bluuebunny/crossref_metadata_2025 huggingface dataset:
Friction and part curvature in parallel‐ jaw grasping
Authors: Rao Anil S. , Goldberg Kenneth Y.
Published: July 2007

When grasping a part with a parallel‐jaw gripper, the part will generally rotate due to kinematic constraints. Predicting the part's final orientation is useful for grasping and for planning sequences of gripper motions to orient parts. For a given part geometry, the grasp function maps initial orientations to final orientations. Previously, we studied polygonal and algebraic parts in the absence of friction. This article considers how Coulomb friction affects the grasp function.

We consider two models of Coulomb friction. For a deterministic model, we show that the grasp function of any polygonal part can be represented with a piecewise linear function that we call a step‐ramp function. We then show that any step‐ramp function is the grasp function of a curved part operating under zero friction. Both contain ranges of orientations where the part does not rotate when grasped. This yields our primary result, that any part with deterministic friction has equivalent grasp mechanics to a “dual” part under zero friction. We then apply previous results to derive grasp plans that orient parts in the presence of friction. We also give an algorithm for planning under a non‐deterministic model of Coulomb friction, and give bounds on the friction coefficient needed to insure the existence of such plans. © 1995 John Wiley & Sons, Inc.
Efficient workspace generation for binary manipulators with many actuators
Authors: Ebert‐uphoff Imme , Chirikjian Gregory S.
Published: July 2007

Binary actuators have only two discrete states, both of which are stable without feedback. As a result, manipulators with binary actuators have a finite number of states. Major benefits of binary actuation are that extensive feedback control is not required, task repeatability can be very high, and two‐state actuators are generally very inexpensive, thus resulting in low‐cost robotic mechanisms. Determining the workspace of a binary manipulator is of great practical importance for a variety of applications. For instance, a representation of the workspace is essential for trajectory tracking, motion planning, and the optimal design of binary manipulators. Given that the number of configurations attainable by binary manipulators grows exponentially in the number of actuated degrees of freedom, 0(2), brute force representation of binary manipulator workspaces is not feasible in the highly actuated case. This article describes an algorithm that performs recursive calculations starting at the end‐effector and terminating at the base. The implementation of these recursive calculations is based on the macroscopically serial structure and the discrete nature of the manipulator. As a result, the method is capable of approximating the workspace in linear time, O(n), where the slope depends on the acceptable error. © 1995 John Wiley b Sons, Inc.
Surgical applications of milli‐robots
Authors: Cohn Michael B. , Crawford Lara S. , Wendlandt Jeffrey M. , Sastry S. Shartkar
Published: July 2007

Minimally invasive surgical techniques, especially endoscopy and laparoscopy, possess many advantages over conventional methods. These include accelerated patient recovery and reduced rate of complications. However, limitations of current operating instruments create difficulties for the surgeon. In this article, we present a design for an improved polypectomy snare for the endoscope, several rotary actuator designs for endoscopic tools, and a prototype endo‐platform, which provides fine motion control for endoscopic tools. We also present several prototypes of more dextrous laparo‐scopic tools based on the human hand. Finally, we present a sensory glove designed as a natural and dextrous human interface. © 2995 John Wiley & Sons, Inc.
New bounds on the number of frictionless fingers requied to immobilize
Authors: Rimon Eton , Burdick Joel W.
Published: July 2007

Using a novel second‐order mobility theory that was recently developed by the authors, this article develops new lower bounds on the number of frictionless fingers required to immobilize generic planar objects. We show that any generic smooth or polygonal planar object can be immobilized with three convex fingers or fixtures that have sufficiently flat curvature. Further, if it is possible to specify the fingertip curvature, then any generic smooth or polygonal object can be immobilized with two, possibly concave, fingers. © 1995 John Wiley & Sons, Inc.
Geometric optimization algorithims for robot kinematic design
Authors: Park Frank C. , Bobrow James E.
Published: July 2007

This article addresses the problem of designing a robotic mechanism such that its end‐effector frame comes closest to reaching a set of desired goal frames. We formulate this as an optimization problem, in which the kinematic parameters are selected to minimize the total distance between the end‐effector frame and each goal frame. The objective function is defined in terms of a class of distance metrics on the rigid body motions that are invariant with respect to choice of fixed reference frame. A main contribution of this article is an explicit expression for the gradient of this objective function with respect to the kinematic parameters. With this analytic gradient, efficient optimization algorithms can now be developed for the design of general spatial mechanisms. Our design methodology is illustrated with an example involving the base positioning of two cooperating robots. © 1995 John Wiley & Sons, Inc.
Identification and resolution of structural and algorithmic singularity in redundancy control of serial manipulations
Authors: Wang Zhaoyu , Kazerounian Kazem
Published: July 2007

