Performance Of Optical QPSK Coherent Receiver With Limited Bandwidth

With the sharp increasing in communication business, such as video sharing and cloud computing, higher requirements are put forward to the performance of backbone network. Therefore,Studies on the performance of coherent optical communication system become more and more important. However, there are only a few researches on the bandwidth limitation of electronic devices in coherent optical system, different from the mature research in direct detection system. In fact, this bandwidth limitation will also affect the performance of coherent optical system in two aspects. One is that the limitation will remove the additive noise outside this bandwidth; the other is that it would reduce the power of the signal and distort the signal due to the generation of ISI. In order to get better performance, it is necessary to find how the limitation affects the performance of the coherent QPSK receiver and to realize a reasonable bandwidth.This thesis introduces the basic theory of coherent optical communication system, including the development, basic structure and advantages. Then the effects of the limited bandwidth on the performance of direct detection receiver are analyzed. A function between the bandwidth and BER is obtained and verified by simulation.Secondly, the model of the transmitted signal in coherent receiver is given without consideration of the bandwidth limitation. Two phase estimation algorithms, M-th power and DAML, are introduced based on the characteristic of laser phase noise. The simulation results of the derived BER show that the additive noise deteriorates BER performance seriously. Therefore, it is very necessary to discuss the bandwidth limitation of the receiver.Finally, the performance of coherent QPSK receiver in the presence of bandwidth limitation are further studied. Models of QPSK signal and coherent receiver with limited bandwidth are established to analyze the transmitted signal with ISI. When considering ISI from the previous symbol only, original QPSK constellation points shift into a similar distribution of 16 QAM. Then the phase references of M-th power and DAML algorithms are re-estimated with consideration of ISI and got the corresponding BER. Monte-Carlo simulations show that the performance of M-th power algorithm has no change even if ISI is considered. However, the performance of DAML algorithm would deteriorate by ISI when OSNR > 10 dB. Furthermore, 0.7Rs ~ 0.9Rs is proved to be a reasonable bandwidth for both phase estimation algorithms and different OSNRs.