Spectrum sensing and communication functionalities verification in cognitive radio

This paper is intended to discuss hardware setup implementation for realizing the spectrum sensing and communication functionalities of a five‐port integrated ultrawideband and narrowband antenna system. The five‐port integrated antenna system consists of one ultrawideband antenna and four narrowband antennas. The ultrawideband antenna is used for spectrum sensing in cognitive radio, whereas the four narrowband antennas are used for communication. In order to validate functionalities of the antennas, their spectrum sensing and communication performance is verified using an arbitrary waveform generator, real‐time signal analyzer, and Universal Software Radio Peripheral. The ultrawideband antenna is able to sense the various frequencies transmitted by the arbitrary waveform generator. These transmitted signals from the arbitrary waveform generator are treated as busy spectrum channels in the cognitive radio environment. The narrowband antennas are able to perform communication by transmitting the signals at identified spectrum holes. The sensed signals are observed on a real‐time signal analyzer, and the communication signals are viewed in LabVIEW software for which a real‐time signal reception algorithm is used. This signal reception is performed using Universal Software Radio Peripheral.     

The simultaneous interpolation of antenna radiation patterns inboth the spatial and frequency domains using model-based parameterestimation

The Pade rational function fitting model commonly used for model-based parameter estimation (MBPE) in the frequency domain is enhanced to include spatial dependence in the numerator and denominator coefficients. This allows the function to interpolate an antenna radiated electric field pattern in both the frequency and spatial domains simultaneously, such that a single set of coefficients can be used to accurately reconstruct an entire radiation pattern at any frequency in the fitting-model range. A simple procedure is introduced for transforming interpolated electric fields into gain patterns using input impedance versus frequency curves also obtained via MBPE. The utility of this method is demonstrated by applying it to a dipole antenna over a frequency range of 150-950 MHz and using a polynomial representation in &thetas; for the coefficient spatial dependence. It is also used to estimate radiation patterns for a three-element Yagi array between the frequencies of 470 and 500 MHz using a binomial representation for the spatial variation that includes terms dependent on &thetas; as well as φ. The use of this method for interpolating radiation patterns has at least two significant advantages; one being large compression ratios for the amount of data that must be stored to accurately reproduce patterns and the other being a significant decrease in the amount of time required for modeling problems with large computational domains     

Calcul de la réponse d’une antenne Yagi à une impulsion de champ électromagnétique

The purpose of this paper is to compute the response of a 100 MHz Yagi antenna to an electromagnetic pulse (rise time 10 ns, pulse width 200 ns). Two different techniques are investigated. With the first one, integral equations are solved in the frequency domain and then a Fourier transform is performed. With the second one, Maxwell’s equations are solved everywhere within the space around the antenna, using a finite difference method in the time domain. To solve the very problem dealt with, the time domain method turns out to be the more efficient. The overall antenna response can be divided into a short time waveform followed by a long time one. The long time response exhibits the same directivity effect as the one expected from the antenna when operated at its nominal frequency. The short time response depends also on the azimuth but in a rather different way. That last point can be accounted for when combining the differential and common modes of the driven folded dipole.     

低频/甚低频电磁脉冲测量系统研究

为了准确地测量电磁脉冲（electromagnetic pulse,EMP）的波形,开展低频/甚低频（low-frequency/very low-frequency,LF/VLF）EMP的形成和传播机理研究,识别不同EMP的特征,并以此为依据来识别、确认雷电及核爆炸等现象,提出了一种高分辨率的EMP原始波形测量系统方案,同时针对LF/VLF这一较低的频段提出了一套有效的系统标校方法,实现了对EMP的连续、准确测量和快速有效判别,并能通过网络实时上传数据.系统采用正交环磁场天线和平板电容电场天线实现信号的接收,设计了低噪声高保真的信道调理电路和高速数据采集电路来实现信号的采集,利用高精度的授时模块为EMP信号标记时间戳,最后结合多点监测波形实现EMP定位估计.实测结果表明:该系统能够给出高分辨率、高精度的LF/VLF EMP波形;利用该系统组网可以实现远距离EMP源的定位,定位精度与目前近距离的定位手段相当.通过长期的运行,验证了该系统具有高可靠、低失真、判别准确、实时性强等特点.     

