Accurate and timely surface precipitation measurements are crucial for water resources management, agriculture, weather prediction, climate research, as well as ground validation of satellite-based precipitation estimates. However, the majority of the land surface of the earth lacks such data, and in many parts of the world the density of surface precipitation gauging networks is even rapidly declining. This development can potentially be counteracted by using received signal level data from the enormous number of microwave links used worldwide in commercial cellular communication networks. Along such links, radio signals propagate from a transmitting antenna at one base station to a receiving antenna at another base station. Rain-induced attenuation and, subsequently, path-averaged rainfall intensity can be retrieved from the signal’s attenuation between transmitter and receiver. Here, we show how one such a network can be used to retrieve the space–time dynamics of rainfall for an entire country (The Netherlands, ∼35,500 km2), based on an unprecedented number of links (∼2,400) and a rainfall retrieval algorithm that can be applied in real time. This demonstrates the potential of such networks for real-time rainfall monitoring, in particular in those parts of the world where networks of dedicated ground-based rainfall sensors are often virtually absent.

Three Years of Country-Wide Rainfall Maps from Cellular Communication Networks

Lightning produces electromagnetic fields and waves in all frequency ranges. In the extremely low frequency (ELF) range below 100 Hz, the global Schumann Resonances (SR) are excited at frequencies of 8 Hz, 14 Hz, 20 Hz, etc. This review is aimed at the reader generally unfamiliar with the Schumann Resonances. First some historical context to SR research is given, followed by some theoretical background and examples of the extensive use of Schumann resonances in a variety of lightning-related studies in recent years, ranging from estimates of the spatial and temporal variations in global lighting activity, connections to global climate change, transient luminous events and extraterrestrial lightning. Both theoretical and experimental results of the global resonance phenomenon are presented. It is our hope that this review will increase the interest in SR among researchers previously unfamiliar with this phenomenon.

https://www.mdpi.com/2073-4433/7/9/116/pdf

ELF Electromagnetic Waves from Lightning: The Schumann Resonances

A design of mobile-phone antenna array with diamond-ring slot elements is proposed for fifth generation (5G) massive multiple-input/multiple-output (MIMO) systems. The configuration of the design consists of four double-fed diamond-ring slot antenna elements placed at different corners of the mobile-phone printed circuit board (PCB). A low-cost FR-4 dielectric with an overall dimension of 75 × 150 mm2 is used as the design substrate. The antenna elements are fed by 50-Ohm L-shaped microstrip-lines. Due to the orthogonal placement of microstrip feed lines, the diamond-ring slot elements can exhibit the polarization and radiation pattern diversity characteristic. A good impedance bandwidth (S11 ≤ −10 dB) of 3.2–4 GHz has been achieved for each antenna radiator. However, for S11 ≤ −6 dB, this value is 3–4.2 GHz. The proposed design provides the required radiation coverage of 5G smartphones. The performance of the proposed MIMO antenna design is examined using both simulation and experiment. High isolation, high efficiency and sufficient gain-level characteristics have been obtained for the proposed MIMO smartphone antenna. In addition, the calculated total active reflection coefficient (TARC) and envelope correlation coefficient (ECC) of the antenna elements are very low over the whole band of interest which verify the capability of the proposed multi-antenna systems for massive MIMO and diversity applications. Furthermore, the properties of the design in Data-mode/Talk-mode are investigated and presented.

Mobile-Phone Antenna Array with Diamond-Ring Slot Elements for 5G Massive MIMO Systems

Abstract. Microwave links in commercial cellular communication networks hold a promise for areal rainfall monitoring and could complement rainfall estimates from ground-based weather radars, rain gauges, and satellites. It has been shown that country-wide (≈  35 500 km2) 15 min rainfall maps can be derived from the signal attenuations of approximately 2400 microwave links in such a network. Here we give a detailed description of the employed rainfall retrieval algorithm. Moreover, the documented, modular, and user-friendly code (a package in the scripting language “R”) is made available, including a 2-day data set of approximately 2600 commercial microwave links from the Netherlands. The purpose of this paper is to promote rainfall mapping utilising microwave links from cellular communication networks as an alternative or complementary means for continental-scale rainfall monitoring.

