O-OFDM System Spectra Efficiency Improvement Research

Orthogonal frequency division multiplexing (OFDM) is a highly developed wireless communication technology. Recently it's been widely researched in optical communication area, and considered as one of the most promising technology for next generation high-speed optical communication systems. Optical OFDM (O-OFDM) has a lot of advantages such as high spectra efficiency, high tolerance to chromatic dispersion (CD) and polarization mode dispersion (PMD), flexible signal modulation and processing.This paper focuses on the research of O-OFDM system spectra efficiency improvement from two different aspects. Firstly we adopted balance receiver in the intensity-modulated directly-detected O-OFDM (IMDD-OOFDM) system to improve its spectra efficiency. This scheme can eliminate the inter-modulation distortion (IMD) noise due to square-law detection of photodiode (PD) effectively, so the spectrum gap between optical carrier and signal can be reduced. We improved the spectra efficiency of traditional Offset-SSB OFDM system by 62.1%. The spectra efficiency can be improved further by employing more complex modulation format than 4QAM.Secondly, to break-through the speed bottleneck of electrical DAC/ADC, we proposed an all-optical sampling OFDM (AOS-OFDM) scheme adopting FBG, and cyclic prefix (CP) was introduced in our system to overcome the CD of fiber link. We also employed subcarrier initial phase pre-emphasis technology to restrain the constructive interference among pulses between neighboring symbols, which can increase the transmission distance to 30km theoretically. By employing our proposed scheme, we realized a real-time AOS-OFDM system with 50Gbit/s data rate and 200GSample/s sampling rate.