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Current Search:
All of 'Hamilton' and 'Institute' in all fields;
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Displaying Results 26 - 50 of 338 on page 2 of 14
Marked
Mark
Joint scheduling and resource allocation in uplink OFDM systems for broadband wireless access networks
(2009)
Huang, Jianwei; Subramanian, Vijay G.; Agrawal, Rajeev; Berry, Randall
Joint scheduling and resource allocation in uplink OFDM systems for broadband wireless access networks
(2009)
Huang, Jianwei; Subramanian, Vijay G.; Agrawal, Rajeev; Berry, Randall
Abstract:
Orthogonal frequency division multiplexing (OFDM) with dynamic scheduling and resource allocation is a key component of most emerging broadband wireless access networks such as WiMAX and LTE (long term evolution) for 3GPP. However, scheduling and resource allocation in an OFDM system is complicated, especially in the uplink due to two reasons: (i) the discrete nature of subchannel assignments, and (ii) the heterogeneity of the users' subchannel conditions, individual resource constraints and application requirements. We approach this problem using a gradient-based scheduling framework. Physical layer resources (bandwidth and power) are allocated to maximize the projection onto the gradient of a total system utility function which models application-layer Quality of Service (QoS). This is formulated as a convex optimization problem and solved using a dual decomposition approach. This optimal solution has prohibitively high computational complexity but reveals guiding principles ...
http://eprints.nuim.ie/2100/
Marked
Mark
Buffer Sizing for 802.11 Based Networks
(2010)
Li, Tianji; Leith , Douglas; Malone, David
Buffer Sizing for 802.11 Based Networks
(2010)
Li, Tianji; Leith , Douglas; Malone, David
Abstract:
Abstract—We consider the sizing of network buffers in 802.11 based networks.Wireless networks face a number of fundamental issues that do not arise in wired networks. We demonstrate that the use of fixed size buffers in 802.11 networks inevitably leads to either undesirable channel under-utilization or unnecessary high delays. We present two novel dynamic buffer sizing algorithms that achieve high throughput while maintaining low delay across a wide range of network conditions. Experimental measurements demonstrate the utility of the proposed algorithms in a production WLAN and a lab testbed.
http://eprints.nuim.ie/2208/
Marked
Mark
Measuring Transmission Opportunities in 802.11 Links
(2010)
Giustiniano, Domenico ; Malone, David; Leith, Douglas J. ; Papagiannaki, Konstantina
Measuring Transmission Opportunities in 802.11 Links
(2010)
Giustiniano, Domenico ; Malone, David; Leith, Douglas J. ; Papagiannaki, Konstantina
Abstract:
We propose a powerful MAC/PHY cross-layer approach to measuring IEEE 802.11 transmission opportunities in WLAN networks on a per-link basis. Our estimator can operate at a single station and it is able to: 1) classify losses caused by noise, collisions, and hidden nodes; and 2) distinguish between these losses and the unfairness caused by both exposed nodes and channel capture. Our estimator provides quantitative measures of the different causes of lost transmission opportunities, requiring only local measures at the 802.11 transmitter and no modification to the 802.11 protocol or in other stations. Our approach is suited to implementation on commodity hardware, and we demonstrate our prototype implementation via experimental assessments. We finally show how our estimator can help the WLAN station to improve its local performance.
http://eprints.nuim.ie/2210/
Marked
Mark
Stabilization and Performance over a Gaussian Communication Channel for a Plant with Time Delay
(2009)
Freudenberg, J.S.; Middleton, R.H.
Stabilization and Performance over a Gaussian Communication Channel for a Plant with Time Delay
(2009)
Freudenberg, J.S.; Middleton, R.H.
