Constantinos I. Votis was born in Arta, Greece, in 1980. He received the B.Sc. degree in Physics from University of Ioannina, Greece in 2002 and the M.Sc. degree in Electronics and Telecommunications from the same University in 2005. His Master Thesis was entitled “Radiofrequency Detector Implementation”. He is, currently, a Ph.D. candidate at Electronics, Telecommunications and Applications Laboratory of Physics Department in University of Ioannina, Greece. He has participated in several research programmes over the last years. He is also a member of various scientific and academic societies. His research interests include mainly digital communications, software radios, smart antennas and MIMO systems. He has twelve publications in international conferences and journals.
Abstract: Ultra-Wideband Impulse Radio (UWB-IR) technologies, although are relatively easy in transmission but
they present difficulties in reception, in fact the reception of such waveform is a quite complicated matter.
The main reason is that in fully digital receiver the received waveform must be sampled at a rate of several
GHz. This paper focuses on the impact of the Analog to Digital (A/D) conversion stage that is used to sample
the received waveform. More specifically we focus on the impact of the two main parameters that affect
the performance of the Software Defined Radio (SDR) system. These parameters are the bit resolution and
the time jittering. The influence of these parameters is deeply examined.
Abstract: 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 dipoleâs 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 dipoleâs 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.
Abstract: In the present paper, design and analysis of a 2.4 GHz printed dipole antenna for wireless communication applications are presented. Measurements on return loss and radiation pattern of this antenna configuration are included in this investigation. The printed dipole is combined with the feeding structure of a microstrip via-hole balun and is fabricated on an FR-4 printed-circuit-board substrate. Two inevitable discontinuities are introduced by this antenna architecture in the form of right-angle bends in the microstrip feed line and in the dipoleâs gap, respectively. The impact of mitering these bends in the reflection coefficient, resonance bandwidth and radiation pattern of antenna has been investigated by means of simulation and experiment.
Abstract: In a 2x2 MIMO antenna array system envelope correlation coefficient âÏâ shows the influence of different propagation paths of the RF signals that reach the antenna elements. The approximated value of this coefficient is based on a simple closed-form equation and also varies from zero to one. Quite perfect performance for MIMO applications is achieved when this parameter approximates to zero. In this paper, we evaluate an an-tenna diversity MIMO system by measuring the envelope correlation coefficient. The corresponding results in our antenna array configurations show that the measured âÏâ has very small values and approximates to zero value. This observation indicates quite perfect behavior and performance of our MIMO antenna array system.
Abstract: Nowadays electromagnetic pollution of non ionizing radiation generated by cellular phones concerns millions of people. In this paper the use of circular antenna array as a means of minimizing the absorbed power by cellular phone users is introduced. In particular, the different characteristics of radiation patterns produced by a helical conventional antenna used in mobile phones operating at 900MHz and those produced by a circular antenna array, hypothetically used in the same mobile phones, are in detail examined. Furthermore, the percentage of decrement of the power absorbed in the head as a function of direction of arrival is estimated for the circular antenna array.
Abstract: 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 varactorâs value is antennaâs VSWR which is monitored by a uC. Additionally, the uC also controls the varactorâs value.
Abstract: 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.
Abstract: Analog-to-Digital Converter (A/D) is a critical component of Software Radio (SWR). Various hardware
restrictions and errors are limiting SWRâs performance. In this paper the performance of a SWR receiver is evaluated considering A/Dâs clipping noise. Significant results are given, in terms of clipping noise, in combination with aperture jitter and quantization error.
Abstract: Orthogonal frequency division multiplexing (OFDM) is a multicarrier data transmission, where a single stream of information is divided over a large number of subcarriers. The primary purpose of this work was to find out the
relationships connecting BER performance in noisy environments and the number of transmitted subcarriers. In order to simulate this kind of environment, various noise types where taken into consideration such as complex Rayleigh fading, complex rician noise, AWGN and phase noise.
Abstract: 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 â 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.
Abstract: 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.
Abstract: 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.
Abstract: Multiple â 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 DDSâs 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.
