Cross-coupled dual-spiral high-temperature superconducting filter

A microstrip bandpass filter implemented with dual-spiral resonators is described in this letter. The dual-spiral resonators make the filter compact and allow implementation of positive and negative inter-resonator couplings. Implementation of the positive and negative couplings is used to introduce a pair of finite frequency transmission-zeros in the filter response. The two zeros are designed to be close to the passband, thus improving the selectivity of the filter. As an example, a four-stage cross-coupled dual-spiral filter is designed and fabricated using superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films deposited on LaAlO/sub 3/ substrate.     

The spatially averaged electric field in the near field and far field of a circular aperture

This paper presents a theoretical and numerical investigation of the spatially averaged electric field in the beam of a circular aperture. The investigation leads to closed-form analytical expressions, based on scalar diffraction theory, which describe the spatially averaged electric field in the Fresnel region of a circular aperture excited by a spatially uniform, harmonic plane wave. The expressions ultimately permit rapid, practical, and efficient prediction of certain routine electromagnetic measurements. Because the expressions are valid in the Fresnel region, they are also valid in the near field, the far field, and the Fraunhofer region of a circular aperture. In fact, it is shown that the closed-form expressions contain, as special cases, classic on-axis and far-field results associated with a circular aperture. The analytical expressions are based on a generalization of Fresnel diffraction originally developed by Lommel in the late 1800s. Hence, a thorough review of the literature on the Lommel diffraction formulation is presented. Finally, it is shown that results obtained from the closed-form expressions compare quite favorably to results obtained from the exact solution computed via the dyadic Green's function approach.     

Self‐adapting protocol tuning for multi‐hop wireless networks using Q‐learning

Today's network control systems have very limited ability to adapt to changing network conditions. The addition of reinforcement learning‐based network management agents can improve quality of service by reconfiguring the network layer protocol parameters in response to observed network performance conditions. This paper presents a closed‐loop approach to tuning the layer three protocol based upon current and previous network state observations, specifically the Hello Interval and Active Route Timeout parameters of the AODV routing protocol (AODV‐Q). Simulation results demonstrate that the self‐configuration method proposed here demonstrably improves the performance of the original Ad‐Hoc On‐Demand Distance Vector (AODV) protocol, reducing protocol overhead by 43% and end‐to‐end delay 29% while increasing the packet delivery ratio by up to 11%. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrical characteristics of a 6H-SiC epitaxial layer grown by chemical vapor deposition on porous SiC substrate

Porous SiC (PSC) has been proposed as a buffer layer for reducing defects in epitaxial SiC layers. In this study, electrical characteristics of a 6H-SiC epitaxial layer grown by chemical vapor deposition on a porous SiC substrate (SiC-on-PSC) have been compared to those simultaneously grown on a standard SiC substrate (SiC-on-STD). Schottky barrier diodes (SBDs) have been fabricated on both epitaxial layers and then investigated with temperature-dependent current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The SBDs on both SiC-on-PSC and SiC-on-STD show about the same I-V and C-V characteristics, and at least four electron traps, i.e., B (0.75 eV), C (0.63 eV), D (0.40 eV), and E (0.16 eV), can be identically found in both SBDs by DLTS measurements. Thus, we conclude that the electrical quality of SiC-on-PSC is comparable to that of SiC-on-STD, and that the higher breakdown voltages observed in SBDs on SiC-on-PSC are not obviously related to a different defect structure.     

Call admission control in wireless multimedia networks

Call admission control (CAC) is a mechanism used in networks to administer quality of service (QoS). Whereas the CAC problem in time-division multiple access (TDMA)-based cellular networks is simply related to the number of physical channels available in the network, it is strongly related to the physical layer performance in code-division multiple access (CDMA) networks since the multi-access interference in them is a function of the number of users and is a limiting factor in ensuring QoS. In this article, the CAC issues in multimedia DS-CDMA systems are reviewed by illustrating the basic principles underlying various schemes that have been proposed progressively from the simplest to the complex. The article also introduces SIR as a measure of QoS and describes the relatively simple schemes to administer CAC. The expression for SIR resulting from linear minimum mean-squared error processing is also presented. This article illustrates how CAC for multiple class service can be casted into an optimality framework and then discuss the recent work addressing self-similar multiple access interference.     

