Institute for Systems Research

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Author: search.filters.author.Ephremides, AnthonySubject: search.filters.subject.Intelligent Signal Processing and Communications Systems

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    Cell Size in Hybrid Wireless Systems
    (1999) ElBatt, Tamer A.; Ephremides, Anthony; Ephremides, Anthony; ISR; CSHCN
    In this paper we study, analytically and via simulation, the impact ofreducing the cell size of a wireless system on the system capacity andhand-off failure rate. First, we focus on a pure cellular system. Wedevelop exact and approximate models for cellular systems in order toemphasize the blocking probability/forced termination probabilitytrade-off. This trade-off motivates the problem of optimizing the systemperformance with respect to the cell size. For hybrid systems, weinvestigate how jointly optimizing the number of cells per spot-beam, andthe bandwidth partitioning could improve the QoS parameters of interest.For both systems, the numerical solution is only feasible for smallnumber of cells. For large systems, a simulation study is presented.

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    Frequency Reuse Impact on the Optimum Channel Allocation for a Hybrid Mobile System
    (1999) ElBatt, Tamer A.; Ephremides, Anthony; Ephremides, Anthony; ISR; CSHCN
    In this paper we study the effect of the frequency reuse constraints inboth layers on the optimum channel allocation for a multi-cell/multi-spot-beam hybrid system. We adopt a specific multi-faceted cost function thatincorporates call-dropping due to unsuccessful hand-off attempts, andblocking of new calls. The minimization of the cost function is attemptedby choosing the optimal split of the total number of channels betweenthe cellular and the satellite layers. This complex optimization problemis solved by means of standard clock simulation techniques along with theadaptive partitioned random search global optimization technique and theordinal optimization approach.
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    Enhanced Throughput for Satellite Multicasting
    (1999) Friedman, Daniel E.; Ephremides, Anthony; Ephremides, A.; ISR; CSHCN
    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.
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    Broadband Access via Satellite
    (1999) Hadjitheodosiou, Michael H.; Ephremides, Anthony; Friedman, Daniel E.; Ephremides, A.; ISR; CSHCN
    Satellites are well suited for broadband communications. In this paperwe consider the special features of satellite systems, some of thebroadband applications that are well-suited for satellites and someof the technologies which make possible broadband satellitecommunications, as well as the research programs that led to theirdevelopment. We describe how such technologies, and other factors,have contributed to the evolution of broadband satellite systems,and discuss some of the challenges in establishing such systems.We finish by offering some concluding remarks on the role ofsatellites for broadband access.

    The research and scientific content in this material hasbeen published in Computer Networks, vol. 31, pp. 353-378, 1999.