Optimization Of Optical Orthogonal Frequency Division Multiplexing System And Its Signal To Noise Ratio Monitoring Technique

Ever since the advent of internet technology,the development of communication industry in human society has been promoted by the improvement of production efficiency and the transition of lifestyle.People's endless desire for internet traffic has been aroused by the widespread deployment and application of emerging communication technology and internet service,while how to improve the transmission capacity has always been one of the most popular issues in communication systems.Multiplexing technique such as polarization division multiplexing,wavelength division multiplexing and space division multiplexing can be used to multiply the transmission capacity in optical fiber communication systems,while how to avoid wasting the limited spectrum resources thus improve the spectral efficiency is the enternal persuit in the field of communication research.Orthogonal frequency division multiplexing has already gained the favor of communication industry and been widely deployed and applied in 4G network due to high spectral efficiency,high tolerance to chromatic dispersion and flexible coding.However,the relatively high peak to average power ratio,sensitivity to synchronization errors and digital signal processing complexity are still hampering its application in optical communication.Aiming at solving problems of orthogonal frequency division multiplexing technique in optical fiber communication systems,this dissertation proposed a number of optimization schemes and implementation approaches based on the current modulation/demodulation algorithms,and subsequently conducted simulations and experiments to verify the proposed solutions.Futhermore,we accomplished optical signal to noise ratio monitoring of optical orthogonal frequency division multiplexing systems.The major research contents and innovative achievements of this thesis are summarized as follows:(1)To deal with the high peak to average power issue in orthogonal frequency division multiplexing system,a low complexity linear precoding algorithm was proposed,of which the precoding matrix is generated from a constant amplitude zero autocorrelation sequence.Simulations results indicate that this precoding schem can help reduce the peak to average power ratio of high modulation format like QPSK and 16 QAM based orthogonal frequency division multiplexing signals and improve the transmission performance.It has been proved experimently the receiver sensitivity of direct detection systems and transmission distance of coherent detection systems can be improved with the assist of this algorithm.As for the issue of sensitivity to synchronization errors and digital signal processing complexity,we proposed a sparse fast Fourier transform based quick synchronization algorithm for orthogonal frequency division system.Through simulations and experiments,our proposed quick synchronization algorithm was demonstrated with excellent performance in orthogonal frequency division multiplexing systems,computation complexity of digital signal processing can be greatly reduced while meeting the synchronization accuracy requirements in orthogonal frequency division multiplexing systems.(2)In order to overcome the inherent defects of orthogonal frequency division multiplexing technique,we carried out research on the non-orthogonal multiplexing technique based on optical orthogonal frequency division multiplexing system.Filter bank multi-carrier technique was employed to filter and reshape each subcarrier,reducing spectrum leakage,thus releasing the strigent orthogonal condition.With the absence of cyclic prefix,spectral efficiency can be improved.Linear precoding algorithm above was also introduced to improve the transmission performance of a filter bank multi-carrier system.Simulation and experiment showed a receiver sensitivity improvement of this precoding assisted filter bank multi-carrier system.In addition,power division multiplexing is employed to break the constraints of zero interference in orthogonal systems and reuse the limited spectrum resouces.Two branches of orthogonal frequency division multiplexing signals in the same frequency band with same transmission rate were overlaid together with different power levels at the transmitter side and separated utilizing successive interference cancelling algorithm at the receiver side.Simulation and experiment results show that power division multiplexing technique is compatible with current multiplexing techniques such as polarization division multiplexing,wavelength division multiplexing and orthogonal frequency division multiplexing,thus improve the spectral efficiency.(3)Research of optical signal to noise ratio monitoring technique is indispensable in the application of orthogonal frequency division multiplexing technique in optical communication systems.To achieve OSNR monitoring for both polarization tributaries of polarization division multiplexing signal,a coherent receiver was used to transform OSNR monitoring into ESNR monitoring,which is easy to implement using digital signal processing technique.We proposed a frequency domain correlation method based on the training sequence used for synchronization,signal and noise was separated to compute the signal to noise ratio utilizing the uncorrelation property of the known training sequence and noise.There is no need to demodulate the entire signal or introduce extra overhead to sacrifice the spectra efficiency.However,a flat frequency response in a specific band is required for this method,which is often not the case in time varing systems.To deal with this issue,we proposed a time domain correlation method based on the training sequence used for synchronization.A butterfly filter was used to separate signal and noise in time domain directly.Channel estimation and polarization demultiplexing were avoided,which makes it a low complexity method not affected by frequency response.These research contents and achievements above are expected to provide technical supports for the application of orthogonal frequency division multiplexing technique in optical fiber transmission systems.