Department of Electrical Engineering and Automation

Distributed Systems Group

Research

My research interests mainly focus on the design and analysis of (wireless) networked control systems that are stable, scalable and energy efficient. The study of such systems involves the interaction of dynamical systems, their communication and the integration of these concepts. As a result, my research is interdisciplinary combining theory and applications from control theory, network and distributed optimization.

The main projects I have been involved can be found below. In an attempt to separate them into different research strands (even though due to their multidisciplinary nature some of them are interconnected), the projects are divided as follows:

  1. Projects with a control and/or optimization theoretic background/flavor, in which communication aspects may appear. These include:
  2. Projects in the field of wireless networks and communications, even though control theoretic tools/approaches may have been used. These include:
  3. I have been also participating in research for some exciting applications, which combine tools and knowledge from control theory, optimization and communications. These include:


Distributed coordination and estimation

Distributed coordination under uncertainties. Classical approaches for asymptotic convergence to the global average in a distributed fashion typically assume timely and reliable exchange of information between neighboring components of a given multi-component system. These assumptions are not necessarily valid in practical settings due to varying delays that might affect transmissions at different times, as well as possible changes in the underlying interconnection topology (e.g., due to component mobility). In this project, we study ways to overcome these limitations. Application of such settings include: the provision of ancillary services in power networks, coordination of large-scale systems (such as the cloud computing paradigm).

Finite-time discrete-time coordination. Distributed algorithms are typically asymptotic, but finite-time algorithms are, in general, more desirable; besides the fact that they converge in finite-time, it is reported that closed-loop systems under finite-time control usually demonstrate better disturbance rejection properties; this can be important in applications where the averaging operation is a first step towards a control or regularization task. In this work, distributed methods are investigated with which various coordination problems can be implemented in a final number of steps.

Distributed weight-balancing in digraphs. Average consensus plays a key role in a number of applications and is closely associated with two classes of digraphs, weight-balanced (for continuous-time systems) and bistochastic (for discrete-time systems). In this work, we study distributed approache that solve the weight balancing problem and the bistochastic matrix formation problem for a distributed system whose components can exchange information via interconnection links that form an arbitrary, possibly directed, strongly connected communication topology. Convergence rates and asynchronous operation are also investigated.

MapReduce

Large-scale load balancing in cloud data centers. One of the fast emerging Cloud applications is big-data computing that handles massive- volumes of data. Modern applications in this domain employ the MapReduce paradigm to execute scalable and parallel processing of data, leveraging on the scalable infrastructure of Cloud data centers. For example, Google uses MapReduce to execute popular applications such as web search indexing, Google News and Google Maps. Facebook relies on the MapReduce paradigm to perform data warehousing operations. In this project, we address the problem of balancing the processing load of MapReduce tasks running on heterogeneous clusters, i.e., clusters composed of nodes with different capacities and update cycles.

We aim at designing fully decentralized algorithms, 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 usage and capacity.

Selected Publications
Journal papers:
[22] Themistoklis Charalambous, Michael G. Rabbat, Mikael Johansson and Christoforos N. Hadjicostis: "Distributed Finite-Time Computation of Digraph Parameters: Left-Eigenvector, Out-Degree and Spectrum", IEEE Transactions on Control of Network Systems, vol. 3, no. 2, pp. 137-148, June 2016.
Abstract | Full text | Bibtex
[20] Themistoklis Charalambous, Ye Yuan, Tao Yang, Wei Pan, Christoforos N. Hadjicostis and Mikael Johansson: "Distributed Finite-Time Average Consensus in Digraphs in the Presence of Time-Delays", IEEE Transactions on Control of Network Systems, vol. 2, no. 4, pp. 370-381, December 2015.
Abstract | Full text | Bibtex
[9] Apostolos Rikos, Themistoklis Charalambous and Christoforos N. Hadjicostis: "Distributed Weight Balancing over Digraphs", IEEE Transactions on Control of Network Systems, Volume 1 (2), pp.1-12, June 2014.
Abstract | Full text | Bibtex
[7] Christoforos N. Hadjicostis and Themistoklis Charalambous: "Average Consensus in the Presence of Delays in Directed Graph Topologies", IEEE Transactions on Automatic Control, Volume 59 (3), pp.763-768, March 2014.
Abstract | Full text | Bibtex