Abstract:
Two problems that have received much attention are those of finding the minimum channel signal to noise ratio compatible with closed loop stability, and of finding the optimal performance, in terms of disturbance attenuation, for a channel with specified signal to noise ratio. In this paper, we study these problems for the case in which the plant has relative degree greater than one, and thus introduces a delay greater than one time step. We show that, unlike the relative degree one case, for the problem of stabilization linear time varying control and communication strategies may have advantages over linear time invariant strategies. We derive a lower bound on optimal disturbance response at a fixed terminal time. If the encoder has access to the state of the plant, then this bound is achievable using linear time varying communication and control.
http://eprints.nuim.ie/1643/
Marked
Mark
Near Threshold Graphs
(2009)
Kirkland, Steve
Near Threshold Graphs
(2009)
Kirkland, Steve
Abstract:
A conjecture of Grone and Merris states that for any graph G, its Laplacian spectrum, A(G), is majorized by its conjugate degree sequence, D∗(G). That conjecture prompts an investigation of the relationship between A∗(G) and D∗(G), and Merris has characterized the graphs G for which the multisets A∗(G) and D∗(G) are equal. In this paper, we provide a constructive characterization of the graphs G for which A(G) and D∗(G) share all but two elements.
http://eprints.nuim.ie/1642/
Marked
Mark
A quadratic stability result for singular switched systems with application to anti-windup control
(2009)
Shorten, R.; Corless, M.; Middleton, R.; Klinge, S.; Wulff, K.
A quadratic stability result for singular switched systems with application to anti-windup control
(2009)
Shorten, R.; Corless, M.; Middleton, R.; Klinge, S.; Wulff, K.
Abstract:
In this note we consider the problem of determining necessary and sufficient conditions for the existence of a common quadratic Lyapunov function for a pair of stable linear time-invariant systems whose system matrices are of the form A, A−ghT , and where one of the matrices is singular. We then apply this result in a study of a feedback system with a saturating actuator.
http://eprints.nuim.ie/1641/
Marked
Mark
Quadratic Stability and Singular SISO Switching Systems
(2009)
Shorten, Robert; Corless, Martin; Wulff, Kai; Middleton, Richard H.
Quadratic Stability and Singular SISO Switching Systems
(2009)
Shorten, Robert; Corless, Martin; Wulff, Kai; Middleton, Richard H.
Abstract:
In this note, we consider the problem of determining necessary and sufficient conditions for the existence of a common quadratic Lyapunov function for a pair of stable linear time-invariant systems whose system matrices are of the form A, A-ghT, and where one of the matrices is singular. A necessary and sufficient condition for the existence of such a function is given in terms of the spectrum of the product A(A-ghT). The technical note also contains a spectral characterization of strictly positive real transfer functions which are strictly proper. Examples are presented to illustrate our results.
http://eprints.nuim.ie/1639/
Marked
Mark
Feedback Stabilization Over a First Order Moving Average Gaussian Noise Channel
(2009)
Middleton, Richard H.; Rojas, Alejandro J.; Freudenberg, James S.; Braslavsky, Julio H.
Feedback Stabilization Over a First Order Moving Average Gaussian Noise Channel
(2009)
Middleton, Richard H.; Rojas, Alejandro J.; Freudenberg, James S.; Braslavsky, Julio H.
Abstract:
Recent developments in information theory by Y.-H. Kim have established the feedback capacity of a first order moving average additive Gaussian noise channel. Separate developments in control theory have examined linear time invariant feedback control stabilization under signal to noise ratio (SNR) constraints, including colored noise channels. This note considers the particular case of a minimum phase plant with relative degree one and a single unstable pole at z = ø(with |ø| >1) over a first order moving average Gaussian channel. SNR constrained stabilization in this case is possible precisely when the feedback capacity of the channel satisfies Cfb ≥ log2 |ø|. Furthermore, using the results of Kim we show that there exist linear encoding and decoding schemes that achieve stabilization within the SNR constraint precisely when Cfb ≥ log2 |ø|.
http://eprints.nuim.ie/1611/
Marked
Mark
A Problem in Positive Systems Stability Arising in Topology Control
(2009)
Knorn, Florian ; Stanojević, Rade; Corless, Martin; Shorten, Robert
A Problem in Positive Systems Stability Arising in Topology Control
(2009)
Knorn, Florian ; Stanojević, Rade; Corless, Martin; Shorten, Robert
Abstract:
We present a problem in the stability of switched positive systems that arises in network topology control. Preliminary results are given that guarantee stability of a network topology control problem under certain assumptions. Roughly speaking, these assumptions reduce the underlying stability problem to a nonlinear consensus problem with a driving term, that eventually becomes a Lur'e problem. Simulation results are given to illustrate our algorithm. While these results indicate that our assumptions can be removed, a proof of the general stability problem remains open.
http://eprints.nuim.ie/1607/
Marked
Mark
On a Buffering Hypothesis in 802.11 Analytic Models
(2009)
Huang, K.D.; Duffy, K.R.
