Design And Performance Study Of Covert Communication System Based On Noise Frequency Modulation Signal

With the continuous improvement of modern communication monitoring technology,the demand for communication security in wireless covert communication systems has gradually increased.At the same time,new requirements have been proposed for the low probability of detection(LPD)characteristics of modulation waveforms.In recent decades,spread spectrum communication technology has made significant progress,especially in the field of military covert communication.However,this development has also increased the difficulty level of communication surveillance.Due to the use of high-speed frequency hopping,complex modulation,and channel coding technologies in covert communication systems,surveillance personnel need to have higher professional skills and more advanced surveillance equipment to effectively detect such systems.Since the existing LPD communication waveforms represented by Direct Sequence Spread Spectrum(DSSS)and Chirp Spread Spectrum(CSS)have obvious convergence characteristics in the communication transformation domain,to effectively counter the third party’s detection of target information and identify useful information,this paper proposes a new type of covert communication system based on the waveform modulation.Starting from the waveform modulation,and with the consideration of the high reliability and anti-interception performance of covert communication,the paper proposes to use Noise Frequency Modulation(NFM)as the waveform basis,and improve it for communication purposes.Binary Orthogonal Keying(BOK)modulation and Varied Time Width(VTW)architecture are adopted to propose two new types of covert communication system schemes,and the theoretical derivation,communication performance,and covert performance of the two communication systems are studied.Firstly,since the NFM communication system is a new type of communication system,improvements need to be made to the NFM interference signal.The theoretical basis and mathematical model of NFM signal are derived,and the feasibility of using NFM signal for communication is demonstrated.The time-domain waveform and frequency spectrum of frequency-modulated Gaussian noise are simulated.Secondly,the mathematical expressions,mathematical models,and coherent demodulation communication system diagrams based on the NFM-BOK communication system scheme are derived.The waveform likelihood function of the NFM-BOK communication system is derived,as well as the theoretical expression of the bit error rate in a high white noise channel and a multipath channel.The power spectrum expression,autocorrelation function expression,and cyclic spectrum expression of the NFM-BOK communication system are also derived.At the same time,the communication and covert performance of the NFM-BOK communication system are analyzed by simulation to verify the correctness of the derivation.Finally,the VTW-NFM-BOK communication system is proposed and the relevant mathematical expressions,mathematical models,communication system block diagram,waveform likelihood function,theoretical expression for bit error rate in high white noise channel,theoretical expression for bit error rate in multipath channel are derived.In addition,simulation analysis of the VTW-NFM-BOK communication system is conducted to verify the correctness of the obtained results and evaluate the communication and stealth performance of the system.