| With the rapid development of High Definition Television(HDTV),smart handheld terminals,big data storage and cloud computing etc.,the data traffic in fixed and mobile access network grows at a Compound Annual Growth Rate(CAGR)of 50 percent and 57 percent,respectively.According to the Cisco forecast about the global mobile data traffic from 2014 to 2019,monthly global mobile data traffic will surpass 24.3 exabytes by 2019.And this number is almost 10 times than the global mobile traffic per month in 2014.In order to address these huge data traffic,ITU Telecommunication Standardization Sector and IEEE 802.3 Ethernet Working Group has also started next generation 40-Gb/s and 100-Gb/s passive optical network standard based on the Wavelength Division Multiplexing(WDM)technology.At the same time,the fifth mobile communication system(5G)has also received much attenuation,which aims at almost 1-Gb/s data rate,1000 times higher mobile data volume per area,10 to 100 times higher number of connected devices and millisecond end-to-end delay.However,with the rapid increase of the wireless data rate,the number of base stations,the backhaul capacity and its cost would also increase significantly.Moreover,the transmission distance of high frequency wireless signal is limited due to the large attenuation.Thus,with the help of large capacity fiber communications,the fixed mobile converged optical access network is the development direction of the future high speed access network,which can realize broadband and flexible information access and reduce the CAPEX and OPEX of base stations.Based on this situation,China mobile proposed a novel Cloud Radio Access Network(C-RAN)to improve the transmission capacity between multiple Remote Radio Units(RRHs)and Baseband Unit(BBU)pool,which is termed as the mobile fronthaul.Recently,extensive research on fronthaul transmitted passive optical network has been started However,there still exist some problems in the future fixed mobile converged optical access network:1)In the last mile access,one single fiber would be used to carry various services including fixed and mobile access services.Thus,in the last mile access,more and more wavelengths will be used in the future fixed mobile converged optical access network to improve its transmission capacity.However,as long as the wavelengths of different wavebands and different services are co-existed in the last mile fiber,the fiber nonlinearity impairment would happen,which could influence the transmission performance of wireless signals;2)In the future 5G communication system,the number of base stations and the number of antennas would be increased significantly.If the digital transmission is used in mobile fronthaul,i.e.the received 5G radio signal is sampled,quantized and coded before transmission,the fronthaul capacity of base stations will be increased rapidly.In addition,the mobile fronthaul based on analog signal would suffer from the fiber dispersion,especially for the high frequency 5G radio signals.Thus,how to adopting the low-cost,large capacity and reliable transmission to resolve the increased capacity requirement of base stations is a problem.3)In the future fixed mobile converged optical access network,the CoMP and massive MIMO would impose strict requirement on the delay,jitter and synchronization of transmission links.So,how to satisfy this requirement in the future converged access network is still a problem.Thus,as to the future fixed mobile converged optical access network,this paper mainly investigates the fiber nonlinearity impairment caused by large amount of wavelengths,and proposed corresponding solutions.The detailed work is as follows:1.The nonlinearity impairment caused by stimulated Raman scattering between passive optical network and mobile fronthaulFor fixed mobile converged access network,we firstly introduced a new passive optical network,i.e.TWDM-PON,and two fronthaul transmission mechanisms:one is the fronthaul based on analog modulation,the other is the fronthaul based on digital signals.Then,detailed theoretical analysis and experiments are conducted to study the nonlinearity impairment caused by stimulated Raman scattering between TWDM-PON and digital fronthaul networks.The main contributed work includes:1)We analyze the stimulated Raman scattering(SRS)induced impairment between multiple TWDM-PON downstream wavelengths in L band and multiple fronthaul wavelength channels in C band theoretically,and investigate its impact on fronthaul signals through simulation.The SRS-caused impairment includes two aspects,one is SRS-induced power depletion and the other is the eye diagram distortion caused by the nonlinear Raman crosstalk.2)An experiment is conducted to measure the SRS-induced power depletion and the signal distortion,i.e.the deterioration of fronthaul signal's eye diagram.Due to the“1”level broadening of the non-return-to-zero on-off keying(NRZ-OOK)signal's eye diagram,the receiving sensitivity with 20-km standard single mode fiber(SSMF)transmission is decreased by?1-dB.2.SRS-induced power depletion and crosstalk noise on bidirectional fronthaul transmission with large amount of wavelengthsLarge amount of data traffic in mobile access network would result in the increase of wavelengths in mobile fronthaul networks.For the research background and related research situation about mobile fronthaul network,we introduced the system architecture and the wavelength allocation in multi-wavelength mobile fronthaul networks,and analyzed the interaction between fronthaul wavelengths through experiments and simulation.The main contributed work includes:1)We analyzed the power depletion and crosstalk induced by SRS in the mobile fronthaul link for the first time to the best of our knowledge.We also analyzed the impact of the direction of SRS on fronthaul signal.As for the Raman crosstalk,it occurs only in the same direction for example the downlink transmission or the uplink transmission.There is almost no Raman crosstalk noise between downstream and upstream transmission owing to the directional dependence of the SRS effect.2)Furthermore,to explore the worst-case influence,we assume the fronthaul transmission occupies the whole unoccupied wavelengths in C+L wavebands.Simulation implies that up to 5-dB extra power of fronthaul upstream signals would be attenuated after 20-km fiber transmission.The results provide a significant reference for the wavelength allocation in the future optical wireless converged access networks.3.Nonlinear Raman crosstalk noise suppressionFor the Raman problem caused by the coexistence of many services in next generation fixed mobile converged optical access network,we introduced the impact of SRS on traditional CATV signals and EPON signals.And we also introduced the traditional Raman noise suppression methods.Moreover,an experiment is also conducted to measure its effect on CATV signals.At last,we proposed a novel Raman noise suppression method named dicode coding.The main contributed work includes:1)The dicode coding features ultra-low power spectral density in the low frequency region,which can reduce the nonlinear Raman crosstalk on the RF-video signal effectively.We analyzed different Raman noise suppression method such as 8B10B,Miller code and dicode coding by experiments.2)Dicode coding is realized by simple delay and subtraction operation,which can alter the signals' spectrum.Experimental results show that,compared with traditional nonreturn-to-zero on-off keying(NRZ-OOK)signals,the crosstalk on RF-video signal can be reduced significantly.And we also measured the bit error ratio(BER)performance of dicode signals. |
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