On a Buffering Hypothesis in 802.11 Analytic Models
(2009)
Huang, K.D.; Duffy, K.R.
Abstract:
Using detailed statistical analysis of data taken from simulations and test-bed experiments, we have recently reported the inappropriateness of a queue-decoupling approximation that has been implicitly adopted by many distinct authors to incorporate queueing behavior in analytic models of finite load 802.11 networks. In the present paper we show that this flawed hypothesis leads to network throughput prediction errors in the presence of stations with asymmetric offered loads. We suggest that care should be taking in drawing deductions from models that adopt this hypothesis and that further modeling innovation is necessary.
http://eprints.nuim.ie/1606/
Marked
Mark
On admissible pairs and equivalent feedback - Youla parameterization in iterative learning control
(2006)
Verwoerd, Mark; Meinsma, Gjerrit; de Vries, Theo
On admissible pairs and equivalent feedback - Youla parameterization in iterative learning control
(2006)
Verwoerd, Mark; Meinsma, Gjerrit; de Vries, Theo
Abstract:
This paper revisits a well-known synthesis problem in iterative learning control, where the objective is to optimize a performance criterion over a class of casual iterations. The approach taken here adopts an infinite-time setting and looks at limit behavior.The first part of the paper considers iterations without current-cycle-feedback (CCF) term. A notion of admissibility is introduced to distinguish between pairs of operators that define a rebustly converging iteration and pairs that do not. The set of admissible pairs is partitioned into disjoint equivalence classes. Different members of an equivalence class are shown to correspond to different realizations of a feedback controller. Conversely, every stabilizing controller is shown to allow for a (non-unique) factorization in terms of admissible pairs. Class representatives are introduced to remove redundancy. The smaller set of representative pairs is shown to have a trivial parameterization that concides with the Youla param...
http://eprints.nuim.ie/941/
Marked
Mark
On linear co-positive Lyapunov functions for sets of linear positive systems.
(2009)
Knorn, Florian ; Mason, Oliver; Shorten, Robert
On linear co-positive Lyapunov functions for sets of linear positive systems.
(2009)
Knorn, Florian ; Mason, Oliver; Shorten, Robert
Abstract:
In this paper we derive necessary and sufficient conditions for the existence of a common linear co-positive Lyapunov function for a finite set of linear positive systems. Both the state dependent and arbitrary switching cases are considered. Our results reveal an interesting characterisation of "linear" stability for the arbitrary switching case; namely, the existence of such a linear Lyapunov function can be related to the requirement that a number of extreme systems are Metzler and Hurwitz stable. Examples are given to illustrate the implications of our results.
http://eprints.nuim.ie/1644/
Marked
Mark
Determining the expected variability of immune responses using the Cyton Model
(2008)
Subramanian, Vijay G.; Duffy, Ken R.; Turner, Marian L.; Hodgkin, Philip D.
Determining the expected variability of immune responses using the Cyton Model
(2008)
Subramanian, Vijay G.; Duffy, Ken R.; Turner, Marian L.; Hodgkin, Philip D.
Abstract:
During an adaptive immune response, lymphocytes proliferate for ve to twenty cell divisions, then stop and die over a period of weeks. The cyton model for regulation of lymphocyte proliferation and survival was introduced by Hawkins et al. [17] to provide a framework for understanding this response and its regulation. The model assumes stochastic values for division and survival times for each cell in a responding population. Experimental evidence indicates that the choice of times is drawn from a skewed distribution such as the lognormal, with the fate of individual cells being potentially highly variable. For this reason we calculate the higher moments of the model so that the expected variability can be determined. To do this we formulate a new analytic framework for the cyton model by introducing a generalization to the Bellman-Harris branching process.
http://eprints.nuim.ie/1645/
Marked
Mark
Low-Delay Dynamic Routing Using Fountain Codes
(2009)
Badarla, Venkataramana ; Subramanian, Vijay G.; Leith, Douglas J.
