Ionospheric distortions of high-frequency pseudorandom signals

The ionospheric propagation of shortwave signals with pseudorandom frequency shift are considered with allowance for their coherent and incoherent reception. The frequency ranges within which 1D propagation is realized over ionospheric radio links 2–8 Mm long are investigated, and the dispersion characteristics of these propagation channels are determined. The effect of the dispersion distortions on the reception of pseudorandom signals is investigated, and the maximal permissible signal parameters are estimated.     

Frequency bands for optimum propagation of complex signals through shortwave radio channels

A radio-engineering approach based on the dispersion characteristic and impulse response of a radio channel is proposed to describe the dispersion distortion of complex shortwave radio signals propagating through ionospheric radio channels. The impulse response of a wideband ionospheric radio channel is investigated using the polynomial representation of the dispersion characteristics, which are determined from oblique sounding of the radio channel by ultrawideband chirp signals. Frequency bands for optimal propagation of complex signals over radio links 3600 to 5700 km long are estimated.     

高频电离层信道4F4T4PSK系统性能

本文提出了高频电离层信道４Ｆ４Ｔ４ＰＳＫ通信系统性能分析方法，给出了其误码率表达式。４Ｆ４Ｔ４ＰＳＫ系统在时变信道上传输出为加性噪声和多条具有不同时延，不同频移和不同到达角传播路径信号的总和。误码率表达式考虑了信道中存在的瑞利衰落，频率与时间选择性衰落，四重频率分集及白噪声的影响。给出了该“不可克服误码率”与信道中存在的归一化最大多径时延和归一化多普勒散布的关系曲线，并根据其结果，讨论了该本制对多     

宽带短波信道建模综述

宽带短波通信系统作为中远程主要通信手段受到了人们的广泛关注,研究宽带短波系统的重点和难点是宽带短波的信道建模。Watterson信道模型作为比较经典的窄带模型,从它的基础上衍生出2种宽带短波信道模型,Watterson后接高斯随机延迟模型和Watterson后接群延迟特性滤波器模型。另外还介绍了电离层物理模型、Volger决定性模型、ITS信道模型及其改进、基于并行子路径结构的宽带信道模型、伪决定性信道模型和子带并行-宽带窄带化模型。对这几种模型的建模思想以及基本原理进行了叙述,对其复杂度进行了比较并且对可行性进行了分析。     

一种短波信道中数字信号调制方式识别算法

短波天波信道存在多径效应及冲击和高斯噪声.针对短波信道特点,提出先利用零记忆非线性系统抑制冲击噪声,后分段盲均衡,基于最大后验概率识别的分类方法.零记忆非线性系统可抑制冲击噪声,而让信号无失真通过.分段均衡减弱信道时变和参数估计误差对均衡器影响.由于冲击噪声的瞬时性和信道时变性,使每段信号均衡效果并不一样,采用最大后验概率的方法选用均衡效果最好的一段信号识别.仿真表明,在典型短波信道且含有冲击噪声情况下,信噪比10 dB时,平均识别概率大于90%,该算法适合在实际中应用.     

The effect of ionospheric reflected noise on the performance of anorbital-debris radar system

An orbital-debris radar system was designed to detect the presence of small objects in low Earth orbit by reflecting radio waves off the objects. The author provides a rigorous derivation of the ionospheric reflected noise power and provides an integration over the complete pulse period. Free electrons and ions contained in the ionosphere cause incoherent scatter of the radar signal. This ionospheric reflection tends to increase the noise at the terrestrial radar receiver. A parameter called the ionospheric scattering cross section per unit volume, which is a function of altitude, is useful for computing the power of the ionospheric reflection signal. The Doppler frequency speed of the ionospheric reflected signal is also a function of altitude. The ionospheric noise of a 9-GHz orbital-debris radar receiver is computed using these concepts. Annual and diurnal variations of the noise are included     

On the Capacity of Mid-latitude High Frequency Ionospheric Channel

In this paper we consider the theoretical characterization of the ionospheric transmission. More accurately, we derive a closed form expression of the average capacity for Mid-latitude High Frequency (HF) ionospheric channels. Heretofore, this problem has been studied for Rayleigh channels when each tap of the impulse response has a Rayleigh distribution without characterizing the variance of this distribution. In this paper, we extend these works to HF ionospheric channels by evaluating the variance of the amplitude attenuation versus the Doppler spread and then the channel capacity. For a multipath HF ionospheric channel, we model the Doppler phenomenon as a Gaussian profile which is suggested for HF environments. Finally, we derive a closed form expression of the average channel capacity using the probability density function (pdf) of the instantaneous impulse response. Numerical results on both simulated and real measured data are derived at the end of the paper.     

