Conference Presentations by Vasilis Christofilakis
UWB communications have attracted considerable interest, targeting applications in high-speed dat... more UWB communications have attracted considerable interest, targeting applications in high-speed data transfer wireless communication systems. This paper studies the effects of matched filter receiver in the performance of such a system. Such effects are evaluated in terms of the Bit Error Rate (BER) for a Binary Pulse Position Modulation (BPPM) scheme, considering multipath propagation channel and the presence of noise. The case of imperfect channel estimation is taken into account. Dependence of BER on parameters such as signal to noise ratio, number of estimation pulses and correletor taps is also presented.
This paper presents design issues of a dipole antenna for wireless communications which is printe... more This paper presents design issues of a dipole antenna for wireless communications which is printed on a dielectric substrate and fed by a microstrip line. A discontinuity in form of a right – angle bend close to the gap of the dipole strips provides a crucial parameter for discussion. The impact of mitering these bends in return loss, resonance bandwidth and radiation pattern of antenna has been investigated. Both simulated and experimental results propose this observation.
Multiple – output transmitter implementation based on Direct Digital Synthesis (DDS) architecture... more Multiple – output transmitter implementation based on Direct Digital Synthesis (DDS) architecture is presented and investigated. A particular number of identical and independent units compose the proposed device. DDS technology is applied on the first stage of these units and provides amplitude, phase and frequency adjustments on the corresponding output signals. Quadrature mixers are driven by DDS's outputs and corresponding amplification and filtering circuits are also used to prepare the radiofrequency signals in the outputs of the proposed device. Design issues of analog circuitry and digital control logic are also described. Phase, amplitude and frequency accuracy that DDS technology provides are further discussed. Experimental results indicate that the proposed transmitter architecture can provide independent RF signals for wireless applications.
This paper describes the design and implementation of an UWB Impulse Radio transmitter. This UWB ... more This paper describes the design and implementation of an UWB Impulse Radio transmitter. This UWB transmitter produces very short pulses with possibility of time-shifting to be used in cases of Pulse Position Modulation (PPM). The transmitter, which is based on step recovery diode (SRD), can operate at 50Mbps with high repetition rate. Experimental results show that, ultra short pulses with duration of about 1nsec and spectrum exceeding 2GHz at-10dB, are produced and transmitted.
Latest technologies in wireless communications present new challenges on antenna design. Reduced ... more Latest technologies in wireless communications present new challenges on antenna design. Reduced size antennas with self reconfigurable ability are characteristical of smart antennas. This paper presents the practical implementation issues of making an automatic smart antenna tuning system varying the resonant frequency of the antenna using a varactor. The primary parameter for changing varactor's value is antenna's VSWR which is monitored by a uC. Additionally, the uC also controls the varactor's value.
In modern mobile communication system, low profile antennas capable of broadband operation are re... more In modern mobile communication system, low profile antennas capable of broadband operation are required. Nevertheless there is a problem in changing fast and easy the resonant frequency to achieve effective bandwidth. In this paper we examine in particular how an added capacitive load affects the resonant frequency, the VSWR and the current distribution of a Planar antenna. The resonant frequency dependence from the capacitance load is useful parameter for making full self-tuning antennas.
Wireless systems contain transmitters and receivers in order to exchange information. In the past... more Wireless systems contain transmitters and receivers in order to exchange information. In the past decades, this exchange of information was conducted with the use of only one modulated carrier (frequency). As wireless technology evolved, new techniques emerged. One of them was unique for its capability of significant lower data loss compared to others. The main concept of the new technique, commonly known as Orthogonal Frequency Division Multiplexing, was the utilization of various orthogonal carriers (frequencies) which were multiplexed with each other in order to prevent system breakdown in the case of frequency fading. In this paper we simulated our enhanced OFDM platform based on Turbo Coding (TC) in the presence of a noisy channel (AWGN, Phase noise, Rician noise, ITU PA3) exhibiting excellent BER performance.
Papers by Vasilis Christofilakis
Atmosphere, 2022
ELF recordings, especially in the 0–50 Hz range (Schumann Resonances), have gained great interest... more ELF recordings, especially in the 0–50 Hz range (Schumann Resonances), have gained great interest during the last twenty years because of their possible relation to many geophysical, climatological, solar, and even biological phenomena, which several well-known scientists have reported. A very important question that still has not been answered is whether some particular variations in the Schumann Resonances (SR) band operate as precursors of forthcoming seismic activity. Greece and the wider Mediterranean area are a very seismic territory where medium size earthquakes (4–6.5 Richter) occur very often, contributing to a high percentage of the natural hazards of the area. In our effort to make evident how effective and prerequisite SR recordings are in the detection of forthcoming earthquakes, we analyze data collected for almost five years by two SR stations located in the north and the south edge of the Greek territory, respectively. We have come to the conclusion that particular S...
