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  • 1. Charalambous, Charalambos D.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, Christoforos N.
    Team Optimality Conditions of Differential Decision Systems with Nonclasssical Information Structures2014In: 2014 European Control Conference (ECC), IEEE , 2014, p. 2851-2856Conference paper (Refereed)
    Abstract [en]

    We derive team optimality conditions for differential decision systems with nonclassical information structures. The necessary conditions of optimality are given in terms of Hamiltonian system of equations consisting of a coupled backward and forward differential equations and a Hamiltonian projected onto the subspace generated by the information structures. Under certain global convexity conditions it is shown that person-by-person optimality implies team optimality.

  • 2.
    Charalambous, Charalambos D.
    et al.
    University of Cyprus.
    Tziortzis, Ioannis
    University of Cyprus.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Dynamic programming with total variational distance uncertaint2012In: Decision and Control (CDC), 2012 IEEE 51st Annual Conference on, IEEE , 2012, p. 1909-1914Conference paper (Refereed)
    Abstract [en]

    The aim of this paper is to address optimality of stochastic control strategies via dynamic programming subject to total variational distance uncertainty on the conditional distribution of the controlled process. Utilizing concepts from signed measures, the maximization of a linear functional on the space of probability measures on abstract spaces is investigated, among those probability measures which are within a total variational distance from a nominal probability measure. The maximizing probability measure is found in closed form. These results are then applied to solve minimax stochastic control with deterministic control strategies, under a Markovian assumption on the conditional distributions of the controlled process. The results include: 1) Optimization subject to total variational distance constraints, 2) new dynamic programming recursions, which involve the oscillator seminorm of the value function.

  • 3.
    Charalambous, Charalambos D.
    et al.
    University of Cyprus.
    Tziortzis, Ioannis
    University of Cyprus.
    Loyka, Sergey
    University of Ottawa.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Extremum Problems with Total Variation Distance2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 1204-1209Conference paper (Refereed)
    Abstract [en]

    The aim of this paper is to investigate extremum problems with pay-off the total variational distance metric subject to linear functional constraints both defined on the space of probability measures, as well as related problems. Utilizing concepts from signed measures, the extremum probability measures of such problems are obtained in closed form, by identifying the partition of the support set and the mass of these extremum measures on the partition. The results are derived for abstract spaces, specifically, complete separable metric spaces, while the high level ideas are also discussed for denumerable spaces endowed with the discrete topology.

  • 4. Charalambous, Charalambos D.
    et al.
    Tzortzis, Ioannis
    Loyka, Sergey
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Extremum Problems With Total Variation Distance and Their Applications2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 9, p. 2353-2368Article in journal (Refereed)
    Abstract [en]

    The aim of this paper is to investigate extremum problems with pay-off being the total variation distance metric defined on the space of probability measures, subject to linear functional constraints on the space of probability measures, and vice-versa; that is, with the roles of total variation metric and linear functional interchanged. Utilizing concepts from signed measures, the extremum probability measures of such problems are obtained in closed form, by identifying the partition of the support set and the mass of these extremum measures on the partition. The results are derived for abstract spaces; specifically, complete separable metric spaces known as Polish spaces, while the high level ideas are also discussed for denumerable spaces endowed with the discrete topology. These extremum problems often arise in many areas, such as, approximating a family of probability distributions by a given probability distribution, maximizing or minimizing entropy subject to total variation distance metric constraints, quantifying uncertainty of probability distributions by total variation distance metric, stochastic minimax control, and in many problems of information, decision theory, and minimax theory.

  • 5.
    Charalambous, T.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Power control inwireless ad hoc networks: Stability and convergence under uncertainties2013In: Springer Optimization and Its Applications, Springer, 2013, p. 143-174Conference paper (Refereed)
    Abstract [en]

    A successful distributed power control algorithm requires only local measurements for updating the power level of a transmitting node, so that eventually all transmittersmeet their QoS requirements, i.e. the solution converges to the global optimum. There are numerous algorithmwhich claim to work under ideal conditions in which there exist no uncertainties and the model is identical to the real-world implementation. Nevertheless, the problem arises when real-world phenomena are introduced into the problem, such as uncertainties (such as changing environment and time delays) or the QoS requirements cannot be achieved for all the users in the network. In this chapter, we study some distributed power control algorithms for wireless ad hoc networks and discuss their robustness to real-world phenomena. Simulations illustrate the validity of the existing results and suggest directions for future research.

  • 6.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Charalambos D.
    Rezaei, Farzad
    Optimal Merging Algorithms for Lossless Codes With Generalized Criteria2014In: IEEE Transactions on Information Theory, ISSN 0018-9448, E-ISSN 1557-9654, Vol. 60, no 9, p. 5486-5499Article in journal (Refereed)
    Abstract [en]

    This paper presents lossless prefix codes optimized with respect to a payoff criterion consisting of a convex combination of maximum codeword length and average codeword length. The optimal codeword lengths obtained are based on a new coding algorithm, which transforms the initial source probability vector into a new probability vector according to a merging rule. The coding algorithm is equivalent to a partition of the source alphabet into disjoint sets on which a new transformed probability vector is defined as a function of the initial source probability vector and scalar parameter. The payoff criterion considered encompasses a tradeoff between maximum and average codeword length; it is related to a payoff criterion consisting of a convex combination of average codeword length and average of an exponential function of the codeword length, and to an average codeword length payoff criterion subject to a limited length constraint. A special case of the first related payoff is connected to coding problems involving source probability uncertainty and codeword overflow probability, whereas the second related payoff compliments limited length Huffman coding algorithms.

