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Analysis of Spread Spectrum Signals Based on FRFT

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Tutor: ShaXueJun
School: Harbin Institute of Technology
Course: Information and Communication Engineering
Keywords: the fractional Fourier transform,Qiepu signal,spreading,resource reuse
CLC: TN911.6
Type: Master's thesis
Year:  2008
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Fractional Fourier transform (FrFT) is a generalization of the ordinary Fourier transform with an order parameter a. The signal in FrFT domain concludes the signal¡¯s information both in time domain and frequency domain which makes FrFT have certain property that can be used to handle non-stationary signals. According to the theory that chirp signal is the basis function of FrFT, A method of rejection of frequency sweeping signal in DS spread spectrum system using FrFT has been proposed in this thesis. The received signal is transformed to the Fractional domain to concentrate the energy of chirp interference in narrow band which makes the signal separate from interference and noise. Consider of the computation and BER, two schemes have been discussed in the thesis. Simulation results show that the the method effectively reject the frequency sweeping signal in DSSS system.Chirp signals with different chirp-rates have the best energy concentration in corresponding fractional Fourier domain. The location of the best energy concentration in fractional Fourier domain is different when the chirp-rate of chirp signal changes. As these theories, a new spread spectrum scheme combined with chirp signal based on fractional Fourier transform has been proposed. Chirp signal is applied to spread the spectrum of the binary base band data in this scheme, the signals were made fractional Fourier transform at the receiving end to get the best energy concentration, and the demodulation is made by the judgment of the energy concentration location. Simulation shows although the BER is worse than BPSK in AWGN, the scheme can reject the single-frequency interference and the multiple access performance is good.In order to improve the performance of above scheme, another receive algorithm has been mentioned, the algorithm uses the filter in fractional domain to reduce the interference, after the inverse FrFT, the signal is match demodulated in time domain. This algorithm improve the performance of the receiver, we use the algorithm in multiple access system and design of a communication system to realize band re-uses. In the band re-uses system, we transmit multiplex BPSK signals using cosine and chirp signals as carriers within the same bandwidth. At the receiver, energy is concentrated right around the peak of output of the corresponding FrFT of chirp signal, interfere can be eliminated by narrowband filter in the FRFT domain, and then demodulate the signal in time domain after inverse transformation. Simulation results show that the performance of this system is good.
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