Research On The Performance Of DDO-OFDM Transmission System With The Technology Of Encoding And Pre-emphasis

Due to the high spectrum efficiency and anti-interference ability, theoptical OFDM is considered to be one of the core technologies of4G. Theoptical fiber transmission system based on direct detection has become themain structure of backbone optical transmission network for theadvantages of si mple structure and lower costs, and gets a lot of studiesand applications. The system of direct detection optical OFDMtransmission can provide the majority of the information services wit hhigh-speed, long distance and high-quality for people, and so researchingon the system is of great practical value and practical significance. Thispaper mainly focuses on how to overcome the effect of thesubcarrier-subcarrier mixing interference and the frequency selectivefading in the system of direct detection optical OFDM transmission, an dthen proposes two solutions:1、 The long irregular LDPC code acts as the channel error correctioncoding to resist the impact of the subcarrier-subcarrier interferenc e.Because of the parity check matrix is sparse, the number of informationbits joining in the same parity equation is sma ll and they are dispersed,making the encoding can resist the continuous burst errors. We compare dthe transmission performance of LDPC coded OFDM signals with threedifferent rates in the long-distance transmission through the numericalsimulation. Simulation results show that the coded modulation technique oflong irregular LDPC coded OFDM with0.75coding rate can effectivelyovercome the subcarrier-subcarrier of the OFDM signal and is the mosteffective for long-distance single mode fiber transmission.2、 The LDPC coded OFDM is employed to overcome the bandwidthlimitation of the graded-index multi-mode fiber (GI-MMF). The signaltransmission over GI-MMF with2.5Gb/s or above is severely constrainteddue to the bandwidth limit, and longer transmission distance, moreattenuation. The simulation that10Gb/s LDPC code OFDM signaltransmitted900m GI-MMF without error show the long irregular LDPCcoding can effectively overcome the dispersion and the mode bandwidthlimitation of GI-MMF. 3、The technology of adaptive pre-emphasis is employed to compensatethe attenuation of the OFDM signal caused by the frequency selectivefading. In the proposed system, to compensate the performance degradationcaused by SSMI, FF and other nonlinear effects, the power of the OFDMsubcarriers at the high frequency is adaptive pre-emphasized befor edigital-to-analog (DA) conversion. We comp ared the performance ofregular OFDM and two kinds of pre-emphasized OFDM signals withsimulation on QPSK-OFDM and16QAM-OFDM baseband transmissionsystem, respectively. Lastly, We experimentally demo nstrate that a2.5-Gb/s pre-emphasized QPSK-OFDM system has the receiver sensitivit yof2dB better than that of the regular intensity-modulation BB-OFDMafter100km SMF transmission at a BER of1×10-4.