基于BD预编码和时分复用的MIMO广播传输方案

在MIMO系统中,块对角化(BD)预编码用来消除多用户的共信道干扰(CCI)。BD预编码要求系统发射天线数大于全部用户接收天线数目的总和。当用户数增多时,用户接收天线总数随之增多,BD算法的复杂度将急剧增大。为了解决这个问题,同时使系统支持的用户数最大化,提出了一种基于BD预编码和时分复用(TDM)相结合的广播传输方案。该方案通过对用户进行分组,对组内的用户进行BD预编码,并对组间进行时分复用,从而降低系统对发射天线数目的要求和算法的复杂度。仿真结果表明,这种组合方案在支持更多用户的同时还可以获得较高的吞吐率。     

Hedgerow species and environmental conditions effects on soil total C and N and C and N mineralization patterns of soils amended with their prunings

An important criterium in selecting species for alley cropping is themineralization pattern of their prunings. This study determined effects of 5years of hedgerow pruning applications on soil organic C and total N at threelocations in Haiti and mineralization patterns from soil amended with theprunings during an incubation using micro-lysimeters. Soils (0–5cm) under 5 hedgerows were collected at each site and analyzed fororganic C and total N. In the laboratory, ground leaves and stems (<1cm diameter) of the hedgerow species were mixed with soil at ratesof 3 and 1.5 Mg ha^–1, respectively, andaerobically incubated in the dark at 25 °C. A non-amended soilwas used as control. Soils were leached to determine mineral N at 1, 3, 7, 14,28, 42, 84 and 120 days of incubation. Evolved CO₂ was measuredfollowing each leaching procedure. At the calcareous site, application ofprunings from Leucaena leucocephala (Lam.) De Wit andDelonix regia (Boj. ex Hook. Raf.) resulted in 23 and 13%higher soil N than the control, respectively, after 5 years. There were nodifferences in total N at the other sites but soil N was highest underLeucaena hybrid and Acaciaangustissima (Mill.) Kuntze, respectively at the basaltic and highelevation sites. Soils under D. regia (calcareous) andA. angustissima and Leucaena hybrid(high elevation) had higher organic C than the respective controls. Carbon andNmineralization and C turnover were highest when soils were amended with leavesof Leucaena diversifolia (Schlecht.) Benth (calcareous andbasaltic soils) and A. angustissima (high elevation) andlowest in non-amended control soils. Stem-amended soils showed differences in Cmineralization for calcareous and high elevation soils whereas N mineralizationwas similar among treatments within sites. Carbon and N mineralization (highelevation soil) correlated positively with N concentrations of leaf prunings.Amendments with leaf prunings increased soil C and N mineralization andturnoverrates, suggesting greater nutrient availability for the crop during a shortperiod than in non-amended control soils.     

Use of the 15N natural abundance technique to quantify biological nitrogen fixation by woody perennials

Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N₂ fixed. Some of the problems involved in measuring N₂ fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N₂-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N₂ fixation to trees is the so-called `¹⁵N natural abundance' technique which exploits naturally occurring differences in ¹⁵N composition between plant-available N sources in the soil and that of atmospheric N₂. In this review we discuss probable explanations for the origin of the small differences in ¹⁵N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N₂ fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N₂ (%Ndfa) using the natural abundance procedure requires that both the ¹⁵N natural abundance of the N derived from BNF and that derived from the soil by the target N₂-fixing species be determined. It is then assumed that the ¹⁵N abundance of the N₂-fixing species reflects the relative contributions of the N derived from these two sources. The ¹⁵N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the ¹⁵N abundance of the N acquired from other sources is not several ¹⁵N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the ¹⁵N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N₂-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N₂-fixing perennial. The compromise solution is to evaluate the ¹⁵N abundance of a diverse range of neighbouring non-N₂-fixing plants and to compare these values with that of the N₂-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems.     

