| With the advent of the 5G era,people’s demand for new broadband communication services has increased dramatically,and the information throughput of wireless communication systems has been greatly improved.In order to meet users’ business needs,wireless communication systems use higher carrier frequencies to obtain more abundant spectrum.In the fronthaul part,the traditional CPRI technology will not be able to meet the requirements of high-frequency and large-bandwidth millimeter-wave signal transmission.Radio over Fiber(RoF)technology carries radio frequency signals through lightwave to realize the fronthaul of large-capacity broadband wireless signals,the channel capacity can be greatly improved,and at the same time,RoF technology has the advantages of simple structure,low cost and low power consumption,which has attracted much attention.Compared with the directly modulated RoF technology,the externally modulated RoF technology has a larger bandwidth and dynamic range,but in the fronthaul system based on the externally modulated RoF,due to the nonlinearity of the optical modulator,the optical carrier millimeter-wave signal generated by the electro-optical modulation not only contains the fundamental frequency signal also contains high-order optical sidebands,which are converted into high-order harmonics or intermodulation terms by photoelectric conversion.Even in narrowband systems,the third-order Intermodulation Distortion(IMD3)cannot be directly filtered,thus limiting the spurious free dynamic range(SFDR)of the entire communication link.Effectively suppressing or eliminating the third-order intermodulation interference components in the link is of great significance to realize the linearization of the RoF link and improve the SFDR.At the same time,in the base station of the fronthaul link,due to the close geographical distance between the transmitting antenna and the receiving antenna,especially when the base station is in the same-frequency full-duplex communication mode,the high-power downlink signal sent by the transmitting antenna will affect the the weak uplink signal received by the antenna at the receiving end causes serious interference.In order to reduce the interference of strong downlink signals to weak uplink signals,it is necessary to eliminate co-channel interference components existing in the link.Based on the investigation of the simultaneous co-frequency full-duplex RoF link,this paper studies the elimination of third-order intermodulation interference and co-frequency self-interference using the optical domain method.The specific research contents are as follows:(1)Through the detailed derivation of the causes of the third-order intermodulation interference,a scheme to realize the linearization of microwave photonic links based on double parallel MZM is proposed.In this scheme,after the optical carrier and the optical carrier millimeter wave signal are combined,the optical carrier in the optical carrier millimeter wave signal and the optical carrier can be coherently superimposed,and the amplitude of output light is regulated by adjusting the relative power and phase of the optical carrier.After photoelectric conversion,the fundamental frequency component has a larger amplitude and the third-order intermodulation term can be destructive superposition,which are produced by the beating of the second-order intermodulation term with the third-order intermodulation term;the beating of the fundamental frequency term with the second-order intermodulation term;the beating of the optical carrier with the third-order intermodulation term.The simulation results show that compared with the traditional scheme,the signal-to-interference ratio is improved by 17.4 dB,the SFDR is improved by 9 dB,and the EVM of the 1 Gbit/s 16QAM signal can reach 2.98%and 3.4%in the back-to-back situation and after being transmitted through a 10 km single-mode fiber which have an increase of 11.19%and 11.58%respectively.(2)A scheme to realize the linearization of microwave photonic links based on orthogonal polarization multiplexing technology is proposed.This scheme uses orthogonal polarization multiplexing technology to simultaneously transmit optical carrier radio frequency signals and optical carriers.The two optical signals are recombined,and the two photocurrents generated by photoelectric conversion include the third-order intermodulation interference term of the same amplitude and phase,the fundamental frequency of the same amplitude and opposite phase.Through the differential detection of the balanced detector,the third-order intermodulation interference component can be suppressed and the fundamental frequency component is output.The simulation results show that compared with the traditional scheme,the signal-to-interference ratio is increased by 22.9 dB,the SFDR is increased by 11 dB,and the EVM of the 1 Gbit/s 16QAM signal in the back-to-back situation is increased by 11.47%,reaching 3.1%;after 40 km of optical fiber transmission,the EVM is maintained at within 10.35%.(3)On the basis of in-depth research on the causes and elimination principles of co-channel interference,a scheme for co-channel interference elimination based on dual parallel MZM is proposed.In the dual parallel MZM,the optical signal generated by the upper branch MZM modulation contains the interference signal and the corresponding components of the reference signal generated by the lower branch MZM are superimposed.When the amplitude and phase of the interference components match,the output signal can eliminate co-channel interference.The simulation results show that when both the interference signal and the useful signal are single-frequency signals,the co-channel interference cancellation depth can reach 61.3 dB;when the interference signal is a 1 GHz wideband signal and the useful signal is a single-frequency signal,the interference cancellation depth can reach 33.4 dB;when the interference signal is 1 GHz wideband signal is eliminated,the EVM of the 16QAM useful signal reaches 4.81%which have a decrease of 92.71%. |
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