Stylianos Karapantazis

Broadband communications via high-altitude platforms: a survey

This article is a survey on communication aspects of High Altitude Platforms (HAPs), namely airships or aircraft positioned in the stratosphere between 17 and 22 km. HAPs can be considered as a novel solution for providing telecommunications services. This survey begins with an introduction to HAPs, that is, some historical information and advantages of HAPs compared to terrestrial and satellite networks, followed by information about suitable airships and aircraft, frequency bands allocated to HAPs, possible architectures, and some points on the system structure. We continue with the studies that have been carried out on channel modeling and interference, antennas, transmission and coding techniques. We also refer to access and resource allocation techniques that have been performed so far. Finally, the survey concludes with the types of applications that HAPs are suitable for, in addition to some related projects.

VoIP: A comprehensive survey on a promising technology

The Internet has burgeoned into a worldwide information superhighway during the past few years, giving rise to a host of new applications and services. Among them, Voice over IP (VoIP) is the most prominent one. Beginning more as a frolic among computer enthusiasts, VoIP has set off a feeding frenzy in both the industrial and scientific communities and has the potential to radically change telephone communications. In this article, we survey all these aspects that have the greatest impact on the quality of voice communications over IP networks. The survey begins with the merits and demerits of VoIP, followed by the Quality of Service (QoS) requirements that voice imposes and a description of test methods for the assessment of speech quality. We then proceed with a delineation of the issues related to the conversion of analog voice to packets, namely we spell out the details of the most well-known voice codecs, while light is also thrown on voice activity detection and voice packetization. Header compression schemes receive intense scrutiny as well. We also provide an overview of the signaling protocols that are tailored to the needs of VoIP, and we continue with the comparison of the call admission schemes that are geared towards the QoS constraints of VoIP. The pivotal issue of security is then discussed, pointing out potential threats as well as approaches for tackling them. Finally, the survey concludes with a discussion on the feasibility of providing VoIP over challenging satellite links.

The role of high altitude platforms in beyond 3G networks

High altitude platforms represent a new alternative to terrestrial and satellite communications systems, and have gained considerable interest in the past few years due to some of their outstanding features. Of the multitude of services HAPs can provide, in this article we, focus on the potential role of HAPs in beyond 3G networks. First, a short introduction to HAPs is made. Then we discuss different hybrid system architectures with emphasis on the merits of HAPs and provide a potential mapping of services to the components of a terrestrial-HAP-satellite integrated system. We also examine critical issues that stem from the use of HAPs. Finally, the maximum uplink capacity is evaluated for a multiservice W-CDMA HAP network.

Distributed on-demand routing for LEO satellite systems

Notwithstanding the limited commercial success of the first narrowband low earth orbit (LEO) satellite systems, the interest of the scientific community in this type of systems has been revived on the basis of the current trend toward the migration to all IP-based services. LEO systems can play a pivotal role in providing services to areas where there is no substantial terrestrial infrastructure. Above all, LEO satellite systems can be used as backbone networks to interconnect autonomous systems worldwide. Such an approach provides flexibility in managing the resulting integrated network infrastructure and supporting innovative applications. In this context, routing data from the source all the way to the destination constitutes a daunting challenge. In this paper, a location-assisted on-demand routing (LAOR) protocol is proposed and evaluated. The proposed protocol introduces for the first time in satellite systems the concept of on-demand routing. However, its implementation is tailored to the requirements imposed by the characteristics of the topology of LEO satellite systems. The performance of the LAOR protocol is assessed for different link-cost metrics and compared to the one of centralized routing protocols proposed in the literature so far. Simulation studies further document and confirm the positive characteristics of the proposed protocol.

Satellite handover techniques for LEO networks

Low earth orbit satellite constellations could play an important role in future mobile communication networks due to their characteristics, such as global coverage and low propagation delays. However, because of the non-stationarity of the satellites, a call may be subjected to handovers, which can be cell or satellite handovers. Quite many techniques have been proposed in the literature dealing with the cell handover issue. In this paper, a satellite handover procedure is proposed, that investigates and exploits the partial satellite diversity (namely, the existing common coverage area between contiguous satellites) in order to provide an efficient handover strategy, based always on a tradeoff of blocking and forced termination probabilities for a fair treatment of new and handover calls. Three different criteria were examined for the selection of a satellite. Each one of them could be applied either to new or handover calls, therefore we investigated nine different service schemes. A simulation tool was implemented in order to compare the different service schemes and simulation results are presented at the end of the paper.

Multiservice on-demand routing in LEO satellite networks

In this paper, a distributed on-demand routing protocol for Low Earth Orbit (LEO) satellite systems, named multiservice on-demand routing (MOR), is proposed and evaluated. The proposed protocol adjusts the routing procedure to the QoS requirements of different traffic classes. The performance of the MOR protocol is compared to the unique proposal for traffic class dependent routing in the literature and the good characteristics of the proposed scheme are corroborated by ample simulation experiments, where significant gains in performance are witnessed.

A Detailed Study of Dynamic Manet On-demand Multipath Routing for Mobile Ad hoc Networks

Mobile Ad hoc Networks (MANETs) are networks with dynamic topology and limited resources, where routing is highly correlated to their performance. Most routing algorithms focus on the establishment of a single path between the source and the destination. However, multiple routes have been proven beneficial, when used either for load balancing or as backup routing paths. In this paper we propose a novel routing algorithm which is based on the Dynamic Manet On-demand (DYMO) routing protocol and evaluate its performance in detail. Simulation results show that multiple paths can reduce the delay while decrease the transmitted routing packets.

Impact of imperfect power control multiuser detection on the uplink of a WCDMA high altitude platform system

Within the broad field of communications, considerable interest has been drawn towards High Altitude Platforms (HAPs) the past few years owing to their appealing features. Among the multitude of services that HAP systems are able to provide, HAPs have the potential to deliver 3G communication services. In this letter, we examine the impact of both imperfect power control and multiuser detection (MUD) on the uplink of a WCDMA HAP system.

QoS handover management for multimedia LEO satellite networks

Low earth orbit (LEO) satellite systems gained considerable interest towards the end of the previous decade by virtue of some of the appealing features that are endowed with, such as low propagation delay and the ability to communicate with handheld terminals. However, after the limited commercial success of the first networks of this kind, future satellite networks are now conceived as complementary rather than competitive to terrestrial networks. In this paper, we focus on one of the most influential factors in system performance, that is, the handover of a call. First, we provide a succinct review of the handover strategies that have been proposed in the literature. Then we propose two different satellite handover techniques for broadband LEO satellite systems that capitalize upon the satellite diversity that a system may provide. The proposed schemes cater for multimedia traffic and are based on the queuing of handover requests. Moreover, a deallocation scheme is also proposed according to which capacity reservation requests are countermanded when the capacity that they strive to reserve is unlikely to be used. Simulation studies further document and confirm the positive characteristics of the proposed handover schemes.

Proposal of a Reliable Multicast Protocol in a HAP-Satellite Architecture

Terrestrial and satellite networks have dominated the telecommunications arena for years. In parallel with these two well-established technologies, a new alternative based on platforms located in the stratosphere, termed high altitude platforms (HAPs), has gained considerable interest in the past few years. In this paper, we consider an integrated HAP-satellite architecture and propose a reliable multicast protocol that capitalizes upon the HAP-layer to perform local retransmissions. Our protocol relies on a progressive packet-level forward error correction (FEC) scheme. The performance of this multicast protocol has been supported thorough the comparison with a theoretical bound in the literature.