Research Interests:
Smart antennas increase capacity of wireless systems by allowing channel reuse by several users in the same cell through space division multiple access (SDMA). Orthogonal Frequency Division Multiplexing (OFDM) creates additional... more
Smart antennas increase capacity of wireless systems by allowing channel reuse by several users in the same cell through space division multiple access (SDMA). Orthogonal Frequency Division Multiplexing (OFDM) creates additional challenges in beam-forming when the beams that can be formed are limited due to transceiver unit limitations. We investigate the impact of smart antennas on channel allocation and propose meaningful heuristics to form beams and assign subcarriers and transceivers to users. Numerical ...
Abstract The smart power grid harnesses information and communication technologies to enhance reliability and enforce sensible use of energy through effective management of demand load. We envision a scenario with real-time communication... more
Abstract The smart power grid harnesses information and communication technologies to enhance reliability and enforce sensible use of energy through effective management of demand load. We envision a scenario with real-time communication between the grid operator and the consumers. The operator controller receives consumer power demand requests with different power requirements, durations, and deadlines by which they are to be activated. The objective of the operator is to devise a power demand task scheduling ...
Research Interests:
In the last decade testbeds have been set-up to evaluate network protocols and algorithms under realistic settings. In order to draw solid conclusions about the corresponding experimental results, it is important for the experimenter to... more
In the last decade testbeds have been set-up to evaluate network protocols and algorithms under realistic settings. In order to draw solid conclusions about the corresponding experimental results, it is important for the experimenter to have a detailed view of the existing channel conditions. Moreover, especially in the context of non-RF-isolated wireless testbeds, where external interference severely impacts the resulting performance, the requirement of experimenters for accurate channel monitoring becomes a prerequisite. ...
Abstract In the last decade testbeds have been set-up to evaluate network protocols and algorithms under realistic settings. Moreover, wireless sensors have lately been integrated into testbeds in order to simulate application scenarios... more
Abstract In the last decade testbeds have been set-up to evaluate network protocols and algorithms under realistic settings. Moreover, wireless sensors have lately been integrated into testbeds in order to simulate application scenarios for lowpower sensor networks, such as environment monitoring. Due to the fact that such scenarios are usually designed to run for a relatively long duration, long-term experiments are rarely executed, in order to avoid blocking of testbed facilities. Toward, this direction, we propose the NITOS Sensors Toolkit ...
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Abstract Efficient association of a station with the appropriate access point has always been a challenging problem. The standard approach of considering only the Received Signal Strength, has recently been substituted by more efficient... more
Abstract Efficient association of a station with the appropriate access point has always been a challenging problem. The standard approach of considering only the Received Signal Strength, has recently been substituted by more efficient schemes that consider channel conditions, cell population etc. However, in spite of the large variety of approaches, several factors that determine to a large extent user throughput after association with an access point have been overlooked. In this work, we propose innovative metrics on which ...
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Abstract The use of smart antennas in extending coverage range and capacity of wireless networks dictates the employment of novel media access control protocols, with which the base station (BS) or access point (AP) provides access to... more
Abstract The use of smart antennas in extending coverage range and capacity of wireless networks dictates the employment of novel media access control protocols, with which the base station (BS) or access point (AP) provides access to users by learning their locations. We consider the class of protocols that employ beam forming and use contention-based or contention-free polling methods to locate users residing in or out of coverage range of the AP. Such protocols allow rapid media access and can be embedded in existing MAC ...
Research Interests:
Abstract Wireless broadband access is an appealing solution to the projected trend towards reliable and easily deployable high-speed connections. In order to enhance system capacity and tolerate volatility of the wireless medium,... more
Abstract Wireless broadband access is an appealing solution to the projected trend towards reliable and easily deployable high-speed connections. In order to enhance system capacity and tolerate volatility of the wireless medium, sophisticated adaptation techniques are required. In this paper, we consider the problem of efficient resource allocation with adaptive modulation techniques in a multi-carrier wireless cellular system. We identify the inherent complexity of the problem and propose a heuristic algorithm for carrier frequency ...
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Abstract Wireless broadband access is becoming increasingly popular in the telecommunications market due to the projected demand for high data-rate connections. Given the inherent volatility of the wireless channel, the accurate... more
Abstract Wireless broadband access is becoming increasingly popular in the telecommunications market due to the projected demand for high data-rate connections. Given the inherent volatility of the wireless channel, the accurate estimation of channel conditions and the adoption of sophisticated adaptation techniques is required, so that transmission parameters are selected based on link quality, and user throughput is maximized. We consider a simple wireless link monitoring method, which is based on ...
