Too much mobility limits the capacity of wireless ad hoc networks

We show that for highly mobile ad hoc networks, the benefits of mobility are overshadowed by the cost of mobility in terms of the increased channel uncertainty and network homogeneity. We assume a block-fading channel model with jointly isotropic fading. We allow relays which can transmit and receive simultaneously. Under fairly general assumptions for the users' channel fades and additive noise distributions we show that increasing the number of transmit antennas M at any node beyond the channel coherence time T_c (measured in units of channel uses) does not affect the capacity region of the ad hoc network. For a fast-fading (coherence time T_clesM) homogeneous network, we determine the exact capacity region of the ad hoc network for any partition of the nodes into source, destination, and relay nodes. The optimal strategy is such that only one pair of source-destination nodes is active at a time while all the other nodes are inactive. There is no benefit from relaying and at high signal-to-noise ratio (SNR) the total throughput grows at most double-logarithmically with the number of nodes. Even for the case of slow fading, where the channel variations are slow enough that the receiver can track the channel perfectly, the inability of the transmitter to track the network topology limits the total throughput growth rate to no more than logarithmic in the number of nodes. Spatial correlation is shown to enhance the capacity region of the Rayleigh-fading ad hoc network     

On the capacity of the vector MAC with feedback

In this correspondence, we determine the feedback capacity region of a two user Gaussian multiple-access channel (MAC) with multiple antennas at the base station and a single antenna at each user. The vector MAC with a single antenna at the base station and multiple antennas at each user is shown to be equivalent to a scalar MAC. We also determine the capacity enhancement due to feedback at high signal-to-noise ratio (SNR) for the scalar and vector MAC for any number of users. Extensions of the high SNR results to the vector broadcast channel are also provided.     

A web-based high-performance multicriteria decision support system for medical diagnosis

Multicriteria decision making (MCDM) methods can be powerful aids for evaluating patients' medical information in medical diagnostic systems. Technique ordered preference by similarity to the ideal solution (TOPSIS) is one of the more widely used MCDM methods in decision support systems. For the purpose of this work, the TOPSIS method is modified into a more suitable form and used for the implementation of a web-based medical diagnostic system. In our modified TOPSIS method, we have utilized fuzzy logic so that users can more accurately describe their symptoms. The data given to the modified TOPSIS method are often massive in proportions and may take a considerable amount of time to generate a ranking of alternatives. TOPSIS lends itself to parallel computation because it is virtually a combination of matrix computations. Therefore, computer parallelism is implemented so that a large amount of input data can be handled simultaneously, hence decreasing overall execution time. In addition, to make our MCDM system more accessible, we have designed our system to be web based. The web-based medical diagnosis system includes a dynamically generated web-based user interface, while the parallel implementation of the modified TOPSIS component, in conjunction with the Common Gateway Interface, forms the back end of the system. © 2007 Wiley Periodicals, Inc. Int J Int Syst 22: 1083–1099, 2007.     

Degrees of Freedom Region of the MIMO X Channel

We provide achievability as well as converse results for the degrees of freedom region of a multiple-input multiple-output (MIMO) X channel, i.e., a system with two transmitters, two receivers, each equipped with multiple antennas, where independent messages need to be conveyed over fixed channels from each transmitter to each receiver. The inner and outer bounds on the degrees of freedom region are tight whenever integer degrees of freedom are optimal for each message. With M = 1 antennas at each node, we find that the total (sum rate) degrees of freedom are bounded above and below as 1 les eta*x les 4/3. If M > 1 and channel matrices are nondegenerate then the precise degrees of freedom eta*x = (4/3)M. Thus, the MIMO X channel has noninteger degrees of freedom when M is not a multiple of 3. Simple zero forcing without dirty paper encoding or successive decoding, suffices to achieve the (4/3)M degrees of freedom. If the channels vary with time/frequency then the channel with single antennas (M = 1) at all nodes has exactly 4/3 degrees of freedom. The key idea for the achievability of the degrees of freedom is interference alignment-i.e., signal spaces are aligned at receivers where they constitute interference while they are separable at receivers where they are desired. We also explore the increase in degrees of freedom when some of the messages are made available to a transmitter or receiver in the manner of cognitive radio.     

有限负压下熔体硅中磷的挥发去除(英文)

对有限负压下熔体硅中磷的挥发去除进行研究。采用电子级硅配制Si-P合金,并采用GD-MS来检测实验前后硅中的磷含量。理论计算结果表明:在有限负压下,硅中的磷以P和P2的气体形式从熔体硅中挥发。实验结果显示:在温度1873K、真空度0.6-0.8Pa、熔炼3600s的条件下,熔体硅中的磷从0.046%(460ppmw)下降到0.001%(10ppmw)。实验结果与理论结果一致表明:当熔体硅中磷的含量大且炉腔内气压相对较高时,磷的去除与气压高度相关;而当炉腔气压很低时,磷的去除基本与气压无关。原因是在相对高磷含量的熔体硅中,磷主要以P2气体的形式挥发;在磷含量较低时,磷主要以单原子气体P的形式挥发。     

