An underwater acoustic direct sequence spread spectrum communication system using dual spread spectrum code

With the goal of achieving high stability and reliability to support underwater point-to-point communications and code division multiple access(CDMA) based underwater networks, a direct sequence spread spectrum based underwater acoustic communication system using dual spread spectrum code is proposed. To solve the contradictions between the information data rate and the accuracy of Doppler estimation, channel estimation, and frame synchronization, a data frame structure based on dual spread spectrum code is designed. A long spread spectrum code is used as the training sequence, which can be used for data frame detection and synchronization, Doppler estimation, and channel estimation. A short spread spectrum code is used to modulate the effective information data. A delay cross-correlation algorithm is used for Doppler estimation, and a correlation algorithm is used for channel estimation. For underwater networking, each user is assigned a different pair of spread spectrum codes. Simulation results show that the system has a good anti-multipath, anti-interference, and anti-Doppler performance, the bit error rate can be smaller than 10^(-6) when the signal-to-noise ratio is larger than-10 dB, the data rate can be as high as 355 bits/s, and the system can be used in the downlink of CDMA based networks.

A novel gradient climbing control for seeking the best communication point for data collection from a seabed platform using a single unmanned surface vehicle

A novel controller for finding the best communication point is proposed for collecting data from a seabed platform by a single unmanned surface vehicle(USV) using underwater acoustic communication(UAC). As far as we know, extremum seeking based on climbing control is usually implemented by multiple vehicles or agents because of the large range of measurement and easy acquisition of gradient estimation. A single vehicle cannot rapidly estimate the field because of the limited extent for measurement;therefore, it is difficult for a single vehicle to seek the extremum point in a field. In this study, an oscillation motion(OM) is designed for a single USV to acquire UAC’s link strength data between the seabed platform and the USV. The field for UAC’s link strength is updated using new measurement from an OM of the USV based on a multi-variable weight linear iteration method. A controller for seeking the best UAC’s point of the USV is designed using gradient climbing and artificial potential considering iterative estimation of an unknown field and an OM operation, and the stability is proved. The reliability and efficiency are shown in simulation results.