Research On Equalization Technology For Optical OFDM Transmission System

With the development of digital multimedia technology, optical orthogonal frequency division multiplexing (O-OFDM) transmission system has been widely researched due to its high rate and large capacity. Since the impairments, suffered from dispersion, optical devices and inter-symbol interference (ISI) of OFDM signal in optical OFDM communication system, would result in a serious signal distortion, the equalization technology which is mature and low-cost is worthy to be applied in optical OFDM transmission at the receiver. In this paper, the performance of adaptive equalization was investigated in direct-detection optical OFDM transmission system and radio-over-fiber OFDM (ROF-OFDM) transmission system by simulation and experiment. Specifically including:Firstly, the frequency domain equalization using pilot for OFDM signal was theoretically discussed. Then the channel response of open-loop equalizer was deduced in detail, which is based on least square (LS) in frequency domain. And a closed-loop equalizer, based on least mean square (LMS) in frequency domain, was proposed for optical OFDM transmission system. LMS algorithm has the same complexity with LS algorithm and uses a stochastic gradient to update filter coefficient in a recursive computation of equalizer. Thus, LMS equalizer can effectively decrease the influence of noise and track changes of channels.Secondly, it is investigated that how the OFDM signals transmit in direct-detection optical communication system and what influence have suffered from dispersion. The simulation had been implemented as optical OFDM signals transmitting over100km single mode fiber (SMF) with variable dispersion. The error vector magnitude (EVM) of system using LMS equalizer is smaller than that using LS equalizer by3%on average. The experimental results show that the power penalty of system using LMS equalizer is2dB less than that using LS equalizer on average when the OFDM signals transmitting back to back, and2.5dB less than that when they transmitting over100km standard single mode fiber (SSMF). It is proved that the system using LMS equalizer has a better performance to restore signals than that using LS equalizer.Finally, the performance of equalization in40GHz and60GHz ROF-OFDM system had been researched and analyzed by simulation. The results indicate that the phase of OFDM signal without equalization has been great deflected and the amplitude suffers from serious attenuation, so that we can’t correctly decide signal. The equalizer based on LS algorithm can improve the system performance only when OOFDM signal travels a short distance. Compared to LS equalizer, LMS equalizer has a higher quality to improve the system performance.The simulation and experiment investigations show that the system, using closed-loop adaptive equalizer based on LMS algorithm in frequency domain, can resist chromatic dispersion and decrease the ISI of OFMD signal effectively. Owing to its low complexity, LMS equalizer can be applied to process real-time O-OFDM signal.