| Underwater acoustic communication network has received more and more attention in the field of military and civilian.The narrow bandwidth,and significant Multipath spread and Doppler spread characteristics of underwater acoustic channel brought a great challenge to underwater acoustic multi-user communication.In this thesis,the research on the Frequency-Hopping Multi-Frequency Shift Keying(FH-MFSK)based multi-user communication is carried out to solve the problem mentationed above,and a prototype of underwater acoustic multi-user communication has been created to meet the need of underwater acoustic communication networks.First of all,this paper studies the technology of point to point FH underwater acoustic communication.Theoretical research include the following contents.(1)The generation and selection of anti-multipath interference hopping patterns.(2)Checks and balances between communication rate and the ability to resist of Multi-path and Doppler interference during the design of the FH-MFSK communication system.(3)Basic principle of Convolutional code and its Viterbi decoder and MAP decoder.(4)Principle of coding and decoding of the Low Density Parity Check(LDPC)codes.Simulation results show that compared to the Convolutional code with soft decision Viterbi decoding and MAP decoding,LDPC Mini-mum decoding algorithm obtains a decoding gain of about 3dB,and 5dB gain than the hard decision Viterbi decoding of Convolutional code.At the same time.The signal processing results of Songhua lake experiment show that the performance of FH-MFSK system with LDPC code are better than the Convolution coded system.Secondly,this paper focuses on the research of FH-MFSK based multi-user underwater acoustic communication.Theoretical research include the following contents.(1)The Reed-Solomen(RS)code based method of generating pattern with the ability of anti-collision on both time and frequency domain.(2)Technology of FH signal based multi-user signal detection and synchronization,including Short-Time Fourier Transform(STFT),Goertzel algorithm,Constant False Alarm Rate(CFAR)detection algorithm.(3)Method of Doppler estimation based on FH signal.Simulation research has been taken to study the impact of hopping pattern on the quality of the multi-user communication,and the impact of the CFAR parameters on detection probability.In order to validate the performance of multi-user detection algorithm and error correction code,underwater acoustic multi-user communication experiments have been conducted in the Songhua Lake.The result of the experiments showsthat the FH signal based multi-user detection algorithm can achieve stable detection of two users.Four users can be detected if only each of the energy of each user is substantially identical.When the difference of distance between user and reception point is too large,due to the near-far effect,only users with higher SNR can be detected if the difference between the SNR of users is larger than 23.0dB.Finally,to meet the need of the network layer of the underwater acoustic communication network,a FH-MFSK based physical layer is designed and realized to support two users(data frame and control frame)on a hardware platform with a core of OMAP-L138.The specific contents include the following works.(1)The configuration of the peripherals that related to the signal acquisition and transmission,the message scheduling between the ARM core and the DSP core.(2)The design of interface protocol between the physical layer and the network layer,the formulation of the physical layer workflow.(3)Selection and implementation of modules such as modulator and demodulator,encoder and decoder,the implementation and optimization of multi-user detection.This physical layer detects two users simultaneously,capable of supporting communication network running ALOHA protocol for both computer simulation and Songhua Lake experiment,and further improvements the detection algorithm based on the results of Songhua Lake experiment make the physical layer much more robust. |
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