Research On Underwater Acoustic Communication For Mobile Platforms Based On Chirp Spread Spectrum

Underwater acoustic communication technology is the primary means of underwater information exchange. Underwater acoustic channel, especially shallow water channel is currently one of the most difficult wireless channels. The channel response is time-variant and space-variant. In recent years, the rapid development of underwater mobile platforms such as Autonomous Underwater Vehicle (AUV) brings new requirements for underwater acoustic communication. Multipath interference and Doppler effect are the main obstacles for acoustic communication under the moving condition. Therefore, in order to achieve reliable, low-power underwater communication on mobile platform, we must overcome multipath interference and Doppler effect. According to the characteristics of Chirp Spread Spectrum technology, it fits well with mobile underwater communication requirements.First, this thesis analyzes the characteristics of shallow water acoustic channel and focuses on Doppler effect between mobile platforms. Based on the analysis, the mobile shallow water acoustic channel model is established which lays the foundation for subsequent communication system simulation.Second, Doppler estimation and timing synchronization techniques have been studied for underwater acoustic communication on mobile platforms. The principle of up-down sweep Hyperbolic Frequency Modulation (HFM) signal method is mainly analyzed and the comparison with other methods based on Linear Frequency Modulation (LFM) signal and HFM signal is carried on. The results show that the up-down sweep HFM method has the best performance.Third, this thesis focuses on Chirp Spread Spectrum technique. Three main modulation schemes have been analyzed. One is binary orthogonal keying, and the others are direct modulation, respectively Chirp-PSK and Pattern Time Delay Shift Coding (PDS). The direct modulation schemes are major analyzed in the anti-multipath and anti-Doppler performance. On this basis, new receiver demodulation schemes have been given. The simulation results prove that the PDS proposed in this thesis using coding time difference between the adjacent symbols by time reversal convolution shows the best performance of all.Finally, the experiment to test the Chirp Spread Spectrum has been conducted. We designed a mobile acoustic communication data acquisition system based on the mobile platform AUV. Then the experiment for mobile underwater acoustic communication was conducted at the Mogan Lake. The experimental results demonstrate that when there is a relative motion between sending and receiving nodes, PDS given herein shows a good performance which further validates the simulation results.