Vasilis Christofilakis

University of Ioannina | UOI · Department of Physics

The urgent need for timely and accurate precipitation estimations in the face of ongoing climate change and the increasing frequency and/or intensity of extreme weather events underscores the necessity for innovative approaches. Recently, several studies have focused on estimating the precipitation rate through induced attenuation of radio frequenc...

Schumann resonances (SR) are the result of global lightning activity and occur in the waveguide between the Earth and the ionosphere. SR appear in the extremely low frequency (ELF) region of the spectrum, presenting peaks from 8 to 45 Hz. The electromagnetic field caused by this phenomenon consists of two components: the electric field component, w...

For this Special Issue of Sensors, the main goal is to approach the solutions procedures with a different point of view by emphasizing the essential idea while expressing it in simple words to enhance the endeavors of various scientific teams. The new proposed techniques must be easily implementable because they refer to a pluralism of phenomena an...

We analyzed a large number (77) of low-to-medium-magnitude earthquakes (M3.5–M6.5) that occurred within a period of three years (2020–2022) in the Southern half of Greece in relation to the ELF activity in that region and time period. In most cases, characteristic ELF signals appear up to 20 days before the earthquakes. This observation may add an...

In light of recent extreme weather events, it is imperative to explore innovative methodologies for promptly and accurately measuring various meteorological parameters. The high spatial and temporal variability in precipitation often surpasses the resolution capabilities of traditional rain gauge measurements and satellite estimation algorithms. Th...

Sudden cardiac death (SCD) is one of the leading causes of death worldwide. Many individuals have no cardiovascular symptoms before the SCD event. As a result, the ability to identify the risk before such an event is extremely limited. Timely and accurate prediction of SCD using new electronic technologies is greatly needed. In this work, a new inn...

This paper presents a compact multifrequency reconfigurable patch antenna in terms of design and fabrication for operating in the S and C bands of the RF spectrum, which are overwhelmed by wireless applications. Reconfiguration is achieved by using a single PIN diode on the ground plane. By varying the voltage applied to the diode, three modes can...

This Special Issue aims to gather researchers from different scientific areas to highlight new and future trends in this interdisciplinary field. This particular issue highlights works that propose new or improved traditional or opportunistic sensing methods, in situ and remote rainfall devices, methodologies, and algorithms. Challenges to be cover...

Several scientific and commercial applications require measuring and recording RF signal attenuation under different conditions and environments. The primary aim of this work is to present a signal attenuation testbed which is easy to implement, it is portable and it has a low cost but at the same time is accurate. The signal attenuation testbed is...

The modern era of advanced communications and electronic structures yields with steady steps the enhanced implementation of smart “leadership in energy and environmental design” (LEED) buildings with artificial intelligence (AI) structures. These structures need development through various studied steps. In this paper, we propose a holistic design...

Recording systems that deal with Extra Low Frequency (ELF) data in the Schumann resonance (SR) range exhibit high sensitivity to external noise. In our effort to refine a time series by identifying and removing external disturbances from real data, we analyzed the effect of induced deliberate anthropogenic disturbances. The signals were recorded at...

Voice assessment consists of many methods, amongst them being the Voice Handicap Index (VHI) which is a non-interventional self-reported questionnaire. This research determined and interpreted the cutoff points (COPs) of the Receiver Operating Characteristic (ROC) curves (signal processing) of the standardized VHI Hellenic version along with the Gr...

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...

Background: Currently, there is a vast number of electronic and mobile devices which serve for self-evaluation purposes. Specifically, wearable technology has been vastly developed in the last years in conjunction with the usage of mobile phones’ applications for the convenience of their users. In the context of the aforementioned, a proper segment...

Background: Currently, there is a vast number of electronic and mobile devices which serve for self-evaluation purposes. Specifically, wearable technology has been vastly developed in the last years in conjunction with the usage of mobile phones' applications for the convenience of their users. In the context of the aforementioned, a proper segment...

Voice assessment consists of many methods, amongst them being the Voice Handicap Index (VHI) which is a non-interventional self-reported questionnaire. This research determined and interpreted the cutoff points (COPs) of the Receiver Operating Characteristic (ROC) curves (signal processing) of the standardized VHI Hellenic version along with the Gr...

FSO communication is a viral technology among optical wireless communications, gathering the interest of both researchers and manufacturers. This is because of the many advantages associated with FSO communication, including high data rates, reliability, safety, and economy. However, there are several unavoidable drawbacks that shadow the performan...

Currently, hospitals and health care sectors employ low-cost Internet of Things based remote health monitoring systems and labs in order to collect a subject's or patients’ real-time data. Such a process can be helpful for the early detection of a healthy newborn life and of critical importance for the survival of these lives. In this article, a pr...