A major difficulty that has haunted most researchers in the process of optimal redundancy resolution of robotic manipulators is the instability observed in even very simple numerical simulations. This numerical instability is not related to the structurally singular configurations of the manipulators, and in the literature has been referred to as “algorithmic singularity,” “artificial singularity,” or “unavoidable singularity.” In this work, conditions on both structural and algorithmic singularities are studied based on the Singular Value Decomposition of the Jacobian matrix, and, hence, a singularity‐free control algorithm for redundant manipulators is developed and resolved as the Lagrange problem of optimal control. It is shown that many well‐known methods for optimal redundant manipulation in the literature, including the Extended Jacobian Technique, most of constraint function‐based methods, and most of the previously reported methods on global optimization techniques, are all special cases of the formulation provided here. Further, the necessary conditions of the global optimality for this general formulation are derived in explicit form and the source of “algorithmic singularity” is rigorously identified and resolved. © 2995 John Wiley & Sons, Inc.
Mobile robot path planning for fine‐grained and smooth path spcification
Authors: Shin Dong Hun , Ollero Anibal
Published: July 2007

This article introduces and draws a comparison on B‐splines and clothoids for generating fine‐grained, smooth path specifications as applied to mobile robot path planning. At one level, a sequence of objective points that the robot must attain to avoid obstacles and to progress toward its goal are supplied by a coarse path planner. Using B‐spline and clothoid curves, these objective points are converted into a path of finer detail, as requisite to a robot driving controller/tracker. Through experimental results with the Navlabll, a mobile robot vehicle at Carnegie Mellon University (CMU), we differentiate the advantages of this approach over conventional poly‐line fit/round corner used to date. © 2995 John Wiley & Sons, Inc.
Improved manipulator perfomance through local d‐h calibation
Authors: Everett Louis J. , Lei James
Published: July 2007

This article deals with the subject of increasing the positioning accuracy of a manipulator beyond what is typically obtained via Denavit‐Hartenberg (D‐H) calibration. It is commonly accepted that one method for accomplishing this requires including “extra” terms (such as static deflection) in the D‐H model. Another technique, which is experimentally verified and quantified in this article, is that of local calibration. Rather than continually introducing more and more modeling terms, we restrict the workvolume of the manipulator. One contribution of this article is an experimental investigation of local calibration. Most data indicates that local calibration offers advantages and it quantifies that the approximate improvement on one particular PUMA 560 robot is as much as four times better than global calibration. Some data indicates that it is not always true that “localizing” the calibration is an improvement. The article also discusses the problem of recognizing bad calibration data. We demonstrate a technique for identifying and removing “bad” data. Finally, the article shows that the locality of workvolume must include a consideration of the joint motions as well. For example, it presents results when data is taken with the arm in a single configuration, and in multiple configurations. © 1995 John Wiley & Sons, Inc.
Feedback linearization of robot manipulators and riemannian curvature
Authors: Bedrossian Nazareth S. , Spong Mark W.
Published: July 2007

Several authors have noted that if the robot inertia matrix D(q) can be factored as NT(q)N(q) where N(q) is the Jacobian of a function Q(q), then Q and P = N(q)q define a canonical transformation relative to which the robot dynamics are linear except for gravity terms. In this article, we show that necessary and sufficient condition for the existence of such a factorization is that the Riemannian curvature of the robot inertia matrix D(q) vanish identically. We use this result to generate feedback linearization and approximate feedback linearization control laws that require fewer calculations than the usual method of computed torque. © 1995 John Wiley & Sons, Inc.
Near‐time optimal robot motion planning foe on‐line applications
Authors: Croft E. A. , Benhabib B. , Fenton R. G.
Published: July 2007

Solving current formulations of the time‐optimal point‐to‐point motion problem for robotic manipulators is a computationally intensive task. Thus, most existing solutions are not suitable for on‐line motion planning applications, such as the interception of moving targets, where time‐optimality of the motion is advantageous. A novel technique is proposed in this article that separates the time‐optimal point‐to‐point motion problem into the following two sub‐problems: (1) selection of a near‐time‐optimal path between the two endpoints, and (2) generation of time‐optimal motion along the selected path (i.e., constrained continuous path motion). Although our approach uses known path‐constrained time‐optimal‐motion algorithms for the second sub‐problem, a new method is proposed for the selection of near‐time‐optimal paths. Based on a study of the characteristics of global‐time‐optimal paths, the near‐optimal path is selected as a minimum‐curvature joint spline, tangent to one of the manipulator's acceleration directions at the start point, and tangent to the required manipulator velocity direction at the end point. The algorithm for determining the overall near‐optimal path is described herein, along with an example. Simulation test results and computation‐time studies indicate that the proposed method is suitable for on‐line motion planning applications. © 1995 John Wiley & Sons, Inc.
Generation of discontinuous gaits for quadruped walking vehicles
Authors: Santos Pablo Gonzalez De , Jimenez Maria A.
Published: July 2007