  • 7.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Hadjicostis, C. N.
    Average consensus in the presence of dynamically changing directed topologies and time delays2014In: Proceedings of the IEEE Conference on Decision and Control, IEEE conference proceedings, 2014, no February, p. 709-714Conference paper (Refereed)
    Abstract [en]

    We have recently proposed a robustified ratio consensus algorithm which achieves asymptotic convergence to the global average in a distributed fashion in static strongly connected digraphs, despite the possible presence of bounded but otherwise arbitrary delays. In this work, we propose a protocol which reaches asymptotic convergence to the global average in a distributed fashion under possible changes in the underlying interconnection topology (e.g., due to component mobility), as well as time-varying delays that might affect transmissions at different times. More specifically, we extend our previous work to also account for the case where, in addition to arbitrary but bounded delays, we may have time varying communication links. The proposed protocol requires that each component has knowledge of the number of its outgoing links, perhaps with some bounded delay, and that the digraphs formed by the switching communication topologies over a finite time window are jointly strongly connected.

  • 8.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Hadjicostis, Christoforos N.
    Electrical and Computer Engineering Department, University of Cyprus.
    Distributed Formation of Balanced and Bistochastic Weighted Digraphs in Multi-Agent Systems2013In: 2013 European Control Conference (ECC), IEEE , 2013, p. 1752-1757Conference paper (Refereed)
    Abstract [en]

    We propose two distributed algorithms, one for solving the weight-balance problem and another for solving the bistochastic matrix formation problem, in a distributed system whose components (nodes) can exchange information via interconnection links (edges) that form an arbitrary, possibly directed, strongly connected communication topology (digraph). Both distributed algorithms achieve their goal asymptotically and operate iteratively by having each node adapt the (nonnegative) weights on its outgoing edges based on the weights of its incoming links. The weight-balancing algorithm is shown to admit geometric convergence rate, whereas the second algorithm, which is a modification of the weight-balancing algorithm, leads asymptotically to a bistochastic digraph with geometric convergence rate for a certain set of initial values. The two algorithms perform better than existing approaches, as illustrated by the examples we provide.

  • 9.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, Christoforos N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed Minimum-Time Weight Balancing over Digraphs2014Conference paper (Refereed)
    Abstract [en]

    We address the weight-balancing problem for a distributed system whose components (nodes) can exchange information via interconnection links (edges) that form an arbitrary, possibly directed, communication topology (digraph). A weighted digraph is balanced if, for each node, the sum of the weights of the edges outgoing from that node is equal to the sum of the weights of the edges incoming to that node. Weight-balanced digraphs play a key role in a variety of applications, such as coordination of groups of robots, distributed decision making, and distributed averaging which is important for a wide range of applications in signal processing. We propose a distributed algorithm for solving the weight balancing problem in a minimum number of iterations, when the weights are nonnegative real numbers. We also provide examples to corroborate the proposed algorithm.

  • 10.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Kalyviannaki, Evangelia
    City University London.
    Hadjicostis, Christoforos N.
    Electrical and Computer Engineering Department, University of Cyprus.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Distributed Offline Load Balancing in MapReduce Networks2013In: 2013 IEEE 52ND ANNUAL CONFERENCE ON DECISION AND CONTROL (CDC), IEEE conference proceedings, 2013, p. 835-840Conference paper (Refereed)
    Abstract [en]

    In this paper we address the problem of balancing the processing load of MapReduce tasks running on heterogeneous clusters, i.e., clusters with different capacities and update cycles. We present a fully decentralized algorithm, based on ratio consensus, where each mapper decides the amount of workload data to handle for a single user job using only job specific local information, i.e., information that can be collected from directly connected neighboring mappers, regarding their current workload and capacity. In contrast to other algorithms in the literature, the proposed algorithm can be deployed in heterogeneous networks and can operate asynchronously in both directed and undirected communication topologies. The performance of the proposed algorithm is demonstrated via simulation experiments on large-scale strongly connected topologies. 

  • 11.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Klerides, Evelina
    Imperial College London.
    Wiesemann, Wolfram
    Imperial College London.
    Vassiliou, Angelos
    University of Cyprus.
    Hadjitheophanous, Stavros
    Deliparaschos, Kyriakos
    Cyprus University of Technology.
    On the Minimum Latency Transmission Scheduling in Wireless Networks with Power Control under SINR Constraints2013In: European transactions on telecommunications, ISSN 1124-318X, E-ISSN 2161-3915, Vol. 26, no 3, p. 367--379Article in journal (Refereed)
    Abstract [en]

    In order to alleviate interference and contention in a wireless network, we may exploit the existence of multiple orthogonal channels or time slots, thus achieving a substantial improvement in performance. In this paper, we study a joint transmission scheduling and power control problem that arises in wireless networks. The goal is to assign channels (or time slots) and transmitting powers to communication links such that all communication requests are processed correctly, specified quality-of-service requirements are met, and the number of required time slots is minimised. First, we formulate the problem as a mixed-integer linear programming. Then, we show that the problem considered is non-deterministic polynomial-time hard, and subsequently, we propose non-trivial bounding techniques to solve it. Optimisation methods are also discussed, including a column generation approach, specifically designed to find bounds for the transmission scheduling problem. Moreover, we develop optimisation techniques in which the bounding techniques are integrated in order to derive the optimal solution to the problem faster. We close with an extensive computational study, which shows that despite the complexity of the problem, the proposed methodology scales to problems of non-trivial size. Our algorithms can therefore be used for static wireless networks where propagation conditions are almost constant and a centralised agent is available (e.g. cellular networks where the base station can act as a centralised agent or wireless mesh networks), and they can also serve as a benchmark for the performance evaluation of heuristic, approximation or distributed algorithms that aim to find near-optimal solutions without information about the whole network. 