夏季短波Es场强计算方法研究

Es层是发生在电离层E层高度上的突发不规则结构,短波接收电平与Es和F层反射密切相关,Es对短波通信影响很大,而中国是Es的高发区.利用中国电离层垂测网的Es层临界频率foEs数据,分析了中国地区夏季Es空间分布特性及其对短波通信的影响.研究发现,夏季Es层可提高短波链路最高可用频率,扩宽了可用频段,提高了信号质量.基...     

Methodology and computer tools to semi-automate assessments of shortwave broadcast coverage resulting from multiple transmissions

A computer-assisted methodology, developed for using a point-to-point ionospheric propagation and communications system performance model to assess shortwave broadcast coverage, is presented. The listening area is approximated by a set of geographic coordinates (test points), and the required power gain (RPRG) for the link from each transmitter site to each test point is computed using the IONCAP ionospheric communications analysis and prediction program. The broadcasts may originate simultaneously from a single site or from multiple-transmitter sites (i.e., multicast). Figures of merit are assigned in terms of the RPG values, and area coverage statistics are computed for the simulated broadcasts. Three interactive computer programs were developed to permit a semi-automated multicast broadcast coverage analysis using IONCAP and a computerized equipment database. They are: propagation parameter set-up program; transmission plan set-up program; and multicast analysis program.<>     

短波通信选频中的电离层同化短期预报方法

为实现短波通信链路可用频率的短期预报,提出了一种基于卡尔曼同化技术的电离层特征参数短期预报方法.该方法以电离层观测网实时测量值为基础,对未来1~24 h的电离层参量foF2进行预报.通过对不同太阳活动期、不同季节中国区域预报的分析,结果表明：该方法短期预报均方根误差在0.57~0.72 MHz之间,相对误差在9%~12%之间;且中高纬地区的预报精度优于低纬地区.     

Software-defined radio technology in the problem concerning with the successive sounding of HF ionospheric communication channels

The problem concerning new software-defined radio technology is investigated as applied to the successive sounding of HF ionospheric communication radio channels. The algorithm for quadrature processing of chirp signals subjected to elementwise compression in the frequency domain is developed. It is established that a channel scattering function and a delay power profile are related to a compressed-signal spectrum and their basic parameters. Data on the key characteristic of HF communications modems are presented. The efficiencies of different modems are estimated using the experimental data on channel parameters. It is demonstrated that the aforementioned technology can be used to estimate not only HF communication channel parameters from the pass band of a radio line but also select the optimal one and, furthermore, makes it possible to combine communications and radio sounding systems in the single device.     

拉萨地区电离层长期变化特性研究

电离层环境变化对电子信息系统有重要影响,因此电离层变化特性研究具有重要意义.文中根据拉萨电波环境观测站30余年数据,研究了拉萨地区电离层F层临界频率f_oF₂、F层底高h'F、Es层临界频率f_oEs的日变化、季节变化和长期变化情况,得到一些有意义结论:电离层f_oF₂全年均值介于3~11 MHz,日变化显著,季节变化明显,随太阳活动变化幅度较大;电离层h'F全年均值介于200~420 km,日变化显著,随季节变化不明显,与太阳周期变化呈正相关关系;电离层f_oEs全年均值介于1~7 MHz,日变化显著,季节变化显著（夏季明显高于其他季节）,太阳周期变化不明显.研究电离层环境的长期变化特性,获取电离层参数长期变化规律,对通信导航、航天器等装备的设计和运行维护具有重要意义.     

Multipath-channel estimation and application to ionospheric channels

In this paper, the theory of linear time-variant systems is applied to multipath channels and, in particular, to ionospheric channels. Some useful formulas in time and frequency domains are given for both deterministic and random linear time-variant systems and multipath channels. The fundamental parameters of a multipath channel are the delay and the gain coefficient of each path, and the number of significant paths. We propose a methodology for estimating path-delays and path-coefficients and its application to the ionospheric channel estimation. Also, if path coefficients are stationary random processes, the power spectral density of the coefficients can be estimated from measured data of real (actual) channels. Finally, exhaustive computer simulations have been realized for testing the algorithms and a sample of the results is provided in the paper; also, estimation results for an actual narrowband HF ionospheric channel are provided, where up to four significant paths (rays) are detected with a Doppler spread lower than 0.2 Hz.     

穿过电离层电子密度不均匀体的脉冲平均到达时间

本文利用前向多重散射理论的新进展，推导了穿过电离层电子密度不均匀体脉冲平均到达时间的理论计算公式，其中包括背景介质折射效应的贡献、色散效应的贡献、电离层电子密度不均匀体的散射效应的贡献和色散效应的贡献，在水平分层背景电离层和高频近似的情况下，给出了背景电离层折射效应、一阶色散效应、电子密度不均匀体的散射效应和一阶色散效应对对称脉冲平均到达时间影响的计算公式。针对特定的电离层情况，分别计算了上述几种     

Mesostructured Arrays of Nanometer‐spaced Gold Nanoparticles for Ultrahigh Number Density of SERS Hot Spots

