Non-geostationary orbit satellite networks have dynamic, yet deterministic topologies. This paper deals with the consequences of relying on the use of multiple visible satellites to improve availability with time/locations through satellite diversity techniques. The assumption of independent blockage events in two different satellite-to-mobile links is generally not accurate and information on shadowing correlation should be taken into account in the assessment of availability.In this paper a shadowing correlation study for urban environments is presented and a model for the real environments, as well as for a generic urban environment is inferred. The model proposed here is simple and can be run on a computer. It also relates urban and constellation geometries thus allowing the identification of those cases for which satellite diversity gain should be expected. As a result of this analysis and modelling, we present empirical analytical expressions describing positive correlation coefficient as a function of satellite angular separation (azimuth and elevation). We also show that an angle exists above which correlation can be negative and satellite diversity can sensibly improve link availability. Copyright © 2002 John Wiley & Sons, Ltd.
Integrated services over a shared satellite channel need a MAC protocol that is able to deal with real-time traffic without substantially affecting the efficiency of the shared medium. Due to its bursty nature, MPEG coded VBR video transmission is one of the most challenging real-time applications. In this paper, we analyse the statistical performance of a simple allocation method for VBR traffic by using an accurate statistical model of an MPEG-2 VBR coded movie, whose traffic exhibits a peak to mean ratio of about 4. The results were obtained both analytically and by simulation, and show that our method is suitable for transmitting non-interactive video and best-effort traffic on the same satellite link. The efficiency of the proposed method shows no dependency on the statistical properties of the input traffic. Copyright (C) 2002 John Wiley Sons, Ltd.
Several carrier phase recovery schemes for Gaussian minimum shift keying (GMSK) modulated signals are derived and analysed in terms of squaring loss and acquisition time. Two bandwidths are considered for the GMSK signal: BTb=0.5 and 0.25, following the recommendations of the Consultative Committee for Space Data Systems. Pre-encoding is assumed to remove the inherent differential encoding of the GMSK modulations. The carrier phase recovery schemes are derived from the approximate expression of the GMSK signal as an OQPSK signal or as the superposition of two OQPSK signals maximum a posteriori/maximum likelihood phase estimation is considered, with perfect clock reference; both non-data-aided and decision–directed schemes are analysed. Moreover, in the case of GMSK with BTb=0.25, an equalizer is also included in the synchronizer loop to remove inter-symbol interference. High and low signal to noise ratio (SNR) approximations are considered to simplify the structure of the synchronizers. It is shown that, for the considered BTb values, the complex synchronizers initially derived do not provide any improvements over their simplified versions; the introduction of the equalizer for GMSK with BTb=0.25 leads to a better performance. Moreover, it is shown that GMSK with BTb=0.5 has a better performance than unfiltered OQPSK, which is better than GMSK with BTb=0.25. The SNR values above which the high SNR approximation turns out to be convenient with respect to the low SNR approximation are also given. Copyright © 2002 John Wiley & Sons, Ltd.
In the present concept of the International Space Station ISS, it is planned to provide the communication services between the European Columbus module of the ISS and the ground equipment via the TDRS (tracking and data relay satellite) network provided by the American NASA (National Aeronautics and Space Agency). Especially because of its low uplink data rate, an unacceptable limitation of the communication services with Columbus is to be expected. In order to investigate possible improvements to this situation, the MEDIS proposal studies the feasibility of a 150 Mbps full duplex communication system to the Columbus Module on via two MEO satellites. A mixed topology with optical inter satellite links and Ka-band up/down-links shall be employed. Also possible liaisons with the Artemis mission will be considered. The MEDIS project is a collaboration of Astrium GmbH, which is project manager, Bosch SatCom, GMD-Fokus and the German Aerospace Center (DLR). The preparation phase of the project will investigate the concept of the communications infrastructure and the experiments that could be realized when the MEOs are launched. This stage has been completed in February 2001. An additional study phase followed, which will be completed with a presentation in July 2002. Briefly the following aspects have been studied by DLR in the preparation phase: - the overall concept of experimental communication services for Columbus, - the system requirements and network topology, - the relevant protocol and hardware architectures, - useful satellite constellations and link scenarios. Two phases could follow the preparation phase, in phase one the satellite will be launched and the link will be tested. In phase two the satellites will be connected to the ISS.
