Research On Channel Estimation Algorithm Of Direct Detection Optical OFDM System

The optical orthogonal frequency division multiplexing (O-OFDM) technology hasreceived abroad attention and research in optical communications due to its high spectrumefficiency, strong anti-interference ability and superior resilience against fiber dispersion. Theoptical OFDM transmission system based on direct detection has been popularized andapplied in backbone optical transmission network for the advantages of low cost and simplestructure. This paper mainly focuses on the effect of frequency selective fading (FF) and otherinterference noise during the signal processing and optical transmission. The main researchresults conclude as follows:1. The joint time-frequency channel estimation algorithm is proposed to compensatethe impact of channel distortion. Pilot symbols is generally inserted in frequency-domain ofOFDM signal, the sub-channels frequency responses obtained by using interpolationalgorithm can only compensate the influence of channel distortion in frequency-domain whilethe time-domain adjacent correlation is not taken into consideration. To improve the accuracyof channel estimation, a joint time-frequency channel estimation method is employed. Thesimulation and experimental results verify the feasibility of the proposed method. Theexperimental results show that the receiver’s power penalty of a6.18-Gb/s OFDM systemwith using the joint channel estimation algorithm can reduce0.8dB at a BER of10-3whencompared with that of conventional OFDM system with using the conventional channelestimation algorithm.2. The enhanced Fourth-Power algorithm for phase estimation with frequencyseparation is put forward to estimate and compensate the varying distortions of OFDMsubcarriers at different frequencies caused by FF. Impaired by optical chromaticdispersion and electronic devices such as electric filter in digital signal processing, OFDMsignal is degraded by FF effect which generally shows the attenuation influence onhigh-frequency of OFDM signal. Affected by FF, the phase rotation of OFDM subcarriersvary at different frequencies. Therefore, an enhanced phase estimation method with frequencyseparation is proposed in this paper to estimate and compensate the phase rotation of all thesubcarriers at different frequencies. The experimental results demonstrate that the phaserotation of high-frequency, intermediate-frequency and low-frequency of OFDM signal canbe basically corrected after using the proposed phase estimation algorithm, and a4-Gb/sOFDM system with using the enhanced Fourth-Power algorithm has the receiver sensitivity of 1.8dB better than that of regular OFDM system without using phase estimation algorithmafter100km SSMF (Standard Single Mode Fiber) transmission at a BER of10-4.