Discontinuous gaits for walking machines have not yet been properly studied. Research has focused on the investigation, comprehension, and mathematical formulation of natural gaits. These gaits feature the fact that the body is in constant motion. The results have been significant, but they seem more adequate for animals than machines. On the other hand, discontinuous gaits, executed by animals under extreme conditions, exhibit excellent attributes for implementation in walking machines. This article presents a comparative study of continuous and discontinuous gaits with regard to their maximum achievable velocity and stability. Other aspects such as implementation in real machines, power requirements, and control under terrain difficulties are mentioned briefly. An elemental discontinuous gait is stated, and some variations on deriving crab and turning gaits are performed. Different methods for enlarging the achievable crab angle and improving stability are discussed for discontinuous crab gaits. A similar study is also done for turning gaits. (c) 1995 John Wiley & Sons, Inc.
Simulation of an open chain of flexible links in a mixed formalism
Authors: Boyer FréDéric , Coiffet Philippe
Published: July 2007

This article presents the second part of a research work devoted to modeling and control of robots with flexible links. First, we recall the generalization of the Newton Euler equations for an open chain of flexible links. These equations constitute an original dynamic model because they represent the dynamic behavior of the chain with a mixed formalism, because it is both Lagrangian and Eulerian. This formalism allows us to find the linear intrinsic dynamic model of links. Second, we exhibit the constrained nature of these equations. We use a numerical integration method for a constrained system, and an updating procedure well suited to our particular formalism. Finally, simulation results on spatial kinematics involving up to three links are reported. (c)1995 John Wiley & Sons, Inc.
Path planning for robobt manipulators in polyhhedral objects environment
Authors: Tseng Ching‐Shiow , Lue Tsun‐Sun
Published: July 2007

A recent development in robotics is the increase of intelligence in robots. One of the research fields is to enable robots to autonomously avoid collisions with surrounding objects. This article presents an efficient method for planning collision‐free paths for an articulated robot that is surrounded by polyhedral objects. The algorithm plans a hypothetical Archimedes's spiral path from the initial position to the goal position. When a collision among the arms and obstacles is detected, the hypothetical path will be modified to avoid the collision. The algorithm applies geometric methods to determine the upper and lower bounds of the reachable area of the wrist and then determines a collision‐free path point on that reachable area. Because the equations, which represent the upper and lower bounds, are simple, the algorithm can rapidly determine a collision‐free path. Moreover, with minor modifications, this path planning algorithm can also be applied to other robots such as spherical, cylindrical, and Cartesian types of robots. ©1995 John Wiley & Sons, Inc.
Cad‐based off‐line planning for active vision
Authors: He Xiaoyu , Benhabib Beno , Smith Kenneth C.
Published: July 2007

A novel active‐vision system for 3‐D object recognition was developed. The vision system combines object pre‐marking and active sensing. Therein, an object is modeled by a few of its 2‐D perspective (standard) views, each with a corresponding viewing axis defined by a marker placed on the object. The operation of the active‐vision system is characterized by its off‐line planning and on‐line recognition phases. The primary objective of this article is to address two issues in regard to the former: optimal object pre‐marking and optimal camera placement. The optimal object pre‐marking problem is defined as the determination of the minimum number of markers, and their best locations on a given set of objects to yield maximum “distinctiveness” for the 2‐D standard‐views defined by these markers. The optimal camera‐placement problem targets minimization of camera movements in detecting the markers. Following a brief description of the proposed on‐line recognition process, the techniques developed for the off‐line planning stage are described in this article. For experimental verification purposes, a prototype of the proposed active‐vision system was also implemented and is described herein. ©1995 John Wiley & Sons, Inc.
A dynamic coupling index for underactuated manipulators
Authors: Bergerman Marcel , Lee Christopher , Xu Yangsheng
Published: July 2007

In recent years, researchers have been dedicated to the study of underactuated manipulators, which have more joints than control actuators. In previous work, researchers always assumed that there was enough dynamic coupling between the active and the passive joints of the manipulator for it to be possible to control the position of the passive joints via the dynamic coupling. In this work, the authors aim to develop an index to measure the dynamic coupling, so as to address when control of the under‐actuated system is possible, and how the motion and robot configuration can be designed. We discuss extensively the nature of the dynamic coupling and of the proposed coupling index, and their applications in the analysis and design of underactuated systems, through a detailed mathematical analysis and several illustrative examples. ©1995 John Wiley & Sons, Inc.
Robust contorl of a mamipulator pefoming constrained motion with uncertainties in the constraint
Authors: Ho Yeons Khing , Wang Danwei , Son Yens Chai
Published: July 2007

This article deals with the control of a robot manipulator performing constrained motion in an environment where the constraint function contains uncertainties. The uncertainties are appropriately modeled, and a controller consisting two components, one for positional and velocity tracking, and the other for force regulation, is proposed. The controller contains a nonlinear term that takes the form of a polynomial in the norms of the trajectory and force errors. The coefficients of the nonlinear function are chosen in relation to the bounds on the dynamic parameters of the robot system and an estimate of the bound on the uncertainty in the constraint function. Simulation results show that the modeling is appropriate and the controller is robust against parameter uncertainties ©1995 John Wiley & Sons, Inc.
Optimization of a leg design
Authors: Eltze JÜRgen , Pfeiffer Friedrich
Published: July 2007