Research Interests:
Research Interests:
A fundamental unresolved problem in wireless networks is that of distinguishing packet errors that are caused by deteriorated link conditions and noise, from errors that occur due to packet collisions. In this paper, we develop advanced... more
A fundamental unresolved problem in wireless networks is that of distinguishing packet errors that are caused by deteriorated link conditions and noise, from errors that occur due to packet collisions. In this paper, we develop advanced algorithms based on Cyclic Redundancy Check (CRC)[10] that solve this problem. Specifically, our innovation is that we form multiple CRCs, each of which is responsible for a different segment in a packet. The CRCs are appended after each segment. In this way, we can essentially visualize the ...
ABSTRACT This paper explores the problem of virtual machine (VM) allocation in a network of cloud server facilities which are deployed in different geographical areas. Each cloud server facility is connected to the conventional power grid... more
ABSTRACT This paper explores the problem of virtual machine (VM) allocation in a network of cloud server facilities which are deployed in different geographical areas. Each cloud server facility is connected to the conventional power grid network and in addition it is supported by an attached renewable energy source (RES). We address the problem of energy-efficient task allocation in the system in the presence of a time-varying grid energy price and the unpredictability and time variation of provisioned power by the RES. The objective is to reduce the total cost of power consumption for the operator. The key idea is to match the VM load with the RES provisioned power. Each request for a task to be executed in the cloud is associated with a VM request with certain resource requirements and a deadline by which it needs to be completed. The cloud provider has to create a VM with the resource requirements of the request and to execute the VM before the deadline. We propose an online algorithm with given look-ahead horizon, in which the grid power prices and patterns of output power of the RESs are known a priori and we compare it with a greedy online algorithm. Numerical results on real traces of cloud traffic and renewable source generation patterns are encouraging in terms of the performance of our techniques and motivate further research on the topic.
The gradually growing demand for experimentation of protocols designed for wireless networks in real environments has resulted in the development of experimental network facilities (testbeds). Most currently deployed testbeds have been... more
The gradually growing demand for experimentation of protocols designed for wireless networks in real environments has resulted in the development of experimental network facilities (testbeds). Most currently deployed testbeds have been designed so as to offer services to experimenters that lie within the testbed's premises, thus limiting the accessibility to external users. The requirement for multi-user access of network resources has led several large-scale testbeds to provide remote access services to certified experimenters. ...
Research Interests:
Research Interests:
Several OMF-based testbeds are using Chassis Manager (CM) cards for autonomously controlling and monitoring the status of nodes. CM cards are typically microcontroller boards and can be connected to different kinds of modules, including... more
Several OMF-based testbeds are using Chassis Manager (CM) cards for autonomously controlling and monitoring the status of nodes. CM cards are typically microcontroller boards and can be connected to different kinds of modules, including sensor modules. The NITOS testbed, which has recently adopted the use of CM cards, features various types of sensors connected to them. Measurements can be easily obtained through dedicated web services running on the microcontroller, through a network interface. This demo describes the ...
ABSTRACT Cognitive radio systems have gathered a lot of research interest during the last decade. Accuracy of spectrum sensing and efficiency of free spectrum utilization are considered as the primary objectives in this emerging... more
ABSTRACT Cognitive radio systems have gathered a lot of research interest during the last decade. Accuracy of spectrum sensing and efficiency of free spectrum utilization are considered as the primary objectives in this emerging technology, which promises a boost in wireless network performance, through exploitation of underutilized licensed frequency bands. As the focus of researchers is usually on these two major challenges, other aspects have been in part underestimated. In this work, we consider two factors that are rather important for evaluation of cognitive platforms, namely sensing delay and energy efficiency. The first is related to the latency induced by the spectrum sensing process and its impact on sensing efficiency, which is tightly connected to both the QoS performance of secondary users and the protection of primary users. On the other hand, energy consumption is considered as a crucial issue in all types of wireless communications, due to restricted battery autonomy of mobile devices, as well as for moving towards "greener" solutions in telecommunications. Therefore, it is important to extend existing testbed experimentation tools and develop new ones, in order to equip cognitive testbeds with such advanced monitoring capabilities. In this work, we present a monitoring procedure that has been directly integrated in the experimentation tools of the CREW testbed federation and demonstrate how it aids in the online evaluation of four different cognitive platforms in terms of the aforementioned metrics.
