# Performance Analysis of Hybrid Optical-Acoustic AUV Swarms for Marine Monitoring
## 3 Application Scenarios, Challenges and Goals  
AUVs find their field of application in several missions such as the monitoring of the underwater pipelines (Figure 1), the reaction to ecological disasters, the maritime archeology and the monitoring of the underwater creatures and ecosystem. Advanced usage of swarms of AUV can benefit from the multiple availability of the relatively low cost vessels that can speed up for instance the scanning of a shipwck sunken to the bottom of the sea.  
Figure 1: Example of a AUV mission: The goal is to monitoring a oil pipeline.  
To carry out all these missions, AUVs are usually equipped by a propulsion and guidance system, one or more communication modems and several specific sensors: all these components represent the resources on top of which a control software operates. The communication system plays a pivotal role, especially in missions requiring coordinated group work. Specifically, AUV devices need to communicate for:  
* Knowing the mutual positions
* Discovering the objectives
* Identifying the relative positions of the targets being monitored
* Creating redundancy for the collected data
* Communicating the presence of obstacles.  
Communication technologies and storage capability of the swarm deeply impacts on the ability of the AUVs to accomplish their missions. In particular, the selection the acoustic link or the optic link impacts several system parameters including the communication _bit-rate_, the _delay_ and the _communication range_ given by the propagation of the signal in the medium and also on the _energy consumption_ of the transmitting and receiving chains. Table 1 reports some typical parameters of optical and acoustical communication channels. In our system model, we include specific values obtained from a real implementation described in Section 4.  
Optical communication offers several benefits in terms of available bit-rate and energy consumption, while presents limitations on the communication distance with respect to the acoustic counterpart. These limitations pose some constraints on the possible topologies and arrangements of the AUVs swarm during the missions, as well as to their ability to create the AUV network. For instance, the AUV network can be designed to cover a certain volume. Usually, this configuration is adopted for swarm motion to monitor a certain area or to measure environmental data on a given surface. Obviously, the physical arrangement of the transducers and sensors impacts this configuration management and the protocol development. Typical swarm configurations can be classified according to the nodes topology in high or low density. Low density topologies tend to occupy the maximum volume while high density topologies ensure low AUV-to-AUV delay and offers a greater network resiliency.  
The simplest arrangement is probably the \"pipeline\" configuration where all the nodes are ideally aligned and each node relaunches information to the next one. This topology configuration (sometimes called chain topology) can be used to collaborative mapping or scanning but also to set up a multi-hop communication channel toward a gateway typically placed on the surface of the sea. Thus the distance between robots plays a fundamental role when the communication is carried out by optical modems. The link range determines the number of AUVs that have to be used during the mission and can change also the mission strategy. Anyway, if the water turbidity is excessive or change during the mission, the communication penalty could lead up to a swarm partition, which could compromise the planned mission.