## 2 Model of the System and Problem Setting  
The research presented is motivated by the framework of the European project UAN--Underwater Acoustic Network [14]. For this reason, without entering into details, it is important to define the general scenario to understand what is the background of the proposed methods and approaches. The main objective of the project is that of implementing a generic ad-hoc mobile acoustic network, composed by fixed and mobile nodes for underwater surveillance of off-shore and coastline critical infrastructures. The underwater network has to be integrated within a wider network, including above water nodes and sensors. Figure 1 gives a conceptual overview of the UAN scenario: the Land/Air system represents what can be thought as a traditional radio based sensing and communication infrastructure; the underwater part is composed by fixed and mobile nodes (AUVs) with detection and communication capabilities. The integration of the different systems has to be guaranteed by an appropriate communication and networking infrastructure in order to ensure the exchange of information among the various elements of the protection system. The UAN project hence has to deal with several aspects, including but not limited to:  
* A deep analysis of the underwater communication physical layer in order to use existing or develop new acoustic technologies for efficient communication support and embedded signal processing;
* Development of tools for prediction of the acoustic channel performance, especially from a communication system standpoint;
* Development of reliable and efficient network architectures able to make the underwater communication as effective as possible. Traditional networking schemes are difficult to be taken to underwater scenarios: MAC schemes, routing protocols and network security features are all aspects that need to be investigated in order to find suitable solutions in situations characterized by long delays, frequency selectivity, and low bandwidth;
* Development of coordination strategies for the management of the mobile nodes of the network so that it is possible to dynamically place the nodes in order to maximize some communication related performance cost functions and periodically measure the environment; at the same time, the coordination strategy must guarantee the operation of the intrusion detection payload as appropriate. The approaches presented in this paper are at the application level of the UAN project. While the experimental results have been obtained with the UAN-project implementation of the underlying layers [14], it is remarked that our algorithms can as well be applied with any alternative network infrastructure. Figure 1: Conceptual overview of the UAN scenario: integration of above water and underwater systems. The envisaged scenario consists of:  
* A land station which acts as a command and control (C2) center for the physical defense of a critical infrastructure;
* An underwater base station wired to the shore with a high bandwidth link. This station represents the connection between the above and below water environments; for this reason this element is both a part of the acoustic network and of a traditional wired communication infrastructure;
* Fixed and mobile nodes (n) acoustically connected in an underwater network which includes the base station. Each node is equipped with an on board sonar for intrusion detection and with an acoustic modem for communication purposes. From the cooperative standpoint, the overall mission goal is that of covering with the vehicle detection sonars the largest possible area in the neighbourhood of a given asset where the land station also resides. Since the performance of the AUV devices depends on the local oceanographic conditions, we also assume that each agent or node is equipped with a sensor able to measure the environment variability (e.g., Conductivity, Temperature, and Depth--CTD probe) and with an acoustic channel performance predictor to convert these measurements into communication performance ([6, 8]).