【作者】 黎静；

【导师】 王宏远； 马泳；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2013， 博士

【摘要】 水下无线激光通信在海洋通信、环境监测、矿藏搜寻和水下设施监测等应用中具有十分重要的经济价值和战略意义。尽管光信号在海水中存在传输“窗口”，但是由于海水强烈的前向散射效应，造成了光信号在时域、空域上展宽和极化域特性的改变，使得现有的激光通信系统有效通信距离受到了极大的限制。针对如何抑制散射光提高通信距离的问题，成为现阶段水下激光通信研究的瓶颈。不能简单的通过发射功率的提高增加水下光通信的距离，原因在于有用的信息被散射光信号所淹没。在通信接收端，携带通信信息的光信号在水中未发生或仅发生极少散射，信号强度很小；而散射信号强度大且产生波形畸变。如果能够提出一种方法抑制前向散射信号强度，提高准直信息信号的对比度，就可以实现对通信信息信号的提取，实现水下光通信距离的增加。文章通过分析海水米氏散射理论及其相函数，建立了海水前向散射信道模型。模型分析结果表明，海水前向散射信道中，产生干扰的散射光的幅频响应呈现低通特征，而携带有用通信信息的准直光的幅频响应为均匀函数。在采用高频载波调制发射信息信号后，经过海水信道的传输，在接收端捕获光信号并通过以调制频率为中心频率的带通滤波后，散射噪声在频域特性上可以与有用的准直信号得到显著区分。针对这一理论，文章提出了一种载波调制抑制水下光通信前向散射的方法，并在此基础上展开载波调制通信的海水散射信道传输特性和传输距离的研究。载波调制水下激光通信链路的建立依赖于对载波调制海水光通信信道的具体时、频域传输特性的分析。传统的解析分析的方法误差较大；实验测量的方法受水槽几何尺寸的限制，参数调整也不灵活，另外水槽中模拟海水的水质参数也与真实海水存在差异，实用性不高。文章在传统统计蒙特卡洛模拟的基础上，分析光子每次微观碰撞的直接到达接收器的解析分量，采用H-G函数作为散射拟合相函数构建了一种统计和解析融合的蒙特卡洛仿真方法—解析蒙特卡洛方法，并在基础上逐步建立了单光子、光脉冲和载波调制光脉冲的蒙特卡洛仿真算法模型。该模型有效解决了实际海水信道散射时/频域特征分析工具的问题，相对于传统算法的运算时间有效提高了164.57倍，成为本课题研究现阶段的水下激光通信信道特征的有力工具。通过对比水槽实验测量不同水质参数下接收光强和接收信号带宽与解析蒙特卡洛模型仿真结果，验证了该模型的有效性。在此基础上，将该模型应用于真实海水水质参数的模拟，采用了水下激光通信中广泛应用的Petzold水质参数，分析了三类海水前向散射通信信道的信道冲击响应及其信道容量。结果表明，水下激光信道冲击响应除了以往研究获得的“准直峰+散射拖尾”的线型外，还存在另外两种线型，更新了对海水无线激光通信信道脉冲响应线型特征的认识。另外，通过对信道冲击响应的频域变换，分析了真实海水信道环境在不同衰减长度下的海水信道容量，发现了其信道容量的近似指数衰减规律，为真实海水激光无线通信链路的设计提供了一种简便易行的信道估算方式。然后文章将载波调制信号输入水下无线通信解析蒙特卡洛模型，采用高斯函数调制以更贴近实际应用的激光调制技术获得了载波调制通信信号的输出波形特征。通过载波调制频率在接收信号频谱的可辨识度着重分析了三类海水水质参数条件下载波调制通信距离上限。仿真结果表明，三类海水的通信距离上限分别为：远洋海水140m；近海海水75m，海湾海水19m。可见载波调制通信技术在远洋海水中的通信距离提高了2倍以上。而由于传统通信性能分析不包含近海海水和海湾海水分析，这两种海水载波调制通信性能无法比较。最后分析了组成载波调制激光通信系统的各个环节及其系统参数指标。结果表明，调Q Nd:YAG倍频激光器峰值功率可达114MW，有效满足远距离通信需求；在激光器调制技术上，设计了Fabry-Perot调制器和内调制两种改进的调制方案备选；对于光滤波器件，碘分子滤光器带宽性能最优，在0.1个波数以内，是光滤波器件首选；对光电探测器件的两种方案及其性能参数也进行了分析。更多还原

