Qpsk Modem Research And Design

The issue stems from cooperation projects with the Institute and the aim is to fulfill image transmission in communication system. The technology of complete digital QPSK(Quaternary Phase Shift Keying) modulation and demodulation is suitable for the digital communication in the future for its flexibility and versatility. This paper propose a program of modulation and demodulation system based on ASIC(Application Specific Integrated Circuit) chips: STEL-1109 and STEL-2105, and fulfilled signal's modulate and demodulate. The modulator and demodulator in this paper are both controlled by FPGA. This system use RS code to improve the reliability of transmitted data. Also this system use simple USB interface technology to provide a good communication between PC and itself. This system contains a feature of Software and hardware combination, so it's suitable to be completed with FPGA.First, this paper depicted the principle of QPSK modulation and demodulation and characteristics of complete digital method, then discussed the design of modulator and demodulator respectively, and at last completed the test of the system and gave out some improvement suggestions.In the process of design of modulator and demodulator, some researches of key technology were related. For example, symbol mapping, shape filter and interpolating filter in the modulator and Integrate & Dump Filters, carrier synchronization and symbol synchronization in the demodulator.This paper gives out the setting of key parameters at the same time of discussion the module of modulator and demodulator. Proved by experiment, the system has some reference merits. It can work effectively to support the subsequent and can complete the QPSK modulator and demodulator with the intermediate frequency of 30.72MHz, and the largest bandwidth reaches 4Mbps.