Access Network Study Based On Optical OFDM System

The Internet has become more and more popular during people’s daily life, which brings a significant and revolutionary change in work and life. With only a computer and the Internet access service, we can get in touch with the world news, airline tickets reservation and video chat with friends and relatives, etc. People cannot live without the convenience brought by the Internet. However, with the rapid growing service like high definition (HD) film, Interactive game and multimedia service, etc. we need a Internet with faster speed and much more quality guaranteed. With such background, the next generation high-speed optical access network technology is becoming a research hotspot. Especially with the superior performance, the optical OFDM (Optical Orthogonal Frequency Division Multiplexing) PON (Passive Optical Network) has gained many attentions. Due to the orthogonality of each OFDM subcarrier, the spectrum efficiency can be increased significantly. The dispersion effect can also be overcome at the same time. Hence, optical OFDM technology has become the research emphasis of the next generation optical access network standard. This paper starts with the integration between OFDM and PON technology. The main research contents are listed as follows:1) Optical OFDM system structure and upstream solution selectionThis part mainly contains the generation and coherent reception of optical OFDM signal. Use limited bandwidth A/D and D/A convertor with polarization multiplexing and multistep subcarriers to generate the high-speed optical OFDM signal. By comparing the OFDMA access method with OFDM+TDMA access method, this paper analyze the difficulty in increasing the upstream data rate and choose OFDMA method as the upstream method of optical OFDM system based on the technology realization and simplification.2) Otical OFDM system simulation and results analysisThis part mainly contains the simulation for both the upstream and downstream of the optical OFDM system by using the optical communication simulation software VPI. Based on the built double-sideband suppression upstream system, optimum system parameters are found by comparing the relationship between OSNR (Optical Signal to Noise Ratio), modulated signal intensity, laser spectrum line width, frequency offset and system performance. Based on the built downstream system, optimum system parameters are found by comparing anti-noise performance under different modulation method and system performance brought by laser spectrum line width, optical fiber attenuation, optical modulation and AD/DA convertor.3) Optical OFDM system experimental platform verification and performance improvementThis part mainly contains the experimental platform verification of optical OFDM system based on the transmitting end (OLT) and the receiving end (ONU). The proposed system performance has been verified. The experiment results analysis are conducted based on system BER (Bit Error Rate), system anti-noise performance under different modulation pattern. The system performance results under different influence factors including optical injection power, laser spectrum line width, transmission distance and wavelength interval of upstream laser are analyzed and compared. After fully understand the system performance and parameters of optical OFDM system, this paper proposes the frequency offset compensation method based on FFT (Fast Fourier Transform) and the PAPR (Peak To Average Power Ratio) suppression method to improve the optical OFDM system performance.