is not noticed by the users.
• T4.2 Cross layer optimisation, considering interference management and mitigation techniques on the physical and link layers, as well as routing techniques and resource allocation [Ber04]. Cross layer information exchange permits the routing algorithm to take into account typical lower layer aspects such as radio interference. Similarly, the interference mitigation techniques used on the physical and link layers may take input from and adapt to the routing algorithms. Such joint optimisation has the potential of enhanced performance and capacity over optimisations done separately at each layer.