A set of various short artificial disturbances from rifle firings, car engine operation, car radio, shakings of the apparatus, etc., were generated deliberately near our ELF recording stations in order to identify their footprint on the recordings of atmospheric electromagnetic radiation in the Schumann resonance (SR) band (from about 2–50 Hz). Suc...

A dedicated extremely low frequency (ELF) detector has been constructed and used successfully for Schumann Resonance (SRs) measurements in N.W. Greece. The main objective of this work was to investigate the effect of local lightning activity on the recorded signal of our ELF detector and consequently on the estimated SRs parameters, namely the powe...

We analyzed an extended time series of Schumann Resonance recordings with two multi-parametric statistical methods, the generalized linear Logistic Regression—LogReg and the non-linear Random Forest—RF, in order to test their potential for earthquake prediction within a narrow time-space-magnitude window of 48 h, 250 km from our observing site, and...

Very recent work reported that patients can monitor their heartbeat at home and specify their heart conditions by means of a millimetre wave radar, but there exist serious limitations because the radar sensor is sensitive to significant body motions that cause Doppler frequency shifts. Such limitations do not exist when using a recently constructed...

Many works have been conducted relevant to rainfall measurements, while the first relevant ones were based on the power loss estimation function from wireless links located back to the early 1940s. It is notable, though, that this innovative idea conduced to many theoretical models correlating the signal attenuation to the rainfall intensity. This...

Rain monitoring through signal attenuation has been a significant concern for the scientific community over the last decade. The lowest L-, Sand C-bands in the microwave spectrum are full of potential for environmental monitoring devices. It has not been extensively proven whether signal attenuation can be used for rain estimation throughout the mi...

The research area of optical wireless communication links has attracted significant interest over the last years due to the significant advantages offered by this kind of technology. However, the performance of FSO communication systems depends strongly on the effects, which are related to the atmosphere along the propagation path such FSO links ar...

Two medium-magnitude earthquakes separated by a distance of 230 km occurred within 34 days from each other in Northern Greece. A few hours before the manifestation of seismic activity, significant extra-low-frequency (ELF) perturbations were detected in a nearby Schumann resonance observation site. The typical spectrum of ELF measurements was defor...

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...

This paper presents a prototype test fixture for the absolute calibration and estimation of the equivalent magnetic flux noise of the extremely low frequency (ELF) Schumann resonant (SR) magnetic antenna receiver and rods’ magnetic permeability measurement. The test fixture, for ELF the SR detector’s calibration, consists of a constructed coil, the...

In this paper, a prototype photoplethysmography (PPG) electronic device is presented for the distinction of individuals with congestive heart failure (CHF) from the healthy (H) by applying the concept of Natural Time Analysis (NTA). Data were collected simultaneously with a conventional three-electrode electrocardiography (ECG) system and our proto...

Data security plays a crucial role in healthcare monitoring systems, since critical patient information is transacted over the Internet, especially through wireless devices, wireless routes such as optical wireless channels, or optical transport networks related to optical fibers. Many hospitals are acquiring their own metro dark fiber networks for...

This study investigated the outage performance of a terrestrial FSO communication system that uses mixed series and parallel decode-and-forward (DF) relay-assisted (i.e., cooperative diversity) configurations, taking into account the influence of both atmospheric turbulence and pointing error effects. Turbulence-induced optical signal fading is mod...

Strong ELF perturbations in the Schumann Resonance band (0–50 Hz) were detected in the North of Greece close to the village of Kalpaki during a shallow (17 km) mid-magnitude earthquake (5,3R) and the post-seismic sequence that followed it. The observation site was serendipitously located almost above the main quake epicenter (3,65 km away). It is p...

In recent years, the usefulness of Green Technology along with the implementation of advanced wireless technologies led scientists towards investigating the effects of this advanced Green Technology to Human Resources. A large number of surveys have investigated the impact, of working conditions inside conventional buildings, on health, productivit...

The promotion of green growth has been a strategy for developed countries by improving the energy performance of residences. This tendency will help towards overcoming the continuously increasing greenhouse effect because the contribution of typical residences to the production of air pollutants is extremely high. Housing conditions (indoor air qua...

In this paper we present an experimental validated system for measuring rainfall due to radio frequency (RF) signal attenuation at 2 GHz. Measurements took place in Ioannina, NW Greece, starting in April 2015 and lasting for twelve months. The primary acquired extensive results have shown reliable and accurate measurements for rainfall amounts smal...

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....

Schumann resonance oscillation detection is a complex procedure which requires customized and high-quality measurement systems. The primary objective of this work was to design and implement a stand-alone, portable, and low-cost receiver able to measure as much Schumann resonance harmonics as possible. Design, as well as detailed analysis of the ef...