  • 12.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, I.
    Medium access control via contention-based distributed power control2012In: 2012 8th International Wireless Communications And Mobile Computing Conference (IWCMC), IEEE , 2012, p. 555-560Conference paper (Refereed)
    Abstract [en]

    A successful distributed power control algorithm requires only local measurements for updating the power level of a transmitting node, so that eventually all transmitters meet their QoS requirements. Nevertheless, the problem arises when the QoS requirements cannot be achieved for all the users in the network. In this paper, a distributed algorithm for wireless ad hoc networks which is contention-based and makes use of a back off mechanism is proposed. This algorithm aims to eliminate overhead communication, improve fairness, allow nodes to operate asynchronously while establishing some performance level. The performance of the algorithm is evaluated via simulations.

  • 13.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, I.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Precoding decision for full-duplex X-relay channel with Decode-And-Forward2014In: IWCMC 2014 - 10th International Wireless Communications and Mobile Computing Conference, IEEE , 2014, p. 791-796Conference paper (Refereed)
    Abstract [en]

    In this paper, we study a simple X-relay configuration where the shared relay operates in full-duplex (FD) mode. The relay node may have limited spatial degrees of freedom, and as a result, it may not be able to handle both the loop interference and the multiuser interference. Hence, a decision on the precoding scheme is necessitated. It is often the case that the relay does not have the option of real-Time switching between different precoding schemes, either due to hardware limitations of the relay or increased complexity of the problem. Hence, we investigate a 'static' precoding decision where the relay node decides on its precoding scheme based only on statistical knowledge of the channel conditions. To perform this decision, the behavior of the system is formulated as a Markov chain and the outage probability of the system is derived in a closed-form with the precoding decision as a parameter. The outage probability is minimized by optimally choosing the precoding scheme, using easily verifiable conditions on the statistical knowledge of the channel conditions. Simulations validate the investigated scheme.

  • 14.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Nomikos, Nikolaos
    Krikidis, Ioannis
    Vouyioukas, Demosthenes
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Modeling Buffer-Aided Relay Selection in Networks With Direct Transmission Capability2015In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 19, no 4, p. 649-652Article in journal (Refereed)
    Abstract [en]

    We consider a wireless relay network that consists of a source, half-duplex decode-and-forward buffer-aided relays and a destination. While the majority of previous works on relay selection assume no direct transmission between source and destination in such a setting, we lift this assumption and propose a link selection policy that exploits both the buffering ability and the opportunity for successful reception of a packet directly from the source. The proposed relay selection scheme incorporates the instantaneous strength of the wireless links and adapts the relay selection decision based on the strongest available link. The evolution of the network as a whole is modeled by means of a Markov chain and thus, the outage probability is associated with the steady state of the Markov chain. It is deduced that even if the link between the source and the destination is in principle a very unreliable link, it is always beneficial for the source to multicast a packet to both the relay with the strongest available link and the destination.

  • 15.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rabbat, M. G.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, C. N.
    Distributed Finite-Time Computation of Digraph Parameters: Left-Eigenvector, Out-Degree and Spectrum2016In: IEEE Transactions on Control of Network Systems, ISSN 2325-5870, Vol. 3, no 2, p. 137-148, article id 7100912Article in journal (Refereed)
    Abstract [en]

    Many of the algorithms that have been proposed in the field of distributed computation rely on assumptions that require nodes to be aware of some global parameters. In this paper, we propose algorithms to compute some network parameters in a distributed fashion and in a finite number of steps. More specifically, given an arbitrary strongly connected network of interconnected nodes, by adapting a distributed finite-time approach, we develop distributed strategies that enable nodes to compute the following network parameters: the left-eigenvector, the out-degree, and the spectrum of weighted adjacency matrices.

  • 16.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Yuan, Ye
    University of Cambridge.
    Yang, Tao
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Pan, Wei
    Imperial College London.
    Hadjicostis, Christoforos N.
    University of Cyprus.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Decentralised Minimum-Time Average Consensus in Digraphs2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 2617-2622Conference paper (Refereed)
    Abstract [en]

    Distributed algorithms for average consensus in directed graphs are typically asymptotic in the literature. In this work, we propose a protocol to distributively reach average consensus in a finite number of steps on interconnection topologies that form strongly connected directed graphs (digraphs). The average consensus value can be computed, based exclusively on local observations at each component, by running a protocol that requires each component to observe and store its own value over a finite and minimal number of steps, and to have knowledge of the number of its out-going links (i.e., the number of components to which it sends information). The proposed algorithm is demonstrated via illustrative examples.

  • 17.
    Charalambous, Themistoklis
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Yuan, Ye
    Yang, Tao
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Pan, Wei
    Hadjicostis, Christoforos N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Distributed Finite-Time Average Consensus in Digraphs in the Presence of Time Delays2015In: IEEE TRANSACTIONS ON CONTROL OF NETWORK SYSTEMS, ISSN 2325-5870, Vol. 2, no 4, p. 370-381Article in journal (Refereed)
    Abstract [en]

    Most algorithms for distributed averaging only guarantee asymptotic convergence. This paper introduces a distributed protocol that allows nodes to find the exact average of the initial values in a finite and minimum number of steps on interconnection topologies described by strongly connected directed graphs (digraphs). More specifically, under the assumption that each component has knowledge of the number of its outgoing links (i.e., the number of components to which it sends information), we show that the average value can be computed based on local observations over a finite time interval. The average can be obtained in a finite number of steps even when the information exchange is subject to delays. The proposed algorithm is the first in the literature that allows for distributed computation of the exact average in digraphs in finite time, with and without delays.

  • 18. Deliparaschos, K. M.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Christodoulides, P.
    Klerides, E.
    Reduced power expenditure in the minimum latency transmission scheduling problem2014In: 6th International Symposium on Communications, Control and Signal Processing, ISCCSP 2014, 2014, p. 206-209Conference paper (Refereed)
    Abstract [en]

    In large networks due to the large number of transmissions it is unlikely that a slot will be used only by a single wireless node. In this paper, a Cutting Plane approach is proposed in order to combine the minimum latency transmission scheduling problem with power expenditure minimization. The problem is partitioned into a Master Problem and a Subproblem; the main objective of the Subproblem is to minimize the total power expenditure. The Cutting Plane algorithm used is evaluated with regard to its performance and the outcome is that the power expenditure is reduced significantly, while achieving the minimum latency transmission scheduling. One of the drawbacks of the proposed approach, however, is the fact that although it reduces the power significantly, the computational time required is quite large, pointing to further research towards that direction.

