Error Control for Multicasting in Satellite and Hybrid Communication Networks

A problem inherent in ARQ multicasting over a broadcast channel is thata retransmission typically benefits only a minority of destinationswhile all others wait unproductively. This results in poorthroughput to each receiving station in the network, with thethroughput diminishing as the number of receivers grows.If point-to-point links between the transmitter and each receiver werealso available, then conceivably retransmissions could be sent over suchsecondary links. This would reduce the frequency of retransmissionsinterrupting the flow of new packets on the broadcast link. That is,a hybrid satellite-terrestrial network architecture would allowgreater throughput for multicasting than a pure-satellite network.This work examines ARQ multicasting in such a network, and confirms byanalysis and simulation that, within limits, such a throughput advantagecan be realized. A detailed discussion of implementation aspects forpoint-to-point and point-to-multipoint ARQ protocols in bothpure-satellite and hybrid networks is presented as well. This work alsoconsiders partitioning a fixed amount of bandwidth to maximize throughput,possibly subject to a cost constraint, and the effect of a "poorlistener" upon performance in both pure-satellite and hybrid networks.<p

Error Control for Satellite and Hybrid Communication Networks

Both forward-error correction (FEC) and automatic-repeat-request (ARQ) error control schemes are used for assuring the accuracy of information transferred through imperfect channels. In satellite systems in which propagation times are typically large, ARQ error control can result in poor throughput to the destination. Also, an ARQ protocol for satellite multicast communication must be carefully crafted to assure good throughput to all destinations regardless of which stations require retransmissions.Supplementing a satellite link with a parallel terrestrial link may allow mitigating some problems of using ARQ in satellite communication systems. ARQ acknowledgments, and possibly retransmissions as well, can be sent terrestrially in such a hybrid network, and so avoid the large satellite propagation delay. The satellite transmission capability of a receiving station which communicates with the transmitter exclusively by terrestrial means can be eliminated and the system cost correspondingly reduced. Further, multicasting with a hybrid network may allow retransmissions to be conducted without interrupting the flow of new information to all destinations, so throughput need not drastically suffer if retransmissions are required.The degree to which throughput can be improved by adopting a hybrid network is not clear. A hybrid network's effect on the fidelity of information delivered to the destination(s) is also not clear. An experiment is presented for investigating such error control issues of hybrid networking

A Simple Complete Search for Logic Programming

Here, we present a family of complete interleaving depth-first search strategies for embedded, domain-specific logic languages. We derive our search family from a stream-based implementation of incomplete depth-first search. The DSL\u27s programs\u27 texts induce particular strategies guaranteed to be complete

Enhanced Throughput for Satellite Multicasting

Faithful information delivery in satellite multicasting requires appropriate error control. If multicast automatic-repeat-request (ARQ)is employed, a retransmission does not benefit receivers which do notrequire it, and consequently the throughput suffers greatly as thenumber of receivers increases. This performance degradation might bealleviated substantially by conducting retransmissions through terrestrialpaths from the transmitter to each receiver instead of through themulticast satellite link. By sending a retransmission directly to thereceiver(s) which requires it, higher throughput can be provided in sucha hybrid network than in a pure-satellite network. In this work,we examine the throughput improvement provided by the hybrid network.The research and scientific content in this material hasbeen accepted for presentation at the International Mobile SatelliteConference, Ottawa, June 16-18, 1999. </Center

A Scheme to Improve Throughput for ARQ-Protected Satellite Communication

Automatic-repeat-request (ARQ) error control is often employed to assure high fidelity information transmission. However, ARQ error control can provide poor throughput for satellite multicasting. The throughput in such communication may be improved by the combination of a terrestrial network parallel to the satellite network and a judiciously modified ARQ protocol. In particular, retransmitted ARQ frames can be sent terrestrially in such a hybrid network, allowing higher throughput than in a pure- satellite network. This work presents analytic results to establish the potential for improving the throughput of satellite multicast communication employing ARQ error control by the adoption of such a hybrid network architecture

Hydrostatic Expansion and Spin Changes During Type I X-Ray Bursts

We present calculations of the spin-down of a neutron star atmosphere due to hydrostatic expansion during a Type I X-ray burst. We show that (i) Cumming and Bildsten overestimated the spin-down of rigidly-rotating atmospheres by a factor of two, and (ii) general relativity has a small (5-10%) effect on the angular momentum conservation law. We rescale our results to different neutron star masses, rotation rates and equations of state, and present some detailed rotational profiles. Comparing with recent observations of large frequency shifts in MXB 1658-298 and 4U 1916-053, we find that the spin-down expected if the atmosphere rotates rigidly is a factor of two to three less than the observed values. If differential rotation is allowed to persist, we find that the upper layers of the atmosphere spin down by an amount comparable to the observed values; however, there is no compelling reason to expect the observed spin frequency to be that of only the outermost layers. We conclude that hydrostatic expansion and angular momentum conservation alone cannot account for the largest frequency shifts observed during Type I bursts.Comment: Submitted to the Astrophysical Journal (13 pages, including 4 figures

Metamorphic III-V solar cells: recent progress and potential

Metamorphic semiconductor devices are commonly considered to have inferior electronic quality. However, recent development of compositionally graded buffers and junction structures have led to the achievement of high quality metamorphic solar cells exhibiting internal luminescence efficiencies over 90%. Optimizing the optical design of the solar cell becomes important in order to enhance photon recycling and open circuit voltage in these cells. In this paper we first present recent performance results for 1eV and 0.7eV GaInAs solar cells grown on GaAs substrates. Then an electro-optical model is used to assess the potential performance improvements in current metamorphic solar cells under different realizable design scenarios. The results show that significant improvements can be achieved by improving both the electronic quality and optics of these cells