Satellite communications at KU, KA, and V bands: Propagation impairments and mitigation techniques

This article surveys the alternative fade mitigation techniques for satellite communication systems operating at Ku, Ka and V frequency bands. The specific phenomena influencing the propagation of radiowaves on Earth-space links are also overviewed. Emphasis is placed on modeling, experimental work carried out in the past, and practical implementations related to each mitigation technique.     

The impact of aggregating benefit and cost criteria in four MCDA methods

Multicriteria decision analysis (MCDA) problems (also known as multicriteria decision-making or MCDM) involve the ranking of a finite set of alternatives in terms of a finite number of decision criteria. Often times such criteria may be in conflict with each other. That is, an MCDA problem may involve both benefit and cost criteria at the same time. Although this is a frequent characteristic of many real-life MCDA problems, this subject has not received adequate attention in the literature. This paper examines the use of four key MCDA methods when two approaches for dealing with conflicting criteria are used. The two approaches are the benefit to cost ratio approach and the benefit minus cost approach. The MCDA methods used in this study are the weighted sum model, the weighted product model, and the analytic hierarchy process (AHP) along with some of its variants, including the multiplicative AHP. Not surprisingly, these two approaches for aggregating conflicting criteria may result in a different indication of the best alternative or ranking of all alternatives when they are used on the same problem. As it is demonstrated here, it is also possible for the two approaches to even result in the opposite ranking of the alternatives. An extensive empirical analysis of this methodological problem revealed that the previous phenomena might occur frequently on simulated MCDA problems. The WSM, the AHP, and the revised AHP performed in an almost identical manner in these tests. The contradiction rates in these tests were rather significant and became more dramatic when the number of alternatives was high. Although it may not be possible to know which ranking is the "correct" one, this study also theoretically proved that the multiplicative AHP is immune to these ranking inconsistencies.     

A comparison of beam propagation and coupled-mode methods:application to optical fiber couplers

We analyze the instantaneous power in the local normal modes of an optical fiber coupler with both various three-dimensional paraxial electric field propagation methods and coupled-mode theory. Our calculations show that large improvements in speed result if a low-order, split-operator, finite-difference propagation algorithm is substituted for the standard split-step fast Fourier transform technique. Further, as field evolution procedures incorporate higher-order modes, the calculated power transfer ratios agree far better with experiment than the corresponding coupled-mode predictions. The coupled-mode results however, can be improved by taking into consideration the exact shape of the refractive index profile and higher-order modes, while the approach is simple to program, and yields qualitatively correct predictions for performance trends     

Component sharing in complex products and systems: challenges, solutions, and practical implications

In this paper, we explore whether a multiproject management approach is helpful in complex products and systems (CoPS) environments. CoPS are high cost, engineering-intensive products and systems, tailor-made according to client's specifications. The focus of this paper is on the possibilities for sharing components between projects in CoPS multiproject environments. Component sharing is the process through which technological components and platforms are transferred and shared between projects. The vast majority of evidence about component sharing is based on empirical research in mass-producing companies. Little has been reported in the CoPS literature about similar processes. To contribute to this gap, this paper discusses challenges faced and solutions applied by a supplier of CoPS, an aerospace company, when attempting to share components between projects in its multiproject environment. Three main areas are examined that facilitate sharing between projects given the unique strategic context of CoPS: coordination and communication mechanisms that facilitate linkages across projects; knowledge transfer mechanisms; and an organizational form that centralizes some of the development activities. This paper concludes by offering managers and engineers a set of tools to consider component sharing in a CoPS environment.     