A crucial point in developing walking machines is the load to weight ratio. Leg design and in it the drives essentially determine the leg's weight. Therefore it makes sense to optimize the design as well as the best motor‐gear‐combination with respect to power density. Some typical principles of legs have been derived from biological findings, especially for insects. The technical solution was adapted to these principles in an optimal way. The resulting leg achieves a load to weight ratio of approximately six. © 1995 John Wiley & Sons, Inc.
Reducing flexible base vibrations through local redundancy resolution
Authors: Hanson Mark L. , Tolson Robert H.
Published: July 2007

Future space systems will use teleoperated robotic systems mounted on flexible bases such as the Shuttle Remote Manipulator System. Due to dynamic coupling, a major control issue associated with these systems is the effect of flexible base vibrations on the performance of the robot. If uncompensated, flexible vibrations can lead to inertial tracking errors and an overall degradation in system performance. One way to overcome this problem is to use kinematically redundant robots. Thus, this article presents research results obtained from locally resolving kinematic redundancies to reduce or damp flexible vibrations. Using a planar, three‐link rigid robot example, numerical simulations were performed to evaluate the feasibility of three vibration damping redundancy control algorithms. Results showed that compared to a zero redundancy baseline, the three controllers were able to reduce base vibration by as much as 90% in addition to decreasing the required amount of joint torque. However, similar to locally optimizing joint torques, excessive joint velocities often occurred. To improve stability, fixed weight, multi‐criteria optimizations were performed. ©1995 John Wiley & Sons, Inc.
The Isotropic Design of Two General Classes of Planar Parallel Manipulators
Authors: Mohammadi H. R. , Zsombor‐murray P. J. Daniali , Angeles J.
Published: July 2007

Manipulator designs with isotropic architectures have a number of advantages, including high servo accuracy and dexterity. Using the isotropy criterion, isotropic designs of two general classes of planar, three‐degree‐of‐freedom, parallel manipulators with three legs are produced. One of these classes comprises 20 manipulators, while the other, 4. As special cases for each class, the complete set of isotropic designs of 2 manipulators is found. The parameter space of isotropic design of these manipulators is a continuum of at least one dimension. © 2995 John Wiley & Sons, Inc.
The ISDN U interface and transceiver
Authors: Borns Robert J.
Published: July 2007

This paper describes the integrated services digital network (ISDN) basic rate interface (BRI) two‐wire U interface and the transceiver necessary for the digital transmission of 160 kb/s of information over unshielded twisted pair transmission media. Topics discussed include the U interface structure and framing as described in the American National Standards Institute (ANSI) T1.601 specification, and the VLSI implementation of the ISDN U transceiver. Pertinent transceiver design issues addressed include the type of line code used, such as 2B1Q or 4B3T, echo cancellation, near end crosstalk, equalization and sampling rates.
Survey of switching techniques in high‐speed networks and their performance
Authors: Oie Yuji , Suda Tatsuya , Murata Masayuki , Miyahara Hideo
Published: July 2007

One of the most promising approaches for high speed networks for integrated service applications is fast packet switching, or ATM (asynchronous transfer mode). ATM can be characterized by very high speed transmission links and simple, hard‐wired protocols within a network. To match the transmission speed of the network links, and to minimize the overhead due to the processing of network protocols, the switching of cells is done in hardware switching fabrics in ATM networks.

A number of designs have been proposed for implementing ATM switches. Although many differences exist among the proposals, the vast majority of them are based on self‐routeing multistage interconnection networks. This is because of the desirable features of multi‐stage interconnection networks such as self‐routeing capability and suitability for VLSI implementation.

Existing ATM switch architectures can be classified into two major classes: blocking switches, where blockings of cells may occur within a switch when more than one cell contends for the same internal link, and non‐blocking switches, where no internal blocking occurs. A large number of techniques have also been proposed to improve the performance of blocking and non‐blocking switches. In this paper, we present an extensive survey of the existing proposals for ATM switch architectures, focusing on their performance issues.
The INTELSAT VII/VIIA generation of global communications satellites
Authors: Thompson P. T. , Silk R. , Herridge A.
Published: July 2007

Developed primarily to provide continuation of services from the INTELSAT V satellites in the Pacific Ocean Region, this latest generation of satellites also has utility for operation in other roles. Several new technologies are combined to give cost‐effective international satellite communications on a global, regional and domestic basis. This new higher power satellite will form a fundamental part of the INTELSAT global network, having been designed to match the projected traffic demands and future service requirements.
Personal communications via iNTELSAT Ku‐band transponders
Authors: Fang R. J. F.
Published: July 2007

