Measurement and characterization of the temporal behavior of fixed wireless links

We analyze the methodology and present the interpretation of results obtained in the measurement of the temporal Ricean K-factor and the time-autocovariance function of the received power for fixed narrowband wireless links in urban residential environments. Our main observation is that the time-autocovariance function (and thus both the temporal K-factor and the coherence time) fundamentally depends on a combination of the characterization of the transmit-receive path and whether the street-level moving objects are in the field of view of the antennas. In most practical cases the latter refers only to the customer premises antenna, as the base station is typically placed at a much greater distance from moving scatterers. No significant basis was found for the popular conjecture that the K-factor is mainly influenced by the excess path loss. However, a strong correlation between coherence times and temporal K-factor was observed over a wide range of link conditions. We also found that the time-autocovariance function can be quite accurately modeled by the weighted sum of two exponentially decaying functions: one with a time constant on the order of 1 s and the other typically slower by one order of magnitude. The results will be helpful in establishing design criteria for radio systems such as multiple input-multiple output that need to estimate the channel response and its persistence in time and for media access control (MAC) layer transmission protocols whose efficiency is affected by the depth and duration of channel outages.     

Queueing behavior of statistical multiplexers with correlatedinputs

The author considers an asynchronous time-division-multiplexing system with a finite number of identical users generating data only during their so-called active periods, which alternate with so-called passive periods. Assuming geometric distributions for the active and passive periods of the users, the author derives a functional equation which describes the full steady-state buffer behavior of the multiplexer. The buffer occupancy at various moments can be derived from this equation and explicit functions of the parameters of the system. The results indicate a strong dependency of the mean buffer occupancy on the actual lengths of the active and passive periods of the users, even for a given mean user activity     

Analysis and prediction of cross-polarization on earth-space links

A two-tier Gaussian model for raindrops, which assumes Gaussian distributions both for instantaneous canting angle and time-varying mean canting angle, together with gross features of ice particles, provide a theoretical framework to organize the experimental data and to yield functional dependence of cross-polarization on frequency, polarization and elevation angle. Agreement has been obtained between measured depolarization data and theoretical results which are essentially independent of details of ice clouds. In particular, a linear relation has been found between cross-polarization amplitude and frequency throughout the centimeter wavelengths for a given earth-space path.     

Analysis of multiple-antenna wireless links at low SNR

Wireless channels with multiple transmit/receive antennas are known to provide a high spectral efficiency both when the channel is known to the receiver, and when the channel is not known to the receiver if the signal-to-noise ratio (SNR) is high. Here we analyze such systems at low SNR, which may find application in sensor networks and other low-power devices. The key point is that, since channel estimates are not reliable, it is often not reasonable to assume that the channel is known at the receiver at low SNR. In this unknown channel case, we show that for sensible input distributions, in particular all practical modulation schemes, the capacity is asymptotically quadratic in the SNR, /spl rho/, and thus much less than the known channel case where it exhibits a linear growth in /spl rho/. We show that under various signaling constraints, e.g., Gaussian modulation, unitary space-time modulation, and peak constraints, that mutual information is maximized by using a single transmit antenna. We also show that at low SNR, sending training symbols leads to a rate reduction in proportion to the fraction of training duration time so that it is best not to perform training. Furthermore, we show that the per-channel use mutual information is linear in both the number of receive antennas and the channel coherence interval.     

空—空通信信道的统计特征分析

近年来空—空通信技术受到了高度关注,空—空信道建模与分析是空—空通信领域的研究热点之一。文章依据德国宇航局基于实测数据的空—空信道统计模型,通过仿真实验分析了地面场景切换时路径接收信号的统计分布特征,其实验结果对空-空信道建模具有一定的参考价值。     

Analysis of a full-memory multidestination ARQ protocol overbroadcast links

Based on an assumption that a steady state exists in the full-memory multidestination automatic repeat request (ARQ) scheme, we propose a novel analytical method called steady-state function method (SSFM), to evaluate the performance of the scheme with any size of receiver buffer. For a wide range of system parameters, SSFM has higher accuracy on throughput estimation as compared to the conventional analytical methods     

Predicting the breakdown behavior of microcontrollers under EMP/UWB impact using a statistical analysis

Reproducible prediction of damaging effects is one of the main problems in intentional electromagnetic interference (IEMI). In this paper, the susceptibility of different types of single microcontrollers to unipolar fast rise time pulses [electromagnetic pulse (EMP), ultrawide band (UWB)] is determined. Therefore, pulses with rise times as fast as 100 ps and electric field amplitudes of up to 100 kV/m are applied to the devices. The results are generalized with a novel statistical procedure. Following discussion and rationale, the Weibull distribution is selected to describe the interference behavior. The statistical analysis provides a new test procedure for a confident determination of the interference behavior parameters.     