  • 19.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Asymptotic and Exponential Stability of General Classes of Continuous-Time Power Control Laws in Wireless Networks2013In: Proceedings of 52nd IEEE Conference on Decision and Control December 10-13, 2013. Florence, Italy, IEEE conference proceedings, 2013, p. 49-54Conference paper (Refereed)
    Abstract [en]

    This paper develops a comprehensive stabilityanalysis framework for continuous-time power control algorithmsin wireless networks under bounded time-varyingcommunication delays. Our first set of results establish globalasymptotic stability of power control laws involving two-sidedscalable interference functions, and include earlier work onstandard interference functions as a special case. We thenconsider contractive interference functions and demonstratethat the associated continuous-time power control laws alwayshave unique fixed points, which are exponentially stable evenin the presence of bounded heterogeneous time-varying delays.For this class of interference functions, we derive an explicitbound on the decay rate that allows us to quantify the impactof delays on the convergence time of the algorithm. Numericalsimulations illustrate our theoretical results.

  • 20.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    ASYMPTOTIC STABILITY AND DECAY RATES OF HOMOGENEOUS POSITIVE SYSTEMS WITH BOUNDED AND UNBOUNDED DELAYS2014In: SIAM Journal of Control and Optimization, ISSN 0363-0129, E-ISSN 1095-7138, Vol. 52, no 4, p. 2623-2650Article in journal (Refereed)
    Abstract [en]

    There are several results on the stability of nonlinear positive systems in the presence of time delays. However, most of them assume that the delays are constant. This paper considers time-varying, possibly unbounded, delays and establishes asymptotic stability and bounds the decay rate of a significant class of nonlinear positive systems which includes positive linear systems as a special case. Specifically, we present a necessary and sufficient condition for delay-independent stability of continuous-time positive systems whose vector fields are cooperative and homogeneous. We show that global asymptotic stability of such systems is independent of the magnitude and variation of the time delays. For various classes of time delays, we are able to derive explicit expressions that quantify the decay rates of positive systems. We also provide the corresponding counterparts for discrete-time positive systems whose vector fields are nondecreasing and homogeneous.

  • 21.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Asymptotic Stability and Decay Rates of Positive Linear Systems with Unbounded Delays2013In: Proceedings of the 52nd IEEE Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 1423-1428Conference paper (Refereed)
    Abstract [en]

    There are several results on the stability analysisof positive linear systems in the presence of constant or timevaryingdelays. However, most existing results assume thatthe delays are bounded. This paper studies the stability ofdiscrete-time positive linear systems with unbounded delays.We provide a set of easily verifiable necessary and sufficientconditions for delay-independent stability of positive linearsystems subject to a general class of heterogeneous timevaryingdelays. For two particular classes of unbounded delays,explicit expressions that bound the decay rate of the system arepresented. We demonstrate that the best bound on the decayrate that our results can guarantee can be found via convexoptimization. Finally, the validity of the results is demonstratedvia a numerical example.

  • 22.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Delay-independent Stability of Cone-invariant Monotone Systems2015In: 54th IEEE Conference on Decision and Control (CDC), IEEE , 2015, p. 6361-6366Conference paper (Refereed)
    Abstract [en]

    Recent results in the literature have shown that particular classes of positive systems are insensitive to time-varying delays, giving the impression that the delay-insensitivity property stems from the fact that the system is positive. Nonetheless, it has been lately shown that a linear cone-invariant system is insensitive to time-varying delays, asserting that the property of delay-independence may stem from the fact that the system is cone-invariant rather than positive. In this paper, we provide additional evidence for this claim by analyzing the stability of cone-invariant monotone systems with bounded time-varying delays. We present a set of sufficient conditions for delay independent stability of discrete- and continuous-time cone-invariant monotone systems. For linear cone-invariant systems, we show that thestability conditions we have derived are also necessary.

  • 23.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Exponential Stability of Homogeneous Positive Systems of Degree One With Time-Varying Delays2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 6, p. 1594-1599Article in journal (Refereed)
    Abstract [en]

    While the asymptotic stability of positive linear systems in the presence of bounded time delays has been thoroughly investigated, the theory for nonlinear positive systems is considerably less well-developed. This paper presents a set of conditions for establishing delay-independent stability and bounding the decay rate of a significant class of nonlinear positive systems which includes positive linear systems as a special case. Specifically, when the time delays have a known upper bound, we derive necessary and sufficient conditions for exponential stability of (a) continuous-time positive systems whose vector fields are homogeneous and cooperative, and (b) discrete-time positive systems whose vector fields are homogeneous and order-preserving. We then present explicit expressions that allow us to quantify the impact of delays on the decay rate and show that the best decay rate of positive linear systems that our bounds provide can be found via convex optimization. Finally, we extend the results to general linear systems with time-varying delays.

  • 24.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    On the rate of convergence of continuous-time linear positive systems with heterogeneous time-varying delays2013In: 2013 European Control Conference, ECC 2013, IEEE conference proceedings, 2013, p. 3372-3377Conference paper (Refereed)
    Abstract [en]

    In this work, a set of conditions are presented for establishing exponential stability and bounds on the convergence rates of both general and positive linear systems with heterogeneous time-varying delays. First, a sufficient condition for delay-independent exponential stability of general linear systems is derived. When the time delays have a known upper bound, we present an explicit expression that bounds the decay rate of the system. We demonstrate that the best decay rate that our bound can guarantee can be easily found via convex optimization techniques. Finally, for positive linear systems, we show that the stability condition that we have developed is also necessary. The validity of the results is demonstrated via numerical examples.