The Rice-lognormal (RLN) channel model was recently generalized to include an additive scattering component having constant average power. The generalized Rice-lognormal (GRLN) model includes as limiting cases many well-known narrowband models such as, e.g. Rice, lognormal (and combinations), and Loo's. The paper provides the GRLN first-order statistical description of envelope and phase, and the second-order statistics in terms of level crossing rate. The paper then provides a fitting procedure to extract model parameters from experimental data. Finally, it presents a new simulation procedure and validates it through comparison with theoretical results. Copyright (C) 2002 John Wiley Sons, Ltd.
In this paper. an algorithm and its practical implementation to activate adaptive modulation as a fade countermeasure (FCM) is presented. Characteristics of the algorithm are derided front the propagation Studies and the algorithm Uses a simple four parameter model. A neural network architecture as used to implement the decision making, block of the controller. The algorithm has been implemented on a fixed point DSP. An experimental set-up With an emulated K-a-band satellite link and a terrestrial return path connection has been used with previously recorded propagation data for experimental verification and performance analysis, Performance of the implemented FCM system is compared with that of fixed. non-adaptive systems. Over concatenated rain events, the adaptive system yields better throughout at or below a given BER than any fixed mode system and only marginally worse BER availability than the most robust scheme in the system. The FCM is thus essential if throughout is of importance as well as availability. The added complexity of the FCM system is not great by contemporary technology standards and is, in authors' opinion. well worth the investment. Copyright (C) 2002 John Wiley Sons. Ltd.
The concept behind path diversity is that a user who can access several satellites simultaneously will be able to communicate more effectively than a user who could only access one. The success of this method depends on the environment, the satellite constellation, and diversity combining technology. This paper explores the path diversity effects of non-geostationary orbit (NGO) satellite personal communication services, for different degrees of user mobility, under various scenarios, using the constellation of the global positioning system (GPS). Measurements are taken near downtown Taipei. Three types of mobilities (fixed-point, pedestrian, and vehicular) are examined, and the switch diversity and maximum ratio combining method are applied to determine the path diversity gain and calculate bit error probability. The error probability performance of applying diversity schemes in coherent binary phase shift keying (BPSK) and non-coherent differential phase shift keying (DPSK) modulations over Rician fading channels are also analysed and evaluated by using the characteristic function method. The results show that fading can be significantly reduced and diversity greatly increased. A significant diversity gain and improvement in bit error rate (BER) can be expected in all cases by simply applying switch diversity scheme. Besides, for the maximum ratio combining method, the results imply that summing two satellite signals suffices to increase diversity and improve the bit error rate performance. Copyright © 2002 John Wiley & Sons, Ltd.
This is an investigation to determine if a low-Earth telecommunications constellation can be used to support data services for small satellites also in Low-Earth Orbit. The intent is to use the telecommunications service provider satellites to supply low-rate command and telemetry services as well as inter-satellite data communications. In this study, we determine access potential via simulations of the orbital contact time using current telecommunications constellation configurations and the assumed configuration for a nanosatellite user. From the access times, we conclude that there are sufficient numbers of contacts to permit command and telemetry services for the nanosatellite constellation. In addition to the access time, we consider Doppler effects, antenna effects, and regulatory restrictions on such communications. However, the regulations do not currently permit such operations on a regular basis. From this analysis, we conclude that the desired communications are possible but the nanosatellite user will need to work closely with the service provider. Copyright © 2002 John Wiley & Sons, Ltd.
In this paper we investigate various issues related to the design of satellite–cellular hybrid systems. First, we review the fundamental problems of channel partitioning and call admission/assignment. Second, we study the impact of different frequency reuse constraints, in both layers, on the optimum channel partitioning. Third, we investigate, analytically and via simulation, the effect of reducing the cell size. We emphasize the blocking-forced termination probabilities trade-off for pure cellular and satellite–cellular hybrid systems. Accordingly, an optimization problem with respect to the cell size is formulated. Finally, we search for the optimum dynamic call re-assignment policy that improves the system capacity at the expense of the complexity associated with tearing down a connection in one system and setting-up an alternative one in the other system. For a small hybrid system, we characterized the optimum re-assignment policies that minimize the blocking probability, dropping probability, and a weighted cost function of these probabilities. Copyright © 2002 John Wiley & Sons, Ltd.
