Research On Downlink Transmission Of Satellite Based On CWTDM And Its FPGA Implementation

CWTDM is a time division multiplexing method of the continuous wave,including two kinds of forms,one is sample intersection,the other is sub-frame overlap-adding.It can change multiplex band-limited continuous signals into one signal to transmit in a continuous channel.The technique of CWTDM can be used in the satellite processor to construct a non regeneration signal processor,without having to demodulate and decode the signal,which can reduce the complexity of signal processing.But the latter kind of CWTDM system is very sensitive to the frequency offset,even a small residual frequency offset will lead to phase hop between the frames of the demultiplexing CWTDM signals.This will deteriorates the phase noise.The main goal of this thesis is to decrease the residual frequency offset and achieve a low phase noise CWTDM system.The main research results are summarized as follows:1.The principle of Continuous Wave Time-Division Multiplexing technique based on sub-frame overlap-adding is studied.A efficient CWTDM frame format is designed,which can synchronize and estimate frequency offset correctly in several kinds of channels under the acquired SNR.And a satellite downlink transmission system is proposed,which has low transmission complexity and high effiency and higher power effiency comparing to FDM.The system is consist of the models of the signal modulation,CWTDM multiplexer,travelling-wave tube amplifier,satellite channel in Ka band,synchronizing,estimating frequency offset,demultiplexing signal and demodulating the signal.The satellite channel based on the rain fading channel is analyzed.The simulation of the influence of the channel on QPSK signals is given.2.The frequency offset and three classical algorithms of estimating frequency offset are analyzed,including Kay algorithm,Fitz algorithm and L&R algorithm.The three algorithms are based on maximum likelihood theorem and they cant estimate the frequency offset in low SNR with the required accuracy.In order to reduce the residual frequency offset as far as possible,the thesis proposes a improved phase difference method which can eliminate the phase ambiguity,and a three-stage frequency offset estimate method based on the improved phase difference method and FLL is proposed.The method can decrease the residual frequency offset into 3Hz through three times estimate even when the SNR is 3d B.Therefore,the phenomenon of phase hop will be eliminated.By simulations based on the QPSK and the 16 APSK system,the results show that the system is robust and has low phase noise.3.The system is implemented with FPGA,including the modules of shaping filter,multiplexing,match filter,synchronizing,estimating frequency offset and phase offset,and demultiplexing signals.The platform consists of ZCU102 and AD9371.The effectiveness of the scheme is verified.