Conference papers:
[42] Themistoklis Charalambous, Christoforos N. Hadjicostis, Michael Rabbat and Mikael Johansson: "Totally asynchronous distributed estimation of eigenvector centrality in digraphs with application to the PageRank problem", in the IEEE Conference on Decision and Control (CDC), December 2016.
Abstract | Full text | Bibtex | Presentation
[33] Themistoklis Charalambous and Christoforos N. Hadjicostis: "Average Consensus in the Presence of Dynamically Changing Directed Graph Topologies and Time Delays", in the 53rd IEEE Conference on Decision and Control (CDC), December 2014.
Abstract | Full text | Bibtex | Presentation
[29] Themistoklis Charalambous, Christoforos N. Hadjicostis and Mikael Johansson: "Distributed Minimum-Time Weight Balancing over Digraphs", in the IEEE International Symposium on Communications, Control, and Signal Processing (ISCCSP), May 2014.
Abstract | Full text | Bibtex | Presentation
[26] Themistoklis Charalambous, Ye Yuan, Tao Yang, Wei Pan, Christoforos N. Hadjicostis and Mikael Johansson: "Decentralised Minimum-Time Average Consensus in Digraphs", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation
[22] Themistoklis Charalambous, Evangelia Kalyvianaki, Christoforos N. Hadjicostis and Mikael Johansson: "Distributed Offline Load Balancing in MapReduce Networks", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation
[17] Themistoklis Charalambous and Christoforos N. Hadjicostis: "Distributed Formation of Balanced and Bistochastic Weighted Digraphs in Multi-Agent Systems", in the 12th biannual European Control Conference (ECC), July 2013.
Abstract | Full text | Bibtex | Presentation
[15] Iman Shames, Themistoklis Charalambous, Christoforos N. Hadjicostis and Mikael Johansson: "Distributed Network Size Estimation and Average Degree Estimation and Control in Networks Isomorphic to Directed Graphs", in the 50th Allerton Conference on Communication, Control and Computing, October, 2012.
Abstract | Full text | Bibtex | Presentation
[9] Christoforos N. Hadjicostis and Themistoklis Charalambous: "Asynchronous Coordination of Distributed Energy Resources for the Provisioning of Ancillary Services", in the Allerton Conference, September 2011.
Abstract | Full text | Bibtex | Presentation


Insensitivity of cone invariant (e.g., positive) systems to time delays

A dynamical system is said to be positive if its states are non-negative for all time whenever the initial conditions are non-negative. Positive systems constitute a special case of cone-invariant systems. A plethora of real world physical systems involve variables that are positive by nature. Such variables can be the power levels in wireless transmitters, population levels, probabilities and concentrations of substances.

Models of physical systems are often derived under the assumption that the system evolution depends only on the current values of the state variables. However, in many cases, the system state is also affected by previous values of the states. For example, in distributed systems where exchange of information or materials is involved, delays are inevitable. For this reason, the study of stability and control of dynamical systems with delayed states is essential and of practical importance. In general, time delays limit the performance of closed-loop control systems and may even render an otherwise stable system unstable. However, an astonishing property of positive linear systems is that they are insensitive to certain classes of time delays in the following sense: a positive linear system with delays is asymptotically stable if the corresponding delay-free system is asymptotically stable. Hence, a natural question is if non-linear positive systems are insensitive also to time-varying delays.

Selected Publications
Journal papers:
[15] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Asymptotic Stability and Decay Rates of Homogeneous Positive Systems With Bounded and Unbounded Delays", SIAM Journal on Control and Optimization, Volume 52 (4), pp. 2037-2054, September 2014.
Abstract | Full text | Bibtex
[8] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Exponential Stability of Homogeneous Positive Systems of Degree One With Time-Varying Delays", IEEE Transactions on Automatic Control, Volume 59 (6), pp.1594-1599, June 2014.
Abstract | Full text | Bibtex
[6] Hamid Reza Feyzmahdavian, Mikael Johansson and Themistoklis Charalambous: "Contractive Interference Functions and Rates of Convergence of Distributed Power Control Laws", IEEE Transactions on Wireless Communications, Volume 11 (12), pp.4494-4502, December 2012.
Abstract | Full text | Bibtex