Synthetic aperture inversion for arbitrary flight paths and nonflat topography

The paper considers synthetic aperture radar (SAR) and other synthetic aperture imaging systems in which a backscattered wave is measured from positions along an arbitrary (known) flight path. The received backscattered signals are used to produce an image of the terrain. We assume a single-scattering model for the radar data, and we assume that the ground topography is known but not necessarily flat. We focus on cases in which the antenna footprint is so large that the standard narrow-beam algorithms are not useful. We show that certain artifacts can be avoided if the antenna and antenna footprint avoid particular relationships with the ground topography. We give an explicit backprojection imaging algorithm that corrects for the ground topography, flight path, antenna beam pattern, source waveform, and other geometrical factors. For the case of a non-directional antenna, the image produced by the above algorithm contains artifacts. For this case, we analyze the strength of the artifacts relative to the strength of the true image. The analysis shows that the artifacts can be somewhat suppressed by increasing the frequency, integration time, and the curvature of the flight path.     

Resistivity Surveying as an Aid in Sanguine Site Selection

A practical extremely low frequency (ELF) horizontal antenna requires a site of low effective conductivity. This paper describes the application of certain dc resistivity sounding methods in rapidly evaluating potentially useful areas within geologically interesting regions, and in determining both basement anisotropy and the direction of the principal (maximum) resistivity axis. Apparent resistivity data from dc array expansion is interpreted with a horizontal layered earth model using a "generalized inverse" method of fit. With layered models over principal axes the effective conductivity to be experienced with a horizontal wire antenna of arbitrary orientation is readily calculated. Salient results for three geographical regions are presented.     

Antenna pulsewidth distortion paradox explained

A paradox of why there can be no pulsewidth distortion for an antenna array which is receiving or transmitting a wide-bandwidth chirp waveform is presented and explained. When a very narrow pulse is received at an off-boresite angle by an antenna array using phase steering, the pulse becomes distorted, that is, dispersed. The pulsewidth spreading is equal to the difference in the times of arrival of the pulse at the opposite ends of the antenna. However, if a wideband chirp waveform instead is incident on the antenna, no dispersion can be made to occur if the antenna is resteered toward the signal source during the time the chirp waveform is being received. The steering prevents frequency scanning of the antenna beam because of the changing carrier frequency of the linear FM chirp waveform. This represents a paradox because the dispersion still exists over the antenna. The author explains why the chirp signal is received undistorted at the output of the antenna in spite of the dispersion     

基于开口环谐振器的双频通带可调滤波天线

该文提出了一款通带独立可调的高选择性双频滤波天线。首先设计了一款微带耦合式馈电的双频带通滤波器,通过在微带馈线上引入一个金属通孔,实现源负载耦合,改善裙边选择性。同时,给出了滤波器对应的等效电路模型。在此基础上设计了一款椭圆形单极子天线,利用滤波天线综合设计方法与滤波器集成实现滤波天线。单极子天线带宽覆盖滤波器带宽,仅改变开口环电长度即可实现中心频率可调。为了验证设计加工和测试了相应天线。结果表明,天线工作在2.31 GHz与3.27 GHz时,相对带宽分别为10.3%和7.7%,增益分别为3.2 dBi和4.1 dBi,有4个辐射零点,中心频率可调范围为1.8~4.8 GHz。测量与仿真结果基本吻合。     

Fast frequency sweep technique for the efficient analysis of dielectric waveguides

This paper describes a new approach to spectral response computations of an arbitrary two-dimensional (2-D) waveguide. This technique is based on the tangential-vector finite-element method (TVFEM) in conjunction with the asymptotic waveform evaluation (AWE) technique. The former is used to obtain modes characteristics for a central frequency, whereas the latter employs an efficient algorithm to compute frequency moments for each mode. These moments are then matched via Pade approximation to a reduced-order rational polynomial, which can be used to interpolate each mode over a frequency band with a high degree of accuracy. Furthermore, the moments computations and subsequent interpolation for a given set of frequency points can be done much more rapidly than just simple simulations for each frequency point.     

