Placeholder image for staff profiles

Dr Dionysia Triantafyllopoulou


Senior Research Fellow
PhD, MSc, BSc
+44 (0)1483 683606
05 CII 02

Biography

Research

Research interests

Research projects

My publications

Publications

Triantafyllopoulou D, Passas N, Kaloxylos A (2009) Integration of Handover in a Cross-Layer Mechanism for Mobile Multimedia Systems, 2009 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-8 pp. 4973-4977 IEEE
Triantafyllopoulou D, Kollias K, Moessner K (2015) QoS and Energy Efficient Resource Allocation in Uplink SC-FDMA Systems, IEEE Transactions on Wireless Communications 14 (6)
In this paper we present and evaluate the performance of a resource allocation algorithm to enhance the Quality of Service (QoS) provision and energy efficiency of uplink Long Term Evolution (LTE) systems. The proposed algorithm considers the main constraints in uplink LTE resource allocation, i.e., the allocation of contiguous sets of resource blocks of the localized Single Carrier ? Frequency Division Multiple Access (SC-FDMA) physical layer to each user, and the imperfect knowledge of the users' uplink buffer status and packet waiting time. The optimal resource allocation is formulated as a discrete connected cake-cutting problem, where different agents are allocated consecutive subsequences of a sequence of indivisible items. This problem is NP-hard, therefore a suboptimal algorithm is introduced, which performs resource allocation using information on the estimated uplink packet delay, the average delay and data rate of past allocations, as well as the required uplink power per resource block. Based on simulation results, the proposed algorithm achieves significant performance improvement in terms of packet timeout rate, goodput, and fairness. Moreover, the effect of poor QoS provision on energy efficiency is demonstrated through the evaluation of the performance in terms of energy consumption per successfully received bit.
Triantafyllopoulou D, Guo T, Moessner K (2014) Energy-Efficient WLAN Offloading through Network Discovery Period Optimization, IEEE Transactions on Vehicular Technology 64 (6)
In this paper we present an analytical framework to improve the energy consumption of mobile nodes through traffic offloading via Wireless Local Area Networks (WLANs), taking into account the energy consumption for both data transmission and network discovery operations. More specifically, we formulate an optimization problem, according to which the network scanning period is optimized in order to minimize the total energy consumption and the energy consumption per transmitted bit in a scenario where a user moves with a constant, either pedestrian or vehicular, speed along a road covered by a long range cellular network and a number of randomly deployed WLANs. The performance of the system that employs the proposed framework, which uses information on the user speed as well as on the availability and the load level of neighboring networks and performs periodic network scanning with the optimal period, is compared against a sub-optimal system that does not take into consideration the user and network context information when determining the network scanning period. According to performance evaluation results, the use of the optimal network scanning period achieves significant improvement in terms of total energy consumption, energy efficiency and network detection delay.
Triantafyllopoulou D, Xenakis D, Tsolkas D, Passas N (2011) Medium Access Control Layer, In: Bader F, Zorba N (eds.), Advances on Processing for Multiple Carrier Schemes
Merentitis A, Triantafyllopoulou D (2010) Transmission power regulation in cooperative cognitive radio systems under uncertainties, IEEE 5th International Symposium on Wireless Pervasive Computing pp. 134-139
An algorithm for power control in cooperative Cognitive Radio networks is proposed. The algorithm utilizes Medium Access Control layer mechanisms for message exchange between the nodes, in order to achieve interference mitigation. A fuzzy logic reasoner is used to cope for uncertainties that appear in real life systems, caused from parameters such as non-ideal message exchange or high user mobility. The proposed algorithm is applied in Filter Bank Multicarrier as well as Orthogonal Frequency Division Multiplexing systems under various scenarios and its performance is evaluated through extensive simulations. Experimental results indicate improved behavior compared to previous schemes, especially in the case of uncertainties that cause underestimation of the interference levels.
