| With the development of communication and aerospace technology,the requirements of service quality of satellite TT&C?Tracking,Telemetry and Command?system are increasing.Spread spectrum system is an effective way to integrate the communication and ranging functions of TT&C system which has become a trend of application.The performance of spread spectrum TT&C system is mainly determined by two aspects:one is the design performance of spread spectrum code,while the other is the accuracy of the actual receiver,which is mainly determined by the performance of signal synchronization.The anti-interference and multiple access multiplexing features of the spread spectrum system require accurate carrier,spreading code and clock synchronization,while the synchronization of the spread spectrum signal includes both acquisition and tracking process.Therefore,based on chaotic sequences,the paper focuses on the design and performance evaluation of compound chaotic spreading codes and the acquisition and tracking algorithm of non-coherent spreading signals.In order to improve the synchronization ability of spread spectrum ranging system so that ultimately improves the service quality of satellite TT&C system,the main contents of this paper are as follows:First,to overcome the shortcoming of conventional PN code that have bad security,and the conventional binary quantification chaotic code who have poor code balance,a chaotic compound code which can optimize the balance of spread spectrum code is proposed.The performance criteria of spreading code acquisition,tracking,anti-interference and code balance based on welch boundary is studied.And the performance evaluation of the proposed code and the ranging code of common ranging system such as GPS and Beidou has been made.The theoretical analysis and simulation results show that the balance of chaotic compound code proposed in this paper satisfy strong balance constraints and has great interception resistance.The correlation characteristics of the proposed code meet the requirements of acquisition,tracking and anti-interference,while keeping a large code word space.Second,study the acquisition algorithm of non-coherent spread spectrum signal,based on the conventional parallel acquisition algorithm,an analysis and improvement of PMF-FFT?Partial Match Filter Fast Fourier Transform?acquisition algorithm has been made.Simulation results show that the PMF-FFT algorithm can effectively increase the acquisition range of high dynamic signal and solve the SNR loss caused by incoherent spread spectrum signal bit flip.Finally,study the tracking algorithm of non-coherent spreading signals.A bit tracking loop that can dynamically change the coherent integration time of the tracking loop is proposed for the non-coherent spread spectrum signal that the information bits and the spreading code clocks are not homologous,and the spread spectrum ratio is variable.On the basis of the conventional carrier tracking loop and the spreading code tracking loop,the bit tracking loop is combined to overcome the problem of variable bit flipping time and the uncertainty of loop update time.The system simulation results show that the bit tracking loop designed in this paper can effectively make up the SNR loss of coherent demodulation caused by bit flipping.Under the condition of normalized carrier-to-noise ratio of 51dB·Hz,the bit error rate can satisfy 10-5 when the information rate is lower than 17.5Kbps. |
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