【Abstract】 Underwater wireless optical communication has very important economic value andstrategic significance in marine communications, environmental monitoring, mineralsearch and underwater facilities monitoring applications. Although in the presence oftransmission "window" of optical signals in seawater, the transmission distance of theexisting laser communication systems has been greatly limited, mainly due to the stronglyforward scattering of seawater, resulting in a broadening effect in the time/spatial and thepolarization domain. The problem of how to suppress the scattered light to improvecommunication distance becomes the bottleneck of the underwater laser communicationstudies currently.It is not practical to increase the distance of the underwater optical communicationthrough simply increasing the transmitter power. The reason is that the useful informationis drowned by scattered optical signal. The strength of the optical signal carrying thecommunication information Communication is very small at the receiving end and did notor occurs only minimal scattering in the water. On the other hand, the scattering signalintensity is relavively large and causes the distortion of receiving waveform. If there is amethod suppressing the forward scattering signal intensity to improve the contrast of thecollimation signal that carries the information, it is possible to achieve the increase of thedistance of the underwater optical communication.Through the analysis of seawater Mie scattering theory and its function, the seawaterforward scattering model is established in this paper. The results of model analysis showthat the amplitude-frequency response of seawater prior to the scattering channelinterference of scattered light showing a low-pass characteristics, and theamplitude-frequency response of collimated light carring useful communication information is uniform function. By ultilizing a high-frequency carrier to modulate thetransmitting information signal, the useful ballistic optical signal in the frequency domaincharacteristics can be detected out of the scattering noise after the transmission in seachannel by a bandpass filter with its center frequency equals the modulation frequency atthe receiving end. Thus, the article proposes a carrier modulated carrier method tosuppress forward scattering in underwater optical communication. Based on this method,the scattering channel transmission characteristics and the transmission distance arestudied.The establishment of the the carrier modulation underwater laser communication linkdepends on the time/frequency domain transmission characteristics analysis of the carriermodulation seawater optical communication channel. Traditional analytic method existslarger error. On the other hand, the experimental measurement method is limited by thesink’s geometric size. The parameter adjustment is not flexible. Moreover, the waterquality parameters differ from a real seawater. Thus, theses methods are not practical.Based on the traditional statistical Monte Carlo simulation, the paper proposed a analyticalMonte Carlo method by analysis of the photon parsing component that directly reachingthe receiver after each microscopic collision. The HG function is used as the scatteringfitting phase function. After that the single light pulse and the carrier modulation lightpulse model are build based on the single photon Monte Carlo simulation model. Thismodel is an effective solution to the problem of time/frequency domain analysis of theactual seawater scattering channel. With respect to the traditional algorithm, thecomputing time effectively increased164.57times. The model become a powerful tool ofthe underwater laser communication channel characteristics studies at this stage.By comparing the receiving the light intensity and the received signal bandwidth oftank experiments and the analytical Monte Carlo model simulation results with a variatyof water quality parameters, the effectiveness of the model is verified. On this basis, the model is applied to the simulation of real seawater quality parameters. The widely usedPetzold’s three types water quality parameters of seawater forward scattering channel isapplied. In addition to the traditional pattern of collimation peak+scattering tailing inprevious studies, the simulation results show that there are two other patterns exist. Theresult renews our understanding of the characteristics of the channel impulse response ofwireless laser communication in seawater. After that, the channel capacity under differentattenuation length of seawater is analyzed through the FFT transform. It is found that itschannel capacity approximately obeys the law of exponential decaying. It provides asimple channel estimation method for real seawater laser wireless communication link.Finally, the article carrier modulation signal input underwater wirelesscommunication to resolve the Monte Carlo model closer to the actual application of lasermodulation technique using Gaussian function modulated carrier modulatedcommunication signal output waveform characteristics. Conditions download wavemodulation maximum communication distance of the three types of marine water qualityparameters analyzed in the received signal spectrum can be recognizable by the carriermodulation frequency. Simulation results show that the maximum communication distanceof the three types of seawater was: ocean seawater140m; coastal waters75m Gulfseawater19m. The visible carrier modulation communications technology incommunication distance ocean seawater increased more than2times. Traditionalcommunication performance analysis does not contain the offshore waters and the Gulfseawater analysis, these two the seawater carrier modulation communication performancecan not be compared.Finally, the analysis of the composition carrier modulation laser communicationsystem in all aspects of its system parameters indicators. The results showed that the tuneQ-switched Nd: YAG frequency doubling laser peak power of up to114MW, to effectivelymeet the demand for long-distance communications design; the laser modulation technically, two improved modulation of the Fabry-Perot modulator and within themodulation scheme alternative; optical filter member, the filter bandwidth of the bestperformance of the molecular iodine, wave number of0.1or less, the first choice of theoptical filter member; two programs and the performance parameters of the photodetectormember also analyzed.更多还原