Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item Joint Scheduling and Routing for Ad-hoc Networks Under Channel State Uncertainty(2007) Pantelidou, Anna; Ephremides, Anthony; Tits, Andre L.; ISRWe determine a joint link activation and routing policy that maximizes the stable throughput region of time-varying wireless ad-hoc networks with multiple commodities. In practice, the state of the channel process from the time it is observed till the time a transmission actually takes place can be significantly different. With this in mind, we introduce a stationary policy that takes scheduling and routing decisions based on a possibly inaccurate estimate of the true channel state. We show optimality of this policy within a broad class of link activation processes. In particular, processes in this class may be induced by any policy, possibly non-stationary, even anticipative and aware of the entire sample paths, including the future, of the arrival, estimated and true channel state processes, as long as it has no knowledge on the current true channel state, besides that available through the estimated channel state.Item Routing Session Traffic in Fixed All-Wireless Networks under Energy and Bandwidth Limitations(2001) Michail, Anastassios; Ephremides, Anthony; ISR; CSHCNIn this paper we study the effects of limited bandwidthresources in the development of energy-efficient routing algorithmsfor connection-oriented traffic in fixed wireless ad-hoc networks. Afrequency division multiple access scheme is considered, in whichnodes must schedule their transmissions by selecting frequencychannels from a limited set in an interference-free fashion.In ourearlier work, we had developed a set of algorithms for determiningend-to-end unicast paths based on link metrics. We argue that inorderto address the effects of limited frequency resources,such algorithms must be coupled with channel allocation mechanismsfor providing conflict free frequency assignments over selectedroutingpaths.
To these ends, we propose a set of link metricsfor selecting candidate routing paths and a set of heuristics forfrequency allocation. We evaluate their performance using ourdetailedsimulation model.
Item Energy-Efficient Routing for Connection-Oriented Traffic in Wireless Ad-hoc Networks(2001) Michail, Anastassios; Ephremides, Anthony; ISR; CSHCNWe address the problem of routing connection-oriented traffic in wireless ad-hoc networks with energy efficiency. We outlinethe trade-offs that arise by the flexibility of wireless nodes totransmit at different power levels and define a framework forformulating the problem of session routing from the perspective ofenergy expenditure.A set of heuristics are developed for determiningend-to-end unicast paths with sufficient bandwidth and transceiverresources, in which nodes use local information in order to selecttheir transmission power and bandwidth allocation. We propose a setofmetrics that associate each link transmission with a cost andconsiderboth the cases of plentiful and limited bandwidth resources, thelatter jointly with a set of channel allocation algorithms.
Performance is captured by call blocking probability and averageconsumed energy. A detailed simulation model has been developed andused to evaluate the algorithms for a variety of networks.
Item Cell Size in Hybrid Wireless Systems(1999) ElBatt, Tamer A.; Ephremides, Anthony; Ephremides, Anthony; ISR; CSHCNIn 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.Item Frequency Reuse Impact on the Optimum Channel Allocation for a Hybrid Mobile System(1999) ElBatt, Tamer A.; Ephremides, Anthony; Ephremides, Anthony; ISR; CSHCNIn 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.Item Enhanced Throughput for Satellite Multicasting(1999) Friedman, Daniel E.; Ephremides, Anthony; Ephremides, A.; ISR; CSHCNFaithful 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. Item Broadband Access via Satellite(1999) Hadjitheodosiou, Michael H.; Ephremides, Anthony; Friedman, Daniel E.; Ephremides, A.; ISR; CSHCNSatellites 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. Item Optimization of Connection-Oriented, Mobile, Hybrid Network Systems(1998) ElBatt, Tamer A.; Ephremides, Anthony; ISR; CSHCNIn this paper we consider the extension of a cellular system by means of satellite channels. Specifically, we consider an area covered by a number of cells that is also covered by a number of spot-beams. We consider connection-oriented service and call durations are assumed to be exponentially distributed. Also, users are mobile and, as such, they may cross cell and/or spot- beam boundaries, thus necessitating hand-offs. We incorporate the possibility of call-dropping due to unsuccessful hand-off attempts, in addition to satellite propagation delays along with the probability of new call blocking and formulate a specific cost function that must be ultimately minimized. The minimization is to be carried out by choosing (1) the optimal split of the total number of channels between the cellular and the satellite systems, and (2) the call admission and assignment policy, subject to the constraints of a demand vector that consists of an exogenous (new-call) generation process and an internal (hand- off-based) process that results from the mobility model. This complex optimization problem is solved by means of both numerical and standard clock simulation techniques along with the ordinal optimization approach. This paper was presented at the "17th AIAA International Communications Satellite Systems Conference and Exhibit", February 24-26, 1998, Yokohama, Japan.Item Multicast Routing and Resource Allocation in a Mobile Wireless Network Like the Digital Battlefield(1997) Bhattacharya, Rupla; Ephremides, Anthony; ISR; CSHCNﲍulticasting refers to the transmission of the same information to several destinations. In this paper we are addressing the issue of multicast routing in a wireless network that consists of an arbitrarily large number of nodes, each of which is mobile in an unpredictable manner. Most existing multicast algorithms have been developed for non-wireless, stationary networks in which there is an abundance of bandwidth and where intended destinations initiate their connection to the multicast tree. In the Digital Battlefield of the future, bandwidth may be limited if not scarce and, in addition to destination-initiated connections, there will be purely source-initiated multicasts that correspond to typical command or reconnaissance messages. In this paper, we establish the beginnings of a complete multicast algorithm that is capable of adapting to topological changes. More importantly, the algorithm is combined with dynamic channel allocation procedures that are capable of reassigning bandwidth resources on an as-needed basis throughout the network. Power control is applied to tradeoff between routing delays and number of connection requests satisfied. The goal of the algorithm is to establish and maintain the maximum number of connection requests while making efficient use of available bandwidth and avoiding congestion which might lead to network collapse.Item A Scheme to Improve Throughput for ARQ-Protected Satellite Communication(1997) Friedman, Daniel E.; Ephremides, Anthony; ISR; CSHCNAutomatic-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.