Research On Key Technology Of Signal Processing Of Spaceborne Spread Spectrum Transponder

TT&C(Telemetry,Track and Command)transponder is the core equipment of space TT&C system,which is mainly responsible for the transmission of telemetry and remote control signals between satellites and ground stations.With the continuous development of space TT&C system,an incoherent TT&C system based on CDMA(Code Division Multiple Access)has been widely used.In order to meet the communication requirements of remote control and telemetry,higher and higher requirements are put forward for the receiving performance of spaceborne spread spectrum transponder.Aiming at the spaceborne incoherent spread spectrum transponder with one remote control signal and four ranging signals,this thesis studies and realizes the acquisition and tracking algorithm of multi-channel incoherent sequence spread spectrum signal.The main work is as follows:(1)Aiming at the problem of incoherent data jump and large Doppler frequency offset,the block-double zero-padding algorithm,half-cycle pseudo-code inversion algorithm and the method of dynamically adjusting the center frequency are proposed respectively.Because the spread spectrum of incoherent sequence destroys the periodicity of pseudo code,there is data jump problem,and peak fission will occur in acquisition.In order to solve this problem,this thesis proposes a block-double zero-padding algorithm and halfcycle pseudo-code inversion algorithm to suppress the influence of data jump without reducing the signal-to-noise ratio.Apart from this,in order to solve the problem of long search time under large Doppler frequency offset,a method of dynamically adjusting the center frequency is proposed,which transforms the large Doppler frequency offset into a small range of frequency search and improves the signal acquisition range and speed.(2)A signal acquisition unit of incoherent spread spectrum transponder based on FFT half-cycle pseudo-code inversion algorithm is designed,and the correlation integration unit,detection decision unit and acquisition verification unit are analyzed and designed in detail.In the correlation integration unit,the semi-periodic pseudo-code flip technology is used to suppress the influence of data jump and increase the coherent integration time to ensure the acquisition performance in the case of low signal-to-noise ratio.In the detection and decision unit,the cross-correlation interference of CDMA system is analyzed,and a twolevel threshold decision method is proposed to suppress the cross-correlation interference between channels and improve the detection probability.Finally,a capture verification unit is added to suppress sidelobe interference,which makes the capture result more accurate.(3)The channel signal tracking and processing unit of incoherent spread spectrum transponder is designed.The tracking processing unit uses the combination of carrier loop and pseudo-code loop to track and lock the signal in the form of closed-loop feedback.The carrier loop uses a third-order phase-locked loop and a second-order frequency-locked loop in parallel to deal with dynamic interference.The code loop adopts delay-locked loop structure,and the carrier is added to assist the rate correction of the code loop.Finally,combined with incoherent spread spectrum measurement and control equipment,the designed scheme is tested systematically on FPGA platform,which verifies the feasibility of the scheme.