Low-Delay Dynamic Routing Using Fountain Codes
(2009)
Badarla, Venkataramana ; Subramanian, Vijay G.; Leith, Douglas J.
Abstract:
This paper considers augmenting current maximum throughput routing algorithms to use fountain coding at senders. We demonstrate that this joint routing/coding approach is able to achieve significantly improved delay performance.
http://eprints.nuim.ie/1646/
Marked
Mark
Improving Performance for CSMA/CA Based Wireless Networks
(2007)
Li, Tianji
Improving Performance for CSMA/CA Based Wireless Networks
(2007)
Li, Tianji
Abstract:
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based wireless networks are becoming increasingly ubiquitous. With the aim of supporting rich multimedia applications such as high-definition television (HDTV, 20Mbps) and DVD (9.8Mbps), one of the technology trends is towards increasingly higher bandwidth. Some recent IEEE 802.11n proposals seek to provide PHY rates of up to 600 Mbps. In addition to increasing bandwidth, there is also strong interest in extending the coverage of CSMA/CA based wireless networks. One solution is to relay traffic via multiple intermediate stations if the sender and the receiver are far apart. The so called “mesh” networks based on this relay-based approach, if properly designed, may feature both “high speed” and “large coverage” at the same time. This thesis focusses on MAC layer performance enhancements in CSMA/CA based networks in this context. Firstly, we observe that higher PHY rates do not necessarily translate into corresponding in...
http://eprints.nuim.ie/1647/
Marked
Mark
A Strategy for Fair Coexistence of Loss and Delay-Based Congestion Control Algorithms
(2009)
Budzisz, Lukasz; Stanojević, Rade; Shorten, Robert; Baker, Fred
A Strategy for Fair Coexistence of Loss and Delay-Based Congestion Control Algorithms
(2009)
Budzisz, Lukasz; Stanojević, Rade; Shorten, Robert; Baker, Fred
Abstract:
Delay-based TCP variants have attracted a large amount of attention in the networking community because of their ability to efficiently use network resources, control queuing delays, exhibit virtually zero packet loss, etc. One major issue that discourages the wider deployment of delay-based TCP variants is their inability to co-exist fairly with standard loss-based TCP. In this note we propose a simple mechanism that allows delayand loss-based (AIMD) TCP flows to compete fairly with each other. Further, our approach ensures that delay-based flows automatically (and swiftly) switch to a low-delay regime if no lossbased flows are present. We provide analytical and simulation results to validate presented algorithm.
http://eprints.nuim.ie/1648/
Marked
Mark
Applications of Linear Co-positive Lyapunov Functions for Switched Linear Positive Systems
(2009)
Knorn, Florian ; Mason, Oliver; Shorten, Robert
Applications of Linear Co-positive Lyapunov Functions for Switched Linear Positive Systems
(2009)
Knorn, Florian ; Mason, Oliver; Shorten, Robert
Abstract:
In this paper we review necessary and sufficient conditions for the existence of a common linear co-positive Lyapunov function for switched linear positive systems. Both the state dependent and arbitrary switching cases are considered and a number of applications are presented.
http://eprints.nuim.ie/1649/
Marked
Mark
On the impact of correlation between collaterally consanguineous cells on lymphocyte population dynamics
(2009)
Duffy, Ken R.; Subramanian, Vijay G.
On the impact of correlation between collaterally consanguineous cells on lymphocyte population dynamics
(2009)
Duffy, Ken R.; Subramanian, Vijay G.
