The Research And Implementation Of QAM Modem

With the development of engineering, communication secrecy and informationsafety have been taken great importance. The demand on the performance of contract totelephone is much more intense than before by those confidential departments such asParty, government, armed forces, enterprises and individuals. The development ofdigital technology of voice signal provides a theoretical basis for the realization ofdigital encryption telephone. Digital encryption telephone mainly includes thetechnologies such as: speech coding technology, digital encryption technology, channelcoding, modulation&demodulation technology, etc. Digital encryption telephone usesthe latest achievements of digital communication, which has much potential. It hasbecome a leading and preferred direction for telephone confidentiality.The thesis is aimed at studying the research of one of the realization of digitalencryption telephone—the design and implementation of the modem. By the standard ofITU-TV.32and according to the software program, the thesis designs and realizes aMODEM system based on32QAM technologies. The system uses the relatively maturemodulation&demodulation technology in the field of algorithms, which considers thecomprehensive aspects of each part of QAM MODEM. The essay has made a furtheranalysis of the steps of QAM MODEM by software and had a test for its function andachievement by the principle that takes theory into practice. To illustrate it in detail, themain work of the thesis includes the following several aspects:The thesis first demonstrates the framework of branch modules of QAM modulatorsystem from two aspects: modulation and demodulation, and it illustrates the functionof each module and the procedure of data processing, including Scrambler, DifferentialCode, Convolutional Coding, Signal Mapping, Pulse Shape Filtering and QuadratureModulator, etc.The thesis then discusses the algorithms principle of each module of MODEM andthe detailed steps of project implementation. The thesis especially studies and discusseswhether the initial value of the register influence the sequence of performance, theimplementation process and parameter of symbol synchronization as well as theimplementation of the scheme of Carrier Recovery and Viterbi decoding. The thesisputs forward a real-time fitting calculation based on the data table producing sine/cosinecarrier method, which directs at the fact that the carrier frequency and sampling rate arenot of the actual integer times.Furthermore, the thesis proposes the exact function of the realization in the projectand analyzes the functions of its branch modules. According to the experimental data, itverifies the performance of the system modules that meet the requirements of thetechnical indicators. After realizing the implementation of the whole system, the thesis tests and analyzes the functions of the Bit Error Rate, its curve under the systemsatisfies the theoretical performance limit and the project.The thesis finally summarizes the advantages and disadvantages of the researchmethod and specifies the direction of further study. The MODEM system of the thesishas studied, in spite of the part of enciphered telephony, is of an important part ofcommunication and uses widely and has a broad market prospect.