Vasilis Raptis

In depth analysis of noise effects in orthogonal frequency division multiplexing systems, utilising a large number of subcarriers

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.

Enhanced PAPR in OFDM without Deteriorating BER Performance

Orthogonal frequency division multiplexing (OFDM) is vastly used in wireless networks. Its superiority relies on the fact that information can be split in large amount of frequencies. Each frequency is called information subcarrier. OFDM exhibits excellent annotation in channel fades and interferers as only a few subcarriers can be affected and consequently a small part of the original data stream can be lost. Orthogonality between frequencies ensures better spectrum management and obviates the danger of intersymbol interference. However, an essential problem exists. OFDM systems have high peak to average power ratio. This implies large fluctuations in signal power, ending up in increasing complexity of ADCs and DACs. Also, power amplifiers must work in a larger linear dynamic region. In this paper we present two new techniques for reducing Peak to Average Power Ratio (PAPR), that can be added in any OFDM system and we compare them with other existing schemes.

Design and Measurements of Ultra-Wideband Antenna

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.

Performance of UWB‐Impulse Radio Receiver Based on Matched Filter Implementation with Imperfect Channel Estimation

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.

Design and Implementation of Ultra‐Wideband Impulse Radio Transmitter

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 50 Mbps with high repetition rate. Experimental results show that, ultra short pulses with duration of about 1nsec and spectrum exceeding 2 GHz at −10 dB, are produced and transmitted.

A/D Restrictions (Errors) in Ultra-Wideband Impulse Radios

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.

Active Tuning Antennas for Wireless Communication

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.

Tuning Techniques for Planar Antennas in Wireless Communication

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.

Effects of Variations on Geometrical Parameters on a 2.4 GHz Printed Dipole Antenna architecture

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.

Absorbed Power Minimization in Cellular Users with Circular Antenna Arrays

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 900 MHz 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.