Research On Sampling Clock Offset Compensation Algorithms In Optical Fiber NHS-OFDM Transmission Systems

With the rapid development of network information technologies such as the Internet of Thing(IOT)and Fifth-generation(5G)mobile networks,people's requirements for information transmission rates are getting higher and higher.In order to meet people's increasing bandwidth requirements for access networks,orthogonal frequency division multiplexing(OFDM)has been considered as one of the most promising modulation format for the next generation passive optical network(PON),due to its high spectral efficiency,strong resistance to multipath fading and great resistance to optical fiber dispersions.As a special modulation technology,OFDM has been widely used in traditional wired and wireless communication systems.However,OFDM-based PON system has many problems,such as the sampling clock frequency offset(SFO)between digital-to-analog converters and analog-to-digital converters always exists in asynchronous direct-detection OFDM system.Non-Hermitian symmetric OFDM(NHS-OFDM)can make full use of the fast Fourier transform(FFT)function than Hermitian symmetric OFDM(HS-OFDM).Hardware implementation of NHS-OFDM has a low complexity under maintaining the same bandwidth granularity.Therefore,NHS-OFDM technology is more suitable for cost-sensitive PON system.However,similar to the HS-OFDM system,the optical NHS-OFDM transmission system is also very sensitive to SFO,which seriously affects the performance of the receiver.This paper focuses on NHS-OFDM system and studies the efficient SFO compensation algorithm based on digital signal processing method.The specific research content and results of this paper are as follows:Firstly,the SFO compensation scheme based on inter-symbol differential decoding(ISDD)combined with residual phase compensation(RPC)algorithm is investigated in the NHS-OFDM system.On this basis,an enhanced residual phase estimation algorithm is also proposed to provide more accurate residual phase estimation.The bit error rate(BER)performance of ISDD-based SFO compensation is investigated by both numerical simulation and offline experiment.The results exhibited that the ISDD with RPC algorithm can well compensate SFO-induced phase rotations.Moreover,the BER performance can be further improved with the help of ISDD with the enhanced residual phase compensation(ERPC)algorithm.A similar BER performance can also be observed for the HS-OFDM and NHS-OFDM.Furthermore,up to 200 ppm of SFO can be compensated by using the ISDD with the ERPC algorithm compared to the SFO-free case,and the BER performance penalty is negligible.Secondly,the SFO compensation scheme based on precoding and digital interpolation(DI)technology is investigated in the NHS-OFDM system.We experimentally demonstrated the 4-th order DI algorithm to compensate SFO in the NHS-OFDM system with intensity modulation and direct detection.Besides,the orthogonal cyclic matrix transform(OCT)precoding is applied to further improve the BER performance.The experimental results indicate that,with the help of the 4-th order DI algorithm,the SFO up to 200 ppm can be well compensated without power penalties in comparison with the SFO-free case.Furthermore,the receiver sensitivity of the NHS-OFDM enabled by OCT precoding can be improved by more than 2 d B at the BER of 1í10^-3 after 20 km standard single-mode fiber transmission.