Research On Blind Equalization Techniques For Optical OFDM Systems

With the development of digital multimedia technology, people become moreurgent to communication quality demand. How to realize information transmissionwith large capacity, high speed and high quality at limited bandwidth draws greatattention of researchers. Optical orthogonal frequency division multiplexing (O-OFDM)transmission system has been widely researched due to its flexible spectrum allocation,high spectrum efficiency and anti-interference ability. Since the impairments,suffered from chromatic dispersion (CD), polarization mode dispersion (PMD) andinter-symbol interference (ISI) of OFDM signal in O-OFDM communication systemswould result in a serious signal amplitude distortion and phase deviation, the adaptiveblind equalization technology which is mature and low-cost is worthy to be applied inO-OFDM transmission at the receiver. The O-OFDM system based on the directdetection is very easy to be implemented and has the merits of low cost and stabilityoperation. In this paper, the performance of adaptive blind equalization wasinvestigated in direct-detection optical OFDM (DDO-OFDM) transmission system bysimulation and experiment. The main research results include:Firstly, it is investigated that how the OFDM signals transmit in direct-detectionoptical communication system without training sequence and pilot, and whatinfluence have suffered from CD, PMD and ISI of OFDM signal. An intensitymodulation (IM) DD-OOFDM without training sequence and pilot experimentalsystem based on standard single-mode optical fiber (SSMF) is established. Then，thesystem’s common blind equalization algorithms are described and performance ofthese algorithms evaluation criteria in OFDM systems are also discussed.Secondly, in order to reduce frequency selective fading(FF) of the O-OFDMsignal due to CD in DDO-OFDM system without training sequence and pilot, a blindequalization method based on separate frequency modified constant modulusalgorithm (SF-MCMA) is proposed. The experimental results show that the powerpenalty of system using SF-MCMA equalization is2dB less at10e-4BER than that noequalization when the OFDM signals transmitting over100km SSMF. The SF-MCMAequalization method can be effectively corrected OFDM signal amplitude distortionand phase offset. It is proved that the method is good at dispersion compensation andthe system using SF-MCMA equalizer has a good performance to restore signals. Finally, the effects of the PMD to optical fiber communications systems aresimply analyzed, and the compensation technologies of the PMD to O-OFDM systemsare generalized. By introducing multi-input multi-output (MIMO) structures inO-OFDM systems, O-OFDM transmission systems with the PMD can be substantiallyequivalent to a1×2single-input dual-output (SITO)-OFDM channel. Based on theseresearch, the imitate model of the optical communication system with the PMD(PMD-OOFDM) is built and sdutied, and a blind equalization method based onconstant modulus algorithm (PMD-CMA) is proposed to eliminate the PMD. Thesimulation results show that the PMD-CMA equalization method is suitable forPMD-OOFDM system, which can overcome the polarization effects. ThePMD-OFDM systems using the PMD-CMA blind equalization method have bettercompensation performance.