bluuebunny/crossref_metadata_2025_split huggingface dataset:
Overview of wideband packet protocols
Authors: Sherif M. H. 
Published: July 2007

AbstractWideband 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

AbstractIn 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

AbstractThis 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

AbstractAutomatic 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

AbstractThis 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

AbstractFuture 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

AbstractAn 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

AbstractThe 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

AbstractKey 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

AbstractA 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

AbstractThe 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

AbstractRadio 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

AbstractAn 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

AbstractNASA 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

AbstractMobile 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

AbstractThis 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

AbstractFor 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

AbstractA 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

AbstractObjectives 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.
Oxidation effects in adsorption bleaching of vegetable oils
Authors: King R. R. , Wharton F. W. 
Published: July 2007

AbstractIn bleaching fatty oils an equilibrium exists between two reactions favorable and two unfavorable in respect to color reduction. Adsorption of color and oxidative decrease in color are favorable. Oxidative increase in color and oxidative stabilization against adsorption are unfavorable. Adsorbents are shown to catalyze these oxidative reactions.Under any comparable set of conditions signifi‐cantly lower colors results when bleaching is carried out under vacuum or in an inert atmosphere. Multiple stage bleaching under atmospherie conditions shows no advantage because of the adverse oxidative effect but may be advantageous under vacuum conditions.Low pH earths respond better than high pH earths to vacuum bleaching with respect to improved color removal and oil stability.