Research On Modulation And Demodulation And SNR Estimation Technology In Telemetry

Telemetry is a technique that transmits measurement results to the remote terminal via wireless communications,which is widely used in flight test missions for missiles,satellites and other aircrafts.With the rapid development of telemetry technology,the technical characteristics of the futu re telemetry system is mainly towards the multi-mode in modulation and high-speed development,but the current domestic telemetry systems is integrally characterized by single function and poor equipment compatibility,so can not adapt to the future development of telemetry tasks.In view of the above situations,the SDR technology provides an effective solution,and the main idea of SDR is to use a unified hardware platform to achieve the communication functions by using the software.Based on the design idea of SDR,this research designs a multi-modulation mode intermediate frequency digital telemetry transmitter by using all-digital method to meet the needs of current and future telemetry tasks.In this paper,the nonlinear characteristic and band-limiting characteristic of the telemetry channel are studied and analyzed.Combined with the channel characteristics and the international telemetry standard IRIG 106,PCM/FM(Pulse Code Modulation/Frequency Modulation),QPSK(Quadrature Phase Shift Keying)and SOQPSK(Shaped Offset QPSK)modulation system are selected as the research objects.Then,the modulation and demodulation algorithms of PCM/FM,QPSK and SOQPSK are researched,and the phase characteristics,bit error rate and spectral efficiency of each modulation and demodulation technology are analyzed through the simulation.Aiming at the problem of difficult carrier recovery in traditional coherent demodulation algorithm of PCM/FM,this paper presents a demodulation method which does not need carrier recovery,so it can reduce the complexity of the system.Meanwhile,due to the slow out-of-band frequency spectrum attenuation,an improved SOQPSK modulation method is proposed by optimizing the frequency pulse function of the existing SOQPSK modulation signal,and the simulation results prove that the improved SOQPSK modulation signal has faster out-of-band attenuation characteristics.In addition,this paper analyzes and evaluates the key parameters —SNR(Signal-to-Noise Ratio)in adaptive modulation under the additive Gaussian white noise channel environment.Aiming at the shortcomings of several typical non-data-aided SNR estimation algorithms with poor estimation accuracy and narrow estimation range,an improved SNR estimation algorithm is proposed.The simulation results show that the SNR estimation performance of the improved algorithm is better than the original algorithms,which provides the basis for the adaptive modulation of the multi-modulation telemetry system.Finally,this study uses Verilog hardware description language to write the FPGA implementation codes of each modulation algorithm,then,completes Modelsim function simulation and actual test s,and the test results show the effectiveness of the design.