Analysis of link-layer backoff schemes on point-to-point Markov fading links

Backoff algorithms are typically employed in multiple-access networks (e.g., Ethernet) to recover from packet collisions. We propose and carry out the analysis for three types of link-layer backoff schemes, namely linear backoff, exponential backoff, and geometric backoff, on point-to-point wireless fading links where packet errors occur non-independently. In such a scenario, the backoff schemes are shown to achieve better energy efficiency without compromising much on the link layer throughput performance.     

出租车自组织网络拓扑特征的研究与评估

由于受到车流量、车辆数目及通信半径等诸多外界因素的影响,使得车辆自组织网络中节点不断移动,造成了网络拓扑结构复杂、变化频繁及网络节点分布不均匀,这对提高车辆自组织网络的可扩展性提出了挑战。文中基于上海市4300多辆出租车的车载装置GPS得到的实时数据,对车辆自组织网络的拓扑特性进行分析及研究,并结合复杂网络理论得出,车辆自组织网络中节点的度服从幂律分布,是无标度网络,且具有很强的异构性,但不具有小世界特征;在某些限定条件下,网络的连通性与节点的通信距离之间呈线性关系。     

Analysis of infrared wireless links employing multibeamtransmitters and imaging diversity receivers

We analyze the improvements obtained in wireless infrared (IR) communication links when one replaces traditional single-element receivers by imaging receivers and diffuse transmitters by multibeam (quasi-diffuse) transmitters. This paper addresses both line-of-sight (LOS) and nonline-of-sight (non-LOS) IR links. We quantify link performance in terms of the transmitter power required to achieve a bit error rate (BER) not exceeding 10^-9 with 95% probability. Our results indicate that in LOS links, imaging receivers can reduce the required transmitter power by up to 13 dB compared to single-element receivers. In non-LOS links, imaging receivers and multibeam transmitters can reduce the required transmitter power by more than 20 dB. Furthermore we discuss the use of multibeam transmitters and imaging receivers to implement space-division multiple access (SDMA). In a representative example with two users transmitting at a power sufficient to achieve a BER not exceeding 10^-9 with 95% probability in the absence of cochannel interference, when SDMA is employed, the system can achieve a BER not exceeding 10^-9 with a probability of about 88%     

斜测链路F层最高观测频率变化特性分析

斜向探测是获取电离层状态信息的重要手段之一,根据斜向探测系统得到包含电离层状态信息的斜测电离图,能用于分析斜测链路上电离层相关参数的变化特性.以苏州—青岛和兰州—青岛两条链路为例,分析了斜向探测链路F层最高观测频率(Maximum Observed Frequency of F layer,MOFF)日变化特征;利用2013年12条斜向探测链路MOFF的观测数据,分析了冬、夏、分季的MOFF月中值变化特征,发现各链路的MOFF与电离层电子密度类似呈现出较为明显的日变化和季节变化特性.选取2013年西安—青岛、广州—海口两条链路的MOFF数据,与利用链路中点附近垂测数据计算的等效MOFF参数进行对比分析.结果表明:实测MOFF数据与等效MOFF数据具有较好的一致性,可为短波链路选频、电离层环境分析等提供参考.     

