The Study Of The Key Technologies For UnderWater Acoustic Spread-Spectrum Communication

Underwater acoustic channel is not only one of the most complex wireless channels, butalso the few medium that undertake the long-distance underwater transmission. However, theseawater medium has a high attenuation for high frequency, what leads to the narrowtransmission bandwidth. In the meantime, the slow speed of the sound wave in the seawatercontributes to the high Doppler shift in the received signal. Then, the coherent multi-pathsignals made the received signal distorted. The complexity of the seawater environment is thereason for the multi-path, Doppler, random time-varying and space-varying of underwateracoustic channel.With its own advantages, spread spectrum technique has the ability to overcome thecomplexity and adverse conditions of the channel. In the field of wireless communication,spread spectrum is a mature technique which has been widely used. However, due to thecomplexity of the underwater acoustic channel, the spread spectrum communicationtechnique that adapted to the underwater acoustic channel is still a hot issue.At present, the underwater acoustic spread spectrum communication technique faces twochallenges:1. Faced with severe time-varying in underwater acoustic channel, spread spectrumsynchronization technique with the ability to track time-varying channel needs to beresearched.2. Faced with severe restricted bandwidth, spread spectrum technique with highbandwidth utilization needs to be researched.The paper which based on the two themes mentioned above has in-depth study on spreadspectrum synchronization technique, high spectral efficiency multi-carrier spread spectrumtechnique and MIMO spread spectrum technique.To adapt to vast time-varying underwater acoustic channel and to improve sequencemodulation efficiency of the spread-spectrum system, a orthogonal-mixed spread spectrumcommunication scheme with efficient M-ary orthogonal sequence modulation technique isproposed and studied. By continuous transmitting synchronization sequence, the receivercould measure channel change in real-time.Doppler in underwater acoustic channel belongs to broadband Doppler. Carrierfrequency offset and code phase offset are two interrelated factors. The research is based onthe time-frequency2-dimensional searching sequence acquisition scheme what carrier frequency offset and symbol offset formed a two-dimensional phase space. In the space,optimal carrier frequency and code phase are found to capture and track the carrier frequencyand chip phase at the same time. By simulation, lake test, sea test, data processing andanalysis, the algorithm of time-frequency spread spectrum synchronization in2-dimensionalsearching is verified effective and with robustness. In conclude, this scheme can adapt tounderwater acoustic channel perfectly well.The paper studied the orthogonal multi-carrier underwater acoustic spread spectrumcommunication technique. Combined with the characteristic of the underwater acousticchannel, the paper defined the compared parameter--chip carrier frequency coefficient toindicate the advantage that single-carrier spread spectrum has over multi-carrier spreadspectrum in performance.(As the ratio of the wireless communication bandwidth to centerfrequency is much larger than underwater acoustic channel, the wireless communicationresearchers do not care about the value of chip carrier frequency coefficients, so the wirelesscommunication does not define this parameter). For orthogonal multi-carrier spread spectrumunderwater acoustic system, not only did the orthogonality between carriers make inter-carriernoninterference from each other, but also improved the spectrum utilization. Chipfrequency-carrier coefficient is controlled to close to the single-carrier ones at the same time.Spread spectrum signal has a long symbol time and wide bandwidth. However, there isnot extra resource in time domain and space domain for underwater acoustic spread-spectrumtechnique. To improve system working rate and performance, there is a way out by makinguse of the spatial MIMO technology resources. The paper present MIMO communicationscheme for underwater acoustic channel and studied the receive and transmit diversity indetail.After all sorts of simulation and research, the paper conducted quite a lot pool, lake andsea test not only to verify the performance in real underwater acoustic channel, but also toverify the effectiveness of various algorithms and schemes. Finally, some good test resultsobtained. It is worth mentioning that in the orthogonal multi-carrier spread spectrumtechnique11.23km remote transmission sea test, the communication speed reached94.1176bps and spectral efficiency increased by50%.