AIS Relay for Autonomous Underwater Vehicle using an Adaptive Bit Rate Multi-Carrier Spread Spectrum Modem
Résumé
Autonomous Underwater Vehicles (AUV) are commonly used in research and military sea applications. Therefore, AUVs are becoming part of the ocean navigation systems. To calibrate their navigational system by GPS, AUVs must regularly surfaced, operation during which collisions with vessels can occur. For security purpose, communicating the locations of boats to AUVs is required.
In this paper, a communication system is developed to gather vessels positions using the AIS system and relay this information to AUVs. By having a buoy or a boat at the surface, it is possible to collect data about the surrounding ships and forward these data to the AUV fleet. Since the bit rate of underwater communication is way lower than the AIS one especially at large distances, reducing the payload for relay purpose is required. By removing irrelevant AIS data and compressing positional ones, the AIS payload can be reduced saving 182 bits on each 256 bits message.
The proposed communication system is a multi-carrier spread spectrum (MC-SS) downlink modem broadcasting the positions of vessels towards the AUVs fleet. MC-SS combines spreading and multi-carrier signal. Using variable length spreading sequences and OFDM, this communication system creates a flexible and robust solution thanks to the frequency diversity and processing gain to transmit at various distances.
The modem was experimented with the Watermark channel simulator considered as a realistic test benchmark for underwater communications. Results show the effectiveness of the spreading factor to improve the error rate at low SNR.