Study Of The Synchronization And Channel Estimation Technique Incoherent Optical Ofdm Communication System

In the era that information technology develop rapidly, as cloud computing and online data transmission technology become more and more popular, the requirement of data transfer rate become higher. In the field of wireless communication, 3G technology has been popular, 4G technology is going into the life of people, the basic research of 5G technology is being studied. In the field of wireless communication, OFDM(orthogonal frequency division multiplexing)has been widely used since its high frequency spectrum utilization. Through the study of optical communication, we found than OFDM technology is very robustness to fiber chromatic dispersion(CD) and polarization mode dispersion(PMD). So OFDM technology is widely attention in the field of optical fiber communication in recent years.In this paper, the synchronization and channel estimation technique of coherent optical OFDM communication system will be mainly studied. The working process of CO-OFDM system will be described as follows: In the transmitter, bit signal will be through the digital modulation(QPSK,QAM). Then the digital signal will be converted into electrical signal through ADC. The electrical signal will modulate the phase of optical signal through Mach-Zehnder modulate to make electrical signal up-convert to optical signal. At the receiver, the optical signal will be down-converted into electrical signal through mixer. Then the electrical signal will be demodulated to bit signal. CO-OFDM system is multi-carrier communication system. The sub-carrier is orthogonal to each other. So it’s very sensitive to frequency offset and phase noise. As its length of the symbol is much longer than single carrier communication system, to gain a complete symbol window, we must use symbol timing synchronization.In view of the above problems, the work of this paper will simulate the influence of symbol timing offset to the CO-OFDM system firstly. We found the constellation graph rotation will more serious as the length of the OFDM symbol become longer.Secondly, we found that the constellation graph will rotate when the laser frequency difference between transmitter and receiver is integer times of the sub-carriers’ frequency difference. If the frequency difference is decimal times of the sub-carriers’, the constellation graph will become very mess as the difference become bigger. This will lead that the received signal can’t be correct detection. At last, we simulate the influence of phase noise which is from laser’s linewidth and chromatic dispersion from fiber channel to the CO-OFDM system.The main work of this paper is the CO-OFDM system simulation platform whichis the joint of VPI optical simulate software and MATLAB. In this platform, we use algorithm which is based on cyclic prefix to study the symbol timing synchronization. Then we use algorithm which is based on training symbol to estimate the frequency offset. At last, we use LS algorithm which is based on the frequency comb pilot to recover the influence of chromatic dispersion from fiber channel and compensate the phase noise. Through the results of the simulation, the algorithms which are studied in this paper can recover the signal well and make the CO-OFDM performance better. Meanwhile, the simulation platform in this paper improves the coding efficiency and is more convenient and integrity than the simulation which is only use MATLAB. And it’s also more flexible as it can use different algorithm than the simulation which is only use VPI. The work of this paper will have some influence to the related study of CO-OFDM study.