Research On Optimization Algorithms For Transmission Performance Of Optical OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) belongs to abroader class of frequency-division multiplexing (FDM). It ischaracterized by mutually orthogonal sub-carriers. The modulatedfrequency spectrum of each sub-channel can overlap to decrease guardinterval and improve spectrum utilization. After discrete Fourier transform(DFT) was used to achieve multi-carrier modulation, the complexity of thesystem was reduced, so OFDM technology has been developed rapidly.OFDM was brought into optical communications in2005. And opticalOFDM has become one of the promising multiplexing technologies foroptical fiber communications due to its advantages of robustness againstchromatic dispersion (CD) and polarization mode dispersion (PMD).In the thesis, the transmission performance of optical OFDM systemsis optimized via several key technologies including symbolsynchronization, channel estimation, and bit and power loading. Symbolsynchronization is aimed to determine the starting position of each OFDMsymbol, which is the guarantee of demodulation accuracy at receiver.Channel estimation obtains the crucial information of the transmissionchannel to enable compensation of various transmission impairments. Bitand power loading improves the bit rate in the premise of goodtransmission quality via adjusting modulation format and transmittedpower. In the thesis, several theoretical and experimental studies arecarried out on the above technologies as follows:Firstly, a new pilot-aided symbol synchronization algorithm isproposed. There is a non-zero part in the training sequence in time domain, and at receiver the symbol synchronization can be obtained by finding outthe special non-zero part in the training sequence. The computationcomplexity is reduced in the premise of high accuracy. The superiorperformance is demonstrated via both simulation and experiment, wherethe synchronization is accurate even in the relatively bad channel.Secondly, a novel channel estimation method based on decisionfeedback is proposed. For different formation reasons of amplitudeattenuation and phase noise (PN), the amplitude and phase are equalizedrespectively. The method prevents the error propagation via pilot symbol.The performance of the scheme is corroborated in terms of error vectormagnitude (EVM) of received signals after transmission over20km ofsingle-mode fiber (SMF). The optimum averaging numbers applied in themethod are discussed via simulation to reduce computation complexity.At last, the scheme aimed to optimize the bit rate of optical OFDMsystems is proposed. According to the equivalent channel property ofoptical OFDM systems, the modulation format and transmitted power areadjusted to improve the transmission efficiency. LC algorithm is improvedvia multiple assignments to expand the application scope. Water fillingalgorithm is improved via threshold. The performance is evaluated andcompared via simulation in terms of transmitted bit rate in the premise ofachieving symbol error ratio (SER) target.In this thesis, the performance of optical OFDM systems is optimizedin terms of above three key technologies, including symbolsynchronization, channel estimation and bit and power loading. Thetheoretical and experimental studies are carried out. It is aimed toguarantee the transmission quality at high bit rate.