  • 25.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Stability and Performance of Continuous-Time Power Control in Wireless Networks2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 8, p. 2012-2023Article in journal (Refereed)
    Abstract [en]

    This paper develops a comprehensive stability analysis framework for general classes of continuous-time power control algorithms under heterogeneous time-varying delays. Our first set of results establish global asymptotic stability of power control laws involving two-sided scalable interference functions, and include earlier work on standard interference functions as a special case. We then consider contractive interference functions and demonstrate that the associated continuous-time power control laws always have unique fixed points which are exponentially stable, even under bounded heterogeneous time-varying delays. For this class of interference functions, we present explicit bounds on the decay rate that allow us to quantify the impact of delays on the convergence time of the algorithm. When interference functions are linear, we also prove that contractivity is necessary and sufficient for exponential stability of continuous-time power control algorithms with time-varying delays. Finally, numerical simulations illustrate the validity of our theoretical results.

  • 26.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sub-homogeneous positive monotone systems are insensitive to heterogeneous time-varying delays2014In: 21st International Symposium on Mathematical Theory of Networks and Systems (MTNS 2014), 2014Conference paper (Refereed)
    Abstract [en]

    We show that a sub-homogeneous positive monotone system with bounded heterogeneous time-varying delays is globally asymptotically stable if and only if the corresponding delay-free system is globally asymptotically stable. The proof is based on an extension of a delay-independent stability result for monotone systems under constant delays by Smith to systems with bounded heterogenous time-varying delays. Under the additional assumption of positivity and sub-homogenousvector fields, we establish the aforementioned delay insensitivity property and derive a novel test for global asymptotic stability. If the system has a unique equilibrium point in the positive orthant, we prove that our stability test is necessary and sufficient. Specialized to positive linear systems, our results extend and sharpen existing results from the literature.

  • 27.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Contractive interference functions and rates of convergence of distributed power control laws2012In: IEEE International Conference on Communications (ICC), Ottawa, Canada., IEEE , 2012, p. 4395-4399Conference paper (Refereed)
    Abstract [en]

    The standard interference functions introduced by Yates have been very influential on the analysis and design of distributed power control laws. While powerful and versatile, the framework has some drawbacks: the existence of fixed-points has to be established separately, and no guarantees are given on the rate of convergence of the iterates. This paper introduces contractive interference functions, a slight reformulation of the standard interference functions that guarantees existence and uniqueness of fixed-points and geometric convergence rates. We show that many power control laws from the literature are contractive and derive, sometimes for the first time, convergence rate estimates for these algorithms. Finally, we show that although standard interference functions are not contractive, they are paracontractions with respect to a certain metric space. Extensions to two-sided scalable interference functions are also discussed.

  • 28.
    Feyzmahdavian, Hamid Reza
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Contractive interference functions and rates of convergence of distributed power control laws2012In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 11, no 12, p. 4494-4502Article in journal (Refereed)
    Abstract [en]

    The standard interference functions introduced by Yates have been very influential on the analysis and design of distributed power control laws. While powerful and versatile, the framework has some drawbacks: the existence of fixed-points has to be established separately, and no guarantees are given on the rate of convergence of the iterates. This paper introduces contractive interference functions, a slight reformulation of the standard interference functions that guarantees the existence and uniqueness of fixed-points along with linear convergence of iterates. We show that many power control laws from the literature are contractive and derive, sometimes for the first time, analytical convergence rate estimates for these algorithms. We also prove that contractive interference functions converge when executed totally asynchronously and, under the assumption that the communication delay is bounded, derive an explicit bound on the convergence time penalty due to increased delay. Finally, we demonstrate that although standard interference functions are, in general, not contractive, they are all para-contractions with respect to a certain metric. Similar results for two-sided scalable interference functions are also derived.

  • 29.
    Gonga, Antonio
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fast Information Exchange in Proximity-Based Multichannel Wireless Networks2015In: Proceedings - 2015 8th IFIP Wireless and Mobile Networking Conference, WMNC 2015, IEEE conference proceedings, 2015, p. 24-31Conference paper (Refereed)
    Abstract [en]

    This paper considers the problem of distributed neighbor discovery in multi-channel wireless networks. We propose a protocol in which nodes randomly select a channel and decide whether to transmit or listen for neighbor discovery beacons. When nodes transmit, they use epidemic information dissemination to spread knowledge about all the nodes they have discovered so far. Theoretical guarantees on discovery times are complemented by extensive simulations and practical implementations. The evaluations show that multi-channel communication effectively reduces the number of collisions between nodes in the network (especially in dense networks) and that epidemic information dissemination yields both significant speed-ups and increased resilience to packet losses. Finally, we also show that our protocol compares favorably to previously proposed solutions in the literature.

  • 30.
    Gonga, António
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Neighbor discovery in multichannel wireless clique networks: An epidemic approach2013In: Proceedings - IEEE 10th International Conference on Mobile Ad-Hoc and Sensor Systems, MASS 2013, IEEE , 2013, p. 131-135Conference paper (Refereed)
    Abstract [en]

    We investigate the problem of neighbor discovery in multichannel wireless ad hoc and sensor networks with epidemic information dissemination. Previous works have considered neighbor discovery in a single channel where at most one node can be discovered per time instant. To reduce the effect of collisions observed in single channel solutions, we formulate models for multichannel neighbor discovery and allow for epidemic dissemination of information. As a result, nodes can discover all their neighbors faster, either directly or indirectly by hopping between orthogonal channels and exploring the neighbors in each of them. We show analytically, by simulations, and by experimental evaluations that the expected neighbor discovery time is reduced considerably compared to single channel neighbor discovery solutions.

