Pulse Position Amplitude Modulation for Time-Hopping Multiple-Access UWB Communications

In this letter, we propose a new modulation scheme called pulse position amplitude modulation (PPAM) for ultra-wideband (UWB) communication systems. PPAM combines pulse position modulation and pulse amplitude modulation to provide good system performance and low computational complexity. The channel capacity of PPAM is determined for a time-hopping multiple-access UWB communication system. The error probability and performance bounds are derived for a multiuser environment.     

光通信中双幅度脉冲位置调制的功率谱分析

在研究数字脉冲调制时,对调制方式的功率谱分析十分必要。详细推导了随机双幅度脉冲位置调制(DAPPM)序列的功率谱密度,并分别对连续谱和离散谱进行了分析。数值结果表明,DAPPM的功率谱形状类似于一般的抽样函数,并且包含直流分量,幅度因子α只影响功率谱的幅度,不影响功率谱的形状;时隙基频分量存在与否与脉冲占空比有关;时隙基频幅度强弱依赖于调制阶数和脉冲占空比。当占空比为0.5时,时隙基频分量最大,调制阶数每增大1,幅度衰减约5 dB。同时指出,当调制阶数较大时,随机DAPPM序列难以直接提取时隙基频分量。     

跳时脉幅脉位混合调制超宽带通信性能研究

针对目前超宽带通信调制方式误码率高的问题，提出正交脉冲脉幅脉位混合调制（OAPPM）改善系统性能的方法。OAPPM调制方式是把信息调制在信号的位置和幅度上，接收端利用最佳检测器进行接收。文中分析了TH—OAPPMUWB系统的信道容量和误符号率并进行了仿真。结果表明：该方法与传统的PPM和PAM相比，降低了误符号率和系统复杂度，较好地改善了UWB通信系统性能。     

微波系统脉冲幅相的宽带自动检测

介绍一种新研制成的宽频带微波系统脉冲信号幅度、相位自动检测系统，它的检测原理、特点和实验结果、应用范围等。     

基于超材料的太赫兹幅度调制器设计

提出了一款基于超材料的太赫兹幅度调制器,其结构由开口"弓"形超材料结构、高电子迁移率晶体管、斜十字馈线和碳化硅衬底四部分构成."弓"形超材料结构开口处的连通和断开两种状态将对通过该结构的太赫兹波产生不同的响应.在开口处添加高电子迁移率晶体管可模拟开口连通和断开的效果.当对晶体管上的栅极不施加偏压时,超材料结构开口相当于导通,对太赫兹波透射系数高;当对晶体管上的栅极施加偏压时,超材料结构开口相当于断开,对太赫兹波透射系数低.仿真结果表明,在0.22 THz处,对晶体管栅极不施加偏压时,调制器的透射系数为0.579;对晶体管栅极施加偏压时,调制器的透射系数为0.040.通过公式计算得到其调制深度为93％,而且对x和y极化入射波具有不敏感的特性.同时,通过分析0.22 THz处的电场分布和表面电流分布研究了该太赫兹调制器的工作原理.所设计的太赫兹调制器具有调制深度高、结构简单和易于加工等特点,在太赫兹通信领域具有广阔的应用前景.     

基于混沌脉冲位置调制的实时超声测距系统串扰抑制

 超声测距系统中各换能器之间存在的干扰(即串扰)会严重制约测距的准确性;为消除串扰,采用混沌脉冲位置调制构造每个超声换能器的短发射序列;相关分析法被用于识别回声序列.分析了超声串扰产生原因,并给出了脉冲占空比与各次谐波能量的对应关系.为获得最佳的相关效果,采用遗传算法对Logistic混沌满映射调制的短序列脉冲间隔做了优化.所提方法在DC-DC(直流－直流)激励的静电型超声测距系统中获得了验证.结果表明,在合理配置脉冲间隔的情况下,混沌脉冲位置调制可用于消除实时超声测距系统的串扰.     

