Research On The Related Digital Signal Processing Techniques For Short Reach Optical Access Networks Based On OFDM

With the continuous improvement of people's demand for information technology,the upgrading of optical access network architecture and improvement of the technology have become an inevitable trend.Passive optical network(PON)has been regarded as main stream of optical access network,and its system bandwidth,spectrum efficiency,transmission rate and security have been put forward higher request.The combination of the direct detection optical orthogonal frequency division multiplexing(DDO-OFDM)technology with the advantages of high frequency spectrum efficiency(SE)and low cost with passive optical network has been regarded as a strong candidate for the key technology of NG-PON,and has attracted widespread attention from the industry.In this paper,based on the theoretical analysis of various impairment existing in DDO-OFDM,the digital signal processing(DSP)technology is used to further improve the performance and safety of communication system.The main contents of this paper and innovation research effort are summarized as follows:Firstly,an optical line terminal(OLT)-centralized adaptive sampling frequency offset(SFO)scheme has been proposed which verified in point-to-multipoint OFDM-PON system.The phase offset of the data symbol on the sub-carriers of the detection signal can estimated by utilizin g least square method,which reduces the complexity of the optical network unit(ONU)architecture and does not reduce the spectrum efficiency of the signal.In addition,by designing the structure of the OFDM frame properly,the SFO induced inter symbol interference(ISI)can be avoided and the computational complexity of the system can be reduced.The experimental results show that,when received Optical power is fix at-8 d Bm,14.29 Gps 16 QAM OFDM signals after 20 km standard single-mode fiber transmission,the SFO up to±200ppm in four ONUs can be effectively compensated and the SFO estimation accuracy can be within±1 ppm.Secondly,a sampling frequency offset compensation scheme based on circular amplitude phase shift keying(APSK)and inter-symbol differential detection(ISDD)is proposed.By using this SFO compensation scheme,without any pilot assistance to reduce the signal frequency efficiency,do not need to estimate the sampling frequency offset,so as to avoid the influence of residual sampling frequency deviation.In addition,this paper uses the sideband phase compensation technology to compensate the algorithm induced sideband phase.However,the algorithm induced phase noise will seriously degrade the performance of transmission system with the increase of signal modulation order.On this basis,this paper proposes a 64 point amplitude and phase shift keying signal modulation scheme,which is robust to phase noise.It is verified by simulation in additive white Gaussian noise(AWGN)channel and direct detection optical fiber transmission system.The simulation results show that,when SFO up to±200 ppm,the employed 64 APSK constellation scheme outperform the conventional 64 QAM modulation format in the presence of the ISDD-induced phase noise.By using the proposed 64 APSK constellation scheme,the SNR performance can be improved by up to 3 d B when the BER is 1E-3 in AWGN channel.Besides,up to 2 d B improvement in the receiver sensitivity in terms of received optical power can be also achieved,compared to conventional 64 QAM modulation format.Thirdly,a scheme of one-dimensional chaotic Chebyshev sequence mapping combined with RSA public key algorithm is proposed and verified by simulation in 60GHz fiber-wireless(Fi-Wi)optical communication architecture.The simulation results show that the using of LDPC that is forward error correction channel coding improves the transmission performance of the system.After 30 km SSMF fiber transmission,the 64 QAM OFDM signal can be can effectively decrypt into the original data stream by using the joint key.Fourthly,a physical layer encryption scheme based on chaos sequence for OFDM-PON is proposed by using Henon mapping to control row-column index of Hadamard precoding matrix.In order to overcome the serious phenomenon of high frequency subcarrier fading in optical communication system,precoding technology based on Hadamard matrix has been used to realize the sharing of noise on subcarriers.In addition,owning to characteristics the correlation betw een the Hadamard matrix and matrix after row-column transformation are not change,the encrypted Hadamard matrix can controlled by chaotic sequence which is generated and verified in OFDM-PON.Experimental results show that the encryption scheme has a lar ge key space(10¹⁷⁸).When the BER is 1E-3,the sensitivity of received optical power of OB2B case and after 20 km single-mode fiber transmission case,there is up to 2.3 d B and 1.2 d B improvement of encrypted OFDM signals,compared to conventional OFDM signals.