Abstract:
During an adaptive immune response, lymphocytes proliferate for five to twenty five cell divisions, then stop and die over a period of weeks. Based on extensive flow cytometry data, Hawkins et al. (PNAS, 2007, 104, 5032{5037) introduced a cell-level stochastic model of lymphocyte population dynamics, called the Cyton Model, that accurately captures mean lymphocyte population size as a function of time. In Subramanian et al. (J. Math. Biol., 2008, 56:6, 861{892), we performed a branching process analysis of the Cyton Model and deduced from parameterizations for in vitro and in vivo data that the immune response is predictable despite each cell's fate being highly variable. One drawback of flow cytometry data is that individual cells cannot be tracked, so that it is not possible to investigate dependencies in the fate of cells within family trees. In the absence of this information, while the Cyton Model abandons one of the usual assumptions of branching processes (the independe...
http://eprints.nuim.ie/1650/
Marked
Mark
Large Deviations Of Max-Weight Scheduling Policies On Convex Rate Regions
(2008)
Subramanian, Vijay G.
Large Deviations Of Max-Weight Scheduling Policies On Convex Rate Regions
(2008)
Subramanian, Vijay G.
Abstract:
We consider a single server discrete-time system with K users where the server picks operating points from a compact, convex and co-ordinate convex set in Rk+. For this system we analyse the performance of a stablising policy that at any given time picks operating points from the allowed rate region that maximise a weighted sum of rate, where the weights depend upon the workloads of the users. Assuming a Large Deviations Principle (LDP) for the arrival processes in the Skorohod space of functions that are right-continuous with left-hand limits we establish an LDP for the workload process using a generalised version of the contraction principle to derive the corresponding rate function. With the LDP result available we then analyse the tail probabilities of the workloads under different buffering scenarios.
http://eprints.nuim.ie/1651/
Marked
Mark
Downlink Scheduling and Resource Allocation for OFDM Systems
(2009)
Huang, Jianwei; Subramanian, Vijay G.; Agrawal, Rajeev; Berry, Randall A.
Downlink Scheduling and Resource Allocation for OFDM Systems
(2009)
Huang, Jianwei; Subramanian, Vijay G.; Agrawal, Rajeev; Berry, Randall A.
Abstract:
Abstract—We consider scheduling and resource allocation for the downlink of a cellular OFDM system, with various practical considerations including integer tone allocations, different subchannelization schemes, maximum SNR constraint per tone, and "self-noise" due to channel estimation errors and phase noise. During each time-slot a subset of users must be scheduled, and the available tones and transmission power must be allocated among them. Employing a gradient-based scheduling scheme presented in earlier papers reduces this to an optimization problem to be solved in each time-slot. Using a dual formulation, we give an optimal algorithm for this problem when multiple users can time-share each tone. We then give several low complexity heuristics that enforce integer tone allocations. Simulations are used to compare the performance of different algorithms.
http://eprints.nuim.ie/1652/
Marked
Mark
A New Hypothesis for Sleep: Tuning for Criticality
(2009)
Pearlmutter, Barak A.; Conor J. Houghton, Conor J.
A New Hypothesis for Sleep: Tuning for Criticality
(2009)
Pearlmutter, Barak A.; Conor J. Houghton, Conor J.
Abstract:
We propose that the critical function of sleep is to prevent uncontrolled neuronal feedback while allowing rapid responses and prolonged retention of short-term memories. Through learning, the brain is tuned to react optimally to environmental challenges. Optimal behavior often requires rapid responses and the prolonged retention of short-term memories. At a neuronal level, these correspond to recurrent activity in local networks. Unfortunately, when a network exhibits recurrent activity, small changes in the parameters or conditions can lead to runaway oscillations. Thus, the very changes that improve the processing performance of the network can put it at risk of runaway oscillation. To prevent this, stimulus-dependent network changes should be permitted only when there is a margin of safety around the current network parameters. We propose that the essential role of sleep is to establish this margin by exposing the network to a variety of inputs, monitoring for erratic behavior, ...
http://eprints.nuim.ie/1653/
Marked
Mark
Achieving fairness in Lossy 802.11e wireless multi-hop Mesh networks.
(2009)
Cao, Qizhi,; Li, Tianji; Leith , Douglas
Achieving fairness in Lossy 802.11e wireless multi-hop Mesh networks.