On Space–Time Coding With Pulse Position and Amplitude Modulations for Time-Hopping Ultra-Wideband Systems

In this work, we propose novel families of space-time (ST) block codes that can be associated with impulse radio ultra-wideband (IR-UWB) communication systems. The carrier-less nature of this nonconventional totally real transmission technique necessitates the construction of new suitable coding schemes. In fact, the last generation of complex-valued ST codes (namely, the perfect codes) cannot be associated with IR-UWB systems where the phase reconstitution at the receiver side is practically infeasible. On the other hand, while the perfect codes were considered mainly with quadrature amplitude modulation (QAM) and hexagonal (HEX) constellations, IR-UWB systems are often associated with pulse-position modulation (PPM) and hybrid PPM-PAM (pulse-amplitude modulation) constellations. In this paper, instead of adopting the classical approach of constructing ST codes over infinite fields or for the perfect codes), we study the possibility of constructing modulation-specific codes that are exclusive to PPM and PPM-PAM. The proposed full-rate codes are totally real, information lossless, and have a uniform average energy per transmit antenna. They permit to achieve a full diversity order with any number of transmit antennas. In some situations, the proposed schemes have an optimal nonvanishing coding gain and satisfy all the construction constraints of the perfect codes in addition to the constraint of being totally real. Simulations performed over realistic indoor UWB channels showed that the proposed schemes outperform the best known codes constructed from cyclic division algebras.     

太赫兹脉冲能量测量

利用太赫兹波的单啁啾脉冲电光探测技术,通过调节实验装置中λ/4波片晶轴与探测激光线偏振方向成45°,可以巧妙地得到太赫兹波的绝对电场强度.利用CCD阵列,通过抽运探测的方法可以得到太赫兹脉冲的光斑尺寸,进而在两步实验的基础上计算得到太赫兹脉冲的能量.提供了一种行之有效的测量太赫兹脉冲能量的实验方案.     

Real-Time Amplitude and Phase Imaging of Optically Opaque Objects by Combining Full-Field Off-Axis Terahertz Digital Holography with Angular Spectrum Reconstruction

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials.     

太赫兹脉冲整形技术研究进展

太赫兹(THz)脉冲整形技术在量子理论、生物医学成像、安全检查、亚毫米波通信等领域都具有重要的学术价值和应用前景。概述了基于飞秒脉冲的整形技术和新型太赫兹辐射材料整形技术,分析了太赫兹脉冲整形器件整形技术目前的研究状况,并对各种整形方法的发展进行了展望。     

A Novel Trellis Coded Overlapping Amplitude and Pulse Position Modulation Scheme for Gamma-Gamma Channel Free-Space Optical Communication

A novel trellis coded-4×8 overlapping amplitude and pulse position modulation(TC-4×8AOPPM) scheme is proposed to enhance bit error rate(BER) performance of free-space optical communication(FSO) system. In addition, an uncoded AOPPM referential scheme is also designed. The schemes manage to decrease BER by designing gamma-gamma(GG) channel applicable decoding and demodulation methods. Simulation results of 8, 16 and 64-state TC-4×8AOPPM show 2.5-3.3 dB SNR gain against traditional TC-4×8AOPPM scheme respectively. Thus significant BER performance improvement is achieved and the reliability of the FSO system is also enhanced.     

太赫兹超分辨率成像评述II:实时成像

为提升成像速度和分辨率,实时太赫兹超分辨率成像方法近年来受到了越多越多的关注。基于此种成像手段构建的超透镜能突破衍射极限,在单次照射下得到微纳米级别分辨率的图像,推动了太赫兹成像的应用拓展。文中介绍了实时超分辨率成像的发展历程,总结了两种太赫兹频段实时超分辨率成像方法的基本原理、研究进展和存在的问题,包括基于金属周期结构和基于石墨烯结构的方法,最后对实时太赫兹超分辨率成像的发展作出展望。     

微弱电磁脉冲信号低 失真度实时检测的FPGA实现

为了实时检测宽带噪声背景中的微弱电磁脉冲信号，研究了基于强耦合Duffing振子的微弱脉冲信号检测算法及其FPGA （Field Programmable Gate Array）实现技术.利用脉冲信号在强耦合Duffing振子间引起的广义"阱内失同步"现象，使用变量差分原理进行微弱脉冲信号检测与识别.首先，利用具有最大稳定域和最小截断误差的实时四阶Runge-Kutta算法求解Duffing微分方程；其次，设计了适合在FPGA上运行的流水线结构并进行了优化；为了提高检测速度，设计了数据分段并行算法；通过增加FIFO存储器将阈值检测过程与波形恢复过程分离，解决了数据分段带来的波形失真问题.最后，闪电电磁脉冲实时检测实验表明系统最小可探测信噪比可达-18dB，同时也证明了系统的有效性.     