  • 31. Hadjicostis, Christoforos N.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Average Consensus in the Presence of Delays in Directed Graph Topologies2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 3, p. 763-768Article in journal (Refereed)
    Abstract [en]

    Classical distributed algorithms for asymptotic average consensus typically assume timely and reliable exchange of information between neighboring components of a given multi-component system. These assumptions are not necessarily valid in practice due to varying delays that might affect computations at different nodes and/or transmissions at different links. In this work, we propose a protocol that overcomes this limitation and, unlike existing consensus protocols in the presence of delays, ensures asymptotic consensus to the exact average, despite the presence of arbitrary (but bounded) delays in the communication links. The protocol requires that each component has knowledge of the number of its out-neighbors (i.e., the number of components to which it can send information) and its proof of correctness relies on the weak convergence of a backward product of column stochastic matrices. The proposed algorithm is demonstrated via illustrative examples.

  • 32. Jarrassé, N.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Burdet, E.
    A Framework to Describe, Analyze and Generate Interactive Motor Behaviors2012In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 7, no 11, p. e49945-Article in journal (Refereed)
    Abstract [en]

    While motor interaction between a robot and a human, or between humans, has important implications for society as well as promising applications, little research has been devoted to its investigation. In particular, it is important to understand the different ways two agents can interact and generate suitable interactive behaviors. Towards this end, this paper introduces a framework for the description and implementation of interactive behaviors of two agents performing a joint motor task. A taxonomy of interactive behaviors is introduced, which can classify tasks and cost functions that represent the way each agent interacts. The role of an agent interacting during a motor task can be directly explained from the cost function this agent is minimizing and the task constraints. The novel framework is used to interpret and classify previous works on human-robot motor interaction. Its implementation power is demonstrated by simulating representative interactions of two humans. It also enables us to interpret and explain the role distribution and switching between roles when performing joint motor tasks.

  • 33. Kalyvianaki, Evangelia
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Hand, Steven
    Adaptive Resource Provisioning for Virtualized Servers Using Kalman Filters2014In: ACM Transactions on Autonomous and Adaptive Systems, ISSN 1556-4665, E-ISSN 1556-4703, Vol. 9, no 2Article in journal (Refereed)
    Abstract [en]

    Resource management of virtualized servers in data centers has become a critical task, since it enables cost-effective consolidation of server applications. Resource management is an important and challenging task, especially for multitier applications with unpredictable time-varying workloads. Work in resource management using control theory has shown clear benefits of dynamically adjusting resource allocations to match fluctuating workloads. However, little work has been done toward adaptive controllers for unknown workload types. This work presents a new resource management scheme that incorporates the Kalman filter into feedback controllers to dynamically allocate CPU resources to virtual machines hosting server applications. We present a set of controllers that continuously detect and self-adapt to unforeseen workload changes. Furthermore, our most advanced controller also self-configures itself without any a priori information and with a small 4.8% performance penalty in the case of high-intensity workload changes. In addition, our controllers are enhanced to deal with multitier server applications: by using the pair-wise resource coupling between tiers, they improve server response to large workload increases as compared to controllers with no such resource-coupling mechanism. Our approaches are evaluated and their performance is illustrated on a 3-tier Rubis benchmark website deployed on a prototype Xen-virtualized cluster.

  • 34.
    Kalyviannaki, Evangelia
    et al.
    Imperial College London.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Fiscato, Marco
    Imperial College London.
    Pietzuch, Peter
    Imperial College London.
    Overload Management in Data Stream Processing Systems with Latency Guarantees2012Conference paper (Refereed)
  • 35. Krikidis, I.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Thompson, J. S.
    Opportunistic relay selection for cooperative networks with buffers2012In: Communications (ICC), 2012 IEEE International Conference on, IEEE , 2012, p. 5578-5582Conference paper (Refereed)
    Abstract [en]

    In this paper, a relay selection policy is proposed that fully exploits the flexibility offered by the buffering ability of the relay nodes in order to maximize the achieved diversity gain. The suggested scheme incorporates the instantaneous strength of the wireless links as well as the status of the finite relay buffers and the relay selection decision is based on the strongest available link. Hence the switching occurs dynamically between relay reception and transmission. We show that the proposed relay selection scheme significantly outperforms conventional relay selection policies for all cases and ensures a diversity gain equal to two times the number of relays for large buffer sizes.

  • 36.
    Krikidis, Ioannis
    et al.
    University of Cyprus.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Thompson, John
    University of Edinburgh.
    Stability Analysis and Power Optimization for Energy Harvesting Cooperative Networks2012In: IEEE Signal Processing Letters, ISSN 1070-9908, E-ISSN 1558-2361, Vol. 19, no 1, p. 20-23Article in journal (Refereed)
    Abstract [en]

    In this letter, we investigate the effects of network-layer cooperation in a wireless three-node network with energy-harvesting nodes and bursty data traffic. By modelling energy harvesting in each node as a queue (buffer) that stores the received energy, we study the interaction between data and energy queues when only knowledge of the arrival rates is available. The maximum stable throughput (in packets/slot) of the source as well as the required transmitted power for both a non-cooperative and an orthogonal decode-and-forward cooperative schemes are derived in closed-form. We prove that cooperation achieves a higher maximum stable throughout than direct link for scenarios with poor energy arrival rates.

  • 37. Krikidis, Ioannis
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Thompson, John S.
    Buffer-Aided Relay Selection for Cooperative Diversity Systems without Delay Constraints2012In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 11, no 5, p. 1957-1967Article in journal (Refereed)
    Abstract [en]

    In this paper, we study the relay selection problem for a finite buffer-aided decode-and-forward cooperative wireless network. A relay selection policy that fully exploits the flexibility offered by the buffering ability of the relay nodes in order to maximize the achieved diversity gain is investigated. This new scheme incorporates the instantaneous strength of the wireless links as well as the status of the finite relay buffers and adapts the relay selection decision on the strongest available link by dynamically switching between relay reception and transmission. In order to analyse the new relay selection policy in terms of outage probability and diversity gain, a theoretical framework that models the evolution of the relay buffers as a Markov chain (MC) is introduced. The construction of the state transition matrix and the related steady state of the MC are studied and their impact on the derivation of the outage probability is investigated. We show that the proposed relay selection scheme significantly outperforms conventional relay selection policies for all cases and ensures a diversity gain equal to two times the number of relays for large buffer sizes.

