STLFM Signal Based Adaptive Synchronization for Underwater Acoustic Communication

In underwater acoustic communications (UAC), signal synchronization plays a key role in the performance. It is usually performed using a known preamble transmitted prior to the data. However, the underwater acoustic (UWA) channel is characterized as time-varying and frequency-varying, which makes the preamble fluctuated as well as the transmitted data. Thus, it contains uncertainty to set a constant threshold for synchronization by using information (e.g., Doppler shift) extracted from the preamble. In this paper, we propose an adaptive scheme for UAC synchronization. The scheme uses the symmetrical triangular linear frequency modulation (STLFM) signal to design a fractional Fourier transform (FrFT)-based detection algorithm. It establishes the frame synchronization by detecting the deviation of the two energy peaks which usually emerge in their "optimal" FrFT domain in pairs. Instead of detecting the absolute peaks, the proposed method performs an initial synchronization and a precise correction based on the relative positional relationship and amplitude attenuation of the two peaks, which makes full use of the two peaks of the STLFM signal in the FRFT domain. The effectiveness of the scheme has been verified by simulations and field works. The results suggest that it is able to peak the time-varying signal amplitude for each frame in UWA channels. Besides, the proposed scheme performs better accuracy and stability in the frame synchronization compared to the traditional LFM method, which is shown as three times less detection error and five-to-ten times dropping of mean square error.