High Speed Digital Modem Digital Research And Realization Of The System

The modulation-demodulation technology in high-speed data transmission system is an important content in modern communications. In digital communication system, if signal will be transmitted over a long distance, especially in the limited bandwidth high frequency channel such as wireless or light transmission channel, it must be made carrier modulation to ensure that the baseband signal power spectrum has been moved to higher on the carrier frequency for the digital baseband signal contains rich low frequency components.Along with the widespread application of the digital technology, the devices such as field programmable gates array(FPGA) whose integration and speed are in high growth get a wide range of applications. FPGA not only has the high logic gate array density and high reliability, but also has the user programmability of programmable logic devices. Its programmable properties bring flexibility of the circuit design, and it is playing a more and more important role in digital circuit design.This paper combined with the requirements of "38GHz Communication System of Maglev Train", finally chooses continuous phase frequency shift keying(CPFSK) which is not sensitive to doppler frequency shift and frequency tolerance, and phase frequency discrimination which has insensitive characteristics to channel decline and strong anti-interference ability as the modulation-demodulation mode of the system based on the detailed comparison to all kinds of binary modulation-demodulation ways.Firstly the paper detailedly introduces both analog design scheme and digital design scheme of the modulator-demodulator respectively, then it makes some improvement to the existing modulator-demodulator achieved in analog way, that makes not only the performance index such as the demodulation sensitivity of the system and probability of error have markedly improved, but also each functional module of the system tend to complete in the practical conditions. On that basis, according to the requirements of new pattern and miniaturization for Communication System of Maglev Train,it makes simulation for the digital design scheme of the modulator-demodulator based on FPGA in Quartus Ⅱ, that provides a certain base for the research to the digitization and miniaturization of the system.At last, the engineering products and its test data of the improved modulator-demodulator achieved in analog way are provided as well as the simulation results of the digital design scheme. the test results show that the improved work makes much promotion to the engineering practicability of the system.