Triantafyllopoulou D, Guo T, Moessner K (2012) Energy Efficient ANDSF-assisted Network Discovery for non-3GPP Access Networks, pp. 297-301 IEEE
The aim of this paper is to improve the energy efficiency during network discovery in heterogeneous networking environments. To this end, we propose a novel network discovery algorithm that exploits both user and network context information in order to efficiently adapt the network scanning period, thus avoiding unnecessary energy-consuming scanning or mis-detection of available networks that can be used as targets of handover. The performance of the proposed algorithm is compared against a system that performs network scanning in a periodic manner, without taking into consideration the user and network context information. According to simulation results, the system that employs the proposed network discovery algorithm achieves significant performance improvement in terms of energy consumption and network detection delay, with no loss in the network detection rate.
Triantafyllopoulou D, Merentitis A (2010) Resource allocation with MAC layer node cooperation in cognitive radio networks, International Journal of Digital Multimedia Broadcasting 2010 458636 pp. 1-11
An algorithm for cooperative Dynamic Spectrum Access in Cognitive Radio networks is presented. The proposed algorithm utilizes Medium Access Control layer mechanisms for message exchange between secondary nodes that operate in license exempt spectrum bands, in order to achieve interference mitigation. A fuzzy logic reasoner is utilized in order to take into account the effect of the coexistence of a large number of users in the interference as well as to cope for uncertainties in the message exchange, caused by the nodes' mobility and the large delays in the updating of the necessary information. The proposed algorithm is applied in Filter Bank Multicarrier, as well as Orthogonal Frequency Division Multiplexing systems, and its performance is evaluated through extensive simulations that cover a wide range of typical scenarios. Experimental results indicate improved behaviour compared to previous schemes, especially in the case of uncertainties that cause underestimation of the interference levels.
Triantafyllopoulou D, Passas N, Merakos L, Sagias NC, Mathiopouios RT (2009) E-CLEMA: A cross-layer design for improved quality of service in mobile WiMAX networks, WIRELESS COMMUNICATIONS & MOBILE COMPUTING 9 (9) pp. 1274-1286 JOHN WILEY & SONS INC
In this paper, an efficient cross-layer design that performs joint adaptation of the physical (PHY) and application layers of a mobile WiMAX network is proposed. The design takes into account channel state and performance information from the PHY and medium access control (MAC) layers, respectively. It uses a decision algorithm to evaluate this information, specify unfavorable conditions regarding low channel quality and increased congestion, and take measures by coordinating modulation order, transmission power, and media encoding rate, toward improved overall quality of service (QoS) offered to the user. Extensive simulation results show that the proposed design achieves considerably reduced packet loss and power consumption, combined with increased throughput as compared to a typical mobile WiMAX system.
Triantafyllopoulou D, Passas N, Salkintzis AK, Kaloxylos A (2007) A heuristic cross-layer mechanism for real-time traffic in IEEE 802.16 networks, 2007 IEEE 18TH INTERNATIONAL SYMPOSIUM ON PERSONAL, INDOOR AND MOBILE RADIO COMMUNICATIONS, VOLS 1-9 pp. 1887-1891 IEEE
Triantafyllopoulou D, Passas N, Kaloxylos A (2008) Cross-layer adaptation for real-time broadband multimedia over IEEE 802.16e networks, 2008 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTIMEDIA SYSTEMS AND BROADCASTING pp. 411-416 IEEE
Tehrani RH, Vahid S, Triantafyllopoulou D, Lee H, Moessner K (2016) Licensed Spectrum Sharing Schemes for Mobile Operators: A Survey and Outlook, IEEE Communications Surveys and Tutorials 18 (4)
The ongoing development of mobile communication networks to support a wide range of superfast broadband services has led to massive capacity demand. This problem is expected to be a significant concern during the deployment of the 5G wireless networks. The demand for additional spectrum to accommodate mobile services supporting higher data rates and having lower latency requirements, as well as the need to provide ubiquitous connectivity with the advent of the Internet of Things (IoT) sector, is likely to considerably exceed the supply, based on the current policy of exclusive spectrum allocation to mobile cellular systems. Hence, the imminent spectrum shortage has introduced a new impetus to identify practical solutions to make the most efficient use of the scarce licensed bands in a shared manner. Recently, the concept of dynamic spectrum sharing has received considerable attention from regulatory bodies and governments globally, as it could potentially open new opportunities for mobile operators to exploit spectrum bands whenever they are underutilised by their owners, subject to service level agreements. Although various sharing paradigms have been proposed and discussed, the impact and performance gains of different schemes can be scenario-specific and vary depending on the nature of the sharing parties, the level of sharing and spectrum access scheme. In this survey, we describe the main concepts of dynamic spectrum sharing, different sharing scenarios, as well as the major challenges associated with sharing licensed bands. Finally, we conclude this survey paper with open research challenges and suggest some future research directions.
