一种基于AODV的链路质量与能效优先的跨层路由协议

针对本振光为高斯分布, 接收信号光经望远镜聚焦后为艾里分布的情况, 首先对高斯和艾里函数用数值计算的方式得到两种光斑最大外差效率: 当艾里斑直径和高斯光束束腰直径之比为1.719时, 最大外差效率为81.45%; 然后介绍了光的标量衍射和Zernike像差理论, 分析了夫琅禾费衍射适用于相干聚焦光场的条件, 计算了平面、高斯、艾里光场和Zernike像差的采样要求, 对存在各种像差的光学系统的外差效率进行了仿真, 分析了倾斜、离焦、像散、慧差、球差等基本像差及组合像差对外差效率的影响, 结果表明: 各种像差对外差效率的影响从低到高分别为像散、倾斜、离焦、慧差和球差; 3dB外差效率损失对应相干系统的指标为跟踪误差优于1μrad(RMS), 组合波像差优于0.1λ。研究结果对相干激光通信系统的链路损耗分配和光机系统的工程设计具有指导意义。     

基于标量衍射的星间相干激光通信外差效率研究

针对本振光为高斯分布,接收信号光经望远镜聚焦后为艾里分布的情况,首先对高斯和艾里函数用数值计算的方式得到两种光斑最大外差效率:当艾里斑直径和高斯光束束腰直径之比为1.719时,最大外差效率为81.45％;然后介绍了光的标量衍射和Zernike像差理论,分析了夫琅禾费衍射适用于相干聚焦光场的条件,计算了平面、高斯、艾里光场和Zernike像差的采样要求,对存在各种像差的光学系统的外差效率进行了仿真,分析了倾斜、离焦、像散、慧差、球差等基本像差及组合像差对外差效率的影响,结果表明:各种像差对外差效率的影响从低到高分别为像散、倾斜、离焦、慧差和球差;3 dB外差效率损失对应相干系统的指标为跟踪误差优于1μrad (RMS),组合波像差优于0.1λ.研究结果对相干激光通信系统的链路损耗分配和光机系统的工程设计具有指导意义.     

合成孔径激光雷达外差探测的影响研究

现阶段合成孔径雷达(SAR)都采用外差探测,因其外差相干探测的优越性,成为研究和应用的热点。然而,在实际探测中,雷达接收系统接收到的回波激光信号与本地振荡光信号进行相干外差时,匹配具有一定难度。文中分析了信号光宇本振光的夹角对外差信号的影响。     

卫星相干光通信跟瞄误差对链路性能的影响

以星间相干激光通信链路的接收机相干效率为研究对象,分析了动态跟瞄误差对相干效率的影响,将相干效率引入到星间相干激光通信链路方程,推导了跟瞄误差波前失配条件下的相干光通信链路的误码率概率表达式,对卫星激光通信链路中跟瞄对准误差导致的相干效率下降、链路误码性能劣化情况进行了仿真计算。结果表明,由于跟瞄对准误差波前失配的影响,相干激光通信接收终端具有一个优化的接收孔径,在本仿真条件下,LEO-GEO星间链路动态跟瞄误差为0.5μrad时,优化接收孔径为0.5 m,链路可获得最佳误码性能。     

复杂水质条件下蓝绿激光通信传输特性研究

由于水下通信信道的复杂性,水下激光通信面临功率衰减和通信距离受限的问题。论文采用蒙特卡洛方法建立了水下高斯光束传输仿真模型,基于该模型重点研究了海水类型、衰减系数、传输距离、接收孔径对接收光功率衰减的影响规律。结果表明：在纯净海水、远海海水、沿岸海水和港口海水中,随着海水浑浊程度的增加,仿真得到的有效传输距离减少,光功率衰减增加;在文中设定的海水衰减系数下,当海水的吸收系数大于散射系数时,有效传输距离减少,光功率衰减增加;当接收孔径小于光束直径时,接收光功率随接收孔径的增加而增加,当接收孔径大于光束直径时,接收光功率不再随接收孔径的增加而变化。     

