The Application Of Fractional Fourier Transform In Underwater Acoustic Communication

With the development of information science and technology, the traditional Fourier Transform exposes its limitations in some research domains. Because the traditional Fourier transform establishes only the one-to-one correspondence between the time domain and the frequency domain of the signal, not the one-to-one correspondence between the local time domain and the local frequency domain of the signal. It means that the traditional Fourier transform is not able to show the statistical characteristics varies with time for non-stationary signal. As a result, the demand is urgent for a new signal analysis theory and method which can reveal the frequency characteristics varies with the time of the signal. Fractional Fourier transform as one of many methods because of its unique characteristics has been paid much more attention.The application of Fractional Fourier Transform (FRFT) in underwater acoustic communication of synchronous detection, coding and decoding is studied based on the analysis of Fractional Fourier Transform theory and characters. However, the underwater acoustic channel is random temporal and spatial varying, and the Intersymbol Interference (ISI) caused by the multipath channel is one of the primary barriers to underwater acoustic communication. In order to acquire high information data rate at low bit error rate (BER), efficient techniques must be adopted to reduce the ISI. This article use RAKE receivers to resolve the problem of multi-path, so as to effectively reduce the underwater acoustic channel Intersymbol Interference.In synchronous communication systems, "Synchronization" is a premise for information transmission. In order to ensure reliable information transmission, the measurements of arrival time about the synchronous signal have a higher accuracy. This article use adaptive notch filter to track synchronous signal (short bursts of CW signal before the LFM signal) and measure channel Doppler frequency offset. This will have two meanings: Firstly, to amend the reference signal of copy correlator and achieve the purpose of precisely tracking synchronous signal; secondly, to reduce the search area of fractional Fourier transform and improve the computation speed.Finally, the detailed realization of fractional Fourier transform which is based on the fixed-point digital signal processor (DSP) is provided. This paper considers comprehensively about calculation cost, memory capacitance and transmitted data amounts, and allocates memory cell reasonably, and simulates the algorithm in CCS. In order to realize the whole process in the hardware platform, a simple but practical display and control interface is provided to transmit signal to the hardware system from the host computer, which is resampled by the hardware system and transmitted to the DSP which in signal processing board and calculated there. Finally, the result is reported.