# Performance Analysis of Hybrid Optical-Acoustic AUV Swarms for Marine Monitoring
## 1 Introduction  
Swarms of Autonomous Underwater Vehicles (AUVs) are expected to be used for various tasks, including monitoring and surveillance of underwater infrastructures, especially for communication and energy usage, and more in general for monitoring the ecology of basins, coasts and continental waters mainly for their ability to explore extreme and dangerous environments [1]. Usually, to properly fulfill the assigned mission, the vessels forming the swarm must communicate each other to exchange location information, collaborate for target discovering and/or share the collected data.  
In many current applications, the communication among devices is usually demanded to acoustic communication channels. Acoustic modems support reliable long-range transmissions but offer a limited transmission capacity [2; 3]. For this reason, many researchers have studied the integration of optical modems to complement acoustic communication link to provide an additional, relatively low power and high capacity communication channel. As a drawback, underwater optical modems use visible light signals transmitted and thus attenuated in water, resulting in a relatively short effective communication range (few meters). Indeed, the performance of optical link is strongly affected by the quality of the water and, in particular, by its turbidity that plays a central role in characterizing the optical signal attenuation.  
Modern AUVs that integrate both acoustic and optical modems can support new classes of underwater missions. However, this high flexibility provides many challenges in the definition ofthe mission strategies since selecting the best communication solution clearly depends on the actual environmental parameters. AUVs controllers integrate smart algorithms and solutions to select the proper communication strategy, according to the application communication requirements and the actual environment status [4]. For instance, if a swarm monitors an area with limited optical visibility, the mission controller has two options: (i) redesign the swarm topology to match the reduced communication range; or (ii) reduce the exchanged information up to the capacity of the acoustical channel.  
In this paper, we analyze possible configurations of acoustic-optical hybrid network with the aim to compare acoustic and optical communication channels for different conditions of water quality. From this analysis, we show the performance of the network configuration resulting from the swarm topology.  
The main contributions of this work are:  
* the evaluation of the network performance in terms of transmission delay, required energy of AUV swarm networks considering parameters taken from a real implementation and specifically from the Venus vessel (described in Section 4), the results of which can be useful to design mission strategies, such as defining the topology to use, and assess the related performance, such as the duration lifetime of the AUVs;
* the extension of the communication model available in the literature [5] to consider omni-directional optical coverage, which follows a technological advance that places three triplets of diode array on the three side faces of a prism, enabling a near omni-directional optical coverage on a plane; and
* the assessment of the effects of the turbidity of the water on the network topologies and system parameters, presenting how the communication models change with the different characteristics of the water.  
The paper is organized as follows: Section 2 describes the state of the art; Section 3 presents the application scenario challenges and goals; Section 4 presents the Venus device, an AUV implementation prototype; Section 5 presents the performance evaluation of different network topologies using the Venus parameters; and, finally, conclusions are drawn in Section 6.