Research On LDPC Codes For Coherent Optical OFDM System

With the fast pace of development of the Internet and information technology, thedemand of the bandwidth for the present communication service and the fourthgeneration(4G) entered into commercial process in2014is greatly increasing. Large capacity,ultra high speed, long distance and lower cost of optical fiber communication has beenmoving towards practical application stage from theoretical research stage gradually, and ithas made remarkable achievements. Coherent optical orthogonal frequency divisionmultiplexing (CO-OFDM) technology is showing its outstanding properties constantly, and ithas become an integral part of the next generation of optical network.Nonetheless, with the transmission rate of CO-OFDM system increasing, people alsopresent more and more requirement for the stability and reliability quality of the transmissionprocess. Channel coding is an effective method to improve the reliability quality of thetransmission system, thereinto, the low density parity check (LDPC) codes has become a hotresearch topic in the field of channel coding in recent years as an important referentialencoding method, which has sparse check matrix, low computational complexity of the codestructure, encoding and decoding, and the powerful error correction performance.Quasi-cyclic LDPC(QC-LDPC) codes has been applied to the IEEE802.16e standard andsome other communication standards widely because of its code length and code ratechoosing flexible, low encoding complexity, structure method which is easy to achieve andsome other advantages. In this dissertation, I have studied CO-OFDM system with LDPCcodes from two aspects of theory and experiment. The main research contents are as follow:1. First of all, it introduces the correlation theory of CO-OFDM system, including thekey technologies of OFDM transmission, the characteristics of optical fiber transmission, thetheory of coherent reception and the composition of CO-OFDM system. Then it introducesthe definition of LDPC codes, the representation method of LDPC codes and several kinds ofmethod to construct the check matrix of LDPC codes. Finally, it summarizes the main methodof encoding and decoding for LDPC codes.2. This dissertation presents a kind of improved QC-LDPC codes, the check matrixs ofthis improved QC-LDPC codes have a quasi dual-diagonal structure similar to QC-LDPC inIEEE802.16e standard. By comparing the coding complexity of improved QC-LDPC codescoding algorithm with the coding complexity of standard coding algorithm of LDPC codes,LU decomposition coding algorithm and approximate lower triangular coding algorithm, weanalysis that the improved QC-LDPC codes coding algorithm can reduce the encoding complexity effectively, and it can achieve complete linear encoding. A simulation withimproved QC-LDPC codes in the wireless OFDM system is built, the improved QC-LDPCcodes are simulated respectively in AWGN and the HiperLAN/29-path fading channel, andthe BER Performance is analyzed. These works lay a solid foundation for both the laterresearch on CO-OFDM system with LDPC codes and the corresponding simulation analysis.3. This dissertation structure simulation platform for CO-OFDM system by using bothMatlab and Optisystem. The simulation results show that BER of the CO-OFDM systempresents a rising trend as the increase of the added noise in optical fiber link with a fixed fiberlength. And BER of the CO-OFDM system also presents a rising trend as the increase oflength of optical fiber with a fixed added noise. Above all, BER performance of the improvedQC-LDPC codes is better than that of IEEE802.16e standard LDPC codes.