Conference papers:
[32] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Sub-Homogeneous Cooperative Systems are Insensitive to Bounded Time-Varying Delays", in the 21st International Symposium on Mathematical Theory of Networks and Systems (MTNS), July 2014.
Abstract | Full text | Bibtex | Presentation
[25] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Asymptotic Stability and Decay Rates of Positive Linear Systems with Unbounded Delays", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation
[24] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Asymptotic and Exponential Stability of a General Class of Continuous-Time Power Control Laws in Wireless Networks", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation
[18] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "On the Rate of Convergence of Continuous-Time Linear Positive Systems with Heterogeneous Time-Varying Delays", in the 12th biannual European Control Conference (ECC), July 2013.
Abstract | Full text | Bibtex | Presentation


Optimality conditions for distributed systems with decentralized information structures

When the system model consists of multiple decision makers and the acquisition of information and its processing are decentralized or shared among several locations, the decision makers' actions are based on different information. We aim at developing a framework addressing optimality conditions for distributed systems with decentralized information structures.

Decentralized information structures
Fig. An example of the architecture of a distributed system with decentralized information structure.

Selected Publications
Conference papers:
[31] Charalambos D. Charalambous, Themistoklis Charalambous and Christoforos N. Hadjicostis: "Team optimality conditions of differential decision systems with nonclasssical information structures", in the European Control Conference (ECC), June 2014.
Abstract | Full text | Bibtex | Presentation


Extremum problems with total variation distance metrics

In this project, we 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. 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.

Approximation of Markov processes by lower dimensional processes via total variation distance metrics. Finite-State Markov (FSM) processes are often employed to model physical phenomena in many diverse areas, such as machine learning, information theory (lossy compression), networked control and telecommunication systems, speech processing, systems biology, etc. In many of these applications the state-space of the Markov process is prohibitively large, to be used in analysis and simulations. One approach often pursue to overcome the large number of states is to approximate the Markov process by a lower dimensional Markov process, with respect to certain measures of discriminating or approximating the distribution of the high dimensional Markov process by a reduced one. The aim of this project is to approximate a finite-state Markov process by another process with fewer states, with respect to a certain pay-off subject to a fidelity criterion defined by the total variation distance metric.

Variable length lossless coding for variational distance class. In many applications the source distribution is unknown. In order to describe the uncertainty, total variation serves as a natural distance metric. In this project, we consider lossless source coding for a class of sources specified by the total variational distance ball centered at a fixed nominal probability distribution. The objective is to find a minimax average length source code, where the minimizers are the codeword lengths while the maximizers are the source distributions from the total variational distance ball.

Dynamic programming with total variation distance uncertainty. This work studies the optimality of stochastic control strategies via dynamic programming subject to total variational distance uncertainty on the conditional distribution of the controlled process.

Selected Publications
Journal papers:
[19] Ioannis Tzortzis, Charalambos D. Charalambous and Themistoklis Charalambous: "Dynamic Programming Subject to Total Variation Distance Ambiguity", SIAM Journal on Control and Optimization, vol. 53, no. 4, pp. 2040-2075, July 2015.
Abstract | Full text | Bibtex
[13] Charalambos D. Charalambous, Ioannis Tzortzis, Sergey Loyka and Themistoklis Charalambous: "Extremum Problems with Total Variation Distance and their Applications", IEEE Transactions on Automatic Control, Volume 59 (9), pp. 2353-2368, September 2014.
Abstract | Full text | Bibtex

Conference papers:
[34] Ioannis Tzortzis, Charalambos D. Charalambous, Themistoklis Charalambous, Christoforos N. Hadjicostis and Mikael Johansson: "Approximation of Markov Processes by Lower Dimensional Processes", in the 53rd IEEE Conference on Decision and Control (CDC), December 2014.
Abstract | Full text | Bibtex | Presentation
[23] Charalambos D. Charalambous, Ioannis Tzortzis, Sergey Loyka and Themistoklis Charalambous: "Extremum Problems with Total Variation Distance", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation


Resource allocation in wireless networks

Power control. Power control has played a fundamental role in wireless networks. It allows transmitters to adapt the power levels at which they send information to the corresponding receivers so as to meet some predefined quality of service (QoS) requirements (e.g. to sustain a given transmission rate) while minimizing the energy spent for the transmissions and thereby extending the battery life time of mobile devices. By selecting power levels just enough to meet the QoS requirements, transmitters also minimize the interference they create, which in turn allows more users to be simultaneously active in the network.