Triantafyllopoulou D, Guo T, Moessner K Optimal Network Discovery Period for Energy-Efficient WLAN Offloading,
In this paper we present an analytical framework that aims to improve the energy efficiency of traffic offloading via Wireless Local Area Networks, taking into account the energy consumption for both data transmission and network discovery operations. More specifically, the network scanning period is optimized in order to minimize the energy consumption in a vehicular scenario where a user moves along a road covered by a long range cellular network and a number of randomly deployed Wireless Local Area Networks. The performance of the system that performs periodic network scanning with the optimal period is compared against a sub-optimal system that does not take into consideration the user and network context information when determining the network scanning period. According to performance evaluation results, the use of the optimal network scanning period achieves significant improvement in terms of energy consumption and network detection delay.
Triantafyllopoulou D, Passas N, Zervas E, Merakos L (2010) Optimized cross-layer adaptation for improved resource management in wireless networks, Proceedings of the 8th International Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks pp. 373-379
The theoretical analysis of a cross-layer mechanism for improving the quality of service of real-time applications in wireless networks is presented. The mechanism coordinates adaptations of the modulation order at the Physical layer and the media encoding mode at the Application layer, to improve packet loss rate, throughput and mean delay. With the use of Continuous Flow Modeling, the system is considered as a ?fluid? queue with inflow and outflow rates representing its traffic generation and service rates, respectively. Each data source is modeled as a Markov chain, from the steady-state of which the optimal adaptation thresholds of the cross-layer mechanism are derived. Performance evaluation results show that the optimized operation of the mechanism attains a significant performance improvement compared to both the sub-optimal mechanism, and a legacy system.
Triantafyllopoulou D, Passas N, Zervas E, Merakos L (2011) Analysis and optimization of a cross-layer adaptation mechanism for real-time applications in wireless networks, WIRELESS NETWORKS 17 (5) pp. 1339-1354 SPRINGER
The theoretical analysis of a cross-layer mechanism for improving the quality of service of real-time applications in wireless networks is presented. The mechanism coordinates adaptations of the modulation order at the Physical layer and the media encoding mode at the Application layer, to improve packet loss rate, throughput and mean delay. With the use of Continuous Flow Modeling, the system is considered as a ``fluid'' queue with inflow and outflow rates representing its traffic generation and service rates, respectively. Each data source is modeled as a Markov chain, from the steady-state of which the optimal adaptation thresholds of the cross-layer mechanism are derived. Extensive performance evaluation results show that the optimized operation of the mechanism attains a significant performance improvement compared to both the sub-optimal case, and a legacy system, which adjusts the modulation order and encoding mode separately and independently of each other.
Triantafyllopoulou D, Passas N, Kaloxylos A (2007) A Cross-Layer Optimization Mechanism for Multimedia Traffic over IEEE 802.16 Networks,
Triantafyllopoulou D, Kollias K, Moessner K (2015) QoS and energy efficient resource allocation in uplink SC-FDMA systems, IEEE Transactions on Wireless Communications 14 (6) pp. 3033-3045
© 2002-2012 IEEE.In this paper, we present and evaluate the performance of a resource allocation algorithm to enhance the Quality of Service (QoS) provision and energy efficiency of uplink Long Term Evolution (LTE) systems. The proposed algorithm considers the main constraints in uplink LTE resource allocation, i.e., the allocation of contiguous sets of resource blocks of the localized Single Carrier - Frequency Division Multiple Access (SC-FDMA) physical layer to each user, and the imperfect knowledge of the users' uplink buffer status and packet waiting time. The optimal resource allocation is formulated as a discrete connected cake-cutting problem, where different agents are allocated consecutive subsequences of a sequence of indivisible items. This problem is NP-hard, therefore a suboptimal algorithm is introduced, which performs resource allocation using information on the estimated uplink packet delay, the average delay and data rate of past allocations, as well as the required uplink power per resource block. Based on simulation results, the proposed algorithm achieves significant performance improvement in terms of packet timeout rate, goodput, and fairness. Moreover, the effect of poor QoS provision on energy efficiency is demonstrated through the evaluation of the performance in terms of energy consumption per successfully received bit.
