The Research On Adaptive Modulation Technique In Optical OFDM System

Nowadays, as the widespread use of a large number of intelligent and integrate services,network flow is growing explosively and the demands for bandwidth will be extremely tremendous.In the case of high-speed and large-capacity, the advantages of optical fiber communication areobvious to all. Multimode fiber (MMF) is widely used in short-distance optical communications forits advantages as follows: high efficiency on coupling with laser source, convenience for fiberconnecting operation, and low system cost. However, the mode dispersion exist in MMF will causethe inter-symbol interference (ISI), which restricts the bandwidth and transmission capacity. Relatedexperimental results demonstrate that Orthogonal Frequency Division Multiplexing (OFDM) canefficiently improve the system transmission performance.Firstly, the transmission characteristics of the multimode optical fiber are investigated. Resultsshow that there are several deep fading nulls in high frequency regions of MMF. Because OFDMcan overcome the effects of frequency-selective fading, it is suitable for the high frequency regionsof MMF. Then the paper introduces the OFDM system and its key technology, and builds theOFDM multimode fiber communication system with Matlab Simulink software.Based on the above model, the coding technique in the OFDM multmode fiber system-a lowdensity parity check (LDPC) code is studied. According to the configuration and the code decodingprinciples of LDPC code, the paper continues to design the LDPC encoder and the decoder module.However, the simulation result shows that simply use of OFDM technology can’t improve systemperformance greatly.To solve this problem, the adaptive modulation and coding technique is proposed in OFDMmultimode fiber systems. The main idea of the adaptive modulation and coding technique is thathigh modulation order with strong channel coding rate can be used to boost up the averagethroughput when the channel state is good, and low modulation order with weak channel codingrate can be used to increase error protection when the channel states is bad. By matching thevariable modulation level with variable channel coding rate, the performance of the system is highlyimproved.Finally, a new rapid-coded irregular quasi-cyclic LDPC codes is designed and its performanceis studied in the OFDM multimode fiber communication system.