MIMO系统中的分集技术

分集技术在MIMO系统中具有十分重要的地位。首先从理论上分析了无线多径衰落信道。在此基础上给出分集技术原理。然后通过介绍分集技术并比较了几种典型的分集技术的优缺点。得出空间分集技术具有提高传输的可靠性,且不会带来带宽利用率上的任何损失的优势。因而成为了MIMO系统中所采用的分集技术。最后通过仿真验证了在慢瑞利衰落信道中采用空间分集技术能够降低系统的误比特率。     

基于SSD的MC-TDMA系统抗衰落技术

为了应对衰落效应对未来无线网稳定性和可靠性的巨大挑战,提出一种应用信号空间分集(signal space diversity,SSD)的多载波时分多址(multicarrier time division multiple access,MC-TDMA)系统。该系统利用MDFT滤波器组的天然特性,挖掘了MC-TDMA系统内部固有的调制分集增益。理论分析及各环境的仿真表明,SSD-MC-TDMA系统相比经典的MC-TDMA系统不仅具有更强的抗多径衰落能力,而且不会改变其优越的低峰均比(peak to average power ratio,PAPR)性能。     

科学与政治互动下的科学家多元角色——以20世纪美国核政策制定中的科学家为对象

20世纪的科学家在美国的核武政策中发挥了重要作用,一度作为代表参加政治谈判。美国的核科学家们通过这一过程被纳入到政治范畴。在政治斗争中,他们也会利用自身的专门知识占据优势地位。这些科学家影响了国家政策的制定,反过来,政治决策活动也深刻地影响了科学家的职业发展。冷战后,美国科学家参与到核电发展之中,经济因素与核安全文化因素直接影响核电公司的运行与技术操作,进而影响到核电科学家的职业行为。科学家表现出追求功利以及社会责任与道德认识多元化等倾向。科学家的社会角色的多元性日趋明显。     

加拿大多元文化主义政策须处理的几大关系

1971年加拿大联邦政府颁布了多元文化主义政策。此后,多元文化主义政策不断发展,在发展过程中遇到了来自多方面的挑战。在挑战面前,多元文化主义政策要沿着良性轨道运行,必须处理好以下几大关系:国家的同一性和族裔多样性的关系、英裔和法裔关系、主流族群与非主流族群的关系。     

下一代移动通信系统中的MIMO开环和闭环发射技术

介绍在下一代移动通信系统中广泛采用的MIMO系统中的开闭和闭环发射技术,对于开环发射分集、开环空间复用、闭环发射分集、闭环空间复用等不同发射方案进行了详细描述;介绍了包括空时分组编码、预编码技术、波束成形技术等相关技术;并通过仿真结果以及分析比较表明需要根据不同传输条件要求合理选择开闭环发射方案。     

湛江高桥红树林贝类资源调查

在2002年和2005年2次调查的基础上,2006年7月对湛江高桥的红树区贝类资源在相同取样点进行了定量和定性调查,调查的红树林区有高桥镇的东村、西村、长田、红坎4个点.此次调查结果表明,高桥红树林区贝类种类组成有3纲6亚纲11目26科43属60种.其中樱蛤科有9种;牡蛎科、帘蛤科各5种;蚶科、贻贝科、织纹螺科各4种;其余各科均在3种以下.通过与前两次的调查结果比较分析,高桥红树林区的贝类无论是从栖息密度还是从生物量上都有明显下降的趋势.因此,采取多方位的保护措施,以使红树林区生物多样性资源得到可持续利用势在必行.     

西藏行道树资源调查与结构特征分析

为摸清西藏地区行道树资源、现状与特征,应用抽样调查法对该地区的行道树进行调查,分析了西藏行道树多样性指数、结构特征、配置模式等.结果表明:1)西藏常用行道树共计20科36属63种,其平均胸径、平均树高、平均冠幅分别为12.32cm、7.21m、3.90m.从总体上看,行道树规格明显偏小.科属上,应用最多的依次为杨柳科、蔷薇科、松科,分别为15、13、7种. 2)西藏行道树基调树种为白柳、北京杨、紫叶李、榆树、雪松,重要值排前3的依次为白柳、北京杨、紫叶李,其重要值依次为47.830、43.459、22.834,且多样性指数较高.3)季相景观上以"春-秋"季、秋季为主,彩叶树种色系上以黄色、白色系季相景观为主.4)垂直分层结构上以一层模式为主(43.93%),行道树复层结构不发达,花灌木、草花及地被植物较匮乏.     

Performance analysis of FFH/BPSK system with partial band noise jamming and channel estimation error in high-mobility wireless communication scenarios

In this paper, we analyze the bit error ratio （BER） of fast frequency hopping （FFH） system with binary phase shift keying （BPSK） in high-mobility wireless channels. By taking into account partial band noise jam- ming and channel estimation errors as Gaussian noise, both of maximal-ratio combining （MRC） and equal-gain com- bining （EGC） schemes are considered, and exact analytical BER expressions for MRC and EGC schemes are obtained. Especially, a closed-form BER expression for the MRC scheme is also presented in high signal-to-noise ratio region. Analytical and simulation results demonstrate that, the FFH/BPSK schemes suffer from performance degra- dation in the presence of channel estimation errors. The worst case jamming factor is shown to be inversely pro- portional to the signal to jamming ratio. With properly designed frequency hopping schemes and channel estima- tion schemes, the performance improvement is also observed as compared with the traditional noncoherent FFH/BFSK system.     

Temporal and seasonal variations in parasites of Metynnis lippincottianus (Characiformes: Characidae), a host from the eastern Amazon (Brazil)

ABSTRACT

This study investigated the influence of temporal and seasonal variation (rainy/dry cycle) on the component community structure and infracommunities of parasites in Metynnis lippincottianus from the eastern Amazon (Brazil). A total of 8,774 parasites representing 9 species and 22,765 parasites representing 12 species was collected in 2011 and 2016 respectively, but only 42.8% of the species were common to both study years. In both years, there was a dominance of Ichthyophthirius multifiliis and the component community was 51% dissimilar. The species richness of parasites and the Berger–Parker dominance were higher in 2016, while the evenness and Brillouin diversity were higher in 2011. The prevalence and abundance of I. multifiliis and P. pillulare, as well as the prevalence of Contracaecum sp. were higher in the rainy season. The abundance of Anacanthorus strongylophalus, Urocleidoides sp., Dadayus pacupeva and Dadaytrema oxycephala was higher in the dry season. Infection with Procamallanus (Spirocamallanus) inopinatus was not influenced by the season, while by Trichodina sp. occurred only in the dry season and Ergasilus xinguensis only in the rainy season. The species richness of parasites, Brillouin diversity index and evenness were higher in the dry season, while the Berger–Parker index was higher in the rainy season. Temporal variation of communities and infracommunities of parasites was influenced by the body size of host populations and increase in anthropogenic impacts. Effect of seasons on environmental quality, host size and availability of infective stages of parasites were the determining factors in structuring the component communities and infracommunities of parasites.     

CERS中集群通信系统调度功能的优化研究

城市应急联动系统(CERS)是政府协调指挥各部门向公众提供社会紧急救助服务的联合行动,是对各种政府资源的整合应用。对城市应急联动系统中数字集群通信系统结构进行了探讨。提出了采用分集接收技术对城市应急联动系统中集群通信的调度功能进行研究。研究数据分析显示采用这种技术系统功能得到了一定的优化。