Research On The Performance Of DDO-OFDM Transmission System With The Training Sequence And Pre-distortion Technique

Orthogonal frequency-division multiplexing （OFDM） as a technology thatenables robust transmission over both wireless and optical channels has already beenemployed in many communication standards. Optical Orthogonal frequency-divisionmultiplexing merges the Orthogonal frequency-division multiplexing technology inwireless with the optical fiber communication technology. It has the advantages ofanti-interference ability and high spectrum efficiency. The optical fiber transmissionsystem based on direct detection has obtained a wide range of applications andresearchs for the advantages of simple structure and lower costs. But there aresubcarrier-subcarrier mixing interference and frequency selective fading existing inDirect-Detection Optical OFDM system. This paper mainly focuses on how toovercome the effect of the subcarrier-subcarrier mixing interference and thefrequency selective fading in the system of direct detection optical OFDMtransmission, and then proposes two solutions:1. The technology of adaptive pre-distortion is employed to compensate theattenuation of the OFDM signal caused by the non-ideal frequency response ofoptic-electro device. In the proposed system, to compensate the performancedegradation caused by FF, the power of the OFDM subcarriers at the high frequency isadaptive pre-distorted before inverse fast fourier transform （IFFT） conversion. Wecompare the regular OFDM signal with the pre-distorted OFDM signal in adirect-detection optical orthogonal frequency-division multiplexing system. Theexperimental results show that the power of high frequency subcarriers of the regularOFDM signal is attenuated severely after200-km standard single-mode fiber （SMF）transmission, but the spectrum is relatively flat by pre-distorting the power of theOFDM subcarriers at the high frequency. In addition, the received sensitivity of thepre-distorted OFDM signal is significantly improved about3dB at a bit of error rateof1×10^-4compared to the regular one without this technique.2. The technology of cascaded variable coefficient training sequences andpre-distortion are employed to resist the influence of the OFDM signal caused bysubcarrier-subcarrier mixing interference and frequency selective fading. Thecascaded variable coefficient training sequences and pre-emphasized configuration ofO-OFDM signal is proposed to decrease the influence of the SSMI, FF and other nonlinear effects. We use two cascaded variable coefficient training sequences, andpre-distort the different sub-carriers of OFDM signal by pre-estimating theamplification factors of each sub-carriers in accordance with the fading of the signalin the optical link transmission system. We compare the performance of the cascadedvariable coefficient training sequences and pre-emphasized configuration of OFDMsignal and the interleaved configuration of training sequence of OFDM signal. Theexperimental results demonstrate that a5Gb/s cascaded variable coefficient trainingsequences and pre-emphasized configuration of OFDM signal has the receiversensitivity of3.5dB better than that of interleaved configuration of training sequenceof OFDM signal after100-km SMF transmission at any dispersion compensation anda BER of1×10^-4.