Research On Covert Communication Scheme Of OFDM System Based On QC-LDPC Code And Symbol Spread Spectrum

Covert communication,as an emerging technology to solve the wireless communication security problem,can not only hide the transmitted secret information,but also hide the communication process,so it has become a hot research issue in wireless communication security in recent years.Foreign research on covert communication focuses on theoretical performance analysis under certain assumptions,and there are relatively few practical studies on the system;domestic research directions tend to design covert communication algorithms,which reduce the detection probability of the system by designing low detection code sequences,these studies has certain practical application significance,but it lacks corresponding theoretical support.Therefore,it is of great theoretical significance and practical value to design a covert communication system model with sufficient theoretical basis and practicality at the same time.Firstly,this paper designs a symbol spreading algorithm,and combines it with OFDM?Orthogonal Frequency Division Multiplexing?technology,proposes an OFDM system scheme based on symbol spreading.Through simulation and theoretical analysis,it is proved that the system can obtain higher spreading gain when the number of OFDM subcarriers is limited,which effectively reduces the power spectral density of the transmitted signal and enhances the concealment of the system.Secondly,based on the problems of the square root law in the practical process,a covert communication scheme of OFDM system based on QC-LDPC?Quasi-Cyclic Low-density Parity-Check?code and symbol spread spectrum is proposed.On this basis,the Gaussian-like nature of the RF?Radio frequency?signal,the low detection probability characteristics of the system and the reliable communication performance under low signal-to-noise ratio is clarified.Meanwhile,we put forward a signal-to-noise constraint condition for measuring the concealment of the system.Finally,in order to obtain the optimal parameter configuration of the model,the system's concealment performance was simulated and analyzed under different parameters such as different code rate,spreading factor,constellation mapping mode,etc.,and the optimal parameters were obtained based on the deduced signal-to-noise ratio constraints.That is,number of OFDM subcarriers N=576,?2304,1152?QC-LDPC coding,16QAM constellation mapping mode,spreading factor 576,Log-BP decoding iteration threshold is 10.Under this parameter,it can achieve a good performance where the total error detection probability of Willie is greater than 0.95 while Bob's bit error rate is lower than10^-5,at the same time,the concealment rate can reach 8.68×10^-4 bit/N_b,which can obtain good system performance.