Research And Implementation Of QPSK Modulation And Demodulation In Coherent Optical Communication System

Coherent optical communication systems have become the focus of research at home and abroad because of their high detection sensitivity and transmission rate.Coherent communication systems that adopt Quadrature Phase Shift Keying(QPSK)modulation methods can increase transmission capacity.But the coherent optical communication system adopting QPSK modulation mode can cause quadrature imbalance,frequency and clock offset,etc.,which affect system performance.This article builds a coherent optical communication experiment platform to demodulate the QPSK intermediate frequency signal,and finally tests the demodulation performance of the QPSK system to verify the effectiveness of the QPSK communication system.The essential work and substance of the thesis are as bellow:1.The principle of coherent optical communication system was introduced,the principle of electro-optical modulation of QPSK and the principle of IF signal demodulation and the block diagram of its realization were researched.Finally,the principle of quadrature imbalance compensation algorithm,frequency offset estimation algorithm,carrier recovery algorithm and bit synchronization algorithm were described,and a specific work flow chart was given.2.QPSK IF signal demodulation algorithm was analyzed and realized.Firstly,the orthogonalization of I and Q intermediate frequency signals was completed by Schmidt orthogonalization algorithm;secondly,the influence of frequency offset on signal demodulation was investigated,and based on phase difference,the the performance of the two frequency offset estimation algorithms of FFT was analyzed;Then a carrier recovery method based on the Matsuo loop was proposed,and the effectiveness of the Matsuo loop was verified;Finally,the Gardner algorithm was used to achieve clock recovery,and the working principle of the Gardner algorithm was investigated.The simulation system verifies the correctness of the Gardner ring.In addition,the influence of noise on the demodulation system was researched.3.A QPSK modem experimental platform was built for experimental research.Firstly,the demodulation of QPSK intermediate frequency signal was realized,and the baseband waveform diagram and constellation diagram were generated;secondly,under the given different signal-to-noise ratio,the actual demodulation of QPSK intermediate frequency signal was analyzed.The value of the bit error rate was compared with the theoretical value.Finally,the relationship between the received optical power of the QPSK signal and the bit error rate was studied.The results show that the Schmidt orthogonalization algorithm can effectively improve the distorted constellation,the Matsuo loop and the Gardner loop can effectively achieve carrier recovery and bit synchronization;the demodulated constellation diagram was distributed in the four corners of the square,and the aggregation was good.The baseband signal waveform envelope was completed,and the system can lock the output in a short time.The output symbol was stable and the error was small;when the QPSK signal had a bit error rate of 10-3,the received optical power after channel transmission was increased by 2.4 dB than the theoretical value;when the bit error rate was 10-2,the two curves have a low signal-to-noise ratio.In this case,the difference was only 1.1dB,which finally showed that the demodulation performance of the system was better.