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Research On Modulation And Detection Techniques In Optical Heterodyne Radio-over-Fiber Downlink

Posted on:2021-05-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y C CaiFull Text:PDF
GTID:1368330626955632Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Radio over fiber(RoF)communication system,which combines the advantages of both optical fiber communication and wireless communication systems,has the characteristics of high bandwidth,low loss,anti-electromagnetic interference,and also supports convenient access at anytime,anywhere.Therefore,the RoF system has broad application prospects in optical fiber wireless access networks,Internet of Things and other fields.Since the RoF system enabled by the optical heterodyne technique shows unique advantages in the generation of radio frequency signal as well as the tunability and scalability of the system,it has caused extensive research by scholars.Generally speaking,in a RoF system,the downlink requires a larger bandwidth and higher data rate than the uplink,hence how to build the RoF downlink to provide better performance is particularly important.The optical heterodyne RoF downlink is currently facing the challenges in transmission distance,system cost,system capacity and energy efficiency.To combat these challenges,different modulation and detection techniques are studied for the optical heterodyne RoF downlink in this dissertation.Focusing on the critical techniques such as single-sideband(SSB)modulation,a single dual-drive Mach-Zehnder modulator(DD-MZM)based optical orthogonal modulation and demodulation,as well as carrier suppression based power detection,the thorough theoretical analyses,simulations and/or experiments have been conducted.The main works and innovatives of this dissertation are listed as follows.1.Research on SSB modulation technique in optical heterodyne RoF linkThe theoretical model of SSB modulation for optical heterodyne RoF link is established.Three kinds of SSB modulation schemes are studied utilizing the DD-MZM and MZM-based IQ modulator.Meanwhile,the advantages and disadvantages of the above three kinds of SSB schemes are analyzed in detail.In addition,the elimination technique of signal-to-signal beating interference(SSBI)based on the Kramers–Kronig algorithm is studied,which can effectively eliminate the SSBI caused by power detection of the SSB signal under the minimum phase condition.These three kinds of SSB modulation schemes have been verified through simulations,and the simulation results are consistent with the theoretical analyses.They provide theoretical basis and practical guidance for the SSB modulation in optical heterodyne RoF links.2.Research on phase noise compensation technique based on the RF pilot(RFP)In the optical heterodyne RoF link enabled by the mixing down-conversion,the phase noise compensation technique based on the RFP is studied.Aiming at the problem that the traditional RFP carrier whole multiplication(CWM)scheme suffers from the second-order nonlinear distortion,a modified RFP carrier signal multiplication(CSM)scheme is proposed.The proposed scheme can effectively compensate the phase noise caused by two free-running lasers in the optical heterodyne RoF link,as well as avoid the influence of second-order nonlinear distortion caused by modulator's nonlinearity.Compared with the traditional RFP-CWM scheme,the proposed RFP-CSM scheme can improve the spectral efficiency by more than 60% and the receiver sensitivity of 1.2 dB.In addition,as compared with the commonly used Viterbi and Viterbi phase estimation method,the proposed RFP-CSM phase noise compensation technique has a higher tolerance limit of linewidth and is more conducive to the deployment of low-cost,wide linewidth lasers in optical heterodyne RoF links.3.Research on optical orthogonal modulation and demodulation technique based on one single DD-MZMThe optical orthogonal modulation and demodulation technique based on one single DD-MZM is proposed.Utilizing the optical orthogonal modulation,two independent signals with the identical frequency are modulated by a DD-MZM which is biased at its quadrature point.Based on the optical heterodyne detection and mixing down-conversion techniques,combined with a novel digital signal processing method,the two independent signals with the identical frequency can be successfully separated and recovered.At the same time,the compensation of laser phase noise can be also realized.Furthermore,based on the optical orthogonal modulation technique,two different modulation schemes,dual double-sideband(DSB)and Twin SSB,are proposed.These two independent co-frequency signals are transmitted simultaneously in the form of two DSBs or two SSBs,respectively.By combining two signals with a complex form to perform dispersion compensation in the frequency domain at the receiving end,the power fading problem of DSB modulated signal is solved.As compared with the conventional DD-MZM DSB or SSB modulation scheme,since the proposed scheme allows simultaneous transmission of two independent wireless signals with the identical frequency on the same RF carrier,it can provide double data rate for the single-wavelength optical heterodyne RoF links.4.Research on carrier suppression based power detection techniqueIn terms of the problem that the optical heterodyne RoF link using traditional power detection reduces the power efficiency and receiver sensitivity due to the existence of high energy-consuming carrier,the carrier suppression based power detection scheme is proposed.In the proposed scheme,the signal precoding technique is used to precode the amplitude and phase of the standard vector signal,thereby allowing power detection to be used directly to restore the desired standard vector signal while suppressing the carrier.In the process of realizing signal precoding,the use of phase unwrapping technique can solve the generation of extra electrical carrier after signal precoded.Meanwhile,the effect of the operation sequence of signal precoding and pulse shaping on system performance is analyzed,and the problem that the system performance of the precoding scheme may be limited by the pulse shaping function is also solved.The proposed scheme inherits the insensitivity of power detection to laser phase noise.Benefiting from the provement of the power efficiency,it can improve the receiver sensitivity by about 5 dB compared with the traditional power detection scheme.The above schemes are verified through theoretical analyses,simulations,and/or experiments in this dissertation.The research results provide references for improving the performance of optical heterodyne RoF downlinks,including extending transmission distance,reducing system cost,increasing system capacity,and improving system energy efficiency and so on.
Keywords/Search Tags:radio over fiber, optical heterodyne, laser phase noise, single-sideband modulation, dual drive Mach–Zehnder modulator
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