Dr .Vasilis Christofilakis is currently a researcher in Siemens Enterprise Communications. He received the B.Sc. degree in Physics from the University of Ioannina, Greece, in 1997 and the M.Sc. degree in Electronics and Telecommunications from the same University in 2000. He has a PhD degree in the field of Software Defined Radios and Smart Antennas from the University of Ioannina. From 2000 to 2004 he was a doctoral scholar at the Mobile Communications Laboratory of the Institute of Informatics and Telecommunications of N.C.S.R. "Demokritos", Athens, Greece. He has published more than 30 papers in refereed journals and conferences. He participated in several research programmes funded by the European community. He is a reviewer of many scientific journals and a member of various academic societies and editorial boards. His research interests include mainly digital and IP communications, software radios, digital signal processing techniques, smart antennas and object-oriented approaches to telecommunication systems.
Abstract: The direction-of-transmission (DOT) resolution of a digital time-delay beamformer (DTDBF) is directly related to sampling intervals. In fact, the smaller the sampling intervals the better the DOT resolution and the larger the number of natural beams. In a DTDBF based on digital signal processor (DSP) the sampling intervals are primarily restricted by the I/O bandwidth of the DSP. In this paper a new and simple digital time-delay beamformer configuration for transmission is proposed, incorporating a programmable delay line (PDL) between DSP and digital-to-analog converter (DAC). The behavior of the resultant system is first analyzed and then compared to the traditional one through simulations. It is proven that the proposed system significantly increases the DOT resolution and overcomes DOT accuracy errors opposed to that of the conventional one. Furthermore, the influences of the additional time jitter effects due to PDL and DAC objects on the performance of the proposed system are investigated.
Abstract: The Forest Fire Risk Management Information System (FFRMIS) presented, constitutes an integrated innovation system consisting of telematics, meteorological, computing equipment and software, which aims to provide information to the appropriate parties of the Region of the Ionian Islands (RII) for the management and handling of the forest fires hazard. The innovation system has been installed and is an essential information and management tool (Control -Command - Coordinate - Communicate) for the Regional Fire Service Administration (RFSA) of the Ionian Islands, which is responsible for facing the problem of forest fires. The FFRMIS is based on state of the art technologies that utilize geographical-spatial data entered by the user, real-time meteorological and vehicle position data and in conjunction with intelligent techniques and algorithms it processes, manipulates and provides a series of useful information to its users for the best co-ordination of the fire tenders before and after the outburst of a forest fire. The FFRMIS consists of aGeographic Information System (GIS), which visualizes and manipulates spatial data of the area in concern, a Forecast Fire Spread Model, which forecasts the fire expansion on time using the BehavePlus simulation model and a Vehicle Track System, which allows the users of the system to know the exact real-time geographical position of every fire tender using satellite tracking technology (Global Positioning System - GPS).
Abstract: The performance of a cellular phone commercial helical antenna at 900~MHz band, both in free space and in the presence of a human head phantom was studied. Numerical simulation of the phone model for the latter case has been performed giving 3D radiation diagrams. The effect of the phantom head on radiation diagrams is presented. The relative amount of the EM power absorbed in the head was obtained for several distances of the phone. Measurements were carried out in a RF anechoic chamber using standard horn antennas and a calibrated measuring system. Absolute radiation patterns of the antenna gain were obtained in the three principal planes. Significant reduction of the absorbed power could be achieved just by moving the phone 1 cm away from the head.
Abstract: Software Radios (SWRs) and Antenna Arrays are the cutting-edge technologies for the future telecommunication systems. The combination of SWRs and Antenna Arrays is expected to release almost completely the hardware dependency of telecommunications systems. Although it may seem simple, in practice there are a variety of restrictions that prevent the achievement of objective. In this paper the time restrictions on a Software Radio receiver which combines the outputs of N individual antenna elements of a linear array are evaluated. Important results are derived concerning the processing speed demanding dynamically steering the radiation pattern (beamforming) of the antenna array
Abstract: A software defined radio (SDR) system is based on state-of-the art technologies covering topics such as: smart antennas, radio frequency (RF) down/up converters, analog to digital converters (ADCs) and digital to analog converters (DACs), digital signal processors (DSPs), modelling and system description languages. This paper is focused in the ADC and studies the various parameters affecting its performance which is critical for the implementation of a SDR receiver. A quantitive analysis is given, in terms of these parameters, followed by a presentation of the basic characteristics of ADCs of the latest technology.
Abstract: The electromagnetic energy absorption by mobile phone users is becoming a very hot point. During the last decade, many concerned organizations as well as individual scientists have taken this issue into consideration as the results of such studies and regulations are of public concern. In this paper we evaluate the reduction of the absorbed electromagnetic energy as a function of the antenna smartness. Conventional (actual) and adaptive (smart) antennas are used in our simulations for cellular communication systems running under 900 MHz. The RF power absorbed by the human head in the case of a conventional antenna is compared with the RF power absorbed by the human head in the case of a smart antenna. Finally a smart antenna model is proposed which satisfies the exposure limits for cellular phones and its easy to use is mobile devices.
