The Research Of Underwater Acoustic Direct Sequence Spread Spectrum Communications Based On LS Codes

In the underwater acoustic(UWA) communications, the transmitted signals are subject to severe multipath interference due to the influences from sea bottom, surface and the changes of propagation speed with the sea depth and areas in the UWA channels. Consequently, inter-symbol interference(ISI) caused by multipath propagation severely limits the transmission rate. Furthermore, other channel factors,such as ambient noise, Doppler effects, frequency selective fading and so on, result in great challenges in robust UWA communications, especially in long-range communications. To account it, direct-sequence spread-spectrum(DSSS) techniques are introduced to the UWA communications, which can not only overcome the frequency selective fading and suppress the noise and multipath interference, but also effectively utilize the multipath signals by joining RAKE receiving technology.However, since the limited-length spread spectrum sequences, represented by m sequence, Gold codes and chaos sequence, are not able to obtain ideal correlation function, it is a great challenge for the UWA DSSS signals based on these spread spectrum sequences to completely remove the multipath interference.LS(Loosely Synchronous) codes, as a kind of complementary series, can construct ideal correlation function in interference-free window(IFW). Therefore,when applying LS-DSSS signals into the UWA spread spectrum communications, the multipath interference could be effectively suppressed, and thus the quality of the UWA communications will be improved.Motivated by this, the paper introduces the time division orthogonal combination LS codes(LS codes) and the carrier-phase orthogonal combination LS codes(CLS codes) into the UWA DSSS communications as the spread spectrum sequences. Main research contents are shown as follows. First, the paper analyzes the correlation properties of LS codes and CLS codes. Then, based on the two spread spectrum sequences, the signal structure, modulation and demodulation techniques of the DSSS/BPSK signals suitable for the UWA communications are given with taking BPSK modulation as an example. Next, the performance of multipath interference suppressing of the both signals, and the influence from demodulation synchronous deviation, are theoretically analyzed in detail. Last, by comparing with msequence-DSSS/BPSK signals, the results from the simulations, lake tests and sea tests verify the advantage of LS- and CLS-DSSS/BPSK signals in multipath interference suppressing.The conclusions from the detailed analysis show that: LS- and CLS-DSSS signals have better performance in multipath interference suppressing than m sequence-DSSS signals; RAKE receiving technology can enhance this advantage of LS- and CLS-DSSS signals.