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In this paper, we study relay selection under outdated channel state information (CSI) in a decode-and-forward (DF) cooperative system. Unlike previous researches on cooperative commu-nication under outdated CSI, we consider that the channel varies continuously over time, i.e., the channel not only changes between relay selection and data transmission but also changes during data transmission. Thus the level of accuracy of the CSI used in relay selection degrades with data transmission. We first evaluate the packet error rate (PER) of the cooperative system under contin-uous time-varying fading channel, and find that the PER performance deteriorates more seriously under continuous time-varying fading channel than when the channel is assumed to be constant during data transmission. Then, we propose a repeated relay selection (RRS) strategy to improve the PER performance, in which the forwarded data is divided into multiple segments and relay is reselected before the transmission of each segment based on the updated CSI. Finally, we propose a combined relay selection (CRS) strategy which takes advantage of three different relay selection strategies to further mitigate the impact of outdated CSI. 相似文献
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C. Bianchi J.A. BaskaradasM. Pietrella U. SciaccaE. Zuccheretti 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
A series of Power Virtual Height measurements (PVH) of radio echoes reflected from the ionosphere were acquired at a given frequency during the period 3–22 January 2008 with the purpose of studying the slow fading variations through time of the ionospheric channel. To obtain PVH data, an ionospheric vertical sounding system was suitably adapted to work at a single fixed frequency. PVH measurements were recorded between two routine ionospheric vertical soundings, providing a data type that enables evaluation of fading fluctuation through time. The time stability of the ionospheric layers is determined by analyzing the level of the received signal power within a chosen threshold. In this paper the fading behaviour and its characteristics are described, considering only temporal periodicity above 0.5 s. In a further analysis a relation is demonstrated between the recorded fading and the time stability of the signal within a fixed interval of values. 相似文献
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S.N.V.S. Prasad P.V.S. Rama Rao D.S.V.V.D. Prasad K. Venkatesh K. Niranjan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
The amplitude scintillations data recorded at 244 MHz from the geostationary satellite, FLEETSAT (73°E) at a low latitude station, Waltair (17.7°N, 83.3°E) during the ten year period of high to low solar activity from 2001 to 2010 is considered to study the occurrence characteristics of the VHF scintillations. A close association between the intense scintillations on VHF signals during pre-midnight hours, associated with range type of spread-F on ionograms and a relatively weak and slow fading scintillations during post-midnight hours associated with frequency type of spread-F is observed during the relatively high sunspot years from 2001 to 2004, whereas during the low sunspot years from 2005 to 2010 the scintillation activity as well as spread-F activity are found to be minimum. During both the high and low sunspot years, it is observed that the maximum scintillation activity occurs during equinoctial months followed by winter with the minimum occurrence during summer months. The annual mean percentage occurrence of scintillations is found to be clearly associated with the variations in the annual mean sunspot number. The nocturnal variations in the occurrence of scintillations show the onset of scintillation activity starts from 19:00 h LT with maximum of occurrence around 21:00 h LT. A clear semiannual variation in the occurrence of scintillations is observed during pre-midnight hours with two peaks in equinoctial months of March/April and September/October. The number of scintillation patches observed is found to be more during pre-midnight hours compared to those during post-midnight hours. The most probable scintillation patch duration lies around 30 min. Further, it is also found that the number of scintillation patches with durations of 60 min and more decreases with the increase in the patch duration. It is also observed in general that the scintillation activity is inhibited during geomagnetic disturbed days. 相似文献
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C. Bianchi J.A. BaskaradasM. Pezzopane M. PietrellaU. Sciacca E. Zuccheretti 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
It is well known that the ionosphere affects radio wave propagation especially in the high frequency (HF) range. HF radio waves reflected by the ionosphere can reach considerable distances, often with changes in amplitude, phase, and frequency. The ionosphere is a dispersive in frequency and time, bi-refractive, absorbing medium, in which multipath propagation due to traveling irregularities is very frequent. The traveling irregularities undulate the reflecting ionospheric layer, introducing variations in signal amplitude (fading). In this multipath time variant channel fading is mainly considered, even though it is not the sole effect. Echo signals from a single reflection, as in ionospheric vertical sounding (VIS) techniques, are affected by a certain degree of variability even in quiet ionospheric conditions. In this work the behavior of the ionospheric channel is studied and characterized by observing the power variation of received echoes using the VIS technique. Multipath fading was analyzed quantifying the power variation of the signal echo due to irregularities on a temporal scale from 0.5 to 256 s. An experimental set-up derived from an ionosonde was implemented and the analysis was performed employing a special numerical algorithm operating off-line on the acquired time sequence of the signal. The gain-loss of the irregularity shapes are determined in some special cases. 相似文献
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针对环绕月球运行的深卒探测航天器提出了一种新的自主导航方案,该方案是以两个环月航天器之间的相对位置矢量信息作为量测量,结合轨道动力学方程,对航天器的轨道自主估计确定,方案中考虑了符合实际的复杂摄动影响,并对描述动力学特性的数学模型与噪声的统计模型不准确而导致的滤波发散问题,提出利用一种经过衰减记忆改进的平方根非线性滤波(SRUKF)算法进行解决。对所提方案做了大量的计算机仿真,同时洋尽的分析和比较了由于衰减因子及采样周期变化所带来的影响。从仿真结果上看,衰减记忆滤波处理能够很好得抑制非线性状态转移条件下的滤波发散趋势。 相似文献
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为了解决无源传感器机动目标跟踪系统非线性较强、传统的跟踪滤波方法不稳定容易发散的缺陷,提出了一种带渐消因子的QKF(FQKF)算法。该算法通过引入时变渐消因子来实时调整状态预测误差协方差阵、量测预测误差协方差阵及状态预测误差和量测预测误差之间的互协方差阵,利用公式推导得出渐消因子实际上是对状态传播积分点和量测传播积分点进行渐消,进而达到实时调整滤波器增益矩阵的目的。并通过算法的机理分析和仿真实验表明FQKF算法具有强跟踪滤波器(STF)的优良性能,能够克服QKF算法的缺陷,对于无源传感器机动目标跟踪中系统的突变状态具有较强的跟踪能力,较QKF算法稳定性有所提高,并且计算量适中。 相似文献
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针对太阳闪烁作用于星际通信链路而产生的幅度闪烁、频谱扩展、相位闪烁、时延扩展问题,提出一种适用于星际通信链路的参数设计方法。该方法根据通信链路与太阳的几何关系,计算太阳风中等离子体作用于无线电波的幅度统计特性、时间谱特性,将计算结果作为通信链路的频率选择、传输信号带宽设计、调制方式选择、锁相环路带宽设计的约束条件,并以某空间探测任务为例进行了S、X、Ka三种频段下的信号传输性能仿真分析,结果表明,在太阳闪烁指数m<0.3的情况下,采用Ka频段+8PSK调制相结合的方式,对于太阳闪烁的抵抗能力最强。该方法设计的参数能够降低太阳闪烁对通信链路的影响,可作为工程实际系统设计的参照。 相似文献
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非合作机动目标天基测角定轨研究 总被引:2,自引:1,他引:2
在对空间非合作目标天基测角卡尔曼滤波定轨中,目标机动会导致常规卡尔曼滤波器发散.提出一种无迹卡尔曼滤波(UKF)结合带次优渐消因子的无迹卡尔曼滤波(SFUKF)的滤波定轨方法,目标无机动时使用UKF,在目标机动时使用SFUKF,给出了目标机动的判断准则.仿真结果表明,此滤波方法在目标无机动时有较高的状态估计精度,在目标... 相似文献