一种高可靠GNSS接收机位同步算法

位同步技术是GNSS接收机技术的重要组成部分,位同步错误会导致接收机伪距解算错误及无法正确解调电文。针对弹载GNSS接收机的高可靠性要求,提出了一种高可靠GNSS接收机位同步算法,对最大似然位同步算法进行了改进。在非相干积分过程中等间隔提取中间结果进行观测,综合多次观测结果和最终非相干积分结果进行位同步判断。数值仿真与分析表明,在弱信号情况下,在不增加估计时间的前提下,可以大幅度减小错误同步概率,对于提高弹载GNSS接收机可靠性有着重要意义。     

A High Throughput Configurable SDR Detector for Multi-user MIMO Wireless Systems

Spatial division multiplexing (SDM) in MIMO technology significantly increases the spectral efficiency, and hence capacity, of a wireless communication system: it is a core component of the next generation wireless systems, e.g. WiMAX, 3GPP LTE and other OFDM-based communication schemes. Moreover, spatial division multiple access (SDMA) is one of the widely used techniques for sharing the wireless medium between different mobile devices. Sphere detection is a prominent method of simplifying the detection complexity in both SDM and SDMA systems while maintaining BER performance comparable with the optimum maximum-likelihood (ML) detection. On the other hand, with different standards supporting different system parameters, it is crucial for both base station and handset devices to be configurable and seamlessly switch between different modes without the need for separate dedicated hardware units. This challenge emphasizes the need for SDR designs that target the handset devices. In this paper, we propose the architecture and FPGA realization of a configurable sort-free sphere detector, Flex-Sphere, that supports 4, 16, 64-QAM modulations as well as a combination of 2, 3 and 4 antenna/user configuration for handsets. The detector provides a data rate of up to 857.1 Mbps that fits well within the requirements of any of the next generation wireless standards. The algorithmic optimizations employed to produce an FPGA friendly realization are discussed.     

SEP calculations for coherent M‐ary FSK in different fading channels with MRC diversity

In this paper, the authors derive symbol error probability (SEP) expressions for coherent M‐ary frequency shift keying (MFSK) modulation schemes in multipath fading channels. The multipath or small‐scale fading process is assumed to be slow and frequency non‐selective. In addition, the channel is also subjected to the usual degradation caused by the additive white Gaussian noise (AWGN). Different small‐scale fading statistics such as Rayleigh, Rician (Nakagami‐n), Hoyt (Nakagami‐q), and Nakagami‐m have been considered to portray diverse wireless environments. Further, to mitigate fading effects through space diversity, the receiver front‐end is assumed to be equipped with multiple antennas. Independent and identically distributed (IID) as well as uncorrelated signal replicas received through all these antennas are combined with a linear combiner before successive demodulation. As the detection is coherent in nature and thus involves phase estimation, optimum phase‐coherent combining algorithms, such as predetection maximal ratio combining (MRC), may be used without any added complexity to the receiver. In the current text, utilizing the alternate expressions for integer powers (1≤n≤4) of Gaussian Q function, SEP values of coherent MFSK are obtained through moment generating function (MGF) approach for all the fading models (with or without MRC diversity) described above. The derived end expressions are composed of finite range integrals, which can be numerically computed with ease, dispenses with the need of individual expressions for different M, and gives exact values up to M=5. When the constellation size becomes bigger (M≥6), the same SEP expressions provide a quite realistic approximation, much tighter than the bounds found in previous literatures. Error probabilities are graphically displayed for each fading model with different values of constellation size M, diversity order L, and for corresponding fading parameters (K, q, or m). To validate the proposed approximation method extensive Monte‐Carlo simulations were also performed, which show a close match with the analytical results deduced in the paper. Both these theoretical and simulation results offer valuable insight to assess the efficacy of relatively less studied coherent MFSK in the context of the optimum modulation choice in wireless communication. Copyright © 2010 John Wiley & Sons, Ltd.     

A prediction-based link availability estimation for routing metrics in MANETs

A Mobile Ad hoc Network (MANET) is a collection of wireless mobile terminals that are able to dynamically form a temporary network without any aid from fixed infrastructure or centralized administration. One critical issue for routing in MANETs is how to select reliable paths that can last as long as possible since terminal mobility may cause radio links to be broken frequently. To solve this problem, a criterion that can judge path reliability is needed. The reliability of a path depends on the number of links and the reliability of each link constituting the path. Many routing metrics in terms of number of links have been proposed, such as the shortest path routing. However, how to measure link availability or reliability in order to find more reliable paths has not been addressed adequately in the literature. (By a link being available, we mean that the radio quality of the link satisfies the minimum requirement for successful communication. Link availability is used to measure probability or degree that a link is available. The terms availability and reliability are used interchangeable in this paper.) This paper first introduces a prediction-based link availability estimation to quantify the link reliability. This quantity makes use of some instantly available information and also considers the dynamic nature of link status in order to properly reflect the link reliability. Then, this quantity has been further used to develop routing metrics for path selection in terms of path reliability to improve routing performances. The proposed schemes have been investigated through computer simulation.     

Wavelet based image adaptive watermarking scheme

An image adaptive watermark casting method based on the wavelet transform is proposed. To increase the robustness and perceptual invisibility, the algorithm is combined with the quantisation model based on the human visual system. The number of factors that affect the noise sensitivity of the human eye are taken into consideration. Experimental results demonstrate the robustness of the algorithm to high compression environments