Giorgos D. Tatsis

Abstract: In a 2 � 2 MIMO antenna array system envelope correlation coefficient ��� shows the influence of different propagation paths of the RF signals that reach the antenna elements. The approximated value of this coeffi-cient is based on a simple closed-form equation and also varies from 0 to 1. 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 ��� has very small values and approximates to zero. This observation indicates quite perfect behavior and performance of our MIMO antenna array system.

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: This paper describes the design, realization and experimental measurements of an antenna element to operate at ultra-wideband (UWB) spectrum. The type of this antenna is a circular disk monopole (CDM), with two notches opposite to each other at two sides of the disk. The feed of the antenna is a coplanar waveguide (CPW). The effect of the presence of the notches is studied through simulations and tested experimentally.

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 varactor�s value is antenna�s VSWR which is monitored by a uC. Additionally, the uC also controls the varactor�s 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 SWR�s performance. In this paper the performance of a SWR receiver is evaluated considering A/D�s 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: 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 � 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.

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 � 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.