(2009)
Cao, Qizhi,; Li, Tianji; Leith , Douglas
Abstract:
We consider achieving max-min fairness in 802.11e based multi-hop wireless networks. We propose an approach which makes use of the TXOP mechanism in combination with an automatic contention window size tuning algorithm based onchannel state sensing. Simulation results show that the proposed approach can provide a good approximation to per-flow max-min fairness and that this is achieved regardless of the active number of flows and when the channel is noisy.
http://eprints.nuim.ie/1654/
Marked
Mark
A framework for decentralised feedback connectivity control with application to sensor networks
(2009)
Knorn, F.; Stanojević, R.; Corless, M.; Shorten, R.
A framework for decentralised feedback connectivity control with application to sensor networks
(2009)
Knorn, F.; Stanojević, R.; Corless, M.; Shorten, R.
Abstract:
In this paper we propose a decentralised algorithm for connectivity maintenance in a distributed sensor network. Our algorithm uses the dynamics of a consensus algorithm to estimate the connectivity of a network topology in a decentralised manner. These estimates are then used to inform a decentralised control algorithm that regulates the network connectivity to some desired level. Under certain realistic assumptions we show the closed loop dynamics can be described as a consensus algorithm with an input, and eventually reduces to a scalar system. Bounds are given to ensure the stability of the algorithm and examples are given to illustrate the efficacy of the proposed algorithm.
http://eprints.nuim.ie/1655/
Marked
Mark
Nonlinear pharmacokinetics of therapeutic proteins resulting from receptor mediated endocytosis
(2009)
Krippendorff, Ben-Fillippo; Kuester, Katharina; Kloft, Charlotte; Huisinga, Wilhelm
Nonlinear pharmacokinetics of therapeutic proteins resulting from receptor mediated endocytosis
(2009)
Krippendorff, Ben-Fillippo; Kuester, Katharina; Kloft, Charlotte; Huisinga, Wilhelm
Abstract:
Receptor mediated endocytosis (RME) plays a major role in the disposition of therapeutic protein drugs in the body. It is suspected to be a major source of nonlinear pharmacokinetic behavior observed in clinical pharmacokinetic data. So far, mostly empirical or semi-mechanistic approaches have been used to represent RME. A thorough understanding of the impact of the properties of the drug and of the receptor system on the resulting nonlinear disposition is still missing, as is how to best represent RME in pharmacokinetic models. In this article, we present a detailed mechanistic model of RME that explicitly takes into account receptor binding and trafficking inside the cell and that is used to derive reduced models of RME which retain a mechanistic interpretation. We find that RME can be described by an extended Michaelis–Menten model that accounts for both the distribution and the elimination aspect of RME. If the amount of drug in the receptor system is negligible
http://eprints.nuim.ie/1658/
Marked
Mark
Sequential activation of metabolic pathways: a dynamic optimization approach
(2009)
Oyarzún, Diego A.; Ingalls, Brian P.; Middleton, Richard H.; Kalamatianos, Dimitrios
Sequential activation of metabolic pathways: a dynamic optimization approach
(2009)
Oyarzún, Diego A.; Ingalls, Brian P.; Middleton, Richard H.; Kalamatianos, Dimitrios
Abstract:
The regulation of cellular metabolism facilitates robust cellular operation in the face of changing external conditions. The cellular response to this varying environment may include the activation or inactivation of appropriate metabolic pathways. Experimental and numerical observations of sequential timing in pathway activation have been reported in the literature. It has been argued that such patterns can be rationalized by means of an underlying optimal metabolic design. In this paper we pose a dynamic optimization problem that accounts for time-resource minimization in pathway activation under constrained total enzyme abundance. The optimized variables are time-dependent enzyme concentrations that drive the pathway to a steady state characterized by a prescribed metabolic flux. The problem formulation addresses unbranched pathways with irreversible kinetics. Neither specific reaction kinetics nor fixed pathway length are assumed. In the optimal solution, each enzyme follows a s...
http://eprints.nuim.ie/1665/
Displaying Results 26 - 50 of 338 on page 2 of 14
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