一种高效动态数字信道化方法

针对宽带数字接收信号中存在多个非均匀分布且动态变化的信道的情况,提出了一种高效动态数字信道化方法.该方法基于信号重构理论,将接收信号均匀划分成多个子带,根据能量检测环节将属于同一信道的相邻子带输入到相应的综合滤波器组,输出即为该信道的基带信号.信道覆盖子带个数变化时,只需根据能量检测结果改变综合滤波器组的系数,因而具有一定的自适应特点.     

A comparison of avian communities and habitat characteristics in floodplain forests associated with valley plugs and unchannelized streams

Channelization of streams associated with floodplain forested wetlands has occurred extensively throughout the world and specifically in the southeastern United States. Channelization of fluvial systems alters the hydrologic and sedimentation processes that sustain these systems. In western Tennessee, channelization and past land‐use practices have caused drastic geomorphic and hydrologic changes, resulting in altered habitat conditions that may affect avian communities. The objective of this study was to determine if there were differences in avian communities utilizing floodplain forests along unchannelized streams compared to channelized streams with valley plugs, areas where sediment has completely filled the channel. During point count surveys, 58 bird species were observed at unchannelized sites and 60 species were observed at valley plug sites. Species associated with baldcypress‐tupelo (Taxodium‐Nyssa) swamps (e.g. Great Egret (Ardea albus) and Black‐crowned Night Heron (Nycticorax nycticorax)) and mature hardwood forests with open midstories (e.g. Eastern Wood‐Pewee (Contopus virens), Yellow‐throated Vireo (Vireo flavifrons), Cerulean Warbler (Dendroica cerulea) and Scarlet Tanager (Piranga olivacea)) were either only found at unchannelized sites or were more abundant at unchannelized sites. Conversely, species associated with open and early successional habitats (e.g. Tree Swallow (Tachycineta bicolor), Northern Mockingbird (Mimus polyglottos) and Blue Grosbeak (Passerina caerulea)) were either only found at valley plug sites or were more abundant at valley plug sites. Results of habitat modelling suggest that the habitat characteristics of floodplain forests at unchannelized sites are more suitable for Neotropical migrant bird species of conservation concern in the region than at valley plug sites. This study, in combination with previous research, demonstrates the ecological impacts of valley plugs span across abiotic and biotic processes and tropic levels. Copyright © 2010 John Wiley & Sons, Ltd.     

一种无盲区数字信道化接收机高效结构方案研究

利用多相滤波器组的成熟理论推导出一种高效的宽带数字信道化接收机结构,并且结合瞬时测频技术,可以实现多通道信号全概率适时接收的跨信道判决,避免虚假信号的产生。利用Matlab进行了仿真测试和验证,仿真结果证明了该实现方法的可行性和有效性。     

AN EXAMINATION OF THE IMPACT OF MULTIPLE DISTURBANCES ON A RIVER SYSTEM: TAXONOMIC METRICS VERSUS BIOLOGICAL TRAITS

River reaches are often subject to multiple co‐occurring anthropogenic stressors. Disentangling the relative impacts of different stressors is important in developing river management strategies. Bio‐assessments using taxonomically based metrics (e.g. taxa richness and diversity) are frequently used as a means of determining the impact of stressors on river reach quality. Although this method has been successful in determining river reach quality, taxonomically based metrics are often limited in their ability to distinguish mechanisms of impact and thus the importance of different stressors. In contrast, biological traits can provide a mechanistic understanding of anthropogenic stressor impacts and therefore have the potential to identify the importance of different co‐occurring stressors. Using both taxonomically based metrics and biological traits, we examined the impact of two common co‐occurring stressors, summer water withdrawal and channelization, on the macroinvertebrate assemblage of a Columbia River tributary Both metrics and traits showed a strong shift at the point where intense channelization began. However, the metrics failed to distinguish whether channelization or co‐occurring water withdrawal was the driver of change. In contrast, biological traits indicated that the assemblage within the heavily channelized area shifted to one dominated by resilience traits often found in river reaches subject to scouring flows. This result indicates that scouring flows associated with intense channelization are the primary driver of change in the macroinvertebrate assemblage and that water withdrawals had little observable impact. Our results demonstrate the utility of biological traits for disentangling the impact of multiple stressors in river systems. Copyright © 2011 John Wiley & Sons, Ltd.     

