| Secure coding is a kind of physical layer information security techniques and does not require the encryption of the information,based on the inherent characteristics of the physical layer.Therefore,the information is transmitted securely while improving the reliability of the communication system.When the difference between the signal-to-noise ratio(SNR)of the legitimate communication channel and the eavesdropping channel reaches a certain condition,the legitimate can transmit the information without error,and the eavesdropper cannot receive the useful information.The relationship between the reliability and the security is defined by the security gap.The smaller the security gap is,the better the performance of security coding has.The transmitter of the cascaded security coding(CSC)system consists of preprocessing coding and channel coding.The receiver completes channel decoding and preprocessing decoding.By channel coding,the private information after the error diffusion,the bit error rate at low SNR can be enlarged and a secure encoding system with small security gap can be obtained.Ideally,the bit error rate of the legitimate receiver is 0.The bit error rate of eavesdropper is 0.5,so it cannot receive any useful information.Low-density parity-check(LDPC)codes and Polar codes are two modern channel codes with excellent performance,which are widely used in communication systems.In CSC system,channel coding with strong error correction ability can reduce the security gap.Therefore,the thesis uses LDPC codes and Polar codes as channel coding,combined with error diffusion encoding and decoding.Design and implementation of CSC system are studied by using software and software-hardware codesign.The main works can be summarized as follows:(1)This thesis studies CSC system based on LDPC codes,and implements secure coding communication system by software mean.For LDPC codes in IEEE802.11n protocol,fast coding,min-sum(MS)and belief propagation(BP)decoding algorithms are studied and implemented.On this basis,a method of error diffusion encoding and decoding is investigated,which decreases the communication quality of eavesdroppers.The simulation results show that when the LDPC code rate is not higher than 3/4,the security gap is less than 2.8dB by using MS decoding algorithm.(2)The LDPC encoder and decoder are designed and implemented by using FPGA.The fast encoding and semi-parallel decoding are carried out to improve the efficiency of LDPC codes.On this basis,a method of combining hardware and software to realize the CSC system is proposed.The error diffusion encoding and decoding are realized by software,and the visual interface is designed based on MFC.Appling the WinPcap mode,the data transmission protocol is designed and implemented in this thesis.Five kinds of data packet formats are presented,including the information of parameters,encoding,decoding and so on.The speed of big data transmission and system efficiency can be improved without the use of IP header and TCP protocol overhead.The experimental results show that when processing the sources with 2M bytes,the efficiency of the software-hardware codesign system can be improved by 7.68 times,and the impact on the security gap is about 0.1 dB.At the same time,with the increase of the source quantity,the working efficiency is improved more obviously.(3)This thesis studies the CSC system based on Polar codes,and proposes a low complexity and resource-saving encoding and decoding method to implement Polar codes.In encoding,in order to reduce the computational complexity and the use of resources,cyclic replication method is applied to generate matrix,and spatial multiplexing method is applied to complete channel estimation.In decoding,the decoding formula is selected according to the symbol number to simplify the calculation process of decoded messages.The experimental results show that the security gap of the Polar code CSC system reaches 2.8dB,when the code length not lower than 2048 and the code rate not higher than 1/2. |
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