多倍频调制在光纤无线通信系统中的研究

【作者】 王文沛；

【导师】 陈林；

【作者基本信息】 湖南大学 ， 信息与通信工程， 2010， 硕士

【摘要】 随着信息时代的到来,人们对多媒体宽带业务的需求日益递增,而对于目前频谱资源紧张的无线通信系统来说,已经不能适应当前的变化,因此就必须开发更高频率的资源,向更高容量和传输速率发展,因此毫米波频段资源成为了未来宽带通信的首选,光纤无线通信系统(ROF)技术的产生正好满足了这一需求,光纤无线通信系统将光纤通信系统大容量,高带宽和无线通信低成本的优势有机的结合在了一起,在未来宽带通信系统中扮演了重要的角色,毫米波的产生是通过中心站的光器件的光学信号处理(包括调制解调)产生的,而基站只需光电转换即可,大大的节约了系统成本,本文论述了采用多倍频调制产生毫米波的过程,并且还将介绍了在宽带传输网络中的运用。本文首先介绍了ROF技术提出的技术背景,分析了国内外目前研究的主要技术趋势,以及ROF系中产生毫米波的几种关键技术。在ROF系统中,光学倍频法利用了光生毫米波的优势,有效的降低本征信号源的频率需求和光学器件的带宽需求,只需要低频的本征信号源就可以产生高频率的毫米波,大大的提高了频谱的资源利用率,本论文着重从以下几个方面讨论多倍频调制的实现方案：1.使用单个调制器实现多倍频调制(1)使用单个双臂调制器产生六倍频毫米波：提出了两种基于单个双驱动调制器实现六倍频毫米波的方案,这两种方案与使用两个调制器方案相比,降低了系统的成本,插入损耗低,容易控制。一种方案是只调节相位偏移,不改变调制深度,另一种方案是既改变相位偏移,又改变调制深度,理论分析和数值模拟结果表明：第二种方案产生的毫米波性能要优于第一种方案。(2)使用单个双驱动调制器产生八倍频毫米波,调节两臂的相移和调制深度,抑制二阶边带和奇阶边带,然后使用FBG将载波滤掉,剩下两个四阶边带,本方案只需要一个调制器,简化了系统的结构。仿真结果表明,该方案能使5GHz的本征信号产生40GHz毫米波信号,在光纤传输40千米时,基带信号眼图清晰。2.使用两个调制器级联实现多倍频调制：首先提出了使用相位调制器和强度调制器级联产生四倍频毫米波信号的方案：使用相位调制器实现载波抑制,将同一个本征信号源的相位偏转90度,驱动强度调制器,产生两个二阶边带,该方案利用了相位调制器没有直流偏置的优点,不需要复杂的直流环路控制,简化了系统的结构,此方案经过实验验证,其实验结果表明系统在单模光纤中传输20km后,功率代价小于0.8dBm。然后使用相位调制器和强度调制器级联实现六倍频毫米波的全双工传输：数据信号和本征信号源进行混频驱动相位调制器,调节相位调制器的深度,使用第一个FBG+CIR将载波分离,将同一个本征信号源的相位偏转90度,调节强度调制器的调制深度，产生两个三阶边带,使用第二个FBG+CIR将三阶边带和载波分离掉,分离的载波送回中心站进行重利用,而分离的三阶边带则下行传输到基站。该方案不仅利用了相位调制器无直流偏置的优点,而且实现了载波的重利用,提高了频率利用率,仿真结果表明,毫米波传输的性能与相位调制深度有关,相位调制深度越大,噪声容限就越大,但是边带的之间的互拍作用也越大。最后提出了使用两个强度调制器级联实现八倍频毫米波的全双工传输,将第一个强度调制器设置在最大偏置点,抑制奇阶边带,使用第一个FBG+CIR将二阶边带和载波分离,将同一个本征信号源的相位偏移180度,驱动第二个强度调制器,调节强度调制器的调制深度,使用第二个FBG+CIR将载波和四阶边带进行分离,分离的载波送回中心站进行重利用,而两个四阶边带通过下行传输到基站,此方案实现了载波重利用,频谱的扩展性强,提高了频谱的利用率,仿真结果表明,5GHz的射频信号源经过调制后产生40GHz毫米波,毫米波经过光纤传输40千米时,眼图依然清晰可见。3.在OFDM-ROF系统中实现多倍频调制正交频分复用(OFDM)具有抗干扰能力强,频谱利用率高,传输容量大的特性,在光通信系统中可以抵抗色散和偏振模色散,将OFDM技术与ROF技术相结合能够发挥各自的优势,降低无线通信系统的成本,而且可以实现通信系统的高带宽,大容量传输,在此基础上,提出了多倍频调制在OFDM-ROF系统产生毫米波的方案并进行了比较,一种方案是使用直接调制OFDM信号实现四倍频信号的传输：将OFDM基带信号驱动直接调制激光源,然后将本征电信号源驱动双驱动调制器实现频率上转换,然后使用IL滤掉载波,经过光纤传输后,剩下的两个二阶边带经过光电二极管拍频产生四倍频毫米波信号。另一种方案是使用外部调制OFDM实现四倍频的方案,采用外部调制器调制OFDM,然后使用双驱动调制器进行频率上转换,IL滤掉载波,剩下的两个二阶边带通过光电转换后产生四倍频毫米波,经过理论分析并进行实验验证,得出经过光纤传输后,直接调制OFDM方案由于受到光源的非线性效应影响,调制带宽小,功率代价比外部调制要大,而外部调制只受到光纤色度色散导致的相位偏移的影响。而经过编码后,毫米波的相位得到了恢复和补偿,功率也得到了一定的改善。更多还原

【Abstract】 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.更多还原