In this project, we develop and analyze classes of distributed power control algorithms for the next generation (5G) of wireless communications.

Transmission scheduling. In this project we consider a scheduling problem with power control in wireless networks. We have already considered the minimum latency transmission scheduling problem in wireless networks with power control under Signal-to-Interference-and-Noise Ration (SINR) constraints. Currently, we have been looking into distributed approaches for scheduling and contention control for accessing the wireless channels in such environments.

Transmission scheduling
Fig. Links are required to be allocated in slots such that simultaneous transmission is possible.

Admission control. 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, i.e., the solution converges to the global optimum. Nevertheless, the problem arises when the QoS requirements cannot be achieved for all the users in the network. The problem addressed in this paper is how a distributed algorithm should behave in these situations, where the network as a system diverges. Some distributed algorithms are restrained to a suboptimal solution that degrades as the number of users increases and finally the whole system collapses. Some others consider admission control, that is, they introduce specific rules to the algorithm so that current users have authorities on the admission of new users. In this way the system favors the older users in the network (taking advantage of the privilege and can potentially keep the channel occupied indefinitely) and have a considerable communication overhead.

The aim of this work is to study the interaction among wireless nodes and use power control to design a contention-based medium access method for the physical model that could stabilize the network around a steady state with fairness, service differentiation and efficiency as targets.

Selected Publications
Book Chapters:
[1] Themistoklis Charalambous: "Power control in Wireless Ad-Hoc Networks: Stability and Convergence under Uncertainties", Optimization, Simulations and Control, Springer Series in Optimization and Its Application (SOIA), vol. 76, pp. 143-174, 2013.
Abstract | Full text | Bibtex

Journal papers:
[16] Themistoklis Charalambous, Wolfram Wiesemann, Evelina Klerides, Angelos Vassiliou, Stavros Hadjitheophanous and Kyriakos M. Deliparaschos: "On the Minimum Latency Transmission Scheduling in Wireless Networks with Power Control under SINR Constraints", Wiley Transactions on Emerging Telecommunications Technologies, Volume 26 (3), pp. 367-379, March 2015.
Abstract | Full text | Bibtex
[12] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Stability and Performance of Continuous-Time Power Control in Wireless Networks", IEEE Transactions on Automatic Control, Volume 59 (8), pp. 2012-2023, August 2014.
Abstract | Full text | Bibtex
[6] Hamid Reza Feyzmahdavian, Mikael Johansson and Themistoklis Charalambous: "Contractive Interference Functions and Rates of Convergence of Distributed Power Control Laws", IEEE Transactions on Wireless Communications, Volume 11 (12), pp.4494-4502, December 2012.
Abstract | Full text | Bibtex
[2] Annalisa Zappavinga, Themistoklis Charalambous and Florian Knorn: "Unconditional stability of the Foschini Miljanic Algorithm", Automatica, Volume 48 (1), pp. 219-224, January 2012.
Abstract | Full text | Bibtex

Conference papers:
[28] Kyriakos Deliparaschos, Themistoklis Charalambous, Paul Christodoulides and Evelina Klerides: "Reduced power expenditure in the minimum latency transmission scheduling problem", in the IEEE International Symposium on Communications, Control, and Signal Processing (ISCCSP), May 2014.
Abstract | Full text | Bibtex | Presentation | Poster
[24] Hamid Reza Feyzmahdavian, Themistoklis Charalambous and Mikael Johansson: "Asymptotic and Exponential Stability of a General Class of Continuous-Time Power Control Laws in Wireless Networks", in the 52nd IEEE Conference on Decision and Control (CDC), December 2013.
Abstract | Full text | Bibtex | Presentation
[13] Angelos Vassiliou, Themistoklis Charalambous, Ioannis Krikidis and Evelina Klerides: "Towards Distributed Transmission Scheduling for Wireless Ad Hoc Networks", in The 8th International Wireless Communications and Mobile Computing Conference (IWCMC), 2012.
Abstract | Full text | Bibtex | Presentation
[12] Themistoklis Charalambous and Ioannis Krikidis: "Medium Access Control via Contention-Based Distributed Power Control", in The 8th International Wireless Communications and Mobile Computing Conference (IWCMC), 2012.
Abstract | Full text | Bibtex | Presentation
[11] Hamid Reza Feyzmahdavian, Mikael Johansson and Themistoklis Charalambous: "Contractive Interference Functions and Rates of Convergence of Distributed Power Control Laws", in The IEEE International Conference on Communications (ICC), 2012.
(Best paper award)
Abstract | Full text | Bibtex | Presentation
[6] Themistoklis Charalambous, Yassine Ariba : "On the Stability of a Power Control Algorithm for Wireless Networks in the presence of Time-Varying Delays", In The 10th European Control Conference (ECC), August, 2009.
Abstract | Full text | Bibtex | Presentation
[4] Evelina Klerides and Themistoklis Charalambous : "Transmission Scheduling in Wireless Networks with SINR Constraints", In The 5th International Conference on Networking and Services (ICNS), April, 2009.
Abstract | Full text | Bibtex | Presentation
[3] Themistoklis Charalambous, Ioannis Lestas and Glenn Vinnicombe : "On the Stability of the Foschini-Miljanic Algorithm with Time-Delays", In The 47th IEEE Conference on Decision and Control (CDC), December, 2008.
Abstract | Full text | Bibtex | Presentation