EEG and MEG: forward solutions for inverse methods

A solution of the forward problem is an important component of any method for computing the spatio-temporal activity of the neural sources of magnetoencephalography (MEG) and electroencephalography (EEG) data. The forward problem involves computing the scalp potentials or external magnetic field at a finite set of sensor locations for a putative source configuration. We present a unified treatment of analytical and numerical solutions of the forward problem in a form suitable for use in inverse methods. This formulation is achieved through factorization of the lead field into the product of the moment of the elemental current dipole source with a "kernel matrix" that depends on the head geometry and source and sensor locations, and a "sensor matrix" that models sensor orientation and gradiometer effects in MEG and differential measurements in EEG. Using this formulation and a recently developed approximation formula for EEG, based on the "Berg parameters," we present novel reformulations of the basic EEG and MEG kernels that dispel the myth that EEG is inherently more complicated to calculate than MEG. We also present novel investigations of different boundary element methods (BEM's) and present evidence that improvements over currently published BEM methods can be realized using alternative error-weighting methods. Explicit expressions for the matrix kernels for MEG and EEG for spherical and realistic head geometries are included.     

Contemporary CMOS aging mitigation techniques: Survey,taxonomy, and methods

The proposed paper addresses the overarching reliability issue of transistor aging in nanometer-scaled circuits. Specifically, a comprehensive survey and taxonomy of techniques used to model, monitor and mitigate Bias Temperature Instability (BTI) effects in logic circuits are presented. The challenges and overheads of these techniques are covered through the course of this paper. Important metrics of area overhead, power and energy overhead, performance overhead, and lifetime extension are discussed. Furthermore, the techniques are assessed with regards to ease of implementation and the ability to cope with challenges such as increase in manufacturing induced process variations. Finally, a taxonomy of the surveyed techniques is presented to facilitate generalization of the discussed approaches and to foster new inspiring techniques for this important reliability phenomenon leading to advancements in the design of defect-tolerant digital circuits.     

Real-time implementation of optical OFDM transmitters and receivers for practical end-to-end optical transmission systems

Real-time optical OFDM transceivers are successfully demonstrated for the first time, which support 1.5 Gbit/s transmission over 500 m 62.5/125 μm multimode fibres in an intensity-modulation and direct-detection system involving a directly modulated DFB laser. The implemented transceivers only use standard commercially available components.     

Transport control protocol in optical burst switched networks: issues, solutions, and challenges

Since its advent in 1981, TCP has been subject to a tremendous amount of research effort and enhancements for achieving better performance over various network environments and application scenarios. Due to the transmission characteristics of optical burst switched networks, such as random burst dropping, retro-blocking (i.e., bursts proceeding or delayed from their actual reservation time slot), burstification delay, and burst signaling delay, TCP could be significantly affected if no corresponding countermeasure and enhancement are developed. In this review article we provide a comprehensive survey on reported studies for TCP enhancements over OBS networks in order to mitigate the numerous side effects due to the buffer- less characteristic of burst transmission. Furthermore, we closely analyze TCP behavior over OBS networks with various burst transmission characteristics while highlighting the open challenges that have not yet been extensively tackled or solved.     

Joint impacts of hardware impairments,imperfect CSIs,and interference constraints on underlay cooperative cognitive networks with reactive relay selection

Telecommunication Systems - In this paper, we investigate joint impacts of hardware impairments, imperfect channel state information (CSI) and interference constraints at secondary users in an...     

Detailed comparison of two approximate methods for the solution of the scalar wave equation for a rectangular optical waveguide

Two approximate methods for the determination of the fundamental mode of an optical waveguide with rectangular core cross section and step refractive-index profiles are presented and analyzed thoroughly. Both methods are based on Galerkin's method. The first method uses Hermite-Gauss basis functions and the second uses the guided and nonguided slab waveguide solutions as basis functions. The results are compared with results from an accurate circular harmonic analysis. Both methods provide values of the normalized propagation constant with errors less than 0.1% for practical rectangular single-mode waveguides. The slab waveguide method is the fastest, and even when only one slab waveguide mode is used the propagation constant for the fundamental mode can be calculated with an error of less than 1%. The slab waveguide method also gives very accurate results for the propagation constant for higher order modes.<>     

Parallel optical interconnects for enterprise class server clusters: needs and technology solutions

We examine the status of enterprise-class server clusters and the communication issues that need to be addressed in future systems. With increasing system performance, new approaches beyond traditional copper-only communication solutions have to be examined. Parallel optics is an attractive solution to overcome copper's shortcomings, but traditional approaches to parallel optics have had their own limitations. We describe a new approach to parallel optics - dense parallel optics - and its relevance to enterprise servers. After discussing system communication needs, we examine dense parallel optics from both the passive cabling and active component views. We explain how dense parallel optics offers a unique way to address the performance, cost, reliability, and scalability of server systems. We also discuss how current approaches to dense parallel optics afford system opportunities beyond simple data transport.     

