George D. Papadopoulos

A Comparative Review on Wildfire Simulators

Phenomena in ecosystems such as forest fires, oil spills, tornados, etc., are complex processes both in time and space. Understanding their dynamics in order to predict their future states is a challenging procedure involving the propagation of several events concerning, e.g., the transfer of energy or material. A common solution used to study such phenomena is the utilization of discrete event models and simulators. Among the phenomena studied by researchers so far, of particular interest and importance, is the forecasting of forest fire propagation since life on earth is greatly dependent on healthy forests. This paper is focused on the investigation of existing simulator models applicable in forecasting forest fire propagation. Twenty-three simulators were found in the literature and are presented here. The comparison discussion concludes with the FARSITE simulator model which is the one that stands out from the rest, and for this reason, it is investigated in depth and evaluated in a test environment.

Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications

Orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC-CDMA) systems are comparatively evaluated for power line communications (PLC) in a frequency-selective fading environment with additive coloured Gaussian noise which is used to model the actual in-home power line channel. OFDM serves as a benchmark in order to measure the performance of various MC-CDMA systems, since multicarrier modulation systems are considered the best candidate for this kind of channel. Both single-user and multi-user cases are taken into account, making use of the appropriate combiner schemes to take full advantage of each case. System efficiency is enhanced by the application of different coding techniques, a fact which shows that powerful coding can make the difference under such a hostile medium. The impact of block interleaving is investigated, while the simulation examines how different modulation schemes fair under the imposed channel conditions as well. The performance of the system is assessed by the commonly used bit error rate vs signal-to-noise ratio diagrams and there is also a comparison regarding throughput efficiency among all the tested systems. As stated in Section 4, a promising PLC application is attained. Copyright

Performance bounds of space-time block coding in rician and log-normal fading channels

Analytical expressions concerning the capacity and bit error rate (BER) of multiple-input multiple-output systems with space-time block coding (STBC) are derived. Two fading environments are examined, log-normal and Rician channels. A tight closed-form upper bound is presented for the BER of systems operating in log-normal fading environments in addition to an upper bound for the capacity of this type of systems. The latter bound applies to systems that operate under Rician fading as well. The analytical results were validated against ample numerical simulations for three STBC schemes and three phase-shift-keying modulations. The proposed bounds proved to be a tractable way to evaluate the system performance when no closed-form expression for the probability density function or the moment generating function is known.

Hybrid ARQ Combined With Distributed Packet Space-Time Block Coding for Multicast Power-Line Communications

In this paper, hybrid ARQ (HARQ) is realized together with distributed packet space-time block coding in order to provide reliable multicast communications over inhome and inbuilding power-line networks. Two HARQ algorithms are examined and their performance is demonstrated via simulations. The proposed retransmission scheme utilizes information from previous transmissions, since the HARQ algorithms update their parameters according to the experienced transmission conditions. Results show that the combination of a physical-layer technique, such as space-time coding, with data-link layer HARQ provides significant gain regarding throughput and mean delay.

Enhancement of transmission security for OFDM based systems

High level of security is essential in wireless communications multimedia applications. The last few years there has been an increase in research interest in the potential of the radio channels physical properties to provide communications security. These research efforts investigate fading, interference, and path diversity to develop security techniques for implementation in wireless systems. In this paper, we propose an enhancement to existing physical layer security schemes, taking advantage of the characteristics of the OFDM technique. An OFDM symbol includes the pilot sub-carriers, essential for the pilot channel estimation process performed at the legitimate receiver. In this work we propose the positions of the sub-carriers to change on every OFDM symbol following a probability distribution known only to the legitimate transmitter and legitimate receiver. An eavesdropper receiver, does not have access to the information of the pilot sub-carriers positions so, it performs blind channel estimation. The theoretical analysis is based on the information theoretic problem formulation and is confirmed by simulations. The performance metrics used are the secrecy capacity, the outage probability, the channel mean square error and the bit error rate. The proposed scheme is very simple and robust, strengthening security in multimedia applications.

Enhancing transmission security for multimedia applications

High levels of security are essential in wireless communications multimedia applications. In this paper, we propose a new approach to performance enhancement of existing physical layer security schemes, taking advantage of the characteristics of the OFDM technique, while exploiting the channel characteristics during the channel estimation process. OFDM is carrying on its subcarriers the pilot symbols, essential for the pilot channel estimation process performed at the legitimate receiver. Conventionally, the position of the subcarriers carrying the pilot symbols is fixed and known to all receivers. In this paper the positions of the subcarriers carrying the pilot symbols change randomly from one OFDM symbol to the next according to a selected distribution around the fixed predefined positions within the OFDM symbols. When a mobile phone communicates to an eNodeB for the first time, the eNodeB and the mobile phone use the imprinting mechanism to establish trust between them. In this phase the eNodeB and the mobile phone share information (e.g. the time-stamp of initial communication) that will allow identification between them in order to set-up the mechanism of non-fixed pilot subcarrier positions using encryption. Thus, an eavesdropper, not capable to decrypt the information related to the non-fixed pilot subcarrier positions is forced to perform blind channel estimation. Our approach is focused on the channel characteristics in order to provide security mechanisms for transmission of multimedia data. The theoretical analysis is confirmed by simulations, and the metrics used are the channel mean square error, the bit-error rate and the secrecy capacity. As stated in the results Section, the proposed scheme is very simple, and robust, strengthening privacy in multimedia applications.

Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications: Research Articles

This paper proposes a non-matrix inversion based algorithm to implement decorrelating detection (DD), namely quasi-decorrelating detector (QDD), which uses truncated matrix series expansion to overcome the problems associated with the matrix inversion in DD, such as noise enhancement, computational complexity and matrix singularity, etc. Two alternative QDD implementation schemes are presented in this paper; one is to use multi-stage feedforward filters and the other is to use an nth order single matrix filter (neither of which involves matrix inversion). In addition to significantly reduced computational complexity if compared with DD, the QDD algorithm offers a unique flexibility to trade among MAI suppression, near-far resistance and noise enhancement depending on varying system set-ups. The obtained results show that the QDD outperforms DD in either AWGN or multipath channel if a proper number of feed-forward stages can be used. We will also study the impact of correlation statistics of spreading codes on the QDDs performance with the help of a performance-determining factor derived in the paper, which offers a code-selection guideline for the optimal performance of QDD algorithm. Copyright