A novel one‐trough synthesis via an air‐water interface is demonstrated to provide hexagonally packed arrays of densely spaced metallic nanoparticles (NPs). In the synthesis, a mesostructured polyoxometalate (POM)‐silicatropic template (PSS) is first self‐assembled at the air‐water interface; upon UV irradiation, anion exchange cycles enable the free‐floating PSS film to continuously uptake gold precursors from the solution subphase for diffusion‐controlled and POM‐site‐directed photoreduction inside the silica channels. NPs ≈ 2 nm can hence be homogeneously formed inside the silica‐surfactant channels until saturation. As revealed via X‐ray diffraction, small‐angle X‐ray scattering (SAXS), grazing incidence SAXS, and transmission electron microscopy, the Au NPs directed by the PSS template are arrayed into a 2D hexagonal lattice with inter‐channel spacing of 3.2 nm and a mean along‐channel NP spacing of 2.8 nm. This corresponds to an ultra‐high number density (≈10¹⁹ NPs cm^?3) of narrowly spaced Au NPs in the Au‐NP@PSS composite, leading to 3D densely deployed hot‐spots along and across the mesostructured POM‐silica channels for surface‐enhanced Raman scattering (SERS). Consequently, the Au‐NP@PSS composite exhibits prominent SERS with 4‐mercaptobenzoic acid (4‐MBA) adsorbed onto Au NPs. The best 4‐MBA detection limit is 5 nm , with corresponding SERS enhancement factors above 10⁸.     

电离层非相干散射谱和自相关函数估计算法

非相干散射谱对电离层各物理参数的研究具有至关重要的作用。现有非相干散射谱模型推导和计算较为复杂,且无法给出相应的自相关函数模型。该文重新推导了电离层非相干谱的简化模型,并提出相应的自相关函数模型。在此基础上,针对非相干散射信号处理中零时延处距离模糊大、分辨率较低,导致自相关函数在不同时延处均衡性较差,影响电离层散射谱估计性能的问题,提出使用自相关函数模型估计零时延自相关值的数据拟合方法。考虑到计算复杂度,提出利用多项式函数逼近自相关函数模型的快速实现方法。最后,通过对实测回波数据处理结果的对比分析,证明了该方法的可行性,对提高电离层探测精度具有重要意义。

     

Estimating the scattering distribution of the received signals in multipath fading channels

This paper presents a novel approach for estimating the distribution of the incoming waves at the mobile unit antenna, i.e. the scattering distribution, in a typical micro-cellular system. This estimate is vital in determining many system parameters of interest as well as designing unbiased estimators for the velocity of mobile units in micro-cellular systems. The proposed approach deploys the zero-crossing rates of the quadrature components and the instantaneous frequency of the received signal at the mobile unit to estimate the scattering distribution. We also propose a new model for simulating multipath fading channels with non-isotropic scattering. We use the channel simulator to evaluate the performance of the proposed estimator for the scattering distribution. Simulation results show that proposed estimator exhibits small bias and root mean square error.     

电离层高频信道互易性研究:中纬度电离层斜向探测实验

该文首次利用武汉电离层斜向探测系统(WIOISS)对武汉-万宁和万宁-武汉两条电离层高频信道的互易性进行了分析。WIOISS是基于GPS时间频率同步手段开发的新型电离层斜向探测系统,通过比较由系统扫频工作模式和定频工作模式得到的两条路径下的斜向传播群时延和电离层高频信道散射函数,可以得出武汉-万宁和万宁-武汉的电离层高频链路有较好的互易性。     

短波信道时间选择性衰落特性的实验研究

分析了大圆距离约770 km的斜向探测电路的实测数据,统计了信号幅度衰落的累积分布、衰落深度、衰落率、基本相关时间、基本不相关时间和多普勒展宽共六个常用于描述短波信道时间选择性衰落特性的参数,还进行了不同积累时间频谱的分析。结果表明:不同传播模式,多普勒域幅度衰落66%较为符合正态分布,时域幅度衰落73%较为符合维布尔分布;电离层稳定时,幅度衰落自相关的特征值较大,而且自相关半径较为接近基本不相关时间;有效积累时间内,积累时间越长,多普勒分辨力越高,信号更加锐化;积累时间大于有效积累时间时,相干性变差,积累效果恶化。     

基于多体制探测数据融合的MUF预报方法

提出了一种基于多体制探测数据融合的最高可用频率（maximum usable frequency，MUF）预报方法.该方法基于数据融合的思想，将电离层垂测数据和斜向探测数据进行融合处理，对短波通信最大可用频率进行预报，试验结果表明，该方法能够有效提升对MUF预报的准确性，预报相对误差降低19.23%，绝对误差降低0.33 MHz；对比实验结果，春秋季和冬季的预报精度提升尤为明显，平均达到25%以上，夏季能达到10%以上.该方法易于工程实现，能够对短波通信选频、短波广播覆盖分析等提供准确的数据支撑.