Science of The Total Environment, Elsevier. 136926 Jan. 2020., Jan 25, 2020
Schumann resonances (SRs) provide a unique tool for exploring continuous and long-term monitoring... more Schumann resonances (SRs) provide a unique tool for exploring continuous and long-term monitoring of global environmental parameters, although their detection and study is a very complex task. This paper presents diurnal and seasonal variations of principal parameters, namely the resonance frequency, the Q-factor, and the power of SRs, in the North-South (NS) magnetic field component for the first five modes from Extra Low Frequency (ELF) measurements. These were acquired at the observation site located at Doliana, Kalpaki, in the Region of Epirus, North West Greece. These variations are the first ones reported for the principal parameters in the South Eastern Mediterranean area and correspond to the one-year measurement period from 2016 to 2017. The comparison with variations at several observations sites is also discussed. Moreover, correlation with global lighting centers, as well as local lightning activity, and SRs parameters variation is attempted.
This paper studies the characteristics of an Ultra-Wideband (UWB) communication system employing ... more This paper studies the characteristics of an Ultra-Wideband (UWB) communication system employing Impulse Radio techniques and, specifically, the effects of the matched filter of a correlator receiver in the performance of such a system. Such effects are evaluated in terms of the Bit Error Rate (BER) for a Binary Pulse Position Modulation (BPPM) scheme, in an indoor multipath propagation channel and in the presence of additive white Gaussian noise (AWGN). The case of imperfect channel estimation is taken into account, while comparisons with perfect channel estimation are performed. The dependence of BER on parameters such as the signal to noise ratio (SNR), the number of estimation pilot pulses and correletor taps, is finally derived. It is shown that there is an optimum number of correlator taps for best BER performance depending on the channel estimation procedure.
International Journal of Communications Network and System Sciences, 2010
Detail experimental measurements of a 2.4 GHz printed dipole antenna for wireless communication s... more Detail experimental measurements of a 2.4 GHz printed dipole antenna for wireless communication systems is presented and discussed. A group of printed dipoles with integrated balun have been designed and constructed on a dielectric substrate. This paper is based on modifications of the known printed dipole architecture. The corresponding printed dipole antennas have differences on their forms that are provided by two essential geometry parameters. The first parameter l is related to the bend on microstrip line that feeds the dipole and the second w corresponds to the form of the dipole's gap. The impact of these parameters on reflection coefficient and radiation pattern of antenna has been investigated. The corresponding measured results indicate that the return loss and radiation pattern of a printed dipole antenna are independent of the w parameter. Instead, variations in the value of the l parameter in the dipole's structure affect the form of the corresponding return loss. These observations are very important and provide interesting considerations on affecting design and construction of antenna elements at frequency range of 2.4 GHz.
The Environmentalist, 2005
The performance of a cellular phone commercial helical antenna at 900 MHz band, both in free spac... more 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.
Journal of Atmospheric and Solar-Terrestrial Physics, 2015
A telecommunication system uses carriers in order to transmit information through a cable or wire... more A telecommunication system uses carriers in order to transmit information through a cable or wirelessly. If each time only one carrier is transmitted, then the system’s signal will not be immune to frequency selective fading. If frequency selective fading includes the working frequency of the system, then the wireless link will not be established. Orthogonal frequency division multiplexing (OFDM) is the primary solution for coping with inter-signal interference and frequency-selective fading. Many carriers can be produced by splitting a fast information stream to slower data series. Different orthogonal frequencies carry slower data series. System’s performance can be further enhanced with the utilization of turbo codes. Turbo codes make the system more immune to noise effects with excellent BER results. This paper presents the thorough analysis of a turbo coded OFDM scheme using a PCCC technique in the presence of a channel which includes AWGN, phase noise, Rayleigh fading, Rician fading and Doppler shift.
Analog-to-Digital Converter (A/D) is a critical component of Software Radio (SWR). In this paper ... more Analog-to-Digital Converter (A/D) is a critical component of Software Radio (SWR). In this paper the performance of a WiMAX SWR receiver is evaluated in terms of A/D's clipping and amplitude distribution error. As a measure of performance we use the total Signal to Noise Ratio (SNR t ) at the output of the A/D. Significant results are derived concerning SNR t as a function of bit resolution, clipping, amplitude distribution and aperture jitter error.
International Journal of Electronics, 2015
Software Radios (SWRs) and Antenna Arrays are the cutting-edge technologies for the future teleco... more 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.
The problem of scattering of electromagnetic waves is one of the most essential problems of elect... more 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 “computing” 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 Se...
Peak to Average Power Ratio (PAPR) is defined as the instantaneous power (maximum value) to the a... more Peak to Average Power Ratio (PAPR) is defined as the instantaneous power (maximum value) to the average power ratio. PAPR is considered to be a major problem in OFDM systems. This problem can cause radical unexpected behavior of the signal fluctuation. This fluctuation is constituted by a large number of power states. The enormous number of these states leads to an additional complexity of ADCs and DACs. This research addresses the previous problem in OFDM systems utilizing Turbo Codes. μLaCP technique is employed for the purpose of decreasing PAPR. Moreover, our OFDM system was simulated in the presence of an AWGN channel with four types of codes (without the presence of ADCs and DACs). These were constituted of PCCC (typical and new), SCCC, and Convolutional Codes. Our Turbo Coded OFDM exhibited unchanged BER performance before and after the use of μLaCP technique. This was accomplished by modifying our previous PAPR reduction technique without sacrificing greatly its attributes.
A software defined radio (SDR) system is based on state-of-the art technologies covering topics s... more 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
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Conference Presentations by Vasilis Christofilakis
Papers by Vasilis Christofilakis