The concept of a fully meshed network of very small terminals is presented for personal communications over Ku‐band satellite transponders. In this concept, undesirable double‐hop delays are avoided for voice communications between these terminals. The bandwidth and power resources of the transponder are efficiently shared by users in a simple demand‐assigned manner via code‐division multiple access (CDMA).1,2 Voice, data and facsimile are statistically multiplexed at each terminal. In order to minamize terminal costs, frequency‐precorrected and level‐preadjusted continuous wave (CW) tones are sent from the central network control station in each beam. The terminals in each down‐link beam can use these pilots as references for antenna acquisition and tracking, as reliable frequency sources, and as indicators of signal fade for up‐link power control (ULPC). The potential CDMA ‘near‐far’ problem due to up‐link fades is mitigated by using ULPC. Quasi‐burst‐mode transmission is employed to minimize the potential loss of clock and pseudorandom number code synchronization. When these terminals are used only to interconnect into the public switched telephony networks (PSTNs) through large, hub‐like gateway stations, they can employ flat‐plate antenna technology. The complete terminal can thereby be packaged into a briefcase so that it is easily transportable for personal use.
Overview of wideband packet protocols
Authors: Sherif M. H.
Published: July 2007

Wideband packet networks operate at rates equal to, or higher than, 64 kb/s, but lower than the basic broadband rate of 150 Mb/s, on cables or satellite links. Wideband packet protocols are the transmission protocols for these networks. They define open interfaces that can be used for public and private ISDNs. The protocols are defined in CCITT Recommendations G.764 and G.765. This paper describes the objectives of the wideband protocols, and how the objectives were achieved.
Advanced RF switching and on‐board regeneration technologies for digital satellite communications
Authors: Gupta R. , Assal F. , Inukai T. , Cotner C.
Published: July 2007

In this paper, functional blocks of a generic on‐board processing payload are identified, and some key technology implementation issues for on‐board RF and/or baseband switching, signal regeneration and processing are addressed. The design and performance characteristics of a microwave switch matrix and a 120 Mb/s coherent quadrature phase shift keying demodulator for satellite applications are presented. This hardware is realized with monolithic microwave integration technology. Significant performance improvement and reduction in size and weight, as well as enhanced subsystem reliability, have been demonstrated, making these technologies attractive for flexible communications satellite payload architectures of the future. Transmission analysis is performed to quantify link budget advantages that could be obtained by using on‐board regeneration.
Modelling and performance of packet synchronization in A data communications system
Authors: Böttcher Axel
Published: July 2007

This paper deals with some problems of packet synchronization in a slotted packet‐radio communications system. The problems considered are bit‐synchronization and unique word detection for bursty systems. The necessary overhead for synchronization purposes is dimensioned for applications in land‐mobile satellite communication systems. The performance of synchronization is evaluated using hardware measurement equipment, including a commercial bit‐synchronizer. Using these results, the performance loss of communication systems due to packet synchronization effects can be estimated. This is done for the additive white Gaussian noise channel and the class of land‐mobile satellite channels.
Statistical description of non‐coherent automatic gain control
Authors: Popken L. , Kriedte W.
Published: July 2007

Automatic gain control (AGC) is an important function in many different systems. It is the objective of this paper to introduce an analysis method for the statistical tracking performance of non‐coherent AGC. The method presented can easily be adapted to a wide variety of different applications and systems, e.g. the power control in communications networks such as CDMA systems, or the design of gain control circuits along receiver down‐conversion chains possibly including analogue‐to‐digital conversions.

We describe the fluctuation of the controlled gain by stochastic differential equations. For some typical operational cases the physical probability density function of the AGC gain variation is compared with different approximation techniques, i.e. the Fokker‐Planck (F–P) approach and the assumption of Gaussian distributions. The limitations of the approximation methods are emphasized because in telecommunications literature the F–P method is often applied without sufficient verification of the actual conditions for the method's applicability.
Ku‐ vs. C‐band and digital vs. analogue television transmission in a tropical climate
Authors: Johannsen Klaus G. , Cuevas Carlos
Published: July 2007

This paper analyses the Ku‐band versus C‐band transmission for digital and/or analogue TV, keeping in mind Ku‐band attenuation in the tropical climate as a function of earth‐station antenna elevation angle. The analysis is done for the case that adjacent satellites illuminate the same converage area and that interfering signals are of same e.i.r.p. and spectral make‐up. It is shown that a multiplex of compressed digital carriers requires smaller receiving antennas than a single analogue TV carrier. It turns out that because of low power requirements of the compressed digital carrier and/or for high e.i.r.p. values, the earth‐station antenna diameter is interference limited. Below the interference limit the aperture can only be reduced by applying an elliptical area to provide for a narrower beamwidth along the geostationary arc.
Performance analysis and measurement of a saw‐based group demodulator for on‐board processing in communications satellites
Authors: Loo Chun , Shaw Michael D.
Published: July 2007

