| Currently,most underwater acoustic communication systems use incoherent underwater acoustic communication when stable and reliable transmission is required,in order to overcome the multipath fading of the underwater acoustic channel and improve the frequency band utilization,multi-carrier frequency shift keying system is proposed.In order to improve the performance of multi-carrier MFSK under shallow sea background,this paper investigates the multi-carrier MFSK communication based on non-binary LDPC compilation code,which includes encoding and decoding algorithm of LDPC,the key technolology of multi-carrier MFSK,encoding and decoding algorithm of concatenation and the implementation of the non-binary LDPC coding and encoding algorithm on the DSP-TMS320C6748.The details are as follows:Firstly,Starting from the non-binary LDPC encoding,this paper gives three construction methods for the non-binary LDPC check matrix.These include the Gallager construction methods,the Mackay construction methods,and qusai-cyclic LDPC construction methods based on RS codes.After introducing QC-LDPC code based on the array dispersion technique.Because of its insufficient internal sparsity,this paper provides two solutions,one is to mask matrix,and the other is to split and combine the matrix and then shield the treatment,both of which are very helpful to improve the sparsity of the matrix.For the non-binary LDPC decoding,this paper uses the Belief Propagation,and the Fast Hadamard transform which can reduce the amount of computation during row updates.Finally,the factors that may affect LDPC decoding under two different modulation modes of BPSK and QAM under Gaussian channel are simulated,including the number of iterations,the check matrix and the number of binary.Secondly,Multi-carrier MFSK system is the core of this paper.This paper examines the key technologies on multi-carrier MFSK systems,including checkout,interleave,channel codes,etc.The cascaded compilation code commonly used in multicarrier MFSK system with Hadamard as the internal code and the non-binary convolutional code as the external code is realized.In view of the improvement of decoding performance,the cascading compilation method of replacing multidecimal convolutional code with a multidecimal LDPC code close to the Shannon limit is realized.Finally,the bit error rate curve under the shallow sea channel is simulated,and the results show that when the bit error rate is 10-4,the performance of the non-binary LDPC code is better than the multidecimal convolutional code by 1-2d B,which confirms the effectiveness of the proposed method.Finally,Based on the good performance of multibase LDPC code in multicarrier MFSK system,this paper implements the non-binary LDPC compilation code algorithm on the TMS320C6748 platform.For the storage and retrieval of check matrix and posterior information,the MMC/SD card controller and FAT file system on the DSP are used.In the process of algorithm implementation,the program optimization implementation method and the for loop optimization mode of FHT are proposed,which reduces the execution time of the algorithm,and the execution time of each step of the compilation code is measured to prove that the algorithm can be decoded in real time.Finally,based on three different cascade coding methods,the experiment is carried out in the channel pool of Harbin Engineering University,the experimental results show that under the same channel and signal-to-noise ratio,the cascade coding scheme with Hadamard as the internal code and the non-binary LDPC as the external code has the best performance,this confirms its effectiveness in the actual underwater acoustic channel. |
