Fotini-Niovi Pavlidou

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.

An overview of the IEEE 802.15.4a Standard

This article presents the IEEE 802.15.4a standard, the first international standard that specifies a wireless physical layer to enable precision ranging. As an amendment to the popular IEEE 802.15.4-2006, the IEEE 802.15.4a-2007 standard introduces new options for the physical layer in order to support higher data rates, extended range, improved robustness against interference, and mobility, while enabling new applications based on distance information of the devices in a low-rate wireless personal area network.

Two-dimensional traffic models for cellular mobile systems

Two-dimensional traffic models for cellular mobile systems are formulated and the main performance measures are evaluated. System analysis in its general form is rather complex but a solution is always attained in closed form or by numerical techniques. >

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.

Modeling the Noise on the OFDM Power-Line Communications System

Power-line communications (PLC) have gained a lot of scientific interest over the past years. In this paper, a practical noise model is proposed that best describes the noise conditions on an orthogonal frequency-division multiplexing PLC system. The noise present on a power-line system is divided in five categories which are grouped into two classes: 1) the generalized background and 2) the impulsive noise. In this paper, all of the components comprising the noise are precisely depicted on a computer simulation system. The statistical properties regarding all component parameters are taken into account and used in our model. By this way, the real conditions on a PLC channel can be portrayed in the most precise way. This model is tested on a PLC channel and its performance in terms of bit-error rate versus the E b/N 0 value is obtained. For reasons of completeness, we examine how two of the models components affect the systems performance by altering their vital parameters. In order to accomplish this, we take various values for these parameters and we check their influence on the system. Furthermore, we apply a popular noise model, such as Middletons noise model and we compare the performance obtained by both noise models.

The Role of High-Altitude Platforms (HAPs) in the Global Wireless Connectivity

Since 1990s, the investigations of aerospace communication segment have not only been concerned with satellites, but increasingly with lower altitude repeaters flying in the stratosphere. They are the so-called high-altitude platforms (HAPs) with important advantages with respect to satellites in terms of reduced cost of implementation, deployment, and launch. However, HAPs are characterized by a reduced coverage, as compared with satellites. Nevertheless, in recent literature, HAPs are not regarded as competitors of the satellite technology. On the contrary, the emphasis is on the effective and seamless integration among heterogeneous aerospace segments (GEO, LEO, and HAP) and aerospace segments with terrestrial wireless networks in order to globally extend the broadband wireless connectivity. This paper is focused on the role of HAPs in providing global connectivity in future communication systems and services. Potentialities, enabling technologies, and challenges are presented from the perspective of the integrated terrestrial/HAPs/satellite communications infrastructure.

Innovative chip waveforms in microcellular DS/CDMA packet mobile radio

The impact of innovative chip waveforms on the performance of a direct sequence code-division multiple-access (DS/CDMA) packet mobile radio network is investigated, assuming either Rayleigh or Nakagami-m frequency selective channels and a differential phase shift keying (DPSK) receiver with selection diversity. A constant improvement of the bit-error rate (BER) and the throughput is presented throughout the study.

A game theoretical approach to clustering of ad-hoc and sensor networks

Game theory has been used for decades in fields of science such as economics and biology, but recently it was used to model routing and packet forwarding in wireless ad-hoc and sensor networks. However, the clustering problem, related to self-organization of nodes into large groups, has not been studied under this framework. In this work our objective is to provide a game theoretical modeling of clustering for ad-hoc and sensor networks. The analysis is based on a non-cooperative game approach where each sensor behaves selfishly in order to conserve its energy and thus maximize its lifespan. We prove the Nash Equilibria of the game for pure and mixed strategies, the expected payoffs and the price of anarchy corresponding to these equilibria. Then, we use this analysis to formulate a clustering mechanism (which we called Clustered Routing for Selfish Sensors—CROSS), that can be applied to sensor networks in practice. Comparing this mechanism to a popular clustering technique, we show via simulations that CROSS achieves a performance similar to that of a very popular clustering algorithm.

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.