Future satellites for ‘thin route’, mobile and portable services will require group demodulators on board the satellite to take advantage of the cost benefits offered with regenerative architectures. The demodulators will translate FDMA up‐links into TDM down‐links. This paper describes the measured performance of a 24‐channel multi‐carrier (64 kb/s QPSK for each carrier) group demodulator which uses SAW chirp Fourier transform (CFT) processing. The recovered data are combined into a single TDM bit stream at the standard T1 rate of 1.544 Mb/s. The 40 dB side‐lobe suppression obtained in the CFT has resulted in less than 0.35 dB combined interference degradation achieved with all channels active. The added degradation over Olympus with a saturated up‐link amplifier is less than 1 dB.
A model for interference studies relating to multisatellite non‐geostationary systems in the mobile satellite service
Authors: Pahl John , Parker John , Singarajah Kumar
Published: July 2007

An option being considered by current and prospective mobile satellite service (MSS) operators is the use of constellations of non‐geostationary orbit (NGSO) satellites to provide global personal communications. The scarcity of free spectrum, together with the bandwidth typically required, means that any new MSS system must consider sharing spectrum with other systems and services. Spectrum sharing must be based on defined interference criteria being satisfied. When multiple NGSO satellites are considered there is no accepted method for assessing interference. This paper describes a model used as the basis for a computer simulation of interference scenarios involving MSS, fixed service (FS) and fixed satellite service (FSS) systems. The simulation includes NGSO dynamics, constellations of MSS satellites with multi‐spot antennas, multiple geostationary FSS satellites and multiple hop FS links. Interference events are based on C/I calculations, and statistics, such as the percentage of time that the C/I falls below a required protection ratio, are calculated. More generally C/I cumulative distribution functions are produced.
Transmission quality of Group III facsimile on IDR satellite links with circuit multiplication equipment
Authors: Sherif M. H. , Cuevas E. , Liebert T. , Dimolitsas S.
Published: July 2007

The focus of this paper is on ways to improve the quality of Group III facsimile on intermediate data rate (IDR) satellite links. First, we present the results of an evaluation of the quality of Group III (G3) facsimile images transmitted on satellite links through circuit multiplication equipment (CME). Based on the results of this study, we propose a model to relate the facsimile image quality requirements to the bit error ratio (BER) on the link. A procedure is introduced to associate the long‐term percentage of error‐free pages in G3 facsimile transmission with various bit error probability (BEP) masks used for satellite link design. The intent of this procedure is to provide comparable end‐to‐end transmission quality for international telephone circuits, irrespective of whether the transmission medium is a satellite link or a fibre‐optic cable. It is concluded that, unless the performance objectives of satellite systems significantly exceed those derived from CCITT/ITU‐T Recommendation G.826, fibre‐optic cables will become the preferred choice for international transmission.
Asynchronous transfer mode (ATM) operation via satellite: Issues, challenges and resolutions
Authors: Chitre D. M. , Gokhale D. S. , Henderson T. , Lunsford J. L. , Mathews N.
Published: July 2007

Key issues regarding the operation of the broadband integrated services digital network (BISDN) via satellite are presented herein. The specific issues, challenges, and their resolutions are detailed. In particular, the impact of error characteristics and propagation delay on the operation of BISDN via satellite is discussed. Solutions are presented for removing adverse effects and providing high‐quality service to users of BISDN via satellite.
An adaptive resource sharing strategy for TDMA
Authors: Von Hugo Dirk , Wilde Andreas
Published: July 2007

A satellite‐based TDMA network consisting of four stations within different rain climatic zones has been operated in the 20/30 GHz frequency range using a recently developed flexible TDMA system allowing for FEC code rate and transmission bit rate variation.

In this paper a strategy is presented to counteract overall link degradations due to atmospheric attenuation by dynamic allocation of resources. A spare time slot within the TDMA frame as a ‘common resource’ for bit rate and code rate switching offers up to 12 dB gain, whereas up‐link power control, as it is implemented in this configuration, can cope with fades of 8 dB at maximum.

For an experimental network configuration the expected long‐term performance in terms of system availability is estimated for a viable version of the resource sharing strategy. Thereby, a model to calculate the probability of concurrent attenuation at the individual earth‐station sites (‘satellite based diversity’) has been applied and the resulting probability to exhaust the resources is considered as a function of the degrading correlation between attenuations.

Simulations with measured data via a ‘channel simulator’ and satellite measurements during the summer months of 1994 with the adaptive TDMA system are planned to test the functionality of the fade countermeasure strategy. Long‐term propagation measurements on large‐scale site diversity are required to verify predictions on the effective utilization of common resources.
Offset Fresnel zone plate antennas
Authors: Guo Y. J. , Barton S. K.
Published: July 2007

