Angelos Michalas

Dual-Hop Communication Over a Satellite Relay and Shadowed Rician Channels

A dual-hop amplify-and-forward (AF) relaying scheme over shadowed Rician fading channels is investigated. Specifically, the source and destination nodes are equipped with N and M antennas, respectively, whereas the relay is equipped with a single antenna. Communication via satellite relaying represents a direct application of the considered infrastructure. To this end, we study the scenario when the source and the destination are terrestrial nodes, whereas the end-to-end communication is established through an intermediate AF relay node, which is a satellite. To fully exploit the spatial diversity provided by multiple antennas, maximum ratio transmission and maximum ratio combining are implemented at the source and the destination, respectively. First, a new closed-form expression for the probability density function (pdf) of the sum of independent and identically distributed (i.i.d.) squared shadowed Rician random variables is derived by assuming integer distribution parameters. Capitalizing on the latter pdf, new closed-form results for the cumulative distribution function (cdf) and the moment function of the end-to-end signal-to-noise ratio (SNR) are obtained. Particularly, the proposed unified analysis includes the channel-state-information (CSI)-assisted and the fixed-gain AF relaying protocols. New expressions for important performance measures, namely, the outage probability, the average symbol error probability (ASEP), and the ergodic capacity of the end-to-end SNR, are presented for both AF schemes. Moreover, some useful engineering insights are manifested, such as simplified asymptotic outage performance results, the diversity order, and the impact on the number of antennas at the source and the destination.

Proportional delay differentiation provision by bandwidth adaptation of class‐based queue scheduling

Supporting quality of service (QoS) over the Internet is a very important issue and many mechanisms have already been devised or are under way towards achieving this goal. One of the most important approaches is the so-called Differentiated Services (DiffServ) architecture, which provides a scalable mechanism for QoS support in a TCP/IP network. The main concept underlying DiffServ is the aggregation of traffic flows at an ingress (or egress) point of a network and the marking of the IP packets of each traffic flow according to several classification criteria. Diffserv is classified under two taxonomies: the absolute and the relative. In absolute DiffServ architecture, an admission control scheme is utilized to provide QoS as absolute bounds of specific QoS parameters. The relative DiffServ model offers also QoS guarantees per class but in reference to the guarantees given to the other classes defined. In this paper, relative proportional delay differentiation is achieved based on class-based queueing (CBQ) scheduler. Specifically, the service rates allocated to the classes of a CBQ scheduler are frequently adjusted in order to obtain relative delay spacing among them. The model presented can also be exploited in order to meet absolute delay constraints in conjunction with relative delay differentiation provision. Simulation experiments verify that our model can attain relative as well as absolute delay differentiation provided that the preconditions posed are satisfied. Copyright

A control-based algorithm for rate adaption in MPEG-DASH

Although MPEG-Dynamic Adaptation Streaming over HTTP (MPEG-DASH) eliminates data packet loss, based on the rate adaptation strategy adopted by the user and the network conditions, still playback interruptions may not be avoided. In this paper an algorithm for rate adaption in MPEG-DASH is proposed that employs fuzzy logic to estimate the resolution of the next video segment that each client should obtain from the server. In this way, the buffering time at each client is kept above a target buffering time and thus buffer overflows and unnecessary bit rate fluctuations are avoided. Simulation results showed that the proposed rate adaptation strategy has a beneficial effect on the quality each user perceives.

Adaptive Task Scheduling in Grid Computing Environments

In this study the task scheduling problem in Grid computing environments has been addressed. To resolve the problem a set of Grid Services are defined and implemented conforming to the OGSA standards. The proposed scheduling architecture is semantically enhanced and provides the most appropriate assignment of tasks to computing resources, given the current load conditions of each computing resource and the network status. The Ant Colony Optimization algorithm (ACO) was used to effectively assign tasks to computing resources. First experimental results indicate that the proposed framework produces good results in comparison to other well known schemes considered (e.g. round robin).

Quality of Service Management in IP Networks Using Mobile Agent Technology

Telecommunication network infrastructures become more and more distributed and heterogeneous, with various network technologies integrated (e.g. ATM, IP, ADSL), and several types of equipment coexisting, typically from different vendors. At the same time, services are becoming more and more demanding in terms of quality of service. In this context, efficient network management is of paramount importance for network operators. The telecommunication business does not any more depend on the type and cost of the services but mostly on their quality and how quickly they can be introduced. In this context, management platforms should be flexible and intelligent enough to dynamically configure, monitor and eventually reconfigure the network. On one hand, these platforms should be able to inform the clients about the quality of the requested services and on the other hand, network administrators must be able to manage their network resources in order to efficiently provide quality of service. Service Level Agreements (SLAs) between network operators / service providers and clients should be honored in a manner absolutely transparent to the users. This paper describes the network management architecture followed by the European IST (Information Society Technology) Project MANTRIP (MANagement, Testing and Reconfiguration of IP based networks using Mobile Software Agents) [1]. This architecture is based on both standard and tailored APIs. The prototype developed during the MANTRIP project is described in detail. Its main objective is to manage the Quality of service in IP Networks, with Mobile Agent Technology (MAT) being used to implement the specified APIs in the QoS management system and IP Differentiated Services (DiffServ) being used as a test case.