智能网联混行环境下交叉口时空资源配置优化

在网联自动车辆（CAVs）与人工驾驶车辆混行环境下,构建混合整数线性规划（MILP）模型,以优化交叉口时空资源配置. 该模型以交叉口通行能力最大化为目标,约束条件主要包括车道渠化、流量分配和信号配时等相关约束. 以典型四车道十字交叉口为例,在网联自动车不同驾驶行为和不同渗透比例的条件下,优化交叉口渠化方案和信号配时方案. 结果表明,随着网联自动车占比和跟驰行为的改变,交叉口最优渠化方案和信号配时方案须相应调整. 网联自动车占比增大和跟车时距减小,均有利于提高交叉口的通行能力,且当网联自动车跟车时距不受前车类型的影响时,交叉口通行能力提高更多.     

美国基西米河渠化工程对河流生态环境的影响

位于美国佛罗里达州的基西米河，所处流域自然条件优越，季节性的水流变化较大，河流沿岸湿地资源丰富，为各种水生生物提供了适宜的生存环境，构成了流域内丰富多彩的生态系统．由于防洪排涝的需要，1961年开展了大规模的渠化工程，将蜿蜒的自然河流变成了几段较为顺直的人工运河，提高了河道的防洪能力，然而渠化后的河道及其下游流域的生态环境也随之出现了一系列负面反应，原有的生态平衡受到破坏，教训是非常深刻的．文中针对该渠化工程的后果及教训进行了总结和介绍。     

A Fast Code-Assignment Strategy for a WCDMA Rotated-OVSF Tree with Code-Locality Capability

In this paper, a new channelization code tree structure, namely an ROVSF (rotated-orthogonal variable spreading factor) code tree, is defined and investigated. Most existing code assignment schemes are investigated on the OVSF (orthogonal variable spreading factor) code tree in WCDMA systems. The main work of this investigation is to exploit and justify the new properties of the ROVSF code tree. We show that the ROVSF code tree offers the same code capability to that of the conventional OVSF code tree, but our ROVSF code tree additionally has the code-locality capability. With the code-locality capability, a fast code-assignment strategy is developed on the ROVSF code tree. Compared to existing code assignment schemes on OVSF code trees, a fast code assignment scheme is developed with lower search costs and a low blocking rate, due to its code-locality capability. Finally, the simulation results illustrate that our proposed scheme on the ROVSF code tree actually has lower search costs and a better blocking rate.This work was supported by the National Science Council of the R.O.C. under grant nos. NSC91-2213-E-194-041 and NSC91-2213-E-194-042.     

偶型排列宽带数字信道化接收机

在讨论了一种无混迭无盲区的宽带数字信道化接收机设计方法的基础上,针对偶型排列宽带数字信道化接收机频带两端输出的两个实数信道不能直接用于信号检测的问题。提出了采用希尔伯特（Hilbert）正交变换的方法将两个实数信道变为复数解析信号,改进信道化接收机的结构,保证实数信道与其它复数信道一样进行信号检测和估计,解决了宽带信道化接收机频带两端的频谱利用问题,提高了宽带数字接收机的频谱覆盖宽度。通过仿真实验,验证了提出的算法的有效性。     

一种中频相同的微波光子信道化接收机

提出一种基于相干光处理的微波信道化接收机,采 用 自由谱范围(FSR)不同的两套光频率梳(OFC,optical frequen cy combs)分别作为 光载波和光本振,使得各个信道的输出具有相同的中频。本文方法将微波信号调制到载波 OFC的每一个 频率分量上,并用法布里-珀罗(F-P)光滤波器对多个信道的调制光边带同时进行带通滤 波,然后与 本振OFC进行相干探测实现频率下转换。若载波OFC比本振OFC的FSR大于1个 信道带宽,并使 F-P滤波器的FSR与本振OFC相等,可得 到等带宽、等中频的微波信道,降低了后续电路的复杂度。对 两个相邻信道进行了实验,将9.74～9.86GHz和9.86～9.98GHz波段的信号分别下转换到两个带宽为120MHz 的中频信道,两个中频信道的中心频率仅相差4MHz分别为900MHz和 896MHz,表明本文方法能够实现等中频的信道输出。     

两种数字信道化实现方法比较

现代电子侦察系统中,数字信道化接收机扮演着十分重要的角色。研究了工程上常用的两种数字信道化实现方法：多相滤波器组法和短时傅里叶变换STFT法,对他们的实现原理和性能进行了分析比较,并通过仿真进一步验证了两者的性能差异,为侦察接收机设计提供一定参考。