The theory of offset Fresnel zone plate antennas is presented. Explicit formulae for the design of the zone boundaries for the offset Fresnel zone plate are given. Theorectical equations and numerical results for the far‐field analysis are presented. It is found that when the offset angle increases, the zone boundaries become more elliptic and the plate becomes more asymmetric in the E‐plane. With the number of full wave zones and the plate width in the W‐plane fixed, the plate width in the E‐plane increases with the offset angle, which keeps the projected aperture area constant. Within a limited range of offset angles, the offset configuration can be employed without degrading the radiation performance much. For large offset angles, however, the asymmetry of the configuration may lead to increased sidelobes and decreased antenna directivity. An experiment with one particular phase reversal zone plate antenna with 30° offset angle shows good agreement between the measured antenna pattern and the theoretical prediction.
A comparative study of solar interference on the iridium and MSS constellations
Authors: Ward Christopher , Chang Tsangmin , Phillips Thomas M. , Acuff Phillip R.
Published: July 2007

Radio frequency cross‐links are the preferred communication medium for low earth orbit (LEO) satellite networks today. However, future cross‐link designs may be expected to deploy laser crosslinks to increase communication bandwidth. These cross‐links are rendered inoperative when incident with high level background radiation sources such as the Sun. The Iridium and Multiple Satellite System (MSS) are two well‐researched constellations in the literature and are ideal candidates for the investigation of solar interference on point‐to‐point constellations. In this paper a comparison of the impact of solar radiation on link availability for these two systems is presented.

Evaluation of link availability based on the motions of the bodies in question is computationally infeasible. It is therefore necessary to reduce the state space to a manageable computing basis by reformulating the problem domain and by simulation. In this report the development of an algorithm and a software simulation tool to analyse the two constellations will be presented, and the results of laser cross‐link availability will be discussed. The technique introduced here may be generalized to study a variety of different constellations.
A multi‐D trellis decoder for A 155 Mb/s concatenated codec
Authors: Pietrobon Steven S. , Kasparian Jeffrey J. , Gray Paul K.
Published: July 2007

An implementation of a 16 state, rate 8/9 six‐dimensional (6‐D) 8PSK rotationally invariant trellis decoder for use in a concatenated codec is described. The concatenated codec allows transmission of STM‐1 signals (at the 155.52 Mb/s information rate) over a 72 MHz satellite transponder. The inner trellis decoder is used with an outer (255,239) RS block decoder. The trellis decoder operates at 165.93 Mb/s and currently has an implementation loss of only 0.2 dB. The concatenated codec achieves a bit error ratio of 10−10 at an Eb/N0 of 8.2 dB (assuming an ideal modem and AWGN channel). Details are given of many Viterbi decoding ‘tricks’ that were used in order to implement the main functions of the decoder on two 10,000 gate equivalent CMOS programmable gate arrays.
On‐board closed‐loop congestion control for satellite‐based packet‐switching networks
Authors: Chu Pong P. , Ivancic William D. , Kim Heechul
Published: July 2007

NASA Lewis Research Center is currently investigating a satellite architecture that incorporates an on‐board packet‐switching capability. Because of the statistical nature of packet switching, arrival traffic may fluctuate, and thus it is necessary to integrate the congestion control mechanism as part of the on‐board processing unit. This study focuses on the closed‐loop reactive control. We investigate the impact of the long propagation delay on the performance, and propose a scheme to overcome the problem. The scheme uses a global feedback signal to regulate the packet arrival rate of the ground stations. In this scheme, the satellite continuously broadcasts the status of its output buffer and the ground stations respond by selectively discarding packets or by tagging the excessive packets as low‐priority. The two methods are evaluated by theoretical queueing analysis and simulation. The former is used to analyse the simplified model and to determine the basic trends and bounds, and the latter is used to assess the performance of a more realistic system and to evaluate the effectiveness of more sophisticated control schemes. The results show that the long propagation delay makes the closed‐loop congestion control less responsive. The broadcast information can only be used to extract statistical information. The discarding method needs carefully‐chosen status information and a reduction function, and normally requires a significant amount of ground discarding to reduce the on‐board packet loss probability. The tagging method is more effective since it tolerates more uncertainties and allows a larger margin of error in status information. It can protect the high‐priority packets from excessive loss and fully use the down‐link bandwidth at the same time.
Trellis‐coded dpsk with multiple‐symbol viterbi decoding over mobile satellite channel atKandLbands
Authors: Zhuang Weihua , Yongacoglu Abbas , Chouinard Jean‐Yves
Published: July 2007

Mobile satellite communication systems atKa/Kband (30/20 GHz) are attractive because of their large bandwidth availability and potentiality to support smaller earth‐stations and satellite antennas compared withL‐band (1·6/1·5 GHz) systems. In this paper, multiple‐symbol Viterbi decoding and dual‐space equal‐gain diversity reception for trellis‐coded differentialM‐ary phase‐shift‐keying (DPSK) modulation are investigated as two mitigation techniques for severe channel impairments expected from a land mobile satellite communication channel atKband and, for comparison purposes, atLband. The multiple‐symbol Viterbi decoder (MSVD) is a modified Viterbi decoder with inputs from multiple differential detectors. The channel is modelled as Rayleigh distributed multipath fading with a lognormally distributed line‐of‐sight (LOS) component due to shadowing. Four trellis‐coded modulation (TCM) schemes are studied. The dependency of the system performance improvement on the decoder structure, the TCM scheme, and the system RF frequency band is presented.
Evaluation of handover mechanisms in shadowed low earth orbit land mobile satellite systems
Authors: Carter P. , Beach M. A.
Published: July 2007

