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1、华中科技大学 博士学位论文 光纤射频传输（ROF）接入系统及无线局域网应用研究 姓名：常宇光 申请学位级别：博士 专业：光电信息工程 指导教师：刘德明 20090514 I 摘 要 随着通信技术的不断发展和社会的不断进步， 未来的通信系统要求实现随时随 地接入网络。这就要求未来的无线通信能够传输更高的数据率，覆盖范围更广，尤 其在室内、建筑物内等中短距离，对数据带宽的需求尤为迫切。如何解决建筑物内 的无线高速数据传输和无线接入覆盖问题就成为无线接入网的技术关键。 光纤射频 传输技术(ROF)充分结合光纤和高频无线电波传输的特点，实现大容量、低成本的 射频信号有线传输和宽带无线接入，为上述问题提
2、供了有效的解决方案。论文主要 研究采用 ROF 技术的无线局域网光纤传输系统，围绕无线局域网 OFDM 信号的特 性和光纤传输链路非线性影响问题深入分析了无线信号在光纤链路中传输的性能， 论文的主要研究内容和创新性成果如下： (1) 详细阐述了IEEE802.11a标准中的主要实现技术正交频分复用(OFDM)技术的 基本原理、系统构成和实现OFDM系统的关键技术，对OFDM信号的特性进行了详细 的分析。给出了IEEE802.11a无线局域网物理层的结构特点和系统工作原理，其中着 重分析了与OFDM系统设计有关的帧结构以及OFDM系统的参数。建立了802.11a无线 局域网数据发送与接收处理部分
3、系统模型。 (2) 研究了无线局域网光纤传输系统架构，并讨论了光纤无线传输链路的主要组 成部分及各部分的优缺点和工作过程。根据马赫曾德调制器的工作原理给出了无线 射频信号调制成光载波信号的过程，并分析了马赫曾德调制器性能对系统的影响， 提供了对无线信号双边带调制和单边带调制的频谱分析结果。总结了光纤无线传输系 统的技术要点并建立了光纤无线传输链路的仿真模型。 (3) 通过具体分析马赫曾德调制器非线性失真的原因和所导致的影响，根据模 拟信号光纤传输链路光载波信号的调制特点，推导出无线光传输链路进行信号无失真 传输的光调制指数上限。 (4) 通过分析无线局域网OFDM信号峰值平均功率比问题以及对光
4、传输系统性能 的影响，提出了一种信号包络软裁剪的方法对 OFDM 信号进行预失真处理，从而抑 制光传输链路非线性失真对 OFDM 信号传输的影响。通过仿真，分析了所提出方案 II 对 OFDM 信号动态范围的控制和系统带内噪声与带外噪声的抑制，验证了该方案在 无线局域网光纤传输系统中的实际效果。 (5) 提出了一种基于检测无线局域网数据帧短训练序列功率，进行马赫曾德调 制器正交偏置相位漂移跟踪与控制的方法，并与常规的导频插入法进行了比较。根据 所提出方法的工作原理建立了马赫曾德调制器偏置电压反馈控制系统模型并通过 仿真计算分析了该系统的性能以及输入光功率，光源相对强度噪声和光检测器响应度 等因
5、素对系统的影响。仿真分析结果表明所提出的马赫曾德调制器偏置电压反馈控 制系统在多种因素影响下均具有较好的性能表现。 (6) 研究了基于以太无源光网络(EPON)的宽带无线接入系统总体设计。包括：系 统总体架构与功能划分，EPON 无线接入终端(ONU-BS)模块结构和功能划分。详细 描述了 ONU-BS 的硬件设计与实现并给出了该系统的实际测试结果。 测试结果表明该 系统具有良好的无线接入性能。 该系统通过基于正交频分复用(OFDM)的 802.11a 无线 接入技术与以太无源光网络技术的融合， 实现了 EPON 光网络与宽带无线接入网络的 无缝融合，达到了光接入网低成本宽带无线接入的目的。
6、关键词关键词关键词关键词：光纤射频传输技术 OFDM 技术 802.11a 无线局域网 马赫曾德调制器 峰值平均功率比 正交偏置相位 非线性失真 III Abstract As the continuously development of telecommunication technology, future telecommunication system will have to provide network service anytime anywhere. This requires wireless communication system support higher bit
7、 rate and wider coverage, especially for short range access in buildings. How to provide high bit rate wireless transmission and access is a problem yet to be deal with. Radio over Fiber combines deployment of optical fiber technology and wireless networks provide great potential for increasing the
8、capacity and the quality of service for network access. By using Wireless on Fiber technology, the capacity of optical networks can be combined with the flexibility and mobility of wireless access networks without significant cost increment. This thesis discusses the effects of using fiber in conjun
9、ction with wireless local area network to distribute RF signal. The main contributions of this paper are as follow: (1) An OFDM based wireless LAN(802.11a) was described in this paper. Basic theory and working mechanism of OFDM technology was presented. We discussed in detail about the characteristi
10、c of OFDM signal and architecture of 802.11a wireless LAN MAC layer. We established the simulation model for the transmitter and receiver of 802.11a wireless LAN system. (2) Techniques for sending wireless signal over fiber were described. We discussed different optical link components and their rol
11、e in analog optical link. Merits and demerits of each component were also discussed. We have described signal modulation techniques using Mach-Zehnder electro-optic modulator and provided figures obtained from optical spectrum analyzer to shown optical double side band and optical single side band m
12、odulation. Wireless over fiber simulation system was setup. (3) Nonlinear distortion of Mach-Zehnder modulator was discussed in detail. A mathematical upper bound of the modulation index for nonlinear distortionless transmission of wireless signal over fiber was derived according to the speciality o
13、f analog optical link modulation process. (4) The high peak to average power ratio problem of OFDM signal and its adverse effects on system performance was analyzed. An OFDM signal predistortion method was proposed, which perform a soft envelope clipping to the OFDM signal to mitigate nonlinear IV d
14、istortions of MZ modulator. Experimental results shown that the proposed method could successfully minish the dynamic range of OFDM signal and restrain both inband and out of band noise of the wireless over fiber system and make OFDM signal more suitable in case of transmission over fiber. (5) A nov
15、el Mach-Zehnder modulator bias control mechanism was proposed, which tracks quadrature bias point drift of MZ modulator by monitoring the second harmonic component of short training sequence in OFDM frame. According to the mechanism proposed, a Mach-Zehnder modulator bias point feedback control syst
16、em was established. Evaluation performance of the proposed system from different aspects like optical power tap ratio, Laser RIN photo diode responsivity etc were give. Experimental results show excellent performance of the proposed system in MZ modulator bias drift tracking in different adverse conditions. (6) EPON broadband wireless access system was designed. The architecture and function-partition of the system was described. Hardware design and implementation of ONU-BS was disc