Technical reports:
[2] Themistoklis Charalambous, Evelina Klerides and Wolfram Wiesemann: "On the Transmission Scheduling of Wireless Networks under SINR Constraints", CUED/F-INFENG/TR.649, February 2010.
Abstract | Full text | Bibtex
[1] Themistoklis Charalambous: "A Lyapunov Krasovskii method for the stability of the Foschini-Miljanic algorithm under time-varying delays: An Independent of Delays approach", CUED/F-INFENG/TR.646, February 2010.
Abstract | Full text | Bibtex


Cooperative relaying in wireless communications
Cooperative relaying

In order to meet the admittedly growing demands, efforts are being made to develop new technologies for the upcoming generation networks. Cooperative relaying is an efficient technique to combat fading and path-loss effects in wireless systems by providing spatial diversity, since it offers alternative and independent transmission paths, thus increasing the diversity gain of the network as multi-path fading is mitigated while also reducing the outage probability. Standards development organizations are considering how to incorporate such techniques into new standards; one such initiative is the IEEE 802.16j standardization activity, which adds relay capabilities to IEEE 802.16 systems. For the 5G mobile communication systems, beam-division multiple access and relays with group cooperation are being developed.

In recent studies, the addition of buffering capability at the relays has been suggested as a way to further improve the diversity of the network. In this project, we develop novel relay selection policies that take advantage of buffering in order to decouple the receiving and transmitting relay, successive opportunistic relaying that considered interference cancellation techniques, as well as power adaptation and hybrid implementation for better performance.

Selected Publications
Journal papers:
[18] Nikolaos Nomikos, Themistoklis Charalambous, Ioannis Krikidis, Dimitrios N. Skoutas, Demosthenes Vouyioukas and Mikael Johansson: "A Buffer-aided Successive Opportunistic Relay Selection Scheme with Power Adaptation and Inter-Relay Interference Cancellation for Cooperative Diversity Systems", IEEE Transactions on Communications, vol.63, no.5, pp.1623-1634, May 2015.
Abstract | Full text | Bibtex
[17] Themistoklis Charalambous, Nikolaos Nomikos, Ioannis Krikidis, Demosthenis Vouyioukas and Mikael Johansson: "Modeling Buffer-Aided Relay Selection in Networks With Direct Transmission Capability", IEEE Communication Letters, Volume 19 (4), pp. 649-652, April 2015.
Abstract | Full text | Bibtex
[11] Nikolaos Nomikos, Demosthenis Vouyioukas, Themistoklis Charalambous, Ioannis Krikidis, Dimitrios Skoutas and Mikael Johansson: "Joint relay-pair selection for buffer-aided successive opportunistic relaying", Wiley Transactions on Emerging Telecommunications Technologies, Volume 25 (8), pp. 823-834, August 2014.
Abstract | Full text | Bibtex
[4] Ioannis Krikidis, Themistoklis Charalambous and John S. Thompson: "Buffer-Aided Relay Selection for Cooperative Diversity Systems without Delay Constraints", IEEE Transactions on Wireless Communications, Volume 11 (5), pp.1957-1967, May 2012.
Abstract | Full text | Bibtex