  • 38. Nomikos, N.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Krikidis, I.
    Vouyioukas, D.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hybrid cooperation through full-duplex opportunistic relaying and max-link relay selection with transmit power adaptation2014In: 2014 IEEE International Conference on Communications, ICC 2014, IEEE Computer Society, 2014, p. 5706-5711Conference paper (Refereed)
    Abstract [en]

    In this work, we study a cooperative network with multiple full-duplex buffer-aided relays. A hybrid cooperative relaying policy is proposed that employs power adaptation and consists of two alternative schemes: (i) full-duplex transmission through the relay which requires the least total power expenditure and loop interference is mitigated through power adaptation; (ii) buffer-aided max - link selection with power adaptation, when full-duplexity is not feasible. Aiming to reduce the overhead of channel state information (CSI) acquisition and processing, we propose a suboptimal distributed method for relay selection, for which the network performance is not degraded significantly. We show that power adaptation offers reduced overhead of CSI acquisition. Numerical results and comparisons with other state-of-the-art relaying schemes are provided and performance evaluation in terms of throughput, power minimization and switching rate, show the benefits of the proposed hybrid scheme.

  • 39. Nomikos, N.
    et al.
    Vouyioukas, D.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, I.
    Skoutas, D. N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Capacity improvement through buffer-aided successive opportunistic relaying2013In: 2013 3rd International Conference on Wireless Communications, Vehicular Technology, Information Theory and Aerospace and Electronic Systems, VITAE 2013 - Co-located with Global Wireless Summit 2013, IEEE , 2013, p. 6617051-Conference paper (Refereed)
    Abstract [en]

    Abstract-In this work, we propose a buffer-aided successive opportunistic relaying scheme that aims to improve the average capacity of the network when inter-relay interference arises between relays that are selected for transmission and reception. In order to exploit the benefits of bufTering at the relays, we propose a relay-pair selection policy that decouples the receiving relay at the previous time slot from being the transmitting relay at the next slot. Furthermore, we impose an interference cancellation threshold allowing the relay that is selected for reception, to decode and subtract the inter-relay interference. The proposed relaying scheme selects the relaying pair that maximizes the average capacity of the relay network. The performance of the proposed scheme is evaluated via simulation and comparisons with other state-of-the-art half and full-duplex relay selection schemes, in terms of outage probability, average capacity and average delay. The results reveal the need for a tradeoff between improving the outage on the cost of reduced capacity and increased delay, and vice versa. Finally, conclusions are drawn and future directions are discussed, including the need for a hybrid scheme incorporating both half and full-duplex characteristics.

  • 40. Nomikos, Nikolaos
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Krikidis, Ioannis
    Skoutas, Dimitrios N.
    Vouyioukas, Demosthenes
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    A Buffer-Aided Successive Opportunistic Relay Selection Scheme With Power Adaptation and Inter-Relay Interference Cancellation for Cooperative Diversity Systems2015In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 63, no 5, p. 1623-1634Article in journal (Refereed)
    Abstract [en]

    In this paper, we present a relay selection scheme which combines the spectral efficiency of successive opportunistic relaying with the robustness of single-link relay selection. More specifically, we propose a scheme that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The new relay selection policy is analyzed in terms of outage probability and diversity by modeling the evolution of relay buffers as a Markov Chain. We construct the state transition matrix of the Markov Chain and obtain its stationary distribution, which in turn, yields the outage probability. The proposed scheme outperforms relevant state-of-the-art relay selection schemes in terms of throughput, diversity, energy efficiency and average delay, as demonstrated via representative numerical examples.

  • 41.
    Nomikos, Nikos
    et al.
    University of Aegean.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Krikidis, Ioannis
    University of Cyprus.
    Skoutas, Dimitrios
    University of Aegean.
    Vouyioukas, Demosthenis
    University of Aegean.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Buffer-aided successive opportunistic relaying with inter-relay interference cancellation2013In: 2013 IEEE 24th International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC), IEEE , 2013, p. 1316-1320Conference paper (Refereed)
    Abstract [en]

    In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward relays characterized by the half-duplex constraint. At each time-slot the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively. When the source and a relay transmit simultaneously, inter-relay interference is introduced at the receiving relay. In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of inter-relay interference through either interference cancellation or mitigation. More specifically, we propose the min - power opportunistic relaying protocol that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The min - power relay-pair selection scheme, apart from minimizing the energy expenditure, also provides better throughput and lower outage probability than existing works in the literature. The performance of the proposed scheme is demonstrated via illustrative examples and simulations in terms of outage probability and average throughput.

  • 42.
    Nomikos, Nikos
    et al.
    University of Aegean.
    Vouyioukas, Demosthenis
    University of Aegean.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, Ioannis
    University of Cyprus, Electrical and Computer Engineering Department.
    Skoutas, Dimitrios
    University of Aegean.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Joint relay-pair selection for buffer-aided successive opportunistic relaying2014In: European transactions on telecommunications, ISSN 1124-318X, E-ISSN 2161-3915, Vol. 25, no 8, p. 823-834Article in journal (Refereed)
    Abstract [en]

    In this work, we present a buffer-aided successive opportunistic relaying scheme that aims at improving the average capacity of the network when inter-relay interference arises between relays that are selected for simultaneous transmission and reception. We propose a relay selection policy that, by exploiting the benefits of buffering at the relays, decouples the receiving relay at the previous time slot to be the transmitting relay at the next slot. Furthermore, we impose an interference cancellation threshold allowing the relay that is selected for reception to decode and subtract the inter-relay interference. The proposed relaying scheme selects the relaying pair that maximises the average capacity of the relay network. Its performance is evaluated through simulations and comparisons with other state-of-the-art half-duplex and full-duplex relay selection schemes, in terms of outage probability, average capacity and average delay. The results reveal that a trade-off has to be made between improving the outage at the cost of reduced capacity and increased delay and vice versa. Finally, conclusions are drawn and future directions are discussed, including the need for a hybrid scheme incorporating both half-duplex and full-duplex characteristics.