This paper presents a quantitative analysis by computer simulation of the active set update (ASU) handover algorithm for a shadowed low earth orbit (LEO) land mobile satellite (LMS) environment. As a precursor to the handover analysis, the mutual visibility statistics for a 66 satellite polar and 48 satellite rosette‐type constellation are presented. These results show the statistical nature of the levels of satellite diversity and mobile‐to‐satellite elevation angles (to the highest satellite) within each network and also indicate the influence of the channel characteristics on the handover strategy. A two‐state Markov modulated channel model is assumed in the handover analysis, and this enables the assessment of increased levels of power and time hysteresis on the quality of service and network signalling load in a shadowed land mobile satellite environment. In particular, attention is given to the different modes of ASU operation for hard handover, switch diversity and soft handover.
Modelling and simulation of a Ka‐band DBPSK/DQPSK mobile satellite channel
Authors: Liu Yi‐Song
Published: July 2007

For the prospective personal satellite communications in the early 21st century, this paper presents a general exploration of a Ka‐band DBPSK/DQPSK land‐mobile satellite channel model and its BER performance from system design, modelling and simulation. A forward‐link (i.e. outbound from the hub) channel design is introduced in brief. A complete model of channel multipath fading and shadowing is proposed for the first time, including a basic model and various derivative submodels corresponding to different actual propagation environments. Computer simulation results are given for a Rician channel model, a shadowed channel model and different combinational‐fading channel models by complete curve sets of BER performance, which coincide with the results of the analysis, and could be used as valuable references in further research and implementation.
Digital audio broadcasting: High‐grade service quality through on‐board processing techniques
Authors: Losquadro G.
Published: July 2007

A digital audio broadcasting (DAB) satellite system is presented here which provides a high‐grade quality of service for some hundreds of channels.

This, with minimum cost of the radio set and of the broadcasting station, greatly improves the radio broadcasting service throughout the world, especially in areas where deployment of conventional relay infrastructure is difficult and not yet cost‐effective.

Potential broadcasters located in the visible hemisphere can directly up‐link their programming to the DAB satellites.

Broadcasters can also copy each up‐link channel and steer flexibility to any beam or combination of satellite beams.

The paper illustrates the advantages and the feasibility of a DAB system based on geostationary satellites with on‐board processing generating signals of MCPC (multiple channel per carrier) type. This multiplexing technique allows large system margins. This solution allows the satellite L‐band TWTA amplifiers to be operated in saturation and eliminates the intermodulation noise associated with the transmission of FDMA channels. A powerful coding scheme has been selected to create a ‘robust’ down‐link.

In this way several high‐power DAB channels of radio programming, with selectable data rate, are directly delivered to the users.
Clinical significance of persistent left superior vena cava diagnosed in fetal life
Authors: Galindo A. , Gutiérrez‐Larraya F. , Escribano D. , Arbues J. , Velasco J. M.
Published: July 2007

Objectives To determine the prevalence and clinical significance of persistent left superior vena cava (PLSVC) in fetuses with and without cardiac and extracardiac anomalies.

Methods Charts and recorded images were reviewed from high‐risk patients who underwent fetal echocardiography between January 2000 and December 2005. This retrospective study included 54 fetuses with confirmed PLSVC who were diagnosed based on the presence of an additional vessel identified to the left of the pulmonary artery in the three‐vessel view of the heart. Associated congenital heart defects (CHDs), extracardiac abnormalities, including first trimester nuchal translucency (NT) thickness, and fetal/postnatal outcome were analyzed.

Results Of 5737 referrals, 5233 had a normal heart, and PLSVC was observed in 10 of these fetuses (0.2%; Group 1). CHDs were present in 504 and PLSVC was observed in 44 of these cases (9%). In the latter group, 18/44 (41%) fetuses had heterotaxy syndrome (Group 2) where the most common structural heart defects were atrioventricular septal defect and double‐outlet right ventricle. Fetuses without heterotaxy syndrome (Group 3) accounted for 26/44 (59%) cases of CHDs associated with PLSVC. In this group of fetuses the most common CHDs were left outflow tract obstructive defects and conotruncal anomalies. Increased NT was observed in 29%, without differences among the three groups. The survival rates among fetuses in Groups 1, 2 and 3 were 100%, 44% and 50%, respectively. After excluding patients who underwent pregnancy termination, there were no significant differences in the survival rates among the groups, probably due to the small size of the samples.

Conclusions PLSVC is associated with CHDs. The identification of PLSVC should prompt a thorough examination of the fetus to identify additional cardiac and extracardiac anomalies. The prognosis of affected fetuses largely depends on whether or not the PLSVC is associated with a CHD. Copyright © 2007 ISUOG. Published by John Wiley & Sons, Ltd.