Research On Orthogonal Modulation System Based On Fractional Fourier Transform

With the booming request of natural resources, human being start to shift their eyes to the ocean. Underwater acoustic communication has been applied to marine environment survey,exploring and many other fields. Nevertheless, the underwater acoustic channel is very complicated and uncertain. The signal transfers through the underwater acoustic channel suffer from path loss, doppler effect and multipath effect, which causes the distortion of the transferred signal. Therefore, it’s very difficult to realize high speed communication underwater acoustic channel.Orthogonal modulation is one of the most wildly-used technologies in wireless communication. The system based on this technology can reduce the multipath effect. However,doppler effect in the channel undermine the performance of the system. Fractional fourier transform is the extension of the traditional fourier transform. It can transform a signal into several orthogonal chirp signal. In underwater acoustic environment, doppler effect will not have a minor effect on chirp signal. Because of the complexity of the underwater acoustic channel, a orthogonal modulation system based on fractional fourier transform is proposed. Combining the merits of the fractional fourier transform and orthogonal modulation, the performance of the system will be improved. The major work of this paper includes:1. A brief introduction of the factors that affect a signal in underwater acoustic channel, including path loss, doppler effect, multipath effect. Based on these factors, two types of underwater channel model are introduced.2. Give a brief induction of the fractional fourier transform and orthogonal modulation, including the definition of fractional fourier transform and its discrete algorithms,the fundamentals of the orthogonal modulations and its advantages and disadvantages.3. A orthogonal modulation system based on fractional fourier transform is proposed. First, chirp signal, the traditional orthogonal system and its equalization algorithms are introduced. Then we used Matlab to simulate the system and the traditional system. Compare the performance of the proposed system and the traditional system. The comparison includes fixed doppler frequency,varied SNR and fixed SNR varied doppler frequency.