  • 43.
    Shames, Iman
    et al.
    University of Melbourne, Australia .
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Hadjicostis, Christoforos N.
    University of Cyprus.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Distributed network size estimation and average degree estimation and control in networks isomorphic to directed graphs2012In: 2012 50th Annual Allerton Conference On Communication, Control, And Computing (Allerton), 2012, p. 1885-1892Conference paper (Refereed)
    Abstract [en]

    Many properties of interest in graph structures are based on the nodes' average degree (i.e., the average number of edges incident to/from each node). In this work, we present asynchronous distributed algorithms, based on ratio consensus, that can be used to accurately estimate the number of nodes in a multi-component system whose communication topology is described by a directed graph. In addition, we describe an asynchronous distributed algorithm that allows each node to introduce or terminate links in order to reach a target average degree in the network. Such an approach can be useful in many realistic scenarios; for example, for the introduction and removal of renewable energy resources in a power network, while maintaining an average degree that fulfils some structural and dynamical properties and/or optimises some performance indicators of the network. The effectiveness of the proposed algorithms is demonstrated via illustrative examples.

  • 44. Tzortzis, I.
    et al.
    Charalambous, C. D.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Hadjicostis, C. N.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Approximation of Markov processes by lower dimensional processes2014In: Proceedings of the IEEE Conference on Decision and Control, IEEE conference proceedings, 2014, no February, p. 4441-4446Conference paper (Refereed)
    Abstract [en]

    In this paper, we investigate the problem of aggregating a given finite-state Markov process by another process with fewer states. The aggregation utilizes total variation distance as a measure of discriminating the Markov process by the aggregate process, and aims to maximize the entropy of the aggregate process invariant probability, subject to a fidelity described by the total variation distance ball. An iterative algorithm is presented to compute the invariant distribution of the aggregate process, as a function of the invariant distribution of the Markov process. It turns out that the approximation method via aggregation leads to an optimal aggregate process which is a hidden Markov process, and the optimal solution exhibits a water-filling behavior. Finally, the algorithm is applied to specific examples to illustrate the methodology and properties of the approximations.

  • 45. Tzortzis, Ioannis
    et al.
    Charalambous, Charalambos D.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Dynamic programming subject to total variation distance ambiguity2015In: SIAM Journal of Control and Optimization, ISSN 0363-0129, E-ISSN 1095-7138, Vol. 53, no 4, p. 2040-2075Article in journal (Refereed)
    Abstract [en]

    The aim of this paper is to address optimality of stochastic control strategies via dynamic programming subject to total variation distance ambiguity on the conditional distribution of the controlled process. We formulate the stochastic control problem using minimax theory, in which the control minimizes the payoff while the conditional distribution, from the total variation distance set, maximizes it. First, we investigate the maximization of a linear functional on the space of probability measures on abstract spaces, among those probability measures which are within a total variation distance from a nominal probability measure, and then we give the maximizing probability measure in closed form. Second, we utilize the solution of the maximization to solve minimax stochastic control with deterministic control strategies, under a Markovian and a non-Markovian assumption, on the conditional distributions of the controlled process. The results of this part include (1) minimax optimization subject to total variation distance ambiguity constraint; (2) new dynamic programming recursions, which involve the oscillator seminorm of the value function, in addition to the standard terms; and (3) a new infinite horizon discounted dynamic programming equation, the associated contractive property, and a new policy iteration algorithm. Finally, we provide illustrative examples for both the finite and infinite horizon cases. For the infinite horizon case, we invoke the new policy iteration algorithm to compute the optimal strategies.

  • 46. Vassiliou, A.
    et al.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Krikidis, I.
    Klerides, E.
    Towards distributed transmission scheduling for wireless ad hoc networks2012In: 2012 8th International Wireless Communications And Mobile Computing Conference (IWCMC), IEEE , 2012, p. 1209-1214Conference paper (Refereed)
    Abstract [en]

    In this paper we study distributed transmission scheduling via power control in wireless ad hoc networks with multiple channels. The target for each node is to manage to be admitted into a channel from the available channels in the network. The aim of this work is twofold: (a) to determine how a wireless node, based on its limited information, will decide which channel to access and (b), to propose a distributed algorithm for each wireless node with which once the channel is chosen a decision is made whether to stay in the channel or not. Here, we propose an algorithm that, if adopted by all the nodes in the network, it converges to a solution that admits most of the wireless nodes in the network, based on limited information only. Simulations in MATLAB justify the good performance of the algorithm.

  • 47.
    Zappavinga, Annalisa
    et al.
    Politecnico di Milano, Italy.
    Charalambous, Themistoklis
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Knorn, Florian
    Hamilton Institute, NUI Maynooth, Ireland.
    Unconditional stability of the Foschini–Miljanic algorithm2012In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 48, no 1, p. 219-224Article in journal (Refereed)
    Abstract [en]

    In this note we prove the unconditional stability of the Foschini-Miljanic algorithm. Our results show that the Foschini-Miljanic algorithm is unconditionally stable (convergent) even in the presence of bounded time-varying communication delays, and in the presence of topology changes. The implication of our results may be important for the design of Code Division Multiple Access (CDMA) based wireless networks.

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