Research On LFM Signal Modulation And Demodulation Performance

Most signals in nature are non-stationary, but people tend to deal withstationary signals instead of non-stationary signal in order to simplify theprocessing of the signal. With the development of time-frequency analysis tool,the application of non-stationary signals in communication system gainsincreasing attention. Chirp signal is a typical non-stationary signal, its frequencychanges linearly with time. It has good autocorrelation properties, wide time-bandwidth product. Fractional Fourier transform is a generalized Fouriertransform, which can be seen as a rotation of the axis by certain anglecounterclockwise. Chirp signal in the corresponding optimal fractional regionshows the pulse aggregation features, and it can be regarded as basic functions offractional Fourier transform, which is similar to the basic functions of theFourier transform, cosine signal. Unlike traditional cosine signal, except for theamplitude, phase, initial frequency, linear FM signal has another parameter themodulation frequency. This paper mainly focuses on the modulation anddemodulation performance of LFM pulse, based on its compression propertiesand aggregation behavior in fractional domain.Firstly, this dissertation briefly introduces the basic concepts and nature oflinear FM signal. Because of its good autocorrelation properties, we propose amatched filter demodulation scheme. We give three definitions of the fractionalFourier transform, and its transform nature and fractional Fourier transform ofsome normal signals. We theoretically derivate and validate the Chirp signalaggregation behavior at optimal fractional domain.Second, we analysis performance of two modulation methods: directmodulation and binary orthogonal key modulation system. Direct linear FMmodulation is to use a Chirp signal with large time-bandwidth product instead ofthe pseudo-random code to spread spectrum of signal to be transmitted to achievespreading. Its performance is similar to direct spread spectrum system. BOKsystem based on Chirp signal modulation has three demodulation means,coherent demodulation, matched filtering demodulation, fractional domaindemodulation. Comparing three kinds of demodulation mode, fractional domainfiltering and coherent demodulation have same performance. Performance ofmatched filtering demodulation is worse. Meanwhile, the fractional domain filterdemodulation is immunity to single-frequency interference and Doppler shiftresistance. Therefore, dealing linear FM signal with fractional Fourier transformhas an unparalleled advantage. In the last of chapter3the author gives two multiple access methods based on linear frequency modulation signal anddemonstrate the feasibility. We prove that multiple access system using the samefrequencies rate and different initial frequency has better performance andgreater system capacity in the same bandwidth than which has differentfrequencies rate and same initial frequency.Finally, based on the foregoing analysis, the author proposes a novelmodulation and demodulation system based on fractional Fourier transform. Thesystem is based on the property that linear FM signal shifts in time-domaincorresponding to frequency shift in fractional domain, i.e. change in position ofthe characteristic peak. Transmitter just need one linear frequency modulationsignal generator, the others can be obtained by the time shift of half-cycle. Thesystem can also support Mary modulation. Meanwhile, we design a multipleaccess system combining binary orthogonal modulation proposed by Chapter IIIand the novel system. Finally, the author gives the theoretical analysis of systemusing different initial frequency modulated binary data.