Research On Channel Estimation Technologies In Optical OFDM Transmissions

Orthogonal frequency division multiplexing (OFDM) has been emerged tononlinearities. Based on the principle of detection, there are two variantsof optical OFDM systems: coherent optical OFDM (CO-OFDM) anddirect detection optical OFDM (DDO-OFDM). For the reason of low cost,easily implementation and could take advantages of well developed digitalsignal processing (DSP) technology, DDO-OFDM has been the mostpromising candidate for the next generation high speed communicationtechnology. Among the technologies in DDO-OFDM, as the assurance ofcorrect demodulation, channel estimation is one of the key technologieswhich featured DDO-OFDM the capability of electronically equalizationvia utilizing DSP techniques to get crucial information of the transmissionchannel and enables efficient digital compensation of various transmissionimpairments.In the thesis, several theoretical and experimental studies are carriedout on the channel estimation in optical OFDM transmission as follows：(1) Pilot patterns are experimentally investigated to characterize thefrequency/time dependence of the OFDM signal transmission. The opticalsignal performance is evaluated in terms of error vector magnitude (EVM).It is shown that, the quality of the OFDM signals can be improved up to4dB in EVM, for10Gb/s,16-QAM-encoded OFDM signals after20km single mode fiber (SMF) transmission with intensity-modulation anddirect-detection (IMDD). The best performance is obtained by applingpilot tones for all subcarriers, which implies that the optical OFDMtransmission is relatively quasi-static with respect to the subcarrierfrequencies. The noise of the OFDM signals originated mainly from theamplitude/phase fluctuation of optical signal with time.(2) Based on the idea of decision feedback, we experimentallydemonstrate a novel data-aided phase equalizer which takes advantages ofoptical channel quasi-static properties thus could adaptively compensatethe phase noise for DDO-OFDM transmission. The bit-error rate (BER) of10-3could be achieved by7dBm less received optical power and thereforecorroborates its superior performance.(3) Based on the idea of constellation distributuion analysis andcombined with the CD properties in fiber channel, we propose a novelapproach to compensate the fiber CD and to adjust the phase/amplitudeusing two pilot tones. Instead of using one or more symbols as the pilottones in the conventional channel estimation methods, the proposed novelmethod improves the performance of the OFDM signal, while using lesspilot tones. The simulations show a significant improvement (~2dB) of theoptical OFDM signal in terms of the bit error rate (BER) penalty comparedwith the conventional least-square (LS) linear interpolation andone-reference-symbol methods. Besides, for short distance opticalOFDM transmisstion, we propose a zero-overhead amplitude and phaseequalization scheme, which shows an improvement (~3dB) of the receivedOFDM signal in terms of the EVM compared with conventionalpilot-aided methods.