A broad-band resistively loaded V-antenna: experimental results

A broadband receiving antenna was realized using a resistively loaded thin-film V-monopole and a 500-Ω oscilloscope probe. The pulse-receiving performance of the antenna was evaluated in a time-domain antenna range, for various directions of the incident pulse. The antenna is capable of receiving a 520 ps electromagnetic pulse, incident within 45° from the boresite direction, with good fidelity. The antenna was also tested in a transverse electromagnetic cell in the time and frequency domains. The frequency-domain receiving transfer function of the antenna was found to be within -57±3 dB in the 22 MHZ to 1.1 GHz frequency range     

Additional degree of freedom in phased-MIMO radar signal design using space-time codes

In this paper, an additional degree of freedom in phased multi-input multi-output (phased-MIMO) radar with any arbitrary desired covariance matrix is proposed using space-time codes. By using the proposed method, any desired transmit covariance matrix in MIMO radar (phased-MIMO radars) can be realized by employing fully correlated base waveforms such as phased-array radars and simply extending them to different time slots with predesigned phases and amplitudes. In the proposed method, the transmit covariance matrix depends on the base waveform and space-time codes. For simplicity, a base waveform can be selected arbitrarily (ie, all base waveforms can be fully correlated, similar to phased-array radars). Therefore, any desired covariance matrix can be achieved by using a very simple phased-array structure and space-time code in the transmitter. The main advantage of the proposed scheme is that it does not require diverse uncorrelated waveforms. This considerably reduces transmitter hardware and software complexity and cost. One the receiver side, multiple signals can be analyzed jointly in the time and space domains to improve the signal-to-interference-plus-noise ratio.     

Waveform optimizations for ultrawideband radio systems

Solutions are presented for various optimizations of transient waveforms and signals used in ultrawideband radio systems. These include the transmit antenna generator waveform required to maximize receive antenna voltage amplitude (with bounded input energy), the transmit antenna generator waveform that provides the "sharpest" received antenna voltage waveform, and the transmit antenna generator waveform that maximizes received energy with an inequality constraint on the radiated power spectral density. Using variational methods, general optimization results are derived for arbitrary antennas, including the effects of generator and load impedances, and numerical examples are provided for lossless dipoles and resistively loaded dipoles using moment method solutions. Closed-form results are provided for short dipole antennas for some special cases.     

一种基于AD9857的任意波形合成器

介绍了一种基于直接数字频率合成（DDS）技术、采用数字正交上变频器AD9857实现的任意波形合成器，该波形合成器可以在6～60MHz范围内合成任意波形。仿真和实验结果证明了该合成器的可行性和实用性。     

Effect of aperture feed and reflector configuration on the time- and frequency domain radiation patterns of reflector impulse radiating antennas

The prompt off-boresight radiation from an arbitrary reflector impulse radiating antenna (IRA) is considered in both the time and frequency domains using a time-domain physical optics formalism. The theory is used to compare the performance between reflector IRAs with common transverse electromagnetic feed configurations that have been designed to maximize the boresight fields. It is found that moving the feed arms of reflector IRAs toward the vertical helps to reduce sidelobe levels, as reflector IRAs with their feed arms oriented at 60/spl deg/ from the horizontal have sidelobes that are 4-5 dB lower in the H plane and 1-2 dB lower in the E plane than more traditional reflector IRAs with feed arms oriented at 45/spl deg/. The lower sidelobes are accompanied by a higher peak gain, albeit with a wider beamwidth. There are corresponding significant differences in the time domain waveforms that result. The theory is verified by comparison with experimental data from a half IRA with feed arms at 45/spl deg/. The peak fields, pulse widths, and overall waveform shapes agree well between theory and experiment, though there are temporal asymmetries in the measured data that are not predicted by the theory.     