光码分多址系统关键技术与发展前景

着重分析光码分多址（ＯＣＤＭＡ）系统实现中的关键技术及其发展动态,并对光码分多址技术存在的问题和发展前景作了深入的探讨。     

A comparison of analog and digital techniques for pattern recognition

Since the computer technology utilized often places limitations on the performance of a particular pattern-recognition task, it is important to compare the state-of-the-art and future trends in both the digital and the analog-computer fields. Electronic, acoustical, and optical analog computers for use in pattern recognition are discussed and their performance compared with that of both general-purpose and special-purpose digital computers. It is shown that the analog computer offers workers using low-precision high-speed one-dimensional or two-dimensional linear discriminant analysis a significant advantage in hardware performance (equivalent bits per second per dollar) over the digital computer in certain limited but important areas. These areas include fingerprint identification, word recognition, chromosome spread detection, earth-resources and land-use analysis, and broad-band radar signal processing. A trend analysis is presented which indicates that the advantages of analog computation will probably be overcome in the next few decades by the rapid performance advances being made in digital-computer hardware.     

Computational cost estimations and comparisons for three methods of applied electromagnetics (MoM, MAS, MMAS)

The computational costs of three numerical techniques used in electromagnetics, namely the moment method (MoM), the method of auxiliary sources (MAS), and its modified version (MMAS), are estimated for various calculation schemes and configurations. Both surface and volumetric problems are considered. The number of multiplications required for the system-matrix fill is calculated and added to the algorithmic cost of the matrix inversion. The Green's function singularity extraction is also taken into account, particularly for the MoM. The original integrals are transformed into the local (area or volume) coordinate systems, and are subsequently evaluated on the basis of standard numerical quadrature schemes. For the surface integral equation (SIE), some calculations using either the well-known Duffy transformations or some analytical-numerical integration schemes are also presented (expressions are available only for the scalar potential integral case). For the MAS and MMAS, the matrix fill is shown to be much faster, since no time-consuming integrations are involved. The analysis is applied to various objects, such as a perfectly conducting (PEC) parallelpiped, a PEC sphere, and a microstrip patch antenna, and useful conclusions are drawn on the relative efficiency of the three methods.     

A comparison of two combining techniques for equal gain, trelliscoded diversity receivers

Trellis coded modulation is widely used for digital transmission over fading channels. Classical diversity techniques are also frequently employed to combat fading. In this paper two different strategies for equal gain combining are compared, One scheme is based on an interleaved code combining technique. The alternative scheme is based on averaged diversity combining. The well known transfer function bounding technique for trellis codes is used to obtain: expressions for the bit error rate performance of the two trellis coded diversity receivers over a slowly fading Rayleigh channel. The analysis of interleaved code combining is a straightforward modification of the analysis for multiple trellis coded modulation. The analysis of averaged diversity combining is accommodated through a more involved, novel modification of the branch labeling of the error state diagram. The analytic techniques presented in this paper are supported by simulation results using a TCM scheme based on QPSK modulation and a rate-l/2 linear convolution code     

Scatter-glare estimation for digital radiographic systems: comparison of digital filtration and sampling techniques

The scatter and veiling glare distribution in images acquired with a digital subtraction angiography imaging system was estimated using a digital filtration and a beam-stop technique. The digital filtration technique utilizes exposure parameters and image gray levels to estimate scatter-glare intensity based on previous phantom measurements. The beam-stop technique uses an array of lead discs in order to sample scatter-glare intensity for each patient. To test the ability of digital filtration and beam-stop techniques to estimate the scatter-glare intensity, they were applied to images of postmortem swine animal models at different projections and beam energies. The systematic and root-mean-square (rms) percentage errors of these estimates were obtained by comparison to directly measured scatter-glare images using a scanning lead strip technique. The average rms percentage error for the digital filtration and beam-stop techniques were 8.07% and 6.67%, respectively. The changes in scatter-glare intensity due to contrast injection during coronary arteriography and ventriculography were also measured using the beam-stop technique. The maximum changes in scatter-glare intensities during coronary arteriography and ventriculography were 19 and 88%, respectively. The results indicate that the digital filtration technique is more suited for applications such as coronary arteriography and ventriculography where the iodinated contrast material significantly changes the scatter-glare intensity.     