相干测风激光雷达望远镜孔径及截断因子的优化分析

基于相干多普勒测风激光雷达外差探测理论,给出自由空间和光纤耦合两种模式下外差效率的解析表达式,并通过参数替换使两者具有统一的形式;结合此表达式以及湍流环境下的信噪比公式,仿真给出了地基及星载相干测风激光雷达系统不同探测距离处的最优望远镜和截断因子。研究结果表明地基系统信噪比随着望远镜孔径变化比较陡峭,存在最优望远镜孔径和截断因子,如若取值不恰当,会造成系统信噪比巨大损失;而星载平台下回波信号的信噪比随着望远镜孔径先增加后保持平稳,望远镜孔径可以综合成本和信噪比进行合理取值,最优化的截断因子为80%。此研究对相干多普勒测风激光雷达探测理论的发展及系统器件的最优化配置提供了重要的理论依据。     

光束准直性对星间相干光通信外差效率的影响

以本振光为高斯光,信号光为艾里斑分布为前提,建立了空间相干光通信系统外差效率的理论模型.分别在信号光垂直入射和存在角失配入射的情况下,对外差效率进行了仿真分析.结果表明,当光敏感面半径与高斯光束腰半径之比为1.53,且高斯光束腰半径与系统F数之比为0.8时,能得到最佳的外差效率为83.3%;信号光与本振光在光敏感面混频时的非准直对外差效率的影响较大,随着失配角的增大外差效率逐渐降低,几毫弧度的失配将使外差效率减小到50%以下.     

使用外差探测和啁啾脉冲压缩的CO2激光测距机

为了阐明有关的设计原理,对 CO2激光波长的外差探测与直接探测系统的灵敏度作了比较。结果表明,当外差系统用在具有内部运动的目标上时,散射辐射的时间相干性的破坏是有重要影响的。然后描述了整个系统和所得到的结果。紧凑的光学头包括一台波导激光器,一台声光调制器和表面声波(SAW)啁啾装置;它有45毫米直径的发射和接收孔径。当对许多反射回波积分时,可测得几公里外的自然目标,精度为10米。利用上啁啾/下啁啾技术,从活动目标的多普勒频移可在测距的同时测出径向速度,试验证实其精度高于1米秒-1。     

微多普勒效应激光平衡外差探测特性研究

面向远距离微多普勒效应探测,建立了激光平衡外差探测输出信噪比的数学模型,并在此基础上仿真对比了平衡外差探测和一般相干探测的信噪比,分析出在合理设置探测器结构参数情况下,平衡外差探测方法在提高信噪比上有明显优势特性。经过实验验证分析,接收信号经时频分析后较一般相干探测可读性更强,减小了特征提取误差,适用于远距离微动探测。研究为目标微动的实际遥感探测提出了一种高效可行的接收方法。     

虚拟子阵平滑算法

主动阵列发射正交信号可以极大地扩展阵列的虚拟孔径,阵列的自由度成几何增长。该文利用主动阵列发射同频带时域正交PCM信号构造的虚拟子阵,提出了一种虚拟子阵平滑(VSS)算法,可解决不同目标回波信号相干的问题。该算法避免了接收阵列物理孔径的损失,当接收阵列具有个阵元时,最大可分辨的相干数目为。仿真实验表明VSS算法的有效性,而且该算法只需要较少的阵元发射信号。     

Performance analysis of a noncoherently combined large apertureoptical heterodyne receiver

The performance of a noncoherently combined, multiple-mirror heterodyne receiver is analyzed. In the absence of atmospheric turbulence, the performance of the noncoherently combined receiver is shown to be inferior to that of a monolithic, diffraction-limited receiver with equivalent aperture area. when atmospheric turbulence is taken into consideration, however, the efficiency of a monolithic aperture heterodyne receiver, is limited by the phase coherence length of the atmosphere, and generally does not improve with increasing aperture size. In contrast, the performance of a noncoherently combined system improves with an increasing number of receivers. Consequently, given a fixed collecting area, the noncoherently combined system can offer a superior performance. The performance of the noncoherently combined heterodyne receiver is studied by analyzing the combining loss of the receiver SNR (signal-to-noise ratio). It is shown that, given a constant collecting area, the performance of the combined receiver is optimized when the diameter of each of the individual receivers is on the order of the phase coherence length r₀ of the atmospheric turbulence     