Abstract: The problem of scattering of electromagnetic waves is one of the most essential problems of electromagnetic theory and occupies a significant interest in both theoretical and practical applications (telecommunications, electromagnetic interaction, biomedicine, remote sensing, etc.). The solution to such a problem varies from a simple analytical solution with serious limitations regarding the shape of the scatterer (spherical, infinitely long cylinder) to complex numerical computations of arbitrary shapes with the use of specialized software packages bound only by the limitations of the required âcomputingâ resources. Due to lack of analytical solutions to such problems, it is very important to evaluate the various numerical methods used for the computational solutions, especially in the case of non-perfect scatterers, with regard to the accuracy and the computational requirements of the method. In the present paper a comparison of the results of the computation of the Radar Cross Section (RCS) of a solid non perfect conducting body obtained by the Finite Elements Method (FEM) to indoors measurements implemented in the RF-Anechoic Chamber of NCSR DEMOKRITOS is examined, presented and analyzed.
Abstract: With the dawn of a new millennium we find ourselves one step before the emergence of the third generation (3G) mobile communications systems in the world market. The implementation of the 3G and furthermore 4G mobile communications systems is included within the intentions of the so-called software defined radio (SDR) systems [1]. The design, development and the implementation of SDR systems are based on a combination and evolution of technologies and techniques including mainly: smart antennas, radio frequency (RF) down/up converters, analog to digital converters (ADCs) and digital to analog converters (DACs), digital signal processors (DSPs), modelling and system description languages. In this paper a quantitative analysis of the basic parameters of one of the most important segments of an SDR receiver, the Analog to Digital Converter, is presented. ADC's of the latest technology and their basic specifications are also indicated.
Abstract: With the dawn of a new millennium we find ourselves one step before the emergence of the third generation (3G) mobile communications systems in the world market. The implementation of the 3G and furthermore 4G mobile communications systems is included within the intentions of the so-called software defined radio (SDR) systems [1]. The design, development and the implementation of SDR systems are based on a combination and evolution of technologies and techniques including mainly: smart antennas, radio frequency (RF) down/up converters, analog to digital converters (ADCs) and digital to analog converters (DACs), digital signal processors (DSPs), modelling and system description languages. In this paper a quantitative analysis of the basic parameters of one of the most important segments of an SDR receiver, the Analog to Digital Converter, is presented. ADC's of the latest technology and their basic specifications are also indicated.
Abstract: Aim of this doctoral dissertation is the study and in-depth analysis of parameters
and functional characteristics of . The combination of these cutting edge technologies is expected to lead to
the creation of a platform that will not depend on hardware, in other words its functional
characteristics will be altered via software. The doctoral dissertation is composed of four
primary parts.
During the accomplishment of the first part, quantitative analysis of the basic
parameters concerning one of the most critical components of Software Radio system, the
Analog to Digital converter, took place. The effect of these in the attribution of our
system was also studied.
In the second part the research was focused in the simulation of SWR system with
capabilities of DBF using a smart antenna as an RF front end. The simulation was
primarily based on Graphical User Interfaces (GUIs) which were created using the
Matlab programming language. Various scenarios regarding the input/output signals, the
geometry, the frequency range, the space-time accuracy were tested. Emphasizing on the
time critical parameters of the digital beamforming for a linear array antenna, important
results were extracted.
During the third part, a new and innovative architecture of a digital time delay
beamformer (DTDBF) was proposed. The specific proposal exceeds the barriers entering
the minimum step angle of the main beam, due to the throughput rate and the DSPâs
input/output interface, without increasing the complexity of the architecture.
The dissertation closes by examining the possibilities of reducing the
electromagnetic pollution. Specifically, the reduction of the absorbed by the human head
electromagnetic energy was estimated as an operation of the intelligent antennas in the
future SWR handheld terminals.
Abstract: The development of a real data transmission network is presented. The
system is based on a wired network, which consists of a central station and
local stations with various control and monitoring devices. Each local station
operates as a source, retransmitter and receiver of real data. The main
component of each local station is the 8-bit microcontroller AT89S8252 of
ATMEL (compatible with the industry standard 80C51 instruction set and
pinout), and in its code memory the firmware that implements communication
protocol and a variety of applications is resident. The central station is based
on a PC and it can not only monitor but also control the local stations. The
software of the central station was implemented with the developmental
program Labview 4.0 of the National Instruments company. The
communication protocol and its features were firstly checked with the
appropriate simulation tools. Eventually, the local stations were developed
made and checked in real-time.