Enhanced fast handovers for PMIPv6 in vehicular environments

In modern wireless networking technologies, the need to support the nodes mobility is growing rapidly. This need is even greater in vehicular environments, in which the nodes are characterized by high mobility. In this paper, we study the basic protocols for mobility support in vehicular environments, such as Fast Handovers for Proxy Mobile IPv6 (PMIPv6) and Transient Binding for PMIPv6, and analyze the handover delay and their performance. Moreover, we present vehicular environment scenarios in which the current protocols fail to provide seamless handovers for the mobile nodes, due to long delays and large percentages of packet loss. Additionally, we propose an enhanced handover scheme for Vehicular Ad Hoc Networks (VANETs), in which data follow the optimal path to the destination and is characterized by a smooth transition of the mobile node to the target access network. Finally, by evaluating the performance of the proposed scheme a significant decrease of the total handover delay compared with the one of reactive and predictive handovers of FPMIP, as well as, in false prediction scenarios of the target access network is observed.

On Effectively Determining the Downlink-to-Uplink Sub-frame Width Ratio for Mobile WiMAX Networks Using Spline Extrapolation

One of the most valuable design issues of the Worldwide Interoperability for Microwave Access (WiMAX) standard is its flexibility. It allows multiple downlink-to-uplink duration definitions concerning the width of the corresponding sub-frames, thus, the telecommunication companies and businesses are able to define the downlink-to-uplink width from 3:1 to 1:1 respectively to meet the traffic demands of the connected subscribers. However, the decision on defining the most appropriate ratio is not yet solved, since current efforts on developing scheduling and mapping schemes consider it as fixed. In this work, a novel adaptive mapping approach -- based on cubic spline extrapolation - is proposed in order to dynamically determine the most appropriate downlink-to-uplink width ratio in accordance to the downlink and uplink traffic demands. The method proposed is evaluated through realistic simulation scenarios, whereas its performance is compared with static approaches maintaining a fixed ratio. The results indicate that the approach suggested succeeds noticeable improvements in terms of network service capabilities and bandwidth utilization.

QoS-aware scheduling in LTE-A networks with SDN control

The 3GPP Long Term Evolution Advanced (LTE-A) standard specifies a set of pioneer features such as relay nodes and carrier aggregation. At the same time, the Software Defined Networks (SDN) have become an emerging technology which provides centralized control and programmability to modern networks. In the current communication environment, cloud computing could combine the advantages of both technologies in order to create a novel cloud assisted Software Defined LTE-A architecture with relay nodes. Moreover, due to the increased requirements of modern services, the optimal resource allocation is a necessity. In such a context, this paper describes a QoS aware cross carrier scheduler for downlink flows, aiming at the optimization of system resources allocation. The proposed scheduler is evaluated against the PF, MLWDF, EXP/PF, EXP RULE, LOG RULE, FLS and FLSA schedulers in a cloud assisted Software Defined LTE-A topology with relay nodes. Simulation results show that the proposed scheduler improves the real time services performance while at the same time maintains an acceptable performance for best effort flows.

Cooperative Relaying in Underlay Cognitive Systems with TAS/MRC, Spatial Correlation and Hardware Impairments

The performance of an underlay cognitive (secondary) dual-hop relaying system with multiple antennas and hardware impairments at each transceiver is investigated. In particular, the outage probability of the end-to-end (e2e) communication is derived in closed-form, when transmit antenna selection with maximum ratio combining (TAS/MRC) is established in each hop. The practical scenario of spatial correlation among the antennas at the received signal is also considered. Simplified asymptotic outage expressions reveal some new useful engineering insights; namely the diversity and array order of the system, the balance on the number of transmit/receive antennas, and the impact of hardware impairments to the e2e communication.

A downlink scheduler supporting real time services in LTE cellular networks

The wide spread of real-time services in wireless networks demands scheduling mechanisms supporting strict Quality of Service (QoS) requirements. Nevertheless, the specifications of the LTE standard for mobile connectivity defined by the 3rd Generation Partnership Project (3GPP) does not impose any specific scheduler for the proper allocation of resources to services. Therefore, several LTE schedulers have been proposed in the literature meeting the QoS requirements of modern services. In this paper a QoS aware scheduler for the LTE downlink is proposed namely the FLS-Advanced (FLSA) aiming at prioritizing real-time traffic. The proposed scheduler has been built on three distinct levels assigning the available radio resources to services according to their requirements. Based on simulation results, the FLSA outperforms in terms of packet loss ratio, attainable throughput and fairness the performance of existing schedulers including PF, MLWDF, EXP/PF, FLS, EXP RULE and LOG RULE.