Maximum Likelihood Phase Demodulation for Spatially Turbulent Channels

The likelihood equation for the maximum likelihood estimate (MLE) of a temporal phase modulation (a sample function of a wide-sense stationary zero-mean Gaussian random process) received in a spatial aperture after transmission through a spatially turbulent channel and addition to a white Gaussian noise field is derived for unknown, Gaussian, and limiting cases of log-normal received signal fields. The received signal field estimation performed by the demodulator structure is explicitly exhibited and other clarifications made. The likelihood equation is linearized and solved and the resulting mean-square estimation error calculated. The Cramer-Rao lower bound is given. In an example it is shown that the linear estimate is efficient and has variance inversely proportional toM, the number of coherence areas required to disjunctly cover the receiver spatial aperture. A heterodyne receiver with an optimized aperture weighting is shown to use essentially only one coherence area; its asymptotic mean-square error is also calculated.     

Optical heterodyne detection of an atmospherically distorted signal wave front

The effect of atmospheric distortion of an optical wave front on the performance of an optical heterodyne detection system is examined theoretically. With the distortion measured by the phase and log-amplitude structure functions, an exact expression is derived relating the distortion to the detector signal-to-noise ratio. It is shown that because of distortion there is a limit to the achievable signal-to-noise ratio no matter bow large the detector collection aperture is. The minimum aperture diameter which will (nearly) achieve this limit, called the efficiency saturation dimension r0, is obtained in terms of the phase and log-amplitude structure fractions. Using the current theory for propagation in a turbulent medium, an expression for r0is obtained for a receiver in the atmosphere looking at a source above the atmosphere. The expression is in terms of the wavelength, the receiver altitude, and the zenith angle of the receiver line-of-sight.     

An Asymptotically Optimal Receiver for Heterodyne Optical Communication

An incoherent receiver is derived for the heterodyne optical channel under the assumption of Phase coherence at the start of each transmitted sequence. The receiver is optimal in the limit of small bit intervals with respect to the coherence time of the laser oscillators, and reduces to that proposed by Jeromin and Chan [1] when no initial phase coherence is present. Computer simulations indicate that the small amount of performance improvement obtained by resolving an initial phase uncertainty may not justify the extra complexity needed, for the data rates currently considered.     

Receiver Performance for Heterodyne Optical Communication

A model for the heterodyne optical channel that is consistent with experimental observations is introduced, and a general maximum-likelihood sequence estimation receiver that accounts for laser phase instabilities is proposed using the generalized log-likelihood functional. The performance of various suboptimal "estimator-correlator" receivers, obtained from optimal, nonimplementable receiver equations, is studied by simulation and compared to that of an incoherent receiver for practical parameter values. It is observed that, for hard decisions, the proposed receiver performs significantly better than an incoherent receiver for bit intervals larger than about half the coherence time of the laser oscillators, with larger gains to be expected when soft decisions on long sequences are made. A way of reducing the complexity of the soft-decisions receiver for long sequences is discussed.     

基于AlGaN/GaN HEMT太赫兹探测器的340 GHz无线通信接收前端

利用天线耦合AlGaN/GaN HEMT太赫兹探测器的自混频和外差混频效应,分别设计并测试了340 GHz频段直接检波式和外差混频式接收机前端。通过接收机信噪比的测量和接收功率的定标,得到了两种接收机的等效噪声功率。直接检波模式下探测器的响应度约为20 mA/W,直接检波模式和外差混频模式下接收机的等效噪声功率分别约为?64.6 dBm/Hz1/2和?114.79 dBm/Hz。在相同的载波功率和接收信号带宽条件下,当本振太赫兹波功率大于?7 dBm时,外差混频接收的信噪比优于直接检波的信噪比。当本振功率大于0 dBm时,外差混频接收机表现出优良的解调特性,其信噪比高出直接检波接收机的信噪比10 dB以上。     

