Retrodirective Terminal Antenna for Satellite and Terrestrial Mobile Communications

The potential for implementation of retrodirective arrays as antenna terminals for future integrated satellite and terrestrial mobile communications is discussed in this paper. Particularly, in the context of the Inmarsat L-band system we address the issues related to array antenna element capacity to produce high-quality circular polarized radiation pattern over large angles of arrival. We also discuss circuitry reduction methodologies and their effect on retrodirected beam characteristics. The possibility of circular polarization modulation of the re-transmit signal is also discussed.

Implementation and Evaluation of Measurement-Based Admission Control Schemes Within a Converged Networks QoS Management Framework

Policy-based network management (PBNM) paradigms provide an effective tool for end-to-end resource
management in converged next generation networks by enabling unified, adaptive and scalable solutions
that integrate and co-ordinate diverse resource management mechanisms associated with heterogeneous
access technologies. In our project, a PBNM framework for end-to-end QoS management in converged
networks is being developed. The framework consists of distributed functional entities managed within a
policy-based infrastructure to provide QoS and resource management in converged networks. Within any
QoS control framework, an effective admission control scheme is essential for maintaining the QoS of
flows present in the network. Measurement based admission control (MBAC) and parameter basedadmission control (PBAC) are two commonly used approaches. This paper presents the implementationand analysis of various measurement-based admission control schemes developed within a Java-based
prototype of our policy-based framework. The evaluation is made with real traffic flows on a Linux-based experimental testbed where the current prototype is deployed. Our results show that unlike with classic MBAC or PBAC only schemes, a hybrid approach that combines both methods can simultaneously result in improved admission control and network utilization efficiency

Received signal characteristics of outdoor body-to-body communications channels at 2.45 GHz

In this paper we conduct a number of experiments to assess the impact of typical human body movements on the signal characteristics of outdoor body-to-body communications channels using flexible patch antennas. A modified log-distance path loss model which accounts for body shadowing and signal fading due to small movements is used to model the measured data. For line of sight channels, in which both ends of the body-to-body link are stationary, the path loss exponent is close to that for free space, although the received signal is noticeably affected by involuntary or physiological-related movements of both persons. When one person moves to obstruct the direct signal path between nodes, attenuation by the person's body can be as great as 40 dB, with even greater variation observed due to fading. The effects of movements such as rotation, tilt, walking in line of sight and non-line of sight on body-to-body communications channels are also investigated in this study. © 2011 IEEE.

Multiuser Cognitive Relay Networks: Joint Impact of Direct and Relay Communications

In this paper, we propose a multiuser cognitive relay network, where multiple secondary sources communicate with a secondary destination through the assistance of a secondary relay in the presence of secondary direct links and multiple primary receivers. We consider the two relaying protocols of amplify-and-forward (AF) and decode-and-forward (DF), and take into account the availability of direct links from the secondary sources to the secondary destination. With this in mind, we propose an optimal solution for cognitive multiuser scheduling by selecting the optimal secondary source, which maximizes the received signal-to-noise ratio (SNR) at the secondary destination using maximal ratio combining. This is done by taking into account both the direct link and the relay link in the multiuser selection criterion. For both AF and DF relaying protocols, we first derive closed-form expressions for the outage probability and then provide the asymptotic outage probability, which determines the diversity behavior of the multiuser cognitive relay network. Finally, this paper is corroborated by representative numerical examples.

Editorial: Special Issue on Secure Physical Layer Communications

In wireless networks, the broadcast nature of the propagation medium makes the communication process vulnerable to malicious nodes (e.g. eavesdroppers) which are in the coverage area of the transmission. Thus, security issues play a vital role in wireless systems. Traditionally, information security has been addressed in the upper layers (e.g. the network layer) through the design of cryptographic protocols. Cryptography-based security aims to design a protocol such that it is computationally prohibitive for the eavesdropper to decode the information. The idea behind this approach relies on the limited computational power of the eavesdroppers. However, with advances in emerging hardware technologies, achieving secure communications relying on protocol-based mechanisms alone become insufficient. Owing to this fact, a new paradigm of secure communications has been shifted to implement the security at the physical layer. The key principle behind this strategy is to exploit the spatial-temporal characteristics of the wireless channel to guarantee secure data transmission without the need of cryptographic protocols.