Triantafyllopoulou D, Moessner K (2015) Uplink Resource Allocation for Real-Time Applications in SC-FDMA Networks,
In this paper we present and evaluate the performance of a resource allocation algorithm to enhance the Quality of Service (QoS) provision and energy efficiency of uplink Long Term Evolution (LTE) systems. The proposed algorithm considers the main constraints in uplink LTE resource allocation, i.e., the allocation of contiguous sets of resource blocks of the Single Carrier ? Frequency Division Multiple Access (SC-FDMA) physical layer to each user, and the imperfect knowledge of the users' uplink buffer status and packet waiting time. Resource allocation is performed using information on the estimated uplink packet delay, the average delay and data rate of past allocations, as well as the required uplink power per resource block. According to simulation results, the proposed algorithm achieves significant performance improvement in terms of packet loss rate, goodput, fairness, and energy efficiency. Moreover, the effect of poor QoS provision on energy efficiency is demonstrated through the evaluation of the performance in terms of energy consumption per successfully received bit.
In this paper we present an analytical framework that aims to improve the energy efficiency of traffic offloading via Wireless Local Area Networks, taking into account the energy consumption for both data transmission and network discovery operations. More specifically, the network scanning period is optimized in order to minimize the energy consumption in a vehicular scenario where a user moves along a road covered by a long range cellular network and a number of randomly deployed Wireless Local Area Networks. The performance of the system that performs periodic network scanning with the optimal period is compared against a sub-optimal system that does not take into consideration the user and network context information when determining the network scanning period. According to performance evaluation results, the use of the optimal network scanning period achieves significant improvement in terms of energy consumption and network detection delay.
Triantafyllopoulou D, Passas N, Lampropoulos G, Kaloxylos A (2008) Joint application and physical layer adaptation for improved performance in wireless networks, 2008 3RD INTERNATIONAL SYMPOSIUM ON WIRELESS PERVASIVE COMPUTING, VOLS 1-2 pp. 256-260 IEEE
Triantafyllopoulou D, Passas N, Salkintzis AK, Kaloxylos A (2007) A heuristic cross-layer mechanism for real-time traffic over IEEE 802.16 networks, International Journal of Network Management 17 (5) pp. 347-361
In this paper we propose and study a cross-layer mechanism that can improve real-time QoS provisioning over IEEE 802.16 metropolitan area networks. This mechanism utilizes information provided by the physical and MAC layers and using a heuristic algorithm it derives new operational parameters for the physical and application layers, which can improve the performance of real-time applications. The main idea is to coordinate the adaptive modulation capability of the physical layer and the multi-rate data-encoding capability of modern real-time applications in order to avoid inefficiencies caused by their independent operation. Simulations show that the proposed mechanism can assist 802.16 systems to better adapt to frequent channel and traffic changes, leading to considerably reduced packet loss rates, especially under heavy traffic conditions. Copyright © 2007 John Wiley & Sons, Ltd.
Triantafyllopoulou D, Moessner K, Alam M, Radwan A, Rodriguez J (2014) Cognitive Means Smart: Knowledge Saves Power, In: Radwan A, Rodriguez J (eds.), Energy Efficient Smart Phones for 5G Networks 6 Springer
The previous chapters mainly examined methods to save energy at the mobile handset, either by using short-range cooperation between mobile terminals, or by performing smart vertical handovers between heterogeneous radio access technologies. These techniques can be beneficial to mobile systems, but they have to be performed based on informed decisions; meaning that mobile devices need to be cognitive. Modern devices already collect significant amounts of information, but they have limited capability to exploit such context/information, and handover decisions are merely based on signal strength, or are network controlled and based on network load. In this chapter, we aim to go beyond the state-of-the-art by envisioning mobile terminals with the capability to make informed decisions based on a reservoir of context information made available through context providers; namely what is referred to as smart phones. We include a survey of the current state of the art for context extraction and management in context-aware systems; besides listing the current context extraction techniques and research efforts, we pinpoint the important properties of good context extraction techniques. Thereafter, we discuss how context information can be exploited in energy saving when performing network or node discovery mechanism, by instructing the mechanisms to scan for certain nodes/networks which are known to be in the vicinity. Finally, we discuss the range of context information that can be used to make informed decisions to save power.