The antenna properties of optical heterodyne receivers

An optical heterodyne receiver is, in effect, both a receiver and an antenna. As an antenna it has an effective aperture or capture cross section AR(Ω) for plane wave signals arriving from any direction Ω. The wavefront alignment between signal and local-oscillator (LO) beams required for effective optical heterodyning may be summarized in the "antenna theorem" ∫∫AR(Ω)dΩ = [η²/η²]λ²where the moments of the quantum efficiency η are evaluated over the photosensitive surface. Thus, an optical heterodyne having effective aperture ARfor signals arriving within a single main antenna lobe or field of view of solid angle ΩRis limited by the constraint ARΩR≈ λ². Optical elements placed in the signal and/or LO beam paths can vary the trade-off between ARand ΩRbut cannot change their product. It is also noted that an optical heterodyne is an insensitive detector for thermal radiation, since a thermal source filling the receiver's field of view must have a temperature T ≈ [In (1 + η)]^-1hf/k to be detected with S/N ≈ 1. Optical heterodyning can be useful in practical situations, however, for detecting Doppler shifts in coherent light scattered by liquids, gases, or small particles. Another antenna theorem applicable to this problem says that in a scattering experiment the received power will be ≲ Nσλ/4π times the transmitted power, where N is the density of scatterers and σ is the total scattering cross section of a single scatterer. The equality sign is obtained only when a single aperture serves as both. transmitting and receiving aperture, or when two separate apertures are optimally focused at short range onto a common volume.     

An improved solution for integrated array optics in quasi-opticalmm and submm receivers: the hybrid antenna

A dielectric lens antenna that is a special case of an extended hemispherical dielectric lens and is operated in the diffraction-limited regime is considered. The dielectric lens antenna is fed by a planar antenna that is mounted on the flat side of the dielectric lens antenna, using it as a substrate, and the combination is termed a hybrid antenna. Beam pattern and aperture efficiency measurements were made at millimeter and submillimeter wavelengths as a function of the extension of the hemispherical lens and of lens size. An optimum extension distance for which excellent beam patterns and simultaneously high aperture efficiencies can be achieved is found experimentally and numerically. At 115 GHz the aperture efficiency was measured to be (76±6)% for a diffraction-limited beam with sidelobes below -17 dB. Results for a single hybrid antenna with an integrated superconductor-insulator-superconductor (SIS) detector and a broadband matching structure at submillimeter wavelengths are presented. The hybrid antenna is space efficient in an array due to its high aperture efficiency, and is easily mass produced, thus being well suited for focal plane heterodyne receiver arrays     

Chirped Lidar Using Simplified Homodyne Detection

A simplified homodyne detection scheme for linear FM modulated lidar is presented in which pulse dechirping is performed in the optical domain. This method provides quantum limited detection sensitivity with much less receiver complexity compared to heterodyne detection systems. Another advantage of this approach is the reduced bandwidth requirement for the photodetector. This removes the limit on the chirp bandwidth, and enables the use of more efficient photodiodes with larger detector area. A field trial using a 5-in aperture diameter commercial telescope and a 370-m target range verified the sensitivity estimation and demonstrated the feasibility of this technique.      

Terahertz Bistatic Synthetic Aperture Radar for 1-D Near-Field High-Resolution Imaging

Considering the difficult transceiver-isolation problem of the monostatic synthetic aperture radar (SAR) in the terahertz (THz) band, this paper proposes a compact THz bistatic SAR (BiSAR) geometry. The system allows the separately distributed transmitter and receivers. At the receiving end, there are a direct-wave receiver and an echo receiver, both operating at the heterodyne and in-phase mode. The echo receiver runs along a linear rail to fulfill the scene scanning, while the direct-wave one is fixed as a reference. Furthermore, assuming that the receivers are synchronized, both the problem of synchronization between the separated transmitter and receivers and the problem of timing at the signal acquisition would be solved by utilizing the high coherence between the echo and the direct wave. Based on such a system, the application of THz BiSAR for one-dimensional imaging is taken into consideration. Then, a high-resolution imaging algorithm is proposed benefitting from the total least squares estimating signal parameters via rotational invariance techniques (TLS-ESPRIT) and the spatial smoothing process (SSP). The imaging performance is then demonstrated by both simulations and the experiments in the 0.183 THz.