Abstract
Direct sequence spread spectrum (DSSS) signal uses phase coherent signals where the information symbols are multiplied with a code sequence, commonly known as chips. The signals are processed at the receiver using the code sequence as a matched filter to extract the information symbols. Taking advantage of the spread processing gain derived from the matched filter, communications can be carried out at low signal levels, which is the preferred method for high quality underwater acoustic (UWA) communication and remote UWA communication. However, in practical applications, DSSS UWA communication will face lots of interferences including multipath interference, Doppler compression interference and phase fluctuation interference, which will seriously affect the performance of DSSS. This paper first analyzes the interferences that DSSS faces in practical applications based on multiple actual received data collected in different water areas. By calculating the gain of the matched filter loss, one can find that influence of phase fluctuation interference has the most serious impact on DSSS. Secondly, the corresponding receiver processing algorithms are given for these interferences, including the differential energy detector and Doppler estimation method. The performance of differential energy detector under the above interferences is analyzed, which shows that the differential energy detector is insensitive to slow phase fluctuation interference and multipath interference and the differential energy detector combined with Doppler compensation can work well based on moving at-sea data. Finally, the parameter design of DSSS system in practical application is discussed and parameter design of DSSS UWA communication system should comprehensively consider the Doppler compression interference and phase fluctuation interference.
Highlights
As a means of underwater information transmission, underwater acoustic communication has a strong military application background, and is indispensable in the civilian field
E2 e j n 2 dn dn 1 22 2 Since the differential energy detector (DED) algorithm compares the energy output results of two correlators, it can be known that when the underwater acoustic (UWA) channel multipath expansion is less than the spread symbol period, the multipath extended component of the UWA channel will become a beneficial contribution of energy
The core of Direct sequence spread spectrum (DSSS) UWA communication is to obtain matched filter processing gain and the fast carrier phase fluctuation interference caused by Doppler Effect is the most important factor affecting the matched filter processing gain
Summary
As a means of underwater information transmission, underwater acoustic communication has a strong military application background, and is indispensable in the civilian field. Direct sequence spread spectrum (DSSS) communication has good anti-interference and multipath resistance and can work under low signal-to-noise ratio conditions, which is the preferred communication method for high-quality UWA communication[2,3,4]. In DSSS UWA communication system, the carrier phase fluctuation will seriously affect the spread spectrum gain resulting in system performance degradation. The multi-path extended interference of the UWA channel will seriously affect the orthogonal characteristics of the pseudo-random sequence, thereby reducing the decoding performance. In response to this problem, related technologies such as DFE and PPC have been proposed[6,7].
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