Conference papers:
[38] Dimitrios Poulimeneas, Themistoklis Charalambous, Nikolaos Nomikos, Ioannis Krikidis, Demosthenes Vouyioukas and Mikael Johansson: "Delay- and diversity-aware buffer-aided relay selection policies in cooperative networks", in the IEEE Wireless Communications and Networking Conference (WCNC), April 2016.
Abstract | Full text | Bibtex | Poster
[30] Themistoklis Charalambous, John Krikidis and Mikael Johansson: "Precoding Decision for Full-Duplex X-Relay Channel with Decode-and-Forward", in the IEEE International Communications and Mobile Computing Conference (IWCMC), August 2014 (invited).
Abstract | Full text | Bibtex | Presentation
[27] Nikolaos Nomikos, Themistoklis Charalambous, Ioannis Krikidis, Demosthenes Vouyioukas and Mikael Johansson: "Hybrid Cooperation through Full-Duplex Opportunistic Relaying and Max-Link Relay Selection with Transmit Power Adaptation", in the IEEE International Conference on Communications (ICC), June 2014.
Abstract | Full text | Bibtex | Presentation
[20] Nikolaos Nomikos, Themistoklis Charalambous, Ioannis Krikidis, Dimitrios Skoutas, Demosthenes Vouyioukas and Mikael Johansson: "Buffer-aided Successive Opportunistic Relaying with Inter-Relay Interference Cancellation", in the IEEE International Symbosium on Personal, Indoor and Mobile Radio Communications (PIMRC), September 2013.
Abstract | Full text | Bibtex | Presentation
[19] Nikolaos Nomikos, Demosthenes Vouyioukas, Themistoklis Charalambous, Ioannis Krikidis, Dimitrios Skoutas and Mikael Johansson: "Capacity Improvement through Buffer-Aided Successive Opportunistic Relaying", in the Global Wireless Summit (GWS), July 2013.
Abstract | Full text | Bibtex | Presentation
[10] Ioannis Krikidis, Themistoklis Charalambous and John Thompson: "Opportunistic Relay Selection for Cooperative Networks with Buffers", in The IEEE International Conference on Communications (ICC), 2012.
Abstract | Full text | Bibtex | Presentation


Energy harvesting in telecommunication networks

In traditional network architectures, the nodes are characterized by limited energy resources and efficient protocols are needed to maximize lifetime or energy savings. The use of Energy Harvesting (EH) nodes (relays) in cooperative networks is a promising and emerging technology in wireless systems, such as wireless sensor networks, addressing the problem of nodes' batteries getting drained in forwarding information to the destination. We investigate the effects of network-layer cooperation in wireless networks with nodes having energy-harvesting capabilities and study the interaction between data and energy queues. Smart exploitation of EH capability and analysis of realistic models are still open issues in the field. Currently, we construct a framework to investigate plethora of scenarios and models for EH, such as Radio Frequency EH.

Radio Frequency Energy Harvesting
Fig. An example where information and energy can be shared among wireless devices in a cellular network. A base station is usually connected to a power supply and hence it does not harvest energy; it only creates RF signals (along with data signals) to be harvested by other devices with RF EH capabilities.
Selected Publications
Journal papers:
[3] Ioannis Krikidis, Themistoklis Charalambous and John S. Thompson: "Stability Analysis and Power Optimization for Energy Harvesting Cooperative Networks", IEEE Signal Processing Letters, Volume 19 (1), pp. 20-23, January 2012.
Abstract | Full text | Bibtex


Intelligent Transportation Systems (ITS)

Decentralized traffic signal control. Traffic conditions in major cities has become an important issue, since congestion results in delays, carbon dioxide emissions, higher energy expenditure and accident risks. Control and coordination of traffic movements has been at the epicenter of intelligent transportation networks from different angles. One approach is to introduce traffic adaptive signaling at intersections, since a fixed-cycle control system might be an inevitable waste of precious green time/phase without incoming traffic demand.

Cooperative intersection collision avoidance. Intersections remain among the most accident-prone sybsystems in modern traffic. With the introduction of ICT technologies it is possible to increase situational awareness and issue warnings to drivers whenever needed.

