Tο αντικείμενο της διδακτορικής διατριβής είναι η μελέτη και σε βάθος ανάλυση και μοντελοποίηση της ηλεκτρομαγνητικής αλληλεπίδρασης του ανθρώπινου σώματος και των κεραιών που χρησιμοποιούν οι φορητές τερματικές συσκευές των σύγχρονων συστημάτων κινητής τηλεφωνίας. Ο στόχος είναι διπλός: αφενός μεν να υπολογιστεί η υποβάθμιση της απόδοσης της κεραίας, που προκαλείται από την παρουσία του σώματος του χρήστη σε μικρή απόσταση από αυτή και αφετέρου να εξεταστεί και να προσδιοριστεί επακριβώς το ποσό της ηλεκτρομαγνητικής ακτινοβολίας που απορροφάται από το ανθρώπινο σώμα και ειδικότερα από τον ανθρώπινο εγκέφαλο κατά τη χρήση του κινητού τηλεφώνου. Ο απώτερος σκοπός είναι η συμβολή στην ανάπτυξη ασύρματων τερματικών (πχ. κινητά τηλέφωνα) που θα είναι πιο αποδοτικά στη λειτουργία τους και ταυτόχρονα περισσότερο ασφαλή για το χρήστη τους. Στην παρούσα διδακτορική διατριβή πραγματοποιήθηκε εκτενής μελέτη και ανάλυση των παραμέτρων που σχετίζονται με την αλληλεπίδραση μεταξύ της κεραίας ασύρματων τερματικών συσκευών και του σώματος του χρήστη. Σχεδιάστηκαν, μοντελοποιήθηκαν, υλοποιήθηκαν και μετρήθηκαν πειραματικά πρωτότυπα τερματικών συσκευών παρουσία ομοιωμάτων του ανθρώπινου κεφαλιού με σκοπό τον υπολογισμό της απορρόφησης ακτινοβολίας από το κεφάλι και της μεταβολής της απόδοσης της κεραίας του τερματικού. Αναπτύχθηκε κατάλληλη μεθοδολογία μετρήσεων στο μακρινό πεδίο για την αξιόπιστη και ακριβή μέτρηση των χαρακτηριστικών των κεραιών του τερματικού. Τα αποτελέσματα έδειξαν έντονη αλλαγή των χαρακτηριστικών της κεραίας και του διαγράμματος ακτινοβολίας της, παρουσία του κεφαλιού του χρήστη. Επίσης, υπολογίστηκε η απότομη πτώση της απορρόφησης ακτινοβολίας και η αύξηση της απόδοσης με την απομάκρυνση του τερματικού από το κεφάλι. Επιτεύχθηκε σημαντική βελτίωση της λειτουργίας μέσω της μορφοποίησης του διαγράμματος ακτινοβολίας που κατορθώνεται με τη χρήση πολλαπλών κεραιών (συστοιχία) στο τερματικό. Τέλος εξετάστηκε η επίδραση του χεριού του χρήστη στην απόδοση ενός ΜΙΜΟ τερματικού και βρέθηκε μείωση της μέσης χωρητικότητας του καναλιού με την παρουσία του χεριού. / Τhe object of this doctoral thesis is the study, in depth analysis and modelling of the electromagnetic interaction between the human body and the antennas used in the handsets of modern wireless telecommunication systems. The aim is twofold. On one hand is the estimation of the antenna efficiency reduction that is caused by the presence of the user’s body in small distance and on the other hand is the study and precise determination of the electromagnetic radiation absorbed by the human body (especially the human head) at the use of a wireless terminal. The final aim is the contribution in the design of wireless terminals (e.g. mobile telephones) that will be more efficient in their operation and simultaneously safer for their user. In this thesis, an extensive study and analysis of the parameters related with the interaction between the wireless terminal antenna and the user’s body were realized. Experimental terminal prototypes were designed, modelled, constructed and measured in the presence of human head models in order to estimate the radiation absorption from the head and the degradation of the antenna efficiency. An appropriate measurements methodology at the far field was developed for the precise measurement of the terminal antenna characteristics. According to the results, an intense change of the antenna characteristics and of its radiation diagram in the presence of the user’s head was observed. Also, the rapid decrease of absorbed power and the increase of the efficiency were calculated after moving the handset away from the head. An important operation improvement was achieved with beamforming, which is realized using multiple antennas at the terminal. Finally, the effect of the user’s hand at MIMO terminal performance was examined and a reduction of the mean capacity of the channel in the presence of the hand was found.