Retrieval algorithm for rainfall mapping from microwave links in a cellular communication network

Due to the growing public awareness of cardiovascular disease (CVD), blood pressure (BP) estimation models have been developed based on physiological parameters extracted from both electrocardiograms (ECGs) and photoplethysmograms (PPGs). Still, in order to enhance the usability as well as reduce the sensor cost, researchers endeavor to establish a generalized BP estimation model using only PPG signals. In this paper, we propose a deep neural network model capable of extracting 32 features exclusively from PPG signals for BP estimation. The effectiveness and accuracy of our proposed model was evaluated by the root mean square error (RMSE), mean absolute error (MAE), the Association for the Advancement of Medical Instrumentation (AAMI) standard and the British Hypertension Society (BHS) standard. Experimental results showed that the RMSEs in systolic blood pressure (SBP) and diastolic blood pressure (DBP) are 4.643 mmHg and 3.307 mmHg, respectively, across 9000 subjects, with 80.63% of absolute errors among estimated SBP records lower than 5 mmHg and 90.19% of absolute errors among estimated DBP records lower than 5 mmHg. We demonstrated that our proposed model has remarkably high accuracy on the largest BP database found in the literature, which shows its effectiveness compared to some prior works.

https://www.mdpi.com/1424-8220/20/19/5668/pdf

Generalized Deep Neural Network Model for Cuffless Blood Pressure Estimation with Photoplethysmogram Signal Only.

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.

Design and analysis of a multiple-output transmitter based on DDS architecture for modern wireless communications

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' performance varies, as well. An important phenomenon which mitigates significantly their performance is the atmospheric turbulence, which causes the scintillation effect and results in fast irradiance fluctuations at the receivers' side, which could be modeled with various statistical distributions depending on the turbulence's strength. Here, we assume weak turbulence conditions and an accurate and with a very compact mathematical expression statistical model is the gamma distribution. Additionally, because of the very narrow optical beam the alignment between the transmitter and the receiver is not perfect and the pointing errors effect should be taken into account. Thus, in this work, we investigate the influence of these effects at the systems' performance, by means of the estimation of their outage capacity, over gamma modeled atmospheric turbulence channels with pointing errors. We derive closed form mathematical expressions for the above mentioned performance metric and we present our results, along with the corresponding simulation outcomes.

Outage capacity estimation of FSO links with pointing errors over gamma turbulence channels

/pdf/design-and-implementation-of-schumann-resonances-sensor-4l9k44hzcm.pdf

Design and implementation of Schumann resonances sensor platform

In 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 complexity measures of the heart rate regarding conditions leading to Sudden Cardiac Death (SCD), according to the “Natural Time Analysis (“N.T.A.”). This analysis demonstrates that when a complex system, like the human heart, approaches the critical state, it can provide us information well before the moment that the event will occur, like the case of SCD which study in this presentation.

Comparison of the R-R intervals in ECG and Oximeter signals to be used in complexity measures of Natural Time Analysis

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 signal generator, and the oscilloscope. The ELF SR detector used has been operating since 2016 near the Doliana village in the Ioannina prefecture, Northwestern Greece. At precisely this spot, far away from electromagnetic noise, the whole setup and experiment took place. The experiments performed with the proposed test fixture showed a sensitivity of 70 nV/pT/Hz and an apparent magnetic permeability at around 250 for the magnetic antenna. The total sensitivity of the ELF receiver was 210 mV/pT near 20 Hz, while the total input noise was around 0.04 pT.

https://www.mdpi.com/2079-9292/9/1/171/pdf

Vasilis Christofilakis

Papers

Design and analysis of a multiple-output transmitter based on DDS architecture for modern wireless communications

Outage capacity estimation of FSO links with pointing errors over gamma turbulence channels

Design and implementation of Schumann resonances sensor platform

Comparison of the R-R intervals in ECG and Oximeter signals to be used in complexity measures of Natural Time Analysis

Design and Implementation of a Test Fixture for ELF Schumann Resonance Magnetic Antenna Receiver and Magnetic Permeability Measurements