# Performance Optimization of Underwater Communication Links at Different Ranges for AIS Relay to AUV
## 1 Introduction  
Autonomous Underwater Vehicles (AUVs) are becoming increasingly popular for a large number of civil and military applications, such as environmental monitoring, oceanography, archaeology, or mine warfare [1]. Increasing the operational safety of AUVs is necessary to fully explore their potential [2]. Operations of AUVs are constrained by the need to guarantee no collision with manned surface vessels and a safe deployment and recovery. An AUV might not be aware of a manned surface vessel passing just above it. Thus, it is urgent to develop strategies to avoid such accidents. The perception issue could be overcome if AUVs and vessels manage to communicate easily. A way to exchange relevant information between these two entities is to use acoustic modem. For example, through a reliable acoustic wireless communication link, AUVs collect live data or exchange information data with ships or surface buoys. One way to address this challenge could be to use existing modems. As an example, a solution is to use JANUS North Atlantic Treaty Organization (NATO) standard [3] which allows interoperability between users. Other modems of the literature could be also suitable for our application. A state of the art is given in the next section. However, in most cases, acoustic modems were designed around one specific application that is defined by a communication range and conditions (relative speed betweenthe transmitter and the receiver, number of sensors used) that can not respond to our requirements. Additionally, our application needs to provide privacy for the link between relay and AUVs, which may be not possible using commercial off-the-shelf modems. These shortcomings represent the motivation behind this study and the proposition of a private waveform. For our application, the distance between the AUVs and the surface buoy can vary a lot during a mission. The system should be able to adapt its characteristics to fit the situation and keep a robust communication. That is why a modem designed to be as flexible as possible is required. Spreading spectrum is particularly well suited for flexible communications at different ranges. By increasing the size of the spreading factor (SF), a processing gain can be added making the communication robust at low SNR. Moreover, the knowledge of the spreading sequence is required to communicate directly with the AUV creating a private and secure communication link. Multi-carrier spread spectrum MC-SS combines spreading and MC signal. In [4], MC-SS was compared to orthogonal frequency-division multiplexing OFDM and DSSS during at sea experiments. Adding a variable length spreading sequence to an orthogonal OFDM signal, creates a flexible solution that provides frequency diversity, time diversity, and adds a processing gain to transmit at various distances. In their results, MC-SS shows better BER (bit error rate) at low signal to noise ratio SNR than both OFDM and DSSS. We propose to extend the conventional MC-SS by a distribution of the spreading code in the time and frequency domains to profit at the best of the channel diversity. In this paper, a MC-SS with two-demensional (2D) spreading (MC-SS-2D) downlink modem for AUVs applications is proposed. The analysis and evaluation of its performance are provided. The optimization of the time/frequency allocation of the SS can be challenging. A method to optimize the time/frequency allocation of the spreading sequence without knowledge of the channel state information is also proposed. This paper is organized as follows. In Section 2, we provide a state-of-the-art of commercial and academic underwater acoustic communication modems and motivate the need to develop a new waveform for our application of relay surface vessel positions to AUVs. In Section 3, we propose a MC-SS waveform with a (2D) spreading code. In Section 4, we analyze the performance of the proposed waveform on theoretical simulated channels and show MC-SS-2D is optimum to profit of the channel diversity.