Research On OFDM Underwater Acoustic Communication Technolgy Based On Chirp Spread Spectrum

In the 21st century,people have increased their exploration of the ocean.In recent years,the wide development and application of underwater mobile platforms such as Autonomous Underwater Vehicles(AUVs)have brought new demands and challenges to underwater acoustic communication technology.Due to the relative motion between the transceivers,in general,the shallow water acoustic channel can be modeled as the delay Doppler dual extended channel model.The multipath effects and Doppler effects in communication are the main factors that restrict the communication quality.In order to meet the requirements for the rate and robustness of underwater acoustic communication,this paper combines the Orthogonal Frequency Division Multiplexing(OFDM)technology and Chirp spread spectrum(CSS)technology to research the underwater acoustic communication technology applicable to the AUVs.This paper first introduces the basic characteristics of shallow water acoustic channel,and then models the static multipath underwater acoustic channel and the delay Doppler dual extended dynamic underwater acoustic channel,which lays a foundation for the subsequent simulation analysis.Then,this paper introduces the implementation of the CSS-OFDM system and introduces the single-array virtual time reversal mirror(VTRM)technology into the receiver.The simulations under the static multipath underwater acoustic channel and fixed-point communication experiments in the pool demonstrate the advantages of CSS-OFDM system over conventional OFDM in resisting channel frequency deep fading points and VTRM technology can be used as a simple and effective equalization method for CSS-OFDM system.For the relative motion of the transceiver,this paper makes an accurate theoretical derivation of the relationship between the transmitter and receiver of the CSS-OFDM symbol in the delay Doppler dual extended underwater acoustic channel model.By using the relationship between the transceiver,the time-varying underwater acoustic channel estimation method based on CSS-OFDM front-end pilot symbols is proposed,where the residual Doppler factor of each multipath is deduced by the change of multipath delay.The estimation of the time-varying channel after resampling is more accurate.Then,using the result of the time-varying channel estimation,the static VTRM equalization technique is extended to the time-varying case.It is assumed that the channel is approximate time-invariant in each symbol block after resampling,and the symbol-by-symbol equalization can effectively avoid channel mismatch under the time-varying channel.The simulations and lake experiments both show the effectiveness of the proposed time-varying channel estimation method and the time-varying VTRM equalization method.Compared with the conventional OFDM system under the same conditions,the CSS-OFDM system can better combat Doppler.For the problem that the Peak to Average Power Ratio(PAPR)is too high in CSS-OFDM symbols,we propose a parallel combination coding method based on cyclic shift Chu sequence,which can reduce the system's upper limit of PAPR effectively.The simulation and lake experiment results verify the feasibility of combining this coding method with CSS-OFDM system.