Research On Modulation And Demodulation Technology For Frequency Shift Channel

The frequency of the signal is different between transmitter and receiver because of thedoppler frequency shift. Doppler frequency shift is caused by the relative motion betweentransmitter and receiver under the wireless communication environment. Hight-speed rail,satellite communication, underwater acoustic communication, etc, these typical dopplerfrequency channels play an important role in human life and scientific research. So theresearch of how to improve the quality of communication in doppler shift channel has greatbig practical significance.There is a serious decease in the performance as the motion speed increases or the appliedcarrier frequency increases. Doppler frequency shift estimation and compensation isperformed before demodulation in the traditional method. When the doppler frequency shift issmall, this method is feasible but it increases receiver complexity; when doppler frequencyshift exceeds the work tolerance of the receiver adaptive equalizer and phase tracker system,this approach is no longer applicable. This paper puts forward two kinds of modulation anddemodulation method for low receiver complexity, low SNR and large doppler frequency shift.These two methods are both base on high precision frequency parameter estimation algorithm.The two methods are respectively:1) Asynchronous communication method based on frequency hopping and multiplesubcarriers. The transmitter modulates carrier amplitude and carrier frequency according tosymbol information. Receiver estimates the carrier amplitude and frequency by using highprecision amplitude estimation and frequency estimation algorithm. Then we can demodulatethe transmitted symbol information according to the mapping relationship. This method usesthe adjacent carrier amplitude and carrier frequency relative jump to transmit information,so the doppler frequency shift is eliminated. We can also use this method to achieveself-synchronization in long delay environment or in sending consecutive identical bitssymbol environment. The simulation results show that this algorithm indeed eliminates theeffect of doppler frequency shift and accesses self-synchronization feature.2) New communication method based on non-orthogonal subcarriers. OFDM spectrumutilization rate is high, but it requires stringent orthogonality. This new communication method is proposed for improvement. The transmitter firstly transforms serial high-speed datainto parallel low-speed data, then modulates subcarriers according to these data. Subcarriersare superimposed in time domain and will be sent together. Receiver estimates the amplitudeand frequency of each subcarrier accurately by using the multi-frequency sinusoidalparameter estimation algorithm, then we achieve non-orthogonal multiple subcarriersdemodulation in frequency shift channel. Compared with OFDM, this method does notrequire maintaining orthogonality between subcarriers. Therefore, the resistance to dopplerfrequency shift is greatly improved. The simulation results show that this algorithm can workproperly in low SNR, large doppler frequency shift environment.