Tailoring Terahertz Propagation by Phase and Amplitude Control in Metasurfaces

Metasurfaces have been very successful at demonstrating the ability to control the wave propagation over the broad electromagnetic spectrum in recent years. The output wavefronts can be controlled at will, by encoding specially designed abrupt changes of electromagnetic parameters into the metasurfaces, such as phase and amplitude. Constituted by a single- or few-layer of planar structures, metasurfaces are straightforward in design and fabrication, thus promising many credible applications. Moreover, such control concept can be further extended to the surface wave regime. In this review, we present our recent progress on metasurfaces capable of tailoring the propagation of both free-space and surface terahertz waves. Following an introduction of the basic concept and theory, a number of unique terahertz metasurfaces are presented, showing the ability to device ultra-thin and compact functional terahertz components.     

Improving Spatial Resolution of Real-Time Terahertz Near-Field Microscope

Terahertz (THz) wave imaging for biomaterial samples such as cells requires real-time acquisition and high spatial resolution beyond the diffraction limit. The existing THz near-field microscopes are based on raster-scanning techniques, and are therefore not able to image and trace morphological changes in a large area. With the recent advances in high-power THz sources, we demonstrated how to achieve high spatial resolution over a large size using a conventional charge-coupled-device (CCD) camera with the electro?Coptic (EO) sampling technique. In this paper, we determine a limiting factor that restricts spatial resolution in our near-field microscope. By calculating the imaging performance of the probe beam together with THz wave diffraction, we show that the most relevant factor is the diffraction inside the EO crystal. Near-field imaging of metal patterns using EO crystals with different thicknesses supports this calculation. A thin EO crystal is essential for achieving THz images with high spatial resolution.     

基于幅度变化点检测的多功能雷达脉冲列解析方法

 在电子情报领域,针对多功能雷达脉冲列解析中的基本波形提取问题,提出了一种新的基于统计变化点检测的方法.该方法利用相控阵天线波束离散扫描的特点,将基本波形提取问题转化成脉冲幅度序列变化点的检测问题.引入生物染色体分析中的循环二分算法来检测并估计幅度序列的多个变化点,进而得到基本波形的集合.该方法不需要任何先验知识,在较低信噪比的情况下仍然能够得到高精度的提取结果.仿真实验表明了该方法的有效性.     

Amplitude Calibration in Terahertz Time-Domain Spectroscopy Using Attenuation Standards

An amplitude calibration method for a terahertz time-domain spectroscopy system (THz-TDS) is proposed and demonstrated for transmittance measurement. The method is based on a direct comparison method using a terahertz attenuator as a reference standard. The measurement uncertainties of the THz-TDS are evaluated in transmittance measurements of metalized film attenuators as test samples. The relative expanded uncertainty is found to be 6.4–14.0% (k = 2) at 1 THz for transmittances ranging from 1 to 0.001. In addition, the calibration capability is extended to the available frequency range of the THz-TDS by considering spectral flatness of the reference standard. Validations of the method are also conducted by comparing the measurements between different systems, and it is confirmed that they agree well with each other.     

Terahertz Parametric Real-Time Imaging of Jade Stone by Terasense

Wireless Personal Communications - The absence of an inspection technique for the jadestone internal structure is a significant problem for identifying density and structure inside a stone. A...     

基于FPGA的多道脉冲幅度分析器的设计

介绍了一种基于FPGA(Field-Programmable Gate Array)的多道脉冲幅度分析器(MCA)的设计。硬件上使用A3P250 FPGA作为核心器件。首先,A3P250 FPGA通过ADC采样模块,将数据采样到FPGA内部,然后利用寻峰模块进行基线判别,判断脉冲是否到来。当有脉冲到来时,寻峰模块使用比较法提取脉冲的最大值,接着成谱模块启动成谱功能,与此同时将成谱结果及实时数据发送给计算机。     

一种改进的脉冲幅度比较测量法

直流电压比较法和高采样数表法可对脉冲幅度进行准确测量,但仅适于周期低频信号,应用受到局限。本文采用常规电子仪器,提出了一种改进的脉冲幅度比较法,对其原理、方法和特点进行了阐述,并结合具体实例给出了不确定度分析。该方法不仅应用方便、准确度高,而且适于高频单次等各类脉冲的准确测量,有广阔的适用性。