On the effects of the packet size distribution on the packet loss process

Real-time multimedia applications have to use forward error correction (FEC) anderror concealment techniques to cope with losses in today’s best-effort Internet. The efficiency of these solutions is known however to depend on the correlation between losses in the media stream. In this paper we investigate how the packet size distribution affects the packet loss process, that is, the distribution of the number of lost packets in a block, the related FEC performance and the average loss run length. We present mathematical models for the loss process of the MMPP+M/D/1/K and the MMPP+M/M/1/K queues; we validate the models via simulations, and compare the results to simulation results with an MPEG-4 coded video trace. We conclude that the deterministic packet size distribution (PSD) not only results in lower stationary loss probability than the exponential one, but also gives a less correlated loss process, both at a particular average link load and at a particular stationary loss probability as seen by the media stream.Our results show that for applications that can only measure the packet loss probability, the effects of the PSD on FEC performance are higher in access networks, where a single multimedia stream might affect the multiplexing behavior. Our results show that the effects of the PSD on FEC performance are higher in access networks, where a single multimedia stream might affect the multiplexing behavior and thus can improve the queuing performance by decreasing the variance of its PSD.     

基于谱相关的直扩信号检测及噪声抑制分析

直扩信号是二阶循环平稳信号,根据谱相关分析方法,利用直扩信号非零循环频率处的谱线特征,可将直扩信号从噪声背景中检测出来。为消除基底噪声,采用电子测量中的3σ准则对直扩信号的循环谱进行了消噪处理,便于实现检测,仿真论证了该方法在低信噪比条件下的有效性。     

INS/CNS/GPS组合导航数据融合算法与仿真研究

研究了INS/CNS/GPS组合导航系统的原理和特点,建立了组合导航系统的数学模型。分别利用集中卡尔曼滤波和联邦滤波数据融合原理对此系统进行了仿真研究,并指出了集中滤波器的不足之处。仿真结果表明采用全球定位系统和天文导航系统对惯导系统误差进行修正不仅可以大幅度提高导航精度,而且通过比较和分析可知联邦滤波可以有效地提高导航系统的可靠性和复原能力,是理想的容错型信息融合算法。     

低剂量CO_2激光对茄子种子活力影响的研究

低剂量激光照射种子对其发芽率、发芽势、酶活性均有一定的刺激效应。郝丽珍等曾在青椒上作过研究。本文在茄子上也得到了类似的结果,进而为蔬菜种子播前处理提供了依据。 1 材料和方法 茄子品种为七叶X紫茄,含水量为8.33％,试验均选用饱满的大小均一的中等种子。将种子置于自制的种盘内照射,CO_2激光扩束后功率为4.05W,光斑直径为2.5cm,照射距离为     

交织的空时分组码级联不对称网格编码调制方法

根据交织的空时分组码级联TCM编码设计标准,提出了一种空时分组码级联不对称网格编码调制(A-TCM)的优化设计方案,并得到了在空时分组码级联不对称8PSK调制的TCM情况下最优的星座图旋转角度.仿真和分析结果表明,在相同的频谱效益和译码复杂度的情况下,相比传统空时分组码级联TCM的方法,新方法可进一步提高系统性能.     

A comparison between different PMD compensation techniques

We quantify the benefits of using different techniques for compensation of polarization mode dispersion (PMD) in fiber-optic communication systems by means of numerical simulations. This is done both with respect to PMD-induced pulse broadening and in terms of system outage probability for different data formats [nonreturn-to-zero (NRZ) and return-to-zero (RZ)]. Attention is focused on simple and relevant single- and double-stage post-transmission compensators with a few degrees of freedom (DOF). It is generally believed that a PMD compensator with a polarization controller and a variable delay line can only compensate the PMD to the first order. We show, from analytical results, the counterintuitive fact that this scheme can also partially compensate for higher order PMD. We also investigate the benefit of using a polarizer as compensation element where the optical average power can be used as a feedback signal     

A comparison between NRZ and RZ data formats with respect toPMD-induced system degradation

We quantify the polarization-mode dispersion (PMD)-induced system outage probability by means of numerical simulations for nonreturn-to-zero (NRZ) and return-to-zero (RZ) data formats with proper comparative conditions and find that RZ performs better than NRZ. We also study the impact of the RZ duty-factor and the tradeoff between power margin and acceptable PMD     

基于点云数据分类的机载雷达地形坡度勘测

为提高变电站的三维设计效果,需勘测变电站地形坡度.提出基于点云数据分类的机载雷达地形坡度勘测方法.采用机载雷达回波扫描技术采集地形坡度勘测数据.结合像素点跟踪识别法,重构地形坡度勘测过程中三维图像.采用三维光学检测技术采样机载雷达地形坡度勘测点云数据,结合中值滤波法和平均值滤波方法,增强地形坡度点云信息分类结果,获取地...     

Design and Implementation of Protocol Library in Repeater Monitoring System

为了兼容众多直放站综合网络监控管理协议,借助C#软件开发平台,采用典型的MVC框架,设计了一种具有统一接口的独立的协议库.该协议库采用多线程发送和接收数据帧及序列存储数据流的方式,通过其统一的协议库处理接口和直放站进行信息交互,处理数据帧的组帧及解析过程.经测试,证明此设计实现了对直放站设备快速有效的监控,具有广阔的应用前景.     

Range and throughput enhancement of wireless local area networks using smart sectorised antennas

At present, wireless local area networks (WLANs) such as HiperLAN2 and 802.11a are being developed and deployed around the world. In this letter, the use of sectorized antennas is considered as a means to improve the physical layer performance of WLANs. Results demonstrate that throughput and range can be enhanced and/or the transmit power can be reduced. However, these benefits are achieved with a small increase in multiple access protocol overhead. Simulations are performed using measured wideband channels in the 5-GHz band. In cases where the channel exhibits strong Rician characteristics, gains as high as 13 dB are demonstrated. These benefits substantially outweigh the associated medium access control overhead.