Research And Implementation Of Digital If Telecommand Demodulation System

As of now,in the process of the gradual advancement of the military-civilian fusion industry's new technologies,there is an increasing urgent need for the new technologies about the airborne missile-borne telemetry and telecommand(TM&TC)communication systems(mainly used in the field of aerospace),especially for the wireless link Telecommand transmission.The system pays more attention to the low SNR large-frequency offset conditions about using the wireless environment,and there are higher interference performance and corresponding multiple access mode(mainly support the multiple users).The spreading spectrum communication technology can be regarded as a good communication system in the field of commercial communications through its capabilities and strengths.However,in the field of measurement and control,the spreading spectrum communication systems should effectively solve the technical difficulties in high-precision,rapid synchronization and other technical conditions under the harsh environments,then they can get the better applications.Generally speaking,its research main point is on carrier synchronization and tracking synchronization.,PN code capture and other issues.At present,one of the main research contents is the implementation of pseudo-code synchronization in this area.Its disadvantages are concentrated in the aspects of the complex circuit structure and long acquisition time.What's more,it is difficult to capture the long PN codesbe under the condition of large Doppler frequency shift and low signal-to-noise ratio conditions.Firstly,this paper mainly studies on the common synchronization methods in direct-sequence demodulation systems.Such as,matched filtering,sliding correlation,etc.Then,derives a theoretical level of its capture performance.Next,from the corresponding analysis,it is not difficult to see that the performing about the pseudo-code acquisition of the two methods are come from the time domain correlation.Although its theory is not complicated and easy to implement.However,in the case of Large Doppler shift and the Low SNR,it is difficult for the above of two to effectively implement pseudo code acquisition.With respect to the problems of the traditional synchronization methods in the time-domain correlation capture,this study proposes to two methods that based on FFT improvement to ensure the efficiency of pseudo code acquisition(under harsh conditions).First of all,aim at the "pure frequency-domain improvement" for Low signal-to-noise ratios,with the help of time domain,the FFT is used to perform the FFT operation on the receive and transmit ends,then instead of the receive and transmit sequences,followed by the frequency domain phase.In a further step,carry out some frequency domain correlations(IFFT operation)to replace the time domain correlation.Because of today's FPGA capacity can increase the amount of computation with the high efficiency,and thus compress the acquisition time;but more importantly:to ensure that the method can be implemented under extremely low signal-to-noise ratio,with the help of coherent and non-coherent accumulation of adjacent pseudo-code period,the correlation peak can be improved effectively.Next,in order to improve the time-frequency domain combination(under-frequency offset),to "paragraphize" local pseudo-codes and received signals,and then to apply corresponding“partial time-domain correlations " to each segment.The correlation value is affected by the frequency offset and the frequency offset estimated by FFT is fed back to the digital frequency converter for multiplication.This article makes a corresponding analysis about the theory that base on the method,and points out the practical theoretical analysis about the impact of FFT points,segmentation numbers,etc.Finally,the implementation process and simulation method(hardware hardware)are given.In the final design activity,the system locates at the spreading sequence as a GOLD sequence.The methods of pseudo code acquisition and adaptive threshold determination are respectively positioned as multi-channel sliding and power instantaneous calibration.After pseudo-code initial synchronization is obtained,it is tracked with a delay-locked loop.The digital Costas loop is responsible for BPSK demodulation and carrier tracking.The corresponding software is designed according to the system plan,and the main functions of the system are realized by FPGA(single chip).Its main components about the platform are include of power management circuits,RS422 interface circuits,FPGA systems,A/D conversion circuits,and so on.The digital signal processing part is located in the FPGA software gate array(field programmable),which can meet the requirements of miniaturization and low power consumption baseband modules.After corresponding system tests,it can be seen that it can capture large Doppler frequency offset and pseudo-code phase,and the parameters are flexible,the bit error rate is low and the capture time isshort,so it can achieve the desired goal well.What's more,there are some advantages of flexible application,convenient debugging and simple structure.For the testing of the actual operating system can get the relevant test data to make the necessary records.It can be seen that the system can meet all the initial design requirements and verify the correctness of the algorithm,too.