Marques P, Rodriguez J, Delaere S, Gundlach M, Triantafyllopoulou D, Moessner K, Delahaye P, Lecroart B, Noguet D (2013) Shared use of radio spectrum in the EU: from research projects to standards, pp. 1249-1254
This paper addresses the current regulatory framework, main research activities and standardization efforts towards a shared use of radio spectrum in the European Union. A survey of emerging spectrum sharing applications and associated standardization paths is described. Remaining challenges for an effective participation of research projects in Cognitive Radio standards are discussed. A new coordination action funded by the European Commission that aims to boost standardization impact of research projects working of cognitive radio and spectrum sharing aspects is presented.
Triantafyllopoulou D, Lampropoulos G, Passas N (2011) Mobility Management for WLANs, In: Zorba N, Skianis C, Verikoukis C (eds.), Cross Layer Designs in WLAN Systems
This book aims to serve as a comprehensive technicalguide and reference material for practitioners, engineers, scientists and researchers providing them withthe state-of-the-art of research work and recentachievements in different aspects ...
Kliks A, Triantafyllopoulou D, De Nardis L, Holland O, Gavrilovska L, Bantouna A (2016) Cross-Layer Analysis in Cognitive Radio - Context Identification and Decision Making Aspects, IEEE Transactions on Cognitive Communications and Networking 1 (4) pp. 450-463
Research on context-aware communications has recently led to the introduction of features and algorithms relying on the presence of rich, accurate context information, and requiring however, the introduction of cross-layer information exchanges. Cognitive radio (CR), in particular, is expected to benefit from context awareness, as the cognitive engine (CE) relies on the availability of multiple information sources to operate efficiently. In this context, this work delivers a detailed, yet concise classification and description of the information exchanged in a CR network between the layers of a generic protocol stack, and between each layer and the CE. For each layer, the key services provided and delivered are presented, followed by a catalogue of exchanged parameters. The analysis, supported by a set of use cases providing a quantitative assessment of the impact of cross-layer information exchanges in a CR framework, is the basis for the discussion of key implementation challenges and the identification of the most promising partition of functions and tasks between layers and CE.
Triantafyllopoulou D, Passas N, Kaloxylos A, Merakos L (2009) Coordinated Handover Initiation and Cross-Layer Adaptation for Mobile Multimedia Systems, IEEE TRANSACTIONS ON MULTIMEDIA 11 (6) pp. 1131-1139 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
A cross-layer mechanism to improve the performance of real-time applications over IEEE 802.16e metropolitan area networks is presented. The proposed mechanism uses channel quality and service quality information from the physical and medium access control layers, respectively, to determine the most suitable burst profile, transmission power level and media encoding rate for a connection, or even initialize a handover execution. The main contribution of this mechanism is the integration of the handover initiation into the cross layer logic, aiming to improve the overall system performance. Extensive simulation results show that the proposed mechanism offers significant performance improvement in terms of packet loss rate, power consumption, throughput, and system capacity.
Marques P, Rodriguez J, Delaere S, Delahaye P, Lecroart B, Gundlach M, Triantafyllopoulou D, Moessner K, Noguet D (2013) Spectrum sharing in the EU and the path towards standardization,
This paper addresses the current regulatory framework, research activities and standardization efforts towards a shared use of radio spectrum in the European Union. Two main research streams are considered: The emerging concept of Licensed Shared Access that ensures a predictable Quality of Service for secondary users of spectrum and the opportunistic Device-to-Device communications that represent a recent and enormous socio-technological trend. The paper presents also research results that support the spectrum sharing standardization path in ETSI and 3GPP.
