Detection And Synchronization For OFDM Signal Over Time-varying Underwater Acoustic Channels

Underwater Acoustic(UWA) channel is one of the most complicated communication channels being research now. As a double-selective channel, it varies in both time and frequency, has severe multipath effect and Doppler Effect, which constrict its application. Orthogonal Frequency Division Multiplexing(OFDM), as a highly spectrum effective modulation technology, its good performance in resisting multipath effect makes it a popular realization in UWA channel. At the same time, OFDM is sensitive to synchronization deviation, and synchronization becomes a critical technology in OFDM system. Besides, signal arrival detection as a basic function of a realistic acoustic system, is very important for subsequent demodulation. This article mainly researches OFDM signal detection and synchronization over underwater acoustic channels. The main works are summarized as follows:1, This paper begins with introducing the fundamental and method of realizing OFDM system, then discusses the differences between wireless communication channel and UWA channel, and introduces some technical difficulties, then it gives a simple model of a OFDM system for underwater acoustic communication.2, This paper research on some OFDM synchronization algorithm proposed so far, discusses some variation between different preamble and different way of decision. It mainly focuses on some practical methods wildly used in realistic system. And then proposes a new synchronization algorithm based on linear frequency modulation sequence which works on bandpass signal and doesn’t need to convert signal to baseband. Simulation shows that its timing performance is consistent with baseband timing in spite of its low complexity compared to traditional baseband signal timing.3, This paper combines double sliding window energy detection and multitone signal detection in signal arrival detection. Compare to traditional single sliding window energy detection, the threshold of double sliding window method is less vulnerable to noise power and easier to get stable detection rate. Multitone signal detection is easy to implement, has low complexity and easy to distinguish between system signal and interference. Simulation shows the combination of the two method can detects signal arrival precisely.4, Apply the proposed detection and synchronization algorithm to underwater acoustic communication system. The Experiments in Qiandao Lake and laboratory pool show the algorithm can successfully filter interference and high-energy chirp noise and detect the arrival of signal, and finally fulfill synchronization of signal.