In this paper, a novel pilot-symbol-aided (PSA) technique is proposed for fading estimation in the land mobile satellite fading channels. The proposed technique combines the fading estimates obtained from a bandwidth-efficient technique and a conventional technique according to the signal-to-noise ratios (SNRs) of the fading estimates. To enhance the transmission quality, trellis-coded modulation (TCM) and diversity reception are employed in the system, and the combined estimates are subsequently used to correct the channel fading effects, to weight the signals from different diversity branches, and to provide channel state information to the Viterbi decoder. Monte Carlo computer simulation has been used to study the bit-error-rate (BER) performance of the proposed technique on trellis-coded 16-ary quadrature amplitude modulation in the frequency non-selective Rician fading channels. Results have shown that the proposed PSA technique requires a very low bandwidth redundancy to provide satisfactory BER performance at low SNRs, and thus is suitable for use with TCM and diversity reception to achieve both bandwidth and power-efficient transmission. Copyright © 2002 John Wiley & Sons, Ltd.
To achieve a global cellular network, integration among segments offering different coverage (indoor, outdoor and global) must be pursued. Of course, the possibility to hold the call switching among different segments must be guaranteed. Hence, efficient algorithms to perform intersegment handover (ISHO) must be implemented.The paper aims at analysing some ISHO procedures developed in the frame of some European projects and other proposed in the literature, in a scenario with satellite and terrestrial segments interworking to achieve a worldwide cellular coverage.Performance evaluation will be carried out for different system configurations utilizing a dynamic satellite constellation simulator in the time domain. The impact of the distance user-gateway on performance will be addressed. For each procedure, the execution delay and its complementary cumulative distribution have been evaluated for different constellation geometries at different distances from the gateway. Copyright (C) 2002 John Wiley Sons, Ltd.
Atmospheric precipitation can seriously affect the propagation of centimeter and millimeter electromagnetic waves. As a consequence, in some applications, it is necessary to make use of a fade countermeasure technique in order to satisfy the system availability and quality requirements. This study analyses the performance of a satellite-based system in geo-stationary orbit operating at 20 GHz, dynamically assigning the antenna directivity pattern to counteract tropospheric attenuation. The on-board power is spatially distributed over the covered region to minimize, at any time, the number of users undergoing outage because of the tropospheric attenuation. Both the aspects of broadcasting and telecommunication services are addressed. The reflector antenna of the system is supposed to be illuminated by a cluster of feeds driven by a set of excitation coefficients, continuously modified and optimized according to the meteorological information derived by processing METEOSAT satellite data and ECMWF (European Centre for Medium-Range Weather Forecasts) data. Copyright © 2002 John Wiley & Sons, Ltd.
International Journal of Satellite CommunicationsVolume 20, Issue 5 p. 307-309 EditorialFree Access Special issue on communications for space missions Michael Hadjitheodosiou, Michael Hadjitheodosiou Guest Editor michalis@isr.umd.edu www.isr.umd.edu/∼michalis Center for Satellite & Hybrid Communication Networks, ISR A.V. Williams Building, University of Maryland, College Park, MD 20742, U.S.A.Search for more papers by this author Michael Hadjitheodosiou, Michael Hadjitheodosiou Guest Editor michalis@isr.umd.edu www.isr.umd.edu/∼michalis Center for Satellite & Hybrid Communication Networks, ISR A.V. Williams Building, University of Maryland, College Park, MD 20742, U.S.A.Search for more papers by this author First published: 13 September 2002 https://doi.org/10.1002/sat.731AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue5Special Issue: Communications for Space MissionsSeptember/October 2002Pages 307-309 RelatedInformation
This paper focuses on the radio resource management in low earth orbit-mobile satellite systems (LEO-MSSs) based on a time division multiple access (TDMA) air interface. A novel demand–assignment medium access control (MAC) protocol, named DRAMA+ (dynamic resource assignment multiple access—enhanced version), is proposed, where voice and Web traffic sources obtain transmission slots through requests sent by means of a random access phase. The round-trip propagation delay (RTD) of LEO-MSSs prevents an immediate feedback for each transmission attempt. Therefore, the main concern of the DRAMA+ scheme is to realize an efficient access phase. All the transmission requests successfully received at the satellite are managed by an on board scheduler. We have shown that DRAMA+ outperforms other techniques appeared in the literature in terms of voice quality, transmission delays for bursty data traffics and resource utilization. Moreover, a performance analysis of an ideal version of the DRAMA+ scheme has permitted us to prove the potentialities of the proposed DRAMA+ technique. Stability issues have been addressed as well as the impact on the DRAMA+ performance of the LEO satellite constellation RTD value. Copyright © 2002 John Wiley & Sons, Ltd.