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Abstract QoS provisioning to users in the presence of volatility of the wireless channel is the most challenging issue in wireless system design. We consider the problem of scheduling constant bit rate (CBR) traffic packets over the... more
Abstract QoS provisioning to users in the presence of volatility of the wireless channel is the most challenging issue in wireless system design. We consider the problem of scheduling constant bit rate (CBR) traffic packets over the wireless channel, subject to packet delivery deadline constraints. We cast the problem as a Markov decision process and derive the optimal scheduling policy, in the sense of minimizing long-term packet loss due to deadline expirations. Performance bounds and design guidelines for general scheduling ...
Research Interests:
Research Interests:
ABSTRACT Queue-based cross-layer optimization algorithms have recently been a subject of intensive research in wireless networks. Their purpose is to guarantee stable operation and to achieve some form of fairness among users, whenever... more
ABSTRACT Queue-based cross-layer optimization algorithms have recently been a subject of intensive research in wireless networks. Their purpose is to guarantee stable operation and to achieve some form of fairness among users, whenever the traffic demand exceeds network capacity. Despite the plethora of work in this field, the scenario where one or more nodes declare false queue backlog values in order to gain throughput advantage remains unexplored. In this paper we examine this type of selfish misbehavior, concentrating on a specific class of algorithms, the so-called quadratic Lyapunov-function-based algorithms (QLA). In particular, the effect of backlog misreporting on a single-hop access network with contending stations is evaluated through simulations. A simple framework for the detection of misbehaving nodes is proposed, under the assumption that the access-point is aware of the utility functions of the stations. The detection approach exploits the fact that under QLA the throughput of a node must be approximately equal to an "expected" value, derived from the reported queue backlogs.
Research Interests:
With the proliferation of access points and termi- nals, wireless access misbehavior emerges as important means of protocol misuse, whereby network entities aim at obtaining higher channel share than the one in legitimate operation. We... more
With the proliferation of access points and termi- nals, wireless access misbehavior emerges as important means of protocol misuse, whereby network entities aim at obtaining higher channel share than the one in legitimate operation. We consider the instantiation of misbehavior in the back-off mechanism in IEEE 802.11. First, we study the competition between two malicious entities that affect each other.
Research Interests:
We study the impact of physical layer (PHY) transmit rate control on energy efficient estimation in wireless sensor networks. A sensor network collects measurements and transmits them to a Fusion Center (FC) with controllable PHY... more
We study the impact of physical layer (PHY) transmit rate control on energy efficient estimation in wireless sensor networks. A sensor network collects measurements and transmits them to a Fusion Center (FC) with controllable PHY transmission rates. The FC performs estimation of an unknown parameter process based on sensor measurements, and it needs to adhere to an estimation error constraint.
Research Interests:
In a wireless ad-hoc network environment, it is important for a mobile host to be able to broadcast control information within a team of nodes in a bandwidth-efficient and timely manner. Although the classical flooding strategy is the... more
In a wireless ad-hoc network environment, it is important for a mobile host to be able to broadcast control information within a team of nodes in a bandwidth-efficient and timely manner. Although the classical flooding strategy is the only reliable broadcast method for high rates of topology change, it leads to unnecessary bandwidth consumption for low rates of topology change.
Research Interests:
Abstract The impetus of the present study is to describe a downlink beamforming method that increase spectrum efficiency and significantly reduces implementation complexity and power consumption compare to beamforming technique at each... more
Abstract The impetus of the present study is to describe a downlink beamforming method that increase spectrum efficiency and significantly reduces implementation complexity and power consumption compare to beamforming technique at each sub-carrier, proposed in the ongoing IEEE 802.11 n standardization. In our scheme, common transmission weight vectors are used at a set of users in all sub-carriers. Our strategy consists of simultaneously designing downlink beamformers to multiple co-channel sets of users under the constraint ...
Research Interests:
Research Interests:
ABSTRACT Advances in statistical physics relating to our understanding of large-scale complex systems have recently been successfully applied in the context of communication networks. Statistical mechanics methods can be used to decompose... more
ABSTRACT Advances in statistical physics relating to our understanding of large-scale complex systems have recently been successfully applied in the context of communication networks. Statistical mechanics methods can be used to decompose global system behavior into simple local interactions. Thus, large-scale problems can be solved or approximated in a distributed manner with iterative lightweight local messaging. This survey discusses how statistical physics methodology can provide efficient solutions to hard network problems that are intractable by classical methods. We highlight three typical examples in the realm of networking and communications. In each case we show how a fundamental idea of statistical physics helps solve the problem in an efficient manner. In particular, we discuss how to perform multicast scheduling with message passing methods, how to improve coding using the crystallization process, and how to compute optimal routing by representing routes as interacting polymers.