Research Of Underwater Acoustic Multiple Access Method Based On Chirp Carrier Quasi-Orthogonality

Different from the wireless network channel characteristics,the underwater acoustic channel is a complex channel with strong multipath,narrow bandwidth,time varying and severe noise interference.Due to the unique characteristics of the underwater acoustic channel,most mature multi-access technologies in land can not be applied simply in the underwater network,only as a reference.Based on this,this paper research the following three aspects of content:traditional multiple access method,multiple access method based on chirp-rate quasi-orthogonality and code division multiple access method based on chirp-rate.First of all,this paper analyzes and reproduces the performance of the classic multiple access method in underwater communication,focusing on the advantages and disadvantages of Orthogonal Frequency Division Multiple Access(OFDMA)and Code Division Multiple Access(CDMA)methods.Then,the chirp signal is introduce as a carrier,because the underwater acoustics channel characteristics causes interference to the single frequency signal.Chirp signal is a pulsed compression signal with anti-multipath and anti-Doppler characteristics,which is widely applied in underwater acoustic communication area.Based on this,this paper introduces three methods of multiple access based on chirp quasi-orthogonality.Firstly,this paper analyzes the chirp carrier access with different center frequency,and ensures that the chirp carrier maintains an approximate orthogonal state by selecting a specific frequency interval,thus minimizing the multiple access interference.Secondly,it analyzes the multiple access interference suppression of single chirp-rate,which verifies that the multiple access method is sufficient to satisfy the application scenario of the different cell user separation in the condition of the small number of users.Finally,it analyzes the multiple access interference suppression of combined chirp-rate,the results show that the system can accommodate more users without significantly sacrificing the system error rate.Based on the above research basis,by referring to the idea of hotspot Sparse Code Multiple Access(SCMA)in the field of wireless communication.This paper proposes to select the user information to form orthogonal codebook by pseudo-random(PN)sequence,and then modulate it into a set of quasi-orthogonal chirp subcarriers to achieve the communication between different clusters of nodes of three Code Division Multiple Access scheme.Firstly,it describes the CDMA scheme based on multi-carrier chirp,by setting the sub-carrier center frequency and signal bandwidth can eliminate the interference between users,to achieve higher transmission rate.Secondly,it describes the CDMA scheme based on single chirp-rate,which utilizes the double orthogonal between PN and subcarriers to achieve better anti-multipath fading performance than traditional single frequency Frequency Hopping-CDMA.Finally,it describes the CDMA scheme based on combined chirp-rate.According to the structure feature of combined slope of chirp,this paper proposes a rapid detection method based on Fractional Fourier Transform of mixed variable slope.The scheme has a better anti-noise effect than traditional Frequency Hopping-CDMA both in Gaussian and Rayleigh fading channels.And the three multiple access scheme are compared by the Bellhop channel model and wuyuan bay channel model.The results show that on the premise of the same communication rate,the CDMA scheme based on single chirp-rate can achieve relatively optimal error performance.It effectively avoid the multiple access interference between the users and channels and maximize the band utilization.