Experimental Study On QPSK Modulation And Demodulation In Coherent Optical Communication System

Coherent optical communication has gained much attention due to its advantages in detection sensitivity,transmission rate and spectrum utilization.In a coherent optical conmmunication system,signal characteristics are deteriorated due to orthogonal imbalance,frequency offset,clock offset,etc,which causes performance degradation of the coherent optical communication system.In this thesis,xwe utilize digital signal processing teclunology to compensate the received signal,restore the baseband signal,and improve the communication quality.The main contributions of this thesis are as follows:1.Analysis and research of the quadrature phase shift keying modulation and demodulation in coherent optical communication systems.We developed an experimental platform of coherent optical communication system.The principle of QPSK modulation was analyzed and experimental research was carried out.The advantages and disadvantages of orthogonal imbalance algorithm and carrier recovery algorithm were summarized.2.Analysis and implementation of QPSK IF signal coherent demodulation.The reason for the quadrature imbalance of the two IF signals output by the coherent receiver was investigated.The impacts of angular imbalance and amplitude imbalance on the system performance were analyzed experimentally,Schmidt orthogonalization algorithm is used to realize the orthogonalization of the non-orthogonal IF signals.The detailed theoretical derivation of the implementation process of Schmidt orthogonalization algorithm,carrier recovery algorithm and bit synchronization algorithm was included.3.Experimental research of QPSK modulation and demodulation.The impacts of noise and frequency offset to the coherent optical communication systems performance were verified by experiments.The impacts of loop bandwidth and noise on the convergence performance of Gardner loop and the performance of coherent optical communication systems were analyzed experimentally.The performances of the loose-tailed loop and Gardner loop were verified.The results show that the constellation of the carrier-band QPSK signal obtained by the IF signal corrected by the Schmidt orthogonalization algorithm is corrected from ellipse to circular.By using the loose-tailed loop,the carrier recovery can be better achieved in the loop capture band from-250 kHz to 300 kHz.When the clock recovery is implemented by the Gardner algorithm,the loop capture and tracking can be realized normally with loop bandwidth of 800 kHz and the signal-to-noise ratio of 15 dB,indicating that the Gardner loop is well performed.