Challenges for GMPLS lightpath provisioning in transparent optical networks: wavelength constraints in routing and signaling - [Topics in optical communications]

GMPLS has introduced several enhancements to the MPLS-TE routing and signaling control plane protocols to handle dynamic lightpath provisioning in wavelength-routed networks. Specifically, the GMPLS signaling protocol has been enhanced to support two new provisioning functionalities, namely, the minimization of the setup delay, and the setup of bidirectional connection requests. In both cases, the source node must perform a wavelength allocation for either minimizing the setup delay (i.e., the suggested label) or requesting a bidirectional connection (i.e., the upstream label). However, these GMPLS provisioning functionalities present important deficiencies when applied to wavelength-routed networks with the wavelength continuity constraint, degrading the network performance considerably. The reason is that the standard GMPLS routing protocols flood link attributes only at bandwidth granularity, that is, no per-wavelength channel granularity is disseminated. Therefore, the source node is unable to perform an optimal wavelength assignment that fulfils the wavelength continuity constraint along the complete route toward the destination. In this article we present and experimentally evaluate an enhanced routing-based solution in the ADRENALINE testbed to handle the wavelength continuity constraint.     

Antiresonant reflecting optical waveguide-type vertical-cavity surface emitting lasers: comparison of full-vector finite-difference time-domain and 3-D bidirectional beam propagation methods

The cold-cavity modal characteristics of an antiresonant optical waveguide-type cylindrical vertical-cavity surface-emitting laser (VCSEL) are investigated through numerical simulations using a three-dimensional (3-D) bidirectional beam propagation method (BD-BPM) and a full-vector axisymmetric finite-difference time-domain (FDTD) method. Good agreement between the BPM- and FDTD-computed radial mode profiles as well as the mode-dependent radiation losses is obtained. The results of this paper establish the accuracy of the BD-BPM technique for simulating this class of devices and confirm effective-index method predictions that antiresonance conditions for cylindrical geometry devices (i.e., VCSELs) differ from those of planar geometry devices (i.e., edge emitters).     

Morphology,optical, and adsorption properties of copper-oxide layers deposited from complex compound solutions

A technique for synthesizing gas-sensing copper-oxide layers by layer-by-layer deposition onto glass and plastic substrates is considered. Deposition is based on the oxidation of a copper–ammonia complex by hydrogen peroxide. Layers synthesized at various numbers of deposition cycles are studied by atomicforce microscopy and transmission spectroscopy methods (300–1000 nm). The spectral shape indicates the mixed nature of the oxide with predominant copper oxide (I) Cu₂O with a corresponding band gap of 2.1 eV. The layers have a continuous granular structure. The surface concentration of Brönsted acid adsorption sites with pK _a ≈ 2.5 (OH acid groups), determined by the indicator method, is 560 μmol/m² for the initial films, and threefold increases after heat treatment at 80°C for 30 min. The results obtained are interesting for the use of the described technology for fabricating copper-oxide active layers in gas-sensing sensors operating at room temperature.     

红外光学系统冷反射分析和定量计算方法

红外光学系统冷反射指制冷型探测器看到自身的反射像,它严重影响红外光学系统的图像质量。光学系统设计中控制冷反射的强度往往与提高光学系统像质和满足边界要求之间是矛盾的。详细介绍了冷反射强度仿真分析和评估方法,提出了冷反射辐射的信号与系统噪声之比的定量计算模型。对两套红外系统的冷反射强度进行了仿真分析和实验测试。研究结果表明,冷反射强度实验测试与仿真计算结果一致,冷反射仿真分析和定量计算在光学设计中能够有效地评估冷反射强度,保证光学系统设计质量。