We present a comparison of two non invasive digital systems. The one is a portable electrocardiograph (ECG) and the second a finger pulse Oximeter that was designed and assembled in our laboratory. The ultimate goal is to use a portable device such as an Oximeter with the prospect of calculating precisely the complexity measures of the heart rate r...

Ιn this paper, we present a comparison of two non invasive digital systems. The one is a portable electrocardiograph (ECG) and the second a finger pulse Oximeter that was designed and assembled in our laboratory. The ultimate goal is to use a simple yet effective portable device such as an Oximeter with the prospect of calculating precisely the com...

In this work we present an accurate and costeffective system for measuring S-band signal attenuation due to rainfall. The system is based on a pair of transmitter and receiver, operating at the lower limit of Sband, i.e. 2 GHz, separated at few meters distance and thus at any instant the rain’s strength is uniform at this short path. The system is...

In this paper a portable detection system for Schumann resonances (SR) measurements is presented. SR are ELF natural electromagnetic eigenmodes in Earth-ionosphere cavity. These electromagnetic waves have been suggested to monitor climate, electrical activity in the atmosphere and other earth — atmosphere phenomena providing observations of great i...

This work presents an experimental study of Single Input Multiple Output (SIMO) channel performance in indoor radio propagation environment. Indoor channel measurements at 2.4 GHz ISM frequency band have been performed using a versatile channel sounder testbed platform. A single transmitting antenna, four receiving antennas with two proposed geomet...

In this paper, a cost-effective prototype data acquisition system specifically designed for Schumann resonance measurements and an adequate signal processing method are described in detail. The implemented system captures the magnetic component of the Schumann resonance signal, using a magnetic antenna, at much higher sampling rates than the Nyquis...

The principal objective of this paper is to present a Schumann resonances detection and measurements platform. Studies and investigation on Schumann resonances are suggested in several research activities that aim to monitor climate, earth and atmospheric phenomena. Schumann oscillations are natural electromagnetic eigenmodes at ELF frequency range...

Sixteen years after the dawn of the new millennium, strong trends in telecommunications drive to adaptable wideband communication systems. This effort is primarily motivated by two technological concepts: software radio (SWR) and ultra wideband (UWB) communications. The design, development and implementation of SWR involve either hardware or softwa...

Abstract Schumann’s Resonance (SR) measurements have been collected in the wider area of Ioannina by an equipment, designed and built in the laboratory of electronics and telecommunication of the University of Ioannina. Remarkable differences among measurements received in various epochs have been detected. At first, SR intensities taken around the...

Wireless systems contain transmitters and receivers in order to exchange information. In the past decades, this exchange of information was conducted with the us e 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 ot...

Schumann's Resonance (SR) measurements have been collected in the wider area of Ioannina by an equipment, designed and built in the laboratory of electronics and telecommunication of the University of Ioannina. Remarkable differences among measurements received in various epochs have been detected. At first, SR intensities taken around the Summer s...

The Free Space Optical (FSO) communication systems, are rapidly gaining popularity as effective, low cost and secure means of transferring data at high rates over relative short distances, with line of sight, through the atmosphere. However, due to the fact that the characteristics of the propagation path do not remain invariable, the systems’ perf...

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....

In this paper, a cost-effective prototype data acquisition system specifically designed for Schumann resonance measurements and an adequate signal processing method are described in detail. The implemented system captures the magnetic component of the Schumann resonance signal, using a magnetic antenna, at much higher sampling rates than the Nyquis...

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 oth...

http://www.tandfonline.com/eprint/RQibMMqFqGk83heMu98t/full This work presents an experimental study of single-input multiple-output (SIMO) channel performance in the indoor radio propagation environment. Indoor channel measurements at the 2.4-GHz ISM frequency band have been conducted using a versatile channel sounder test-bed platform. A single...

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 s...

In this paper, an antenna array structure consisting of a reflector plate and a number of printed dipole radiation elements is investigated. This antenna array implementation is suitable for the globally available 2.4 and 5 GHz ISM frequency band facilitating a variety of wireless applications. S-parameters and radiation characteristics are simulat...

In this paper we perform experimental measurements targeting the UWB wireless channel. For this case we utilize Impulse Radio channel sounding techniques in time domain. The aim is to estimate the channel characteristics such as the path loss, the shadowing effect, the mean excess delay spread, the RMS delay spread and the number of dominant paths....

In this paper we perform experimental measurements targeting the UWB wireless channel. For this case we utilize Impulse Radio channel sounding techniques in time domain. The aim is to estimate the channel characteristics such as the path loss, the shadowing effect, the mean excess delay spread, the RMS delay spread and the number of dominant paths....

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) sch...

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...