Research On Down Converter And Error Correcting Code Of Spread-Spectrum Communication System Of Special Low Earth Orbit Satellite

This paper mainly expounds the techniques of downconvertion and error connection encode in LEO Spread Spectrum Communication System. Downconvertion and error control system based on concatenated code are the critical part of the LEO Spread Spectrum Communication System, and are widely used in the communication system.This paper first introduces the basic theories of direct sequence spread spectrum communication and the realization of digitalization, presenting the overall terminal design framework of the spread communication system, according to which the major contents of the research project is expressed.Then this paper delves into the application of downconvertion as baseband recovery in the spread communication system. As the spread spectrum and despread of digitalization are handled on the digital baseband, it is very important to convert medium frequency signal in analogue to digital baseband signal, and the downconverter functions to convert analogue medium frequency to analogue baseband signal, and to digital baseband signal through A/D transformation and decimation filter. In this process, it is COSTAS that transforms medium signal loaded with data information to baseband signal deprived of carrier. In addition, this paper analyzes the noise-filtering quality of phase locked loop, A/D transformation theory, and the combination and quality of the decimation filter." Invert" phenomenon is inevitable when COSTAS is used to make baseband recovery. Differential modulation can avoid erroneous recovery in the receiver. DPSK is one of the means to represent digital information through relative carrier phase of the neighboring symbols. This paper also touches upon the features of differential modulation and demodulation.Furthermore, this paper analyzes the error control system based on concatenated code, mainly including the formation, encoding, decoding of RS code and convolutional code, interleaving, and deinterleaving. Inner code and outer code are applied in this project. When there are less random errors in the channel, inner code can be utilized to correct them; when there are lasting accidental errors or too many random errors beyond the inner code's capacity,it is up to the outer code to correct the wrong symbols occurred. Being simpler than other single-stage grouping codes that have the same parameter, the error control technique based on concatenated code is widely used in deep space with strong interference.The last part of this paper deals with, as well as experiment wave form, hardware realization of the actual circuit, mainly involving COSTAS, A/D transformation, decimation filter, DMF, modulation, spread spectrum, and the encoding, decoding and interleaving of concatenated code.