Selected Publications


Resource management and scheduling in data centers and cloud infrastructures

Resource provisioning for server consolidation. Server virtualization allows multiple instances of virtual machines (VMs) to run independently on a single server, while live migration enables data center manager to move VMs to different servers while the VM is running, i.e., virtualization not only enables to run multiple isolated applications on a single shared resource, but also enables applications to span multiple physical resources more easily by providing elasticity in their resource usage. This technique, called server consolidation, is developed for efficient use of computer server resources in order to reduce the total number of servers required for a number of software implementations, since more resources are used than necessary to provide the functionality required. 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 towards adaptive controllers for unknown workload types. This work studies the design of feedback controllers that dynamically allocate CPU resources to virtual machines hosting server applications by continuously detecting and self-adapting to unforeseen workload changes.

Enterprise servers
Fig. An example of the operation of enterprise servers.

Efficient resource management and scheduling in data centers and cloud infrastructures is open research challenge that has to be addressed. Novel architectures, technologies and protocols must be developed to ensure efficiency of future cloud computing systems.

DISSP stream processing engine

Data Stream processing. Stream processing systems are becoming increasingly important to analyze real-time data generated by modern applications, such as, online social networks. Their main characteristic is to produce a continuous stream of fresh results as new data are being generated at real-time. Resource provisioning of stream processing systems is difficult due to uknown resource variations of workload data that vary over time. Despite the development of scalable stream processing systems, which aim to provision for workload variations, there still exist cases where such systems face transient resource shortages. During overload, there is a lack of resources to process all incoming data at real-time; data accummulate in memory and their processing latency grows uncontrollably compromising the freshness of stream processing results. In this project, we address the problem of controlling the stream processing latency in periods of resource overload, offering elasticity in time.

Selected Publications
Journal papers:
[10] Evangelia Kalyvianaki, Themistoklis Charalambous and Steven Hand: "Adaptive Resource Provisioning for Virtualized Servers Using Kalman Filters", ACM Transactions on Autonomous and Adaptive Systems, Volume 9 (2), pp. 10:1-10:35, July 2014.
Abstract | Full text | Bibtex
[1] Evangelia Kalyvianaki, Themistoklis Charalambous and Steven Hand: "Resource Provisioning for Multi-Tier Virtualized Server Applications", In the Computer Measurement Group Journal, 2010 (invited).
Abstract | Full text | Bibtex

Conference papers:
[40] Kyriakos Deliparaschos, Themistoklis Charalambous and Evangelia Kalyvianaki: "On the use of Fuzzy Logic Controllers to Comply with Virtualized Application Demands in the Cloud", in the European Control Conference (ECC), June 2016.
Abstract | Full text | Bibtex | Presentation
[14] Evangelia Kalyvianaki, Themistoklis Charalambous, Marco Fiscato and Peter Pietzuch: "Overload Management in Data Stream Processing Systems with Latency Guarantees", in the Feedback Computing Workshop of the 9th IEEE/ACM International Conference on Autonomic Computing and Communications (ICAC), August, 2012.
Abstract | Full text | Bibtex | Presentation
[7] Themistoklis Charalambous and Evangelia Kalyvianaki : "A Min-Max Framework for CPU Resource Provisioning in Virtualized Servers using H-infinity Filters", In The 49th IEEE Conference on Decision and Control (CDC), December, 2010.
Abstract | Full text | Bibtex | Presentation | Poster
[5] Evangelia Kalyvianaki, Themistoklis Charalambous and Steven Hand : "Self-Adaptive and Self-Configured CPU Resource Provisioning for Virtualized Servers Using Kalman Filters", In The 6th IEEE/ACM International Conference on Autonomic Computing and Communications (ICAC 2009), June, 2009.
(15.6% acceptance rate)
Abstract | Full text | Bibtex | Presentation
[2] Evangelia Kalyvianaki, Themistoklis Charalambous and Steven Hand : "Applying Kalman Filters to Dynamic Resource Provisioning of Virtualized Server Applications", In The Third International Workshop on Feedback Control Implementation and Design in Computing Systems and Networks (FeBid), June, 2008.
Abstract | Full text | Bibtex
[1] Evangelia Kalyvianaki and Themistoklis Charalambous: "On Dynamic Resource Provisioning for Consolidated Servers in Virtual Data Centers", In The Eighth International Workshop on Performability Modeling of Computer and Communication Systems (PMCCS-8), September, 2007.
Abstract | Full text | Bibtex