波长共享的WDM无源光网络媒质接入控制算法及其性能分析

结合波长分配和前跳虚时钟算法,提出了一个基于连接的WDM无源光网络媒质接入控制算法.通过理论和仿真,分析了此算法的延时保证和公平性.     

基于100 m色散位移光纤的超连续谱实验研究

利用被动锁模掺铒光纤激光器输出的重复频率13.9 MHz、中心波长1557.28 nm的光脉冲作泵浦源,在100 m的色散位移光纤中产生总宽度达488 nm的超连续谱,不平坦度小于±3.0 dB,利用法布里-珀罗滤波器获得163路多波长脉冲,利用可调谐滤波器得到C波段内脉宽为1.2～2.0 ps的近变换限超短光脉冲.分析了泵浦脉冲的功率、光纤长度等因素对SC光谱展宽的作用,并与色散平坦光纤和较长的色散位移光纤作了比较.     

基于CMMB的外辐射源雷达信号模糊函数分析与处理

根据中国移动多媒体广播(CMMB)信号结构,该文分析了该信号作为雷达辐射源的模糊函数特性,研究了副峰产生的机理,并与欧洲数字视频地面广播(DVB-T)信号特性做了比对;针对因循环前缀引起的模糊函数副峰和同步信号引起的多普勒模糊带对目标探测产生的不利影响,提出了副峰抑制的方法。仿真和实测结果验证了文中副峰产生机理分析的正确性,同时所采用的方法能有效抑制模糊副峰,为目标探测奠定了基础。     

投影小波域MAP估计无源毫米波图像超分辨算法

在无源毫米波成像中,由于系统天线孔径大小的受限而使得成像的分辨率低。为了提高图像的分辨率,该文提出了一种投影小波域最大后验(MAP)估计毫米波图像超分辨算法(PWMAP)。该算法利用基于小波域广义高斯分布的MAP估计来恢复通带内的频谱;然后利用投影的非线性运算实现频谱外推。该算法不仅比以往的算法能提供更准确的先验建模,而且能在每步迭代时自适应地更新正则参数。实验结果验证了该算法的有效性。     

基于相位补偿的BPSK相参脉冲串信号多普勒频率变化率估计算法

目标与观测平台之间径向加速度引起的接收信号多普勒频率变化率的精确估计对于高精度单站无源定位与跟踪具有重要意义。该文针对脉内相位调制信号的频率变化率不能直接表征多普勒频率变化率的问题,首先引入相位补偿的方法消除BPSK相位调制序列的影响,然后针对固定脉冲重复频率和脉冲重复频率抖动两种情况分别提出了径向加速度引起的多普勒频率变化率的精确估计方法,并且给出了多普勒频率变化率与采样频率、观测时间及脉冲重复频率之间的约束关系。仿真结果表明,典型参数条件下该文提出算法的信噪比门限比已有算法低约4~6 dB。     

Numerical evaluation of passive control of shock wave/boundary layer interaction on NACA0012 airfoil using jagged wall

Shock formation due to flow compressibility and its interaction with boundary layers has adverse effects on aerodynamic characteristics, such as drag increase and flow separation. The objective of this paper is to appraise the prac-ticability of weakening shock waves and, hence, reducing the wave drag in transonic flight regime using a two-dimensional jagged wall and thereby to gain an appropriate jagged wall shape for future empirical study. Different shapes of the jagged wall, including rectangular, circular, and triangular shapes, were employed. The numerical method was validated by experimental and numerical studies involving transonic flow over the NACA0012 airfoil, and the results presented here closely match previous experimental and numerical results. The impact of parameters, including shape and the length-to-spacing ratio of a jagged wall, was studied on aerodynamic forces and flow field. The results revealed that applying a jagged wall method on the upper surface of an airfoil changes the shock structure significantly and disinte-grates it, which in turn leads to a decrease in wave drag. It was also found that the maximum drag coefficient decrease of around 17%occurs with a triangular shape, while the max-imum increase in aerodynamic efficiency (lift-to-drag ratio) of around 10%happens with a rectangular shape at an angle of attack of 2.26?.     

Single-walled carbon nanotubes on side polished fiber as a universal saturable absorber for various laser output states

We demonstrated four different laser operational states in the same passive fiber ring laser cavity by controlling solely the polarization state in a saturable absorber. The demonstrated laser operational states were continuous wave, mode-locking, Q-switching, and Q-switched mode-locking. The saturable absorber was fabricated by spin-coating a single-walled carbon nanotubes (SWCNT)/polymer composite on a side polished fiber providing enhanced nonlinear interaction with the SWCNT. The wide tuning range of the saturable absorber allows the simple means to generate different forms of laser output from a single laser cavity.     

地物微波辐射亮温的图像模拟方法

微波辐射亮温图像的模拟在被动遥感领域中具有重要意义。该文根据射线追踪的思想提出了一种三维场景微波辐射图像的模拟方法:亮温追踪法。分析了模拟时必须考虑的天空背景辐射、多次反射造成的极化旋转以及天线平滑作用等影响因素,最终建立了微波辐射亮温图像的模拟方法。应用此方法对典型目标进行了试验模拟,并对模拟结果所表现出的微波辐射现象进行了分析。为验证模拟方法的可靠性,该文对楼层建筑以及飞机跑道等实际场景进行模拟,模拟结果与实地测量图像取得了很好的一致,表明此方法完全可行。     

On the friction drag reduction mechanism of streamwise wall fluctuations

Understanding how to decrease the friction drag exerted by a fluid on a solid surface is becoming increasingly important to address key societal challenges, such as decreasing the carbon footprint of transport. Well-established techniques are not yet available for friction drag reduction. Direct numerical simulation results obtained by Józsa et al. (2019) previously indicated that a passive compliant wall can decrease friction drag by sustaining the drag reduction mechanism of an active control strategy. The proposed compliant wall is driven by wall shear stress fluctuations and responds with streamwise wall velocity fluctuations. The present study aims to clarify the underlying physical mechanism enabling the drag reduction of these active and passive control techniques. Analysis of turbulence statistics and flow fields reveals that both compliant wall and active control amplify streamwise velocity streaks in the viscous sublayer. By doing so, these control methods counteract dominant spanwise vorticity fluctuations in the near-wall region. The lowered vorticity fluctuations lead to an overall weakening of vortical structures which then mitigates momentum transfer and results in lower friction drag. These results might underpin the further development and practical implementation of these control strategies.     

无源波分在5G前传接入中的应用研究

5G基站的大规模建设将消耗大量的前传光纤基础资源,给运营商带来建网成本、能耗的压力.通过比较各种前传方案,从技术、建设周期及成本效益等方面进行分析,提出在纤芯不足的区域采用无源波分方案,可大大节省建设成本、缩短建设周期、减少机房动力改造投资、降低运营成本.该方案能使综合TCO最优,高效解决5G前传难题,提高网络竞争力.