Highly stable Ga-doped ZnO/polystyrene nanocomposite film with narrow-band cyan emission

In the present study, a simple method for the preparation of a luminescent flexible gallium doped zinc oxide (GZO)/polystyrene nanocomposite film was developed. The prepared GZO powder was characterized through different optical and structural techniques. The XRD study revealed the existence of a wurtzite structure with no extra oxide peaks. Elemental-mapping, EDX, FTIR and XPS analyses were used to confirm the presence of elements and the several groups present in the structure. Under excitations of UV, the prepared hybrid nanocomposite showed a strong cyan emission with narrow full width at half the maximum value (20 nm) that has not been reported before. X-ray and laser-induced luminescence results of the hybrid film revealed novel blue-green emission at room temperature. The prepared composite film showed a strong scintillation response to ionizing radiation. The strong emissions, very weak deep-level emissions, and low FWHM of composite indicate the desirable optical properties with low-density structural defects in the GZO composite structure. Therefore, the prepared hybrid film can be considered to be a suitable candidate for the fabrication of optoelectronic devices.     

Effect of Extraordinary Large Floods on at-site Flood Frequency

The peak flow of extraordinary large floods that occur during a period of systematic record is a controversial problem for flood frequency analysis (FFA) using traditional methods. The present study suggests that such floods be treated as historic flood data even though their historical period is unknown. In this paper, the extraordinary large flood peak was first identified using statistical outlier tests and normal probability plots. FFA was then applied with and without the extraordinary large floods. In this step, two goodness-of-fit tests including mean absolute relative deviation and mean squared relative deviation were used to identify the best-fit probability distributions. Next, the generalized extreme value (GEV), three-parameter lognormal (LN3), log-Pearson type III (LP3), and Wakeby (WAK) probability distributions were used to incorporate and adjust the extraordinary large floods with other systematic data. Finally, procedures with and without historical adjustment were compared for the extraordinary large floods in terms of goodness-of-fit and flood return-period quantiles. The results of this comparison indicate that historical adjustment from an operational perspective was more viable than without adjustment procedure. Furthermore, the results without adjustment were unreasonable (subject to over- and under-estimation) and produced physically unrealistic estimates that were not compatible with the study area. The proposed approach substantially improved the probability estimation of rare floods for efficient design of hydraulic structures, risk analysis, and floodplain management.     

如何设计兼具隔离效果和安全性的医疗设备

使用交流电源的医疗设备,可能会因不恰当的接地和电气隔离造成漏电,让患者和医疗人员暴露在危险之下,同时漏电也可能影响到邻近其它医疗设备的性能.对医疗设备来说,EMC是一个相当重要的条件,从2005年11月开始,医疗设备必须符合国际电工委员会(IEC)更新后的IEC 60601-1-2:2001EMC标准.     

Modulation and Detection for Simple Receivers in Rapidly Time-Varying Channels

We investigate the performance degradation of basic modulation schemes in a rapidly time-varying channel using a first-order autoregressive channel model. Various performance metrics are used to indicate the relative advantages of each modulation scheme. We find that noncoherent frequency-shift keying (FSK) is suitable for operating at very high mobility and high signal-to-noise ratio, ideal for some military applications. We then propose a partially coherent detector for FSK and differential phase-shift keying that exploits partial channel knowledge to enable the receiver to operate effectively in both fast and slow fading. The maximum-likelihood rule obtained for the partially coherent FSK turns out to be a linear combination of coherent and noncoherent detection rules. Results demonstrate that significant performance improvement can be achieved over the best of coherent and noncoherent FSK detection. The detector is robust to estimation errors present in the channel statistics. We also propose a few adaptive schemes that employ various combinations of modulation schemes to increase the robustness of the system in fast fading     

Influence of top‐section design and draft‐tube height on the performance of airlift bioreactors containing water‐in‐oil microemulsion

The mixing and mass transfer characteristics of draft‐tube airlift bioreactors (DTAB) for a water‐in‐kerosene microemulsion, as a cold model of petroleum biodesulfurization, were studied. Incomplete gas disengagement at the top‐section of the DTAB and hence high gas recirculation were obtained with the microemulsion system for all the top‐section configurations employed in the present study especially at the high airflow rates. The ratio (S) of the volumes of the riser and the downcomer to the top‐section together with the gas disengagement abilities of the gas separator were both found to affect the mixing performance of the DTAB employed for the microemulsion system. Increase in the draft‐tube height resulted in significant increase in the mixing time (t_m) and a slight increase in the overall volumetric oxygen transfer coefficient (k_La). Increase in the diameter of the top‐section and the height of the liquid above the draft‐tube led to a decrease in k_La, the latter effect being less prominent. New correlations were developed that predicted the mixing time and oxygen transfer coefficients obtained in the present work with reasonable accuracy. Copyright © 2004 Society of Chemical Industry