Triantafyllopoulou D, Moessner K, Kliks A, Bogucka H, Zalonis A, Dagres I, Dimitriou N, Polydoros A (2013) A Context-Aware Decision Making Framework for Cognitive and Coexisting Networking Environments, 19th European Wireless Conference VDE Verlag
One of the major challenges in cognitive and cooperative communication systems that support advanced coexistence technologies for radio resource usage optimization is efficient decision making. However, the high dynamicity of such communication environments, in conjunction with the existence of an increasing amount of diverse context information, result in decision making becoming a non-trivial issue. In this paper, we present a novel decision making framework for modern wireless communication environments. More specifically, the different steps of the decision making cycle and the role they play in the decision making process are presented, while the different fragments that constitute the notion of context, as well as their role and the way they interact with and influence the decision making steps are described in detail. Moreover, two case-studies, on interference management in heterogeneous networks and noise-robust and energy-efficient sensing, respectively, are presented, highlighting their compliance with the described decision making framework.
Triantafyllopoulou D, Passas N, Kaloxylos A (2009) Cross-Layer Design for Cooperative Wireless Communication, In: Zhang Y, Chen H-H, Guizani M (eds.), Cooperative Wireless Communications Auerbach Publications
Triantafyllopoulou D, Kliks A, Rakovic V, Gavrilovska L (2013) Existing Policy Frameworks: An Overview,
Spectrum utilization and resource allocation efficiency in Cognitive Radio networks can be significantly improved by the use of efficient spectrum etiquettes implemented by appropriate spectrum policy frameworks. In such a context, dynamic and flexible policies control the behavior of the terminals and the system as a whole and allow their efficient adaptation to the continuously varying environment conditions. Policies are expressed with the use of appropriate policy description languages that guarantee the efficient communication of the various network entities and the fulfillment of the policy frameworks? objectives. This paper is a survey of the area of policy frameworks in Cognitive Radio networks. More specifically, the characteristics, architecture and main principles of a Generic Policy Framework, are described. Moreover, an overview of the most notable policy frameworks for Cognitive Radio systems is provided.
Triantafyllopoulou Dionysia, Vahid Seiamak, Moessner Klaus (2018) Co-Primary Spectrum Sharing in Uplink
SC-FDMA Networks,
IEEE PIMRC 2018 Proceedings IEEE
In this paper we present a co-primary spectrum
sharing algorithm for the Quality of Service (QoS)
enhancement of uplink Single-Carrier Frequency Division
Multiple Access (SC-FDMA) systems. We consider the
limitations that are resulting from the fact that each
user can only be provided with only contiguous sets of
resource blocks (following the constraints of the localized
SC-FDMA physical layer), and the effect of the limited, or
even lack of, knowledge of each user?s buffer status and
packet delays in the uplink. The sharing of available resources
is based on the operator spectrum access priority,
an estimation of the packet delays in the uplink direction,
the average delay and data rate of earlier allocations, and
the power per resource block. Simulation results show
that the proposed algorithm considerably improves the
performance in terms of packet loss rate, goodput, and
fairness.
Koumaras Harilaos, Tsolkas Dimitris, Gardikis Georgios, Gomez Pedro Merino, Frascolla Valerio, Triantafyllopoulou Dionysia, Emmelmann Marc, Koumaras Vaios, Osma Maria L. Garcia, Munaretto Daniele, Atxutegi Eneko, de Puga Jara Suárez, Alay Ozgu, Brunstrom Anna, Bosneag Anne Marie Cristina (2018) 5GENESIS: The Genesis of a flexible 5G Facility, 2018 IEEE 23rd International Workshop on Computer Aided Modeling and Design of Communication Links and Networks (CAMAD) IEEE
This paper describes the pathway towards the
realisation of a 5G Facility that will allow the validation of the
major 5G Key Performance Indicators (KPIs). It reflects the
approach that the 5GENESIS consortium will adopt in this
direction. More precisely, it describes the key design principles
of such Facility as well as the targeted use cases for the KPIs
validation. The adopted approach for the Facility realisation
includes the design of a common implementation blueprint that
will be instantiated in five Platforms distributed across Europe.
To maximise the diversity and the efficiency of the Facility,
complementary performance objectives have been selected for
the Platforms, while specific characteristics from different
vertical industries have been allocated to each of them.