| For the arrival of information age, peoples'demands for the broadband business of the multimedia are increased progressively day by day and for the fact that spectral resources of wireless communications system are in short at present,they can't adapt to present change,so the higeher frequency resources must be developed,it should develop at higher capacity and transimission rate, So millimetre of waves frequency band resources has become first-selection of future broadband communications. Radio-Over-Fiber(ROF) technology has just meet the requirement,Radio-Over-Fiber system can integrate the advantage of optic communication system that is large capacity.high bandwidth and that of wireless communication that is low-cost organicly,which can play an important role in the broadband communication for future.The production of millimieter-wave is through the optical signal processing of optical devices in central station,and in base station only O/E conversion is needed.which largly reduce the cost of system.This paper has expounded the production of millmeter-wave with optical frequency multiplication modulation and its application in the broadband communications system.This paper has introduced the technological background of ROF technology having put forward and has analyzed the main technological trend that studied both at home and aborad,and the several kinds of key technology of millimeter-wave production in ROF systems.For ROF systems,optical frequency multiplication has take advantage of production millimeter-wave in optical domain and effectively has reduced the frequency requirement of local signal and bandwidth requirement of optical devices,only using low frequency LO signal source can produce high frequency mm-wave signal,which can improve the utilization ratio of sprectal source largly.This paper has discussed some realizational schemes of optical frequency multiplication modulation emphatically as following:1.Using a single modulator to realize optical frequency multiplicational modulation (1)Using a single dual-arm modulator to realize frequency sixupler for millimeter-wave generation:Two schemes based on a single dual-arm modulator to realize frequency sixupler are proposed.These two schemes have reduced the cost and have low insert loss and are easy to control.One scheme is only adjusting the phase shifter and isn't changing the modulation depth,and another scheme is both adjusting and changing the modulation depth.The theorical analysis and numerical simulation results show that the performance of mm-wave for the second scheme is better than that for the first scheme.(2)Using two cascaded modulators to realize frequency eightfold for millimeter-wave generation:Adjusting the phase offset between two arms of the modulator and modulation depth to suppress second order and odd order sidebands,and then use a FBG to filter the carrier,leaving two fourth order sidebands This scheme only requires a single modulator,which can reduce the system cost. The simulation result has showed that the eye diagram of baseband mm-wave signal is still clear after 40 km fiber transimission.2.Using two cascaded modulator to realize optical frqeucny multiplicational modulationAt first,the scheme of frequency quadruple mm-wave generation based on intergrating a phase modulator with an intensity modulator is proposed:using the phase modulator to realize OCS,and the phae fo the same LO signal is shifted by 90 degree, and then drive the intensity modulator to generate two order sidebands,this scheme has utilized the advantage that phase modulator has no dc bias,so that it needs not complex bias circuit,which can simplify the system.This schme is verified by experiment and the experiment results show that penalty is less than 0.8dB with 20km fiber transmission.Then,the scheme of frequency sixupler mm-wave generation and full-duplex transmission based on intergrating phase modulator with intensity modulator is proposed:with data signal mixed with LO signal,phase modulator is driven and phase modulation index is-adjusted,using the first FBG+CIR combiner to split carrier and the phase of the same LO signal is adjusted by 90 degree, and drives the intensity modulator and the modulation index is adjusted,then two third order sidebands is generated and use the second-FBG+CIR to separate carrier and sidebands.The carrier sperated is back to CS to be reused and the two sidebands is downlink transimssing to BS.This scheme not only utilizes the advantage of phase modulator that has no dc bias,but also realize carrier being reused,which can improve spectral utilization efficiency.The simulation results show that the performance of mm-wave is releated to phase modulation index and when phase modulation index is larger,the limitation of noise capacity is_larger,but the beating between sidebands has become serious.At last,the scheme of frequency eightfold mm-wave generation and full-duplex transmission based on two cascaded intensity modulator is proposed:the dc bias of the first intensity modulator is seted by the maximum transimission to supperess odd order sidebands and use the first FBG+CIR to separate carrier and two order sidebands,and the phase of same LO signal is adjusted by 180 degree,and the second intensity modulator is driven and the modulation depth is adjusted,using the second FBG+CIR to separate carrier and four order sidebands,and the carrier separeated is back_to CS to be reused and the two fourth order sidebands is downlink transmission to BS, This scheme has realized carrier being reused,the spectral expansibility of mm-wave is strong,can improve spectral utilization efficiency and the simulation results show that 5GHz can be modulated to 40GHz mm-wave.With 40km fiber transmission,the eye diagram is still clear.3. Optical frequency multiplicaitonal modulation in OFDM-ROF systemsOrthogonal Frequency Division Multiplexing(OFDM) has the advantage of strong resistantance to interference,high spectral utilization efficiency and high transmission capacity. In optical Communication,OFDM can have strong resistance to chromatic dispersion and polarization-mode dispersion.Combining the advantage of OFDM and ROF can give play to their own advantages and reduce the cost of wireless communication system and can realize the high bandwidth of the communication and large capacity transmission.Based on this advisement,we have proposed optical multiplicational modulation schemes for mm-wave generation in OFDM-ROF systems,the schemes proposed in this paper have: (1)OFDM direct modulation for frequency quadplex mm-wave production and transmission:this scheme is based on using a direct modulation laser intergrating with an external modulator,we use OFDM baseband signal to drive direct modulation laser,and then use LO signal to drive dual-arm modulator to realize frequency up-conversion,and then use IL to filter carrier,leaving two second order sidebands,This schme has simplified the system cost and reduced the bandwidth requirement of system,but mm-wave is easily influenced by nonlinear effect of optical source.The experimental result has show that the power penalty is 2dBm after 20km fiber transimission.(2)OFDM external modulation for frequency quadplex mm-wave production:this scheme is based on an external modulator intergrated with a dual-arm modualtor.The intensity modulator is driven by baseband OFDM signal and the dual-arm modulator is driven by LO signal to realize frequency up-conversion,and use IL to filter carrier,leaving two second order sidebands,though O/E conversion,frequeucy quadplex mm-wave is produced.Through Theorical analysis and experiment verification.with fiber transmission,it is concluded that the schemes that direct modulating OFDM has few modulation bandwidth and the penalty is larger than the scheme that external OFDM modulation,and external modulation is only affected by phase offset resulted from chromatic dispersion.but the performance is improved for a certain level after coding. |
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