Research On CO-OFDM System Based On Constant Envelope Modulation Technology

In recent years, with the continuous development of science and technology and the deeper and deeper degree of social information, the traditional optical backbone transmission network that uses intensity modulation/direct detection can not meet the increasing demand. Coherent detection optical orthogonal frequency division multiplexing (CO-OFDM) is a new kind of optical communication technology, which combines the advantages of coherent detection, optical communication technology and wireless OFDM technology. CO-OFDM not only has high spectrum efficiency to achieve high-speed transmission in a single channel, but also can effectively resist dispersion, polarization mode dispersion and other effects in optical fiber. Thus, CO-OFDM system, as a high speed, long distance and large capacity optical transmission system, gets great concern. Constant envelope modulation technique with its constant envelope advantage has been widely used in the traditional wireless communication systems, at the same time, it has a good effect on the inhibition of nonlinear effects, carrier frequency offset and phase noise in optical communication system. Therefore, this article combines both technologies and achieved some results through the following research.1. Research on the basic principles and key technologies of CO-OFDM system. The basic principles of MZ modulator and coherent detection in CO-OFDM technology were analyzed and a CO-OFDM system was built. After a detailed research on the transmitter, optical fiber and the receiver of the system, the following conclusions were drawn. First, in order to avoid the modulation depth of MZ modulator be too deep, the modulated signal must have a low PAPR. Second, in order to reduce the impact of phase noise and carrier frequency offset, the transmission signal should have good resistance to phase noise and carrier frequency offset. 2. Research on constant envelope modulation technology. This part starts from the basic principle of constant envelope modulation technology. Five kinds of non-continuous phase constant envelope modulation(BPSK, DPSK, QPSK, DQPSK, OQPSK) and three kinds of continuous phase constant envelope modulation(CPBFSK, MSK, GMSK) were introduced and the advantages and disadvantages of the two types of constant envelope modulation were compared. Simulations of these constant envelope modulations were done and after comparison and analysis of constellation, constellation trajectories, eye diagram and power spectral density map of them, the following conclusion were drawn. The power spectral density map of continuous phase constant envelope modulation has a concentrating power and a fast attenuating sidelobe, so it have good resistance to phase noise and carrier frequency offset.3. Research on CO-OFDM system based on constant envelope modulation. A CO-OFDM transmission system simulation platform based on constant envelope modulation was built by simulation software and the correctness of the simulation platform was verified by comparing the BER of the simulation platform to theoretical value of BER. The impact of nonlinear effects on the CO-OFDM system is analyzed theoretically, after comparing the bit error rate of CO-OFDM system using different modulation under different input power, it was found that CO-OFDM system based on MSK suffered little effect by nonlinear effects. The impact of carrier frequency offset on the CO-OFDM system is analyzed theoretically, after comparing the bit error rate of CO-OFDM system using different modulation under different carrier frequency offset and cyclic prefix length, it was found that CO-OFDM system based on MSK showing good resistance to carrier frequency offset. The impact of phase noise on the CO-OFDM system is analyzed theoretically, after comparing the bit error rate of CO-OFDM system using different modulation under different laser line-width, it was found that CO-OFDM system based on MSK was less sensitive to phase noise than CO-OFDM system based on phase modulation.