Research On Code Tracking Technology Based On Si-SSCPM

With the increasing development of global satellite navigation technology,the problems of dual limitations of power and bandwidth have gradually become prominent,and new navigation signal modulation systems have become a key research direction.Continuous Phase Modulation(CPM)has the characteristics of constant envelope,high bandwidth efficiency,and high power efficiency.It is widely used in many fields such as military and telemetry.These advantages make the CPM signal a choice of new modulation methods for satellite navigation systems.At present,there are more researches on CPM signals with modulation index less than 1 in the literature,but less about CPM signals with modulation index greater than 1.This paper studies the power spectrum,pseudo-code tracking,and anti-multipath capability of a semi-integer spread spectrum continuous phase modulation(SI-SSCPM)signal with a modulation index greater than 1.The specific research contents of this paper includes: 1.The approximate expression of SI-SSCPM signal is obtained by using the Laurent's decomposition expression of CPM signal.The power spectrum of SI-SSCPM signal with different frequency responses and the binary offset carrier(BOC)are compared.Simulation results show that the power spectrum of the SI-SSCPM signal has similar characteristics to the BOC signal's subcarrier.2.The pseudo-code tracking algorithm of SI-SSCPM signal was studied.And the mathematical expression of the S curve of the signal passing through the delay locked loop was derived For the modulation index greater than 1,there are multiple positive slopes in the half curve of the S curve.A zero-based error lock phenomenon is proposed.A heuristicbased detection and correction mechanism is proposed.Simulations show that the proposed algorithm can effectively detect the error lock state during pseudo-code tracking and correct it.3.Aiming at the pseudo-code tracking error of SI-SSCPM under the multipath effect,an adaptive delay-locked loop suitable for multipath cancellation is proposed.The estimator and separator of multipath signals are designed to separate the multipath signals from the received signal.Simulations show that this method can effectively eliminate multipath signals,and reduce the pseudo-code tracking error of SI-SSCPM signals caused by multipath effects.4.The module design of the adaptive delay locked loop is designed by the FPGA development platform,and the validity of the circuit is verified in conjunction with ISE and Modelsim software.