Research On The Multi-carrier Spread Spectrum Underwater Acoustic Communication Based On Chirp

This dissertation designs a multi-carrier spectrum spread underwater acousticcommunication system based on Chirp signal, aiming at solving the problems offrequency-selective fading caused by multi-path effect and Doppler frequency shift andspread due to relative movement between communication platform. The system is animprovement of the traditional multi-carrier system combined with spectrum spread theory.Compared with the traditional sine-carrier system, this new system preserves advantages ashigh communication rate and bandwidth efficiency and fast algorithm by FFT/IFFT; on theother hand, it conquers adverse effect caused by channel as mention above.The chirped spectrum spread multi-carrier system employs a pair of conjugated chirpsignal as its spread and de-spread basis. In this system, IFFT modulated signals are beingspectrum spread before sending into channel and received signals are being de-spread beforeFFT demodulation, and consequently, the multi-path components are spread in frequencydomain, the time shift transfers into frequency shift, which eliminates frequency-selectivefading. The dissertation works on two types of spectrum spread multi-carrier system,Chirp-OFDM and Chirp-DFSK system, and their performances.The dissertation discusses several types of anti-multipath and anti-Doppler technique infurther application to mobile underwater acoustic communication. Anti-multipath techniqueapplies multi-path components separated from its direct arrival as diversity resource,Rake-Receiver and Virtual-Time-Reversal-Time-Mirror (VTRM) are used to achieve focusinggain and improve system performance. On the other hand, as a common anti-Dopplertechnique, Doppler Diversity uses Doppler frequency shift as a kind of diversity resource,capable of lessening adverse influence of the Doppler Effect and increase available signalenergy to enhance system performance at the same time, combined with multi-path diversityresource will result in better performance. The feasibility and reliability of the scheme areverified through mathematical analysis and simulation experiment.