人体行为特征融合与行为识别的分析

伴随着信息技术的快速发展,人体行为识别技术逐渐被引入到各领域中,如安防监控、运动分析、医学辅助诊断和智能人机交互等,而技术实现的关键在于借助相关的特征融合方法。文章对人体行为识别的相关技术以及兴趣点提取方法、尺度混合特征模型与MKL方法的应用进行分析,以期对人体行为识别技术的发展起到推动作用。     

Monte Carlo Static Timing Analysis with statistical sampling

With aggressive scaling of CMOS technologies, MOSFET devices are subject to increasing amounts of independent local statistical variability. The causes of these statistical variations and their effects on device performance have been extensively studied, but their impact on circuit performance is still difficult to predict. This paper proposes a method for modeling the impact of random intra-die statistical variations on digital circuit timing. The method allows the variation modeled by large-scale statistical transistor simulations to be propagated up the design flow to the circuit level, by making use of commercial STA and standard cell characterization tools. By using statistical sampling techniques, we achieve close to the accuracy of full SPICE simulation, but with a computational effort similar to that of Statistical Static Timing Analysis, while removing some of the inaccurate assumptions of Statistical Static Timing Analysis.     

Optical intersatellite links

Direct communication links between spacecraft will be an important element in the future space communication infrastructure. Optical intersatellite links (OISLs) offer the major advantages over conventional microwave links of low mass, power consumption, size and real estate, and flight-qualified transceivers are now feasible. There are, however, formidable problems in achieving pointing accuracy and stability of the order of a few millionths of a degree. The Japanese ETS VI experiment, the European Space Agency's `SILEX' experiment and NASA's laser communications transceiver are due to fly in the mid-1990s. The authors discuss the status of OISL systems and technology and describe the British Aerospace optical multiple access (OMA) system as an example of a lightweight transceiver attractive to spacecraft designers     

浅析计划统计一体化管理系统的设计与实现

随着互联网的快速发展,数据库技术的进一步成熟,建立具备业务化工作的计划统计一体化管理系统是当前统计管理部门的迫切需求。论文阐述了计划统计一体化管理系统在油田行业的设计与实现。     

Analysis of the convergence behavior of adaptivedistributed-arithmetic echo cancellers

Adaptive distributed-arithmetic echo cancellers are well suited for full-duplex high-speed data transmission. They allow a simpler implementation than adaptive linear transversal filters, since multiplications are replaced by table look-up and shift-and-add operations. Various tradeoffs between the number of operations and the number of memory locations of the look-up tables can be achieved by segmenting the echo canceller delay line into sections of shorter length. Adaptivity is achieved by a decision-directed stochastic gradient algorithm to adjust the contents of the look-up tables. The author adopts the mean-square error criterion to investigate the convergence behavior of adaptive distributed-arithmetic echo cancellers. Under the assumption that the look-up values are statistically independent of the symbols stored in the echo canceller delay line, he obtains an analytical expression for the mean-square error as a function of time. The maximum speed of convergence and the corresponding optimum adaptation gain are also determined. Simulation results for a full-duplex quaternary partial response class-IV system are presented and compared with theoretical results     

Analysis of the settling behavior of an operational amplifier

The settling behavior of a pole-splitting compensated operational amplifier is analyzed using a second-order (two-pole) transfer function. It is shown that although the slewing period of the amplifier is well approximated by the commonly used formula for slew rate, the settling behavior after the slewing period can only be fully explained using a second-order transfer function. Simple criteria relating the circuit parameters to the damping ratio of a second-order feedback system are given. Analytical expressions for the amplifier responses and settling times are derived. The analysis is justified by close correspondence with computer simulations.     

Performance of optical intersatellite links

The performance of optical intersatellite links is described by a model. A significant difference between optical and microwave intersatellite links is the occurrence of so-called burst errors. The statistics of bit error rate for an optical link are derived from simulations taking into account beam mispointing resulting from satellite motions and tracking channel noise. The effect of coding on the bit error rate is also shown. The mutual tracking system between two optical terminals located on different satellites can become unstable if the noise of the tracking loop exceeds a critical value. The results of tracking simulations are presented.     

ASON链路管理机制的研究

自动交换光网络(ASON)的出现是光传送网发展的重大突破,它赋予传统静态的光传送网以智能,使之能动态、自动地实现传送、交换和光连接的功能。实现这些功能的关键是自动交换光网络中的控制平面,通过控制平面的一系列动作,来实现以上功能。控制平面由多种功能元件(目前ITU-T制定了5种结构元件:连接控制器、路由控制器、链路资源管理器、业务量策略和呼叫控制器)组成,这些元件主要用来调用传送网的资源,以提供与连接的建立、维护和拆除有关的功能。要实现这些功能,链路管理功能是必不可少的。ASON链路管理的必要性及特点下一代光网络的网…