Abstract
In underwater acoustic communications, it is difficult to find the starting time of received signals because of noise, multipath, transmission time delay, and Doppler frequency. In this paper, we solve the problem by using 2-STEP frame synchronization in the receiver. The transmitted signal is modulated by using burst PSK. The frame is composed of preamble of PN code and random data. The 2-STEP frame synchronization consist of non-coherent correlation, sliding FFT, and coherent correlation. First, in the STEP 1, the non-coherent correlation and sliding FFT are simultaneously performed to obtain an approximate start time of a frame in time domain and a Doppler frequency in frequency domain. However, through the results of the marine experiments, we can see that it is difficult to guarantee the starting time point in STEP 1 when the received signal power is weak and there is high Doppler frequency. Therefore, in the STEP 2, we use the coherent correlation to get more accurate start time stably after compensating the Doppler frequency of FFT in STEP 1. Finally, we can assure that the performance of the proposed 2-STEP frame synchronization is robust to low power of received signal and high Doppler frequency through marine experiments.
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