基于着色树优化的网络并发链路故障快速恢复方法

为了实现网络并发链路故障的快速恢复,提出一种基于改进人工鱼群算法着色树优化的故障快速恢复方法.首先从备份拓扑构造、着色树生成及流量转发三个方面分析整体恢复方案,在此基础上,建立着色树的生成模型并利用人工鱼群算法对其进行优化以进一步提高其性能,引入变异操作有效解决寻优陷入局部僵局的问题.仿真实验表明,该恢复方案不仅提高了网络在并发链路失效情况下的故障恢复能力,同时在恢复路径及路由备份方面也具有较强的性能.     

非线性减震器的多脉冲轨道和同宿树研究

针对一类非线性减震器,应用能量相位法研究了减震器系统在1∶0内共振,第一阶主共振情形下系统的多脉冲轨道和同宿树.首先,将系统的无量纲动力学控制方程转化为近可积哈密顿系统的标准形式.其次,研究了该系统的未扰动力学行为和扰动动力学行为,分析了耗散因子及相位漂移角对多脉冲轨道脉冲数和层半径的影响,揭示了这类非线性减震器能量从高频模态向低频模态转移的动力学机理.     

Efficient digital certificate verification in wireless public key infrastructure using enhanced certificate revocation list

ABSTRACT

Public key cryptography is widely used for secure data transfer in various applications. The Public key infrastructure provides a foundation that supports public key cryptography, and it is required to deliver the public keys to existing systems or users securely. The public key is exchanged digitally in the form of digital certificates having a certain period of validity. Circumstances may arise under which the validity of the certificates should be ceased, and they need to be revoked. The fast growth of e-commerce demands verification and data transfers to be done in minimum time to maintain high efficiency of communication. In this paper, a new method of developing certificate revocation lists is proposed. The formal model of enhanced PKI for mobile commerce security is put forward. The performance of NTRU is compared with the RSA algorithm and the ElGamal algorithm. The paper also sets forth a comparative analysis of the proposed method with other existing techniques. The results show that the proposed technology has the upper hand over the current methods.     

Evaluation of reservoir sedimentation using data driven techniques

The sedimentation is a pervasive complex hydrological process subjected to each and every reservoir in world at different extent. Hydrographic surveys are considered as most accurate method to determine the total volume occupied by sediment and its distribution pattern in a reservoir. But, these surveys are very cumbersome, time consuming and expensive. This complex sedimentation process can also be simulated through the well calibrated numerical models. However, these models generally are data extensive and require large computational time. Generally, the availability of such data is very scarce. Due to large constraints of these methods and models, in the present study, data driven approaches such as artificial neural networks (ANN), model trees (MT) and genetic programming (GP) have been investigated for the estimation of volume of sediment deposition incorporating the parameters influenced it along with conventional multiple linear regression data driven model. The aforementioned data driven models for the estimation of reservoir sediment deposition were initially developed and applied on Gobindsagar Reservoir. In order to generalise the developed methodology, the developed data driven models were also validated for unseen data of Pong Reservoir. The study depicted that the highly nonlinear models ANN and GP captured the trend of sediment deposition better than piecewise linear MT model, even for smaller length datasets.     

多模卫星信号仿真器的软件设计

本文以多模卫星信号仿真器的研制为背景,对GPS/GLONASS/BD三模仿真器上位机软件设计进行了详细的说明,介绍了仿真器软件开发的总体方案和关键模块,重点对用户界面的设计、上位机与下位机之间的数据通信、多线程的使用进行了详细论述。最后通过实际硬件平台和商用接收机对仿真器进行实验了验证。结果表明,该仿真器产生出的三个导航系统的卫星信号能够被接收机正确接收,验证了仿真器软件的正确性和实用性。     

From Constrained to Unconstrained Maximum Agreement Subtree in Linear Time

We propose and study the Maximum Constrained Agreement Subtree (MCAST) problem, which is a variant of the classical Maximum Agreement Subtree (MAST) problem. Our problem allows users to apply their domain knowledge to control the construction of the agreement subtrees in order to get better results. We show that the MCAST problem can be reduced to the MAST problem in linear time and thus we have algorithms for MCAST with running times matching the fastest known algorithms for MAST. A preliminary version of this paper appears in the Proceedings of the Fifth Workshop on Algorithms in Bioinformatics (WABI 2005). Research of H.F. Ting is supported in part by Hong Kong RGC Grant HKU-7172/06E.