水声通信系统中的Turbo编码方案研究

为了使数据在极其复杂的水声信道中可靠传输,需要采用具有很好纠错性能的纠错码来提高信息传输的可靠性。针对水声信道多径干扰强、信道衰落严重的特点,提出了基于Turbo编码的直接序列扩频水声通信编码方案,并对其在水声多径信道中的性能进行分析。数值仿真及湖试数据处理结果表明,该编码方案具有编码增益高、抗多径能力强的特点,误码性能优于不加直接序列扩频的Turbo编码方案,更适合于强多径、衰落严重的水声信道。     

Effects of Surface State and Applied Stress on Stress Corrosion Cracking of Alloy 690TT in Lead-containing Caustic Solution

The effects of surface state and applied stress on the stress corrosion cracking (SCC) behaviors of thermally treated (TT) Alloy 690 in 10 wt% NaOH solution with 100 mg/L litharge at 330℃ were investigated using C-ring samples with four kinds of surface states and two different stress levels. Sample outer surfaces of the first three kinds were ground to 400 grit (ground), shot-peened (SP) and electro-polished (EP) and the last one was used as the as-received state. Two samples of every kind were stressed to 100% and 200% yield stress of Alloy 690TT, respectively. The results showed that the oxide film consisted of three layers whereas continuous layer rich in Cr was not found. The poor adhesive ability indicated that the oxide film could not protect the matrix from further corrosion. Lead was found in the oxide film and the oxides at the crack paths and accelerated the dissolution of thermodynamically unstable Cr in these locations and also in the matrix. The crack initiation and propagation on Alloy 690TT were effectively retarded by SP and EP treatments but were enhanced by grinding treatment, compared with the cracks on the as-received surface. The cracking severity was also enhanced by increasing the externally applied stress. The accelerated dissolution of Cr and the local tensile stress concentration in the near-surface layer caused by cold-working and higher applied stress reduced the SCC-resistance of Alloy 690TT in the studied solution.     

Tocols of selected spring wheat (Triticum aestivum L.), einkorn wheat (Triticum monococcum L.) and wild emmer (Triticum dicoccum Schuebl [Schrank]) varieties

Wheat contributes significantly worldwide to antioxidant income with beneficial healthy effects. Two varieties of einkorn wheat (Triticum monococcum L. – Escana and Schwedisches Einkorn), two varieties of wild emmer [Triticum dicoccum Schuebl (Schrank)] (Rudico and Kahler Emmer) and three varieties of spring wheat (Triticum aestivum L. – Granny, Kärtner Früher and SW Kadrilj) were evaluated for tocols – tocopherols and tocotrienols by high performance liquid chromatography – electrospray ionisation tandem mass spectrometry (HPLC–ESI/MS/MS). Investigated emmer and einkorn wheat varieties have shown minor lower total tocols content in comparison with spring varieties, however significant typical levels of tocopherols and tocotrienols were found in these groups. In einkorn wheat varieties the major abundance of total tocols is represented by tocotrienols (Schwedisches Einkorn 65.0%, Escana 54.9%), in emmer wheat varieties the abundance of tocotrienols and tocopherols were similar (Rudico 48.9% of tocotrienols, Kahler Emmer 47.6%), whereas in spring bread wheat higher tocopherols (SW Kadrilj 54.5%, Kärtner Früher 60.2%, Granny 55.0%) were recorded.     

多跳无线网络中信道接入与分布式路由的结合

本文为多跳分布式无线网络(Ad Hoc网络)提出了一种信道多址接入与分布式路由相结合的方案.在该方案中,由扩频码实现多信道系统,各节点竞争公共信道,利用RTS/CTS对话形式来预约各业务信道,成功预约后的分组传输不会受到其他节点的干扰.并且该动态预约方法可以提供用于建立分布式路由机制的网络连通性信息.这种方案打破了信道接入层和网络层的分层概念,并以两者的结合带来了更高的带宽利用率和对网络拓扑变化的更快反应.     

建筑物中电气设备安全接地问题的再探讨

从电气设备安全的角度出发，介绍了建筑中电气系统保护接地的形式和常见的几种类型，讨论了其各自的特点。     

Multicarrier Orthogonal-CDMA for Fixed Wireless Access Applications

In this paper, we suggest applying multicarrier spread spectrum (MC-SS) techniques to fixed wireless access applications. Due to the fixed nature of subscribers in these applications, cross-polarization discrimination and directional subscriber antennas can be used to reduce interference in a multicellular system. Power control schemes are applied on both uplink and downlink. The suggested system has a high capacity and uses a simple multiaccess layer. It is robust against multipath effects and can provide service coverage not only to line-of-sight (LOS) but also to nearly LOS subscribers. Only a single carrier frequency is used in the entire network.     

雪崩三极管在扩频通信中的应用

扩频技术是一种新型的通信体制,是通信领域和控制领域中的一个重要的发展方向.在扩展频谱通信系统中,常用SAW抽头延迟线来实现扩频序列的捕捉,利用SAW抽头延迟线可以生成被扩频序列调制的中频BPSK信号.由于SAW器件具有频率选择性及较高的插入损耗,使得它的输入必须为高脉冲且脉冲的频谱宽度能覆盖SAW抽头延迟线的中心频率.笔者采用CPLD和雪崩三极管电路产生了适合要求的脉冲.实验中测得脉冲电压15～25 V,脉冲宽度<7 ns.     

直接序列扩频系统中的一种新PN码跟踪环

提出一种新颖的全数字PN码跟踪．该跟踪环采用符号判决电路和数字序贯滤波器取代经典的环路滤波器来提高衰落环境下的跟踪性能。通过将定时调整过程等效为一时间离散变量离散的齐次马尔可夫链．导出了瑞利衰落下该环路定时误差的稳态概率分布．并对该结果进行了计算机仿真验证。     

DS扩频系统抗多窄带干扰方法研究

针对AR过程的窄带干扰,本文构建了DS扩频系统多窄带干扰下的信号模型,给出了统一的观测模型,并用线性、非线性滤波方法对其进行干扰抑制。仿真结果表明,所建模型是有效的,而且滤波方法对多个窄带干扰仍有很好的抑制性能,且非线性滤波的性能要优于线性滤波。     

DS/FH混合扩频通信应用于矿井移动通信系统的研究

矿井移动通信系统是矿井安全生产、抢险救灾的重要保障。为了提高矿井移动通信系统的可靠性和稳定性,文章提出将DS/FH混合扩频技术应用于矿井移动通信系统的方案,从理论上对DS/FH混合扩频技术进行了详细分析,在此基础上,基于Systemview建立了通信系统的仿真模型。仿真结果和系统的误码率特性分析证明了DS/FH混合扩频技术应用的可行性,为矿井移动通信的发展提供了重要的理论依据。