The Research And Realization Of A ?/4-DQPSK Modem In GEO Satellite Mobile Communication

GEO(Geostationary Earth Orbit)mobile satellite communication system can achieve wide area coverage of mobile communication through the relay node in the geostationary orbit.It has the characteristics of high EIRP(Isotropic Radiated Power)and G/T value,and the frequency reuse is achieved by spot beam.INMARSAT,THURAYA and other mature commercial mobile satellite communication systems have been built in the world.China will also launch the first GEO mobile communications satellite to build a system that supports regional communications and can communicate with the ground cellular mobile communication network.?/4-DQPSK(Differential Quadrature Phase Shift Keying)is a transformation from QPSK modulation technology.The maximum phase change of ?/4-DQPSK is 3?/4.It has the characteristics of low peak to average ratio,fast demodulation,strong ability of anti-multipath fading and the clock information in rule of phase changing,And is a kind of modulation technology which is suitable for GEO satellite mobile communication system and channel characteristics.By studying the specifications of the physical layer in GEO-Mobile Radio Interface,This thesis proposed a prototype realization method of the ?/4-DQPSK modem for the voice traffic channel in GEO mobile satellite communication system.,and give the realization method of modulation symbol mapping,channel coding and decoding,frame structure,shaping filter,digital down conversion,matched filter,and differential demodulation.Based on clock error estimation model proposed by Oerder and Meyr,this thesis puts forward an optimum sampling method for demodulator baseband signal.The method use square operation to the received baseband first,then use FFT operation to complete time domain to the frequency domain transform and get a phase angle of vector signal.By judging the minimum phase value,the best sampling point in each symbol can be located.The computer simulation shows that the optimal sampling point of the base band data can be obtained under the condition of low SNR.Finally,this thesis analyzes the theoretical value and test method of BER.The test results show that the prototype of the?/4-DQPSK MODEM realized using the proposed method in the thesis obtained a good BER performance under AWGN environment,which can provide reference for product development.