The purpose of this paper is to evaluate the impact of propagation effects at 20/30 GHz on the performances of the radio interface of a VSAT videoconferencing system and to demonstrate the interest to implement Fade Mitigation Techniques in order to improve the link availability. Specifications and system requirements of the VSAT network are briefly described. System performances of the physical layer are simulated using time-series data measured with the ESA's OLYMPUS satellite. The simulations enable to implement and optimize Up-Link Power Control and Data Rate Reduction, and to assess the related link performance improvement. Moreover, the introduction of propagation data sets allows to have a better idea of some system parameters which directly impact on the quality of service such as the switching rate or the link availability that such a system should have when deployed in operational conditions. Copyright (C) 2002 John Wiley Sons, Ltd.
This paper presents and evaluates different routing schemes in a constellation of non-geostationary satellites. Routing decisions may or may not take into account the state of the network. While taking into account the network state is likely to bring an improvement in both the network utilization and the quality of service, it also makes the routing architecture more complex. This paper proposes to evaluate the advantages and disadvantages of these different design approaches for routing. Copyright © 2002 John Wiley & Sons, Ltd.
Orbital diversity is considered to be an effective technique to overcome large fade margins in satellite communication links. This paper discusses triple-orbital diversity,, which uses three satellites and an Earth receiving site. A method for calculating the outage probability of a triple-orbital diversity protection scheme is proposed. It is based on a model for convective raincells and the lognormal assumption for point rainfall rate statistics. Numerical results are compared with an available set of experimental data taken from a VSAT Earth-station located in Japan. The agreement was found to be quite encouraging. Some useful conclusions, concerning the relative advantage of using triple- against the double-orbital diversity scheme are also deduced. Copyright (C) 2002 John Wiley Sons, Ltd.
Third generation communication systems will be characterized by full integration between terrestrial and satellite components. To this aim, global coverage along with not severe requirements for user terminals are mandatory for the satellite segment and the use of constellations of satellites in low or medium Earth orbits (LEO or MEO) seems to be a viable solution. Those satellite systems will adopt multibeam antennas to achieve high spectral efficiency and low-cost terminals. Thus, interference becomes one of the most limiting factors in terms of both link availability and capacity. The paper presents a more complete interference model than previously published in literature. The identification of the interfering users set has been introduced and all the factors impacting interference generation or isolation have been considered in case of both FDMA and CDMA access. This model is suitable for instantaneous analysis of multisatellite constellations. It has been implemented and time-domain simulations have been performed to evaluate the impairments due to co-channel interference for different access techniques for LEO configuration. Simulation results on interference and users spatial distribution, on trade-offs between interference and system capacity will be presented. Finally, the main interference mitigation techniques will be listed and discussed. Copyright © 2002 John Wiley & Sons, Ltd.
This paper addresses the performance evaluation of adaptive routing algorithms in non-geostationary packet-switched satellite communication systems. The dynamic topology of satellite networks and variable traffic load in satellite coverage areas, due to the motion of satellites in their orbit planes, pose stringent requirements to routing algorithms. We have limited the scope of our interest to routing in the intersatellite link (ISL) segment. In order to analyse the applicability of different routing algorithms used in terrestrial networks, and to evaluate the performance of new algorithms designed for satellite networks, we have built a simulation model of a satellite communication system with intersatellite links. In the paper, we present simulation results considering a network-uniform source/destination distribution model and a uniform source–destination traffic flow, thus showing the inherent routing characteristics of a selected Celestri-like LEO satellite constellation. The updates of the routing tables are centrally calculated according to the Dijkstra shortest path algorithm. Copyright © 2002 John Wiley & Sons, Ltd.
NASA's future communications services will be supplied through a space communications network that mirrors the terrestrial Internet in its capabilities and flexibility. The notional requirements for future data gathering and distribution by this Space Internet have been gathered from NASA's Earth Science Enterprise (ESE), the Human Exploration and Development in Space (HEDS), and the Space Science Enterprise (SSE). This paper describes a communications infrastructure for the Space Internet, the architectures within the infrastructure, and the elements that make up the architectures. The architectures meet the requirements of the enterprises beyond 2010 with Internet compatible technologies and functionality. The elements of an architecture include the backbone, access, inter-spacecraft, and proximity communication parts. From the architectures, technologies have been identified which have the most impact and are critical for the implementation of the architectures.