基于耦合振荡器阵列的非线性有源天线阵

利用耦合振荡器阵列实现有源集成天线阵电扫描,可以实现相控阵天线高效率、低成本、小型化。文中利用Y-参数对利用任意耦合网络实现的耦合振荡器阵列进行了分析,得到了系统幅度及相位非线性动力系统的微分方程组,然后针对利用耦合振荡器阵列实现非线性有源天线阵列这一实际应用,提出了一种不用移相器实现有源天线阵列波束扫描的方法,并进行了稳定性分析。最后利用计算机仿真验证了该方法的可行性。     

Application of Characteristic Modes and Non-Foster Multiport Loading to the Design of Broadband Antennas

A conceptual framework is proposed to systematically design antennas with broadband impedance and pattern characteristics using multiple reactive loads. Antennas of arbitrary geometry can have their bandwidths expanded using this technique. The technique is applied to a narrow band thin wire dipole antenna to demonstrate its main features. It is shown that the loaded antenna resonates a desired current over a wide frequency band. The loads are shown to require non-Foster elements when realized. Simulations demonstrate the broadband characteristics of both the dipole input impedance and radiation pattern.      

DDS任意波形发生器研究

工程实践中往往会应用到多种信号形式的信号源。为了实现信号源中信号的多样性,采用直接数字额率合成技术设计了一种任意波形发生器。波形存储器中存储的波形数据是任意波形发生器的关键,详细说明了使用Matlab生成波形存储器中波形数据的方法。采用上述方法得到了一种工程实践中常用的扫频信号仿真波形图。实验表明,所设计的任意波形发生器实现简单,可编程,适合广泛应用于工程实践中。     

Waveform analysis of jitter in SRTS using continued fraction

This paper presents a method to analyze jitter waveform in synchronous residual time stamp (SRTS) using continued fraction. It is shown first that the jitter waveform can be decomposed into triangular wave components and that each component corresponds to a rational approximation value of a parameter called residue. Next, quasi-simple continued fraction expansion of the residue is defined to obtain rational approximation values of the residue in a systematic manner. This method gives a series of rational approximation values of the residue. It corresponds to the decomposition of the SRTS jitter waveform into triangular wave components. Many characteristics of the jitter waveform are derived by the quasi-simple continued fraction expansion of the residue. Especially, the average amplitude and period of each triangular wave component are given     

A mathematical model for the impedance of the cavity-backed slot antenna

A mathematical model is derived that relates the impedance of a cavity-backed slot antenna to that of an identical slot which is free to radiate on both sides of a large ground plane. The model, which utilizes empirical constants from a previous experimental investigation, provides a continuously variable function of frequency and cavity depth for the impedance of a cavity-backed slot of fixed length and cavity cross section. This function is then compared with previously found experimental values and two theoretical solutions, one using a variational method and the other using the complex Poynting theorem.     

基于阶梯波形的高效时间调制多波束天线

时间调制阵列(time-modulated array, TMA)可产生不同指向的边带多波束，但存在效率低、波束幅度一致性差等缺陷. 本文基于阶梯波形提出了一种低成本高效率的时间调制边带多波束实现方法. 首先根据任意多波束空间覆盖需求得到TMA的频谱和时域波形，然后基于数控衰减器和0/π移相器的硬件特性实现对理论调制波形的拟合，最终通过阶梯波形优化抑制无用边带功率和边带波动，实现稳定高效的多波束扫描. 以8单元TMA为例实现基频、±1、±2次边带对应的5边带，通过调整数控衰减器的衰减步进值寻求调制波形最优解，最终仿真结果显示边带电平抑制在?31.20 dB，工作边带功率波动仅为0.68 dB. 该多波束时间调制天线具有成本低、效率高、波束幅度一致性好、无用边带抑制度高等优点，提升了时间调制技术在多波束天线领域的应用价值.