The Research Of Double-Differentially Coded Underwater Acoustic Spread-spectrum Communication Based On Hadamard Transform

As the most important and critical technology in modern marine military, underwater acoustic communication has been paid more and more attention. People hope that it is capable to transmit information underwater as fast and reliable as on land. Underwater acoustic (UWA)spread spectrum communication telemetry exchanges realiability with efficiency, which performs steady and reliable over intense time varying and fading channels. In order to deal with multipath and Doppler shift, it is necessary to synchronize the received signal, despread it and search carrier frequency of the received signal. These lead to significant increase of processing burden, and real-time processing cannot be realized with conventional correlation techniques. Fast Hadamard transform can be used to implement fast correlation between vector and matrix. Therefore, real-time processing of received signals can be achieved by applying fast Hadamard transform in underwater spread-spectrum communication, for example spread-spectrum coding and decoding, frequency search and so on.This paper presents a direct sequence spread spectrum UWA communication system scheme based on fast Hadamard transform. Firstly, the paper introduces the technologies involved in the scheme, including: synchronization techniques used in direct sequence spread spectrum(DSSS), receiver synchronization information extraction; the filtering techniques to overcome intersymbol interference.Then this paper puts forward two kinds of design scheme, using the fast Hadamard transform instead of linear convolution to do the correlation operation. And it is applied to spread spectrum coding, search Doppler shift. Both of the schemes passed the numerical experiments and they were drawn the same conclusion with the theory.The Doppler shift exist in most of the underwater acoustic communication systems. In this paper, we use double-differentially coded modulation to eliminate the impact of communication. By using the direct sequence spread spectrum, we can fix up the shortage of signal to noise by this algorithm. It has been proved that this algorithm is useful for Doppler shift.In the end of this paper, we did the pool test to verify the feasibility of the algorithm. And the shortcomings of the algorithm are analyzed.