Research On Time-varying Frequency-hopping Communication System And Synchronization Method Based On Discrete Fractional Fourier Transform

Frequency Hopping(FH)communication is widely used in various military wireless communications due to its low interception and anti-interference capabilities.However,with the continuous development of detection techniques aimed at traditional FH spread spectrum technology,the importance of concealment performance in military communications has gradually increased.To counter means of acquiring critical parameters of the opponent’s communication system,such as periodic detection of signals,variable time-width non-single frequency signals have been gradually applied to covert communication in information warfare.To enhance concealment performance,this paper introduces the Fractional Fourier Transform(Fr FT)to extend the time-domain to frequency-domain conversion of conventional frequency hopping systems to time-domain to fractional-domain conversion.The approach is presented from the perspective of cooperative communication parties.Moreover,the paper introduces the Varied Time Width Chirp signal(VT-Chirp)as the fundamental signal of the system.This signal,by means of pseudo-random time width changes,breaks the periodicity of the communication system and achieves better concealment characteristics.The proposed method,called " Fractional Fourier Transform Frequency Hopping with Variable Time Wide and Fixed Bandwidth(Fr FT-FH-VTFB)",is based on discrete fractional Fourier transform.The paper also explores a time-width that can effectively improve the concealment performance of the system using a variable fractional Fourier transform synchronization method.The main research content of this paper is outlined as follows:Firstly,this paper briefly introduces the basic definition and main properties of Fr FT.The Fr FT discrete algorithm and normalization processing used in this paper are also presented.The system block diagram of the Fr FT-FH-VTFB system proposed in this paper is then provided,with a detailed introduction to key points such as frequency hopping patterns,modulation,and demodulation steps.This lays the foundation for the subsequent research.Next,the mathematical model of both the sending and receiving ends of the Fr FT-FHVTFB system is established.The theoretical bit error rate of the Fr FT-FH-VTFB system under a Gaussian white noise channel is derived and simulated.Communication performance of the Fr FT-FH-VTFB system is then simulated and analyzed under different channel conditions.Additionally,the characteristic results of the Fr FT-FH-VTFB system are simulated and analyzed in different characteristic domains such as power spectrum,cyclic spectrum,and cepstrum.The concealment performance of the system is also analyzed.Finally,the influence of time delay and noise on fractional domain synchronization is introduced.The different changes of the two in the fractional domain after the fractional Fourier transform are demonstrated through formula derivation and numerical analysis methods.The paper introduces the basic principle of the variable time-width joint synchronization method,elaborates on the design ideas of the synchronization head and receiver,and provides the system block diagram of the two along with the derivation process of some key steps.The simulation verifies the performance of the variable time-width joint synchronization method,such as relative synchronization error and the impact of the addition of the synchronization header on the communication and concealment performance of the Fr FT-FH-VTFB system.