In the era of Internet of things (IoT), sensor-equipped devices exchange data over networks. In battery powered IoT devices, the lifespan of the devices is often much longer than the battery life, leading to multiple costly and environmentally hazardous battery replacements during the operational life of the devices. As a result, there is a growing interest in using rechargeable batteries that can be wirelessly fast charged to prolong the lifespan of IoT devices and their batteries. In wireless power transfer based on induction, the transmitter and receiver antennas can be accurately modeled as two coils in separate circuits. The transmitter coil, energized by alternating current, generates an oscillating magnetic field that induces an electric field in the nearby receiver coil, following Faraday's law of induction. By connecting a resistive load to the receiver coil, it is then possible to extract energy from the induced electric field. This project investigates inductive fast charging for IoT devices with a focus on the electromagnetic power transfer. Two different types of coil antennas were simulated in a solver based on the finite element method and tested in lab for verification purpose. One was a transformer-like ETD coil and the other a flat spiral coil. Both the transmitter and receiver coils were compensated with a capacitor in series to allow for increased efficiency and power transfer at the designated frequency of 100 kHz. The compensating capacitors were tuned such that frequency bifurcation or frequency splitting was avoided. Due to the higher quality factor of the ETD coil compared to the spiral coil they were compensated differently to operate at the resonance peak. The simulation and the experimental tests agreed well, and the findings indicate that both types of coils demonstrate the ability to transfer high power with high efficiency. Theoretically there is no limit in the power transfer for both types of coils since it is proportional to the square of the excitation voltage. All tested coils exhibited the ability to transfer a power of at least 30 W with an 86 to 92 % efficiency without experiencing any significant temperature elevation. The advantages of each coil depend on the design of the systems surrounding the power transfer unit and the nature of the built charging system. For scenarios where the equivalent load resistance of the battery charger unit on the receiver remains relatively constant throughout the charging process, the spiral coil proves to be a suitable choice due to its inherent capacity for easy dimensioning, allowing optimal efficiency for a specific load resistance. Conversely, if the equivalent load resistance fluctuates significantly during the charging process, the ETD coil would be a better alternative, since it exhibits small load dependence and high efficiency. Finally, to further increase the validity of the simulation model, the full magnetization curve of the ferrite core and a more general core loss model should be implemented to enhance the accuracy in studying the effects of higher harmonics and when operating closer to saturation.

For many years batteries have been used as the main power sources for portable electronic devices. However, the rate of scaling in integrated circuits and micro-electro-mechanical systems (MEMS) has been much higher than that of the batteries technology. Therefore, a need to replace these temporary energy reservoirs with small sized continuously charged energy supply units has emerged. These units, named as energy harvesters, use several types of ambient energy sources such as heat, light, and vibration to provide energy to intelligent systems such as sensor nodes. Among the available types, vibration based electromagnetic (EM) energy harvesters are particularly interesting because of their simple structure and suitability for operation at low frequency values (&lt / 10 Hz), where most vibrations exits. However, since the generated EM power and voltage is relatively low at low frequencies, high performance interface electronics is required for efficiently transferring the generated power from the harvester to the load to be supplied. The aim of this study is to design low power and efficient interface electronics to convert the low voltage and low power generated signals of the EM energy harvesters to DC to be usable by a real application. The most critical part of such interface electronics is the AC/DC converter, since all the other blocks such as DC/DC converters, power managements units, etc. rely on the rectified voltage generated by this block. Due to this, several state-of-the-art rectifier structures suitable for energy harvesting applications have been studied. Most of the previously proposed rectifiers have low conversion efficiency due to the high voltage drop across the utilized diodes. In this study, two rectifier structures are proposed: one is a new passive rectifier using the Boot Strapping technique for reducing the diode turn-on voltage values / the other structure is a comparator-based ultra low power active rectifier. The proposed structures and some of the previously reported designs have been implemented in X-FAB 0.35 &micro / m standard CMOS process. The autonomous energy harvesting systems are then realized by integrating the developed ASICs and the previously proposed EM energy harvester modules developed in our research group, and these systems have been characterized under different electromechanical excitation conditions. In this thesis, five different systems utilizing different circuits and energy harvesting modules have been presented. Among these, the system utilizing the novel Boot Strap Rectifier is implemented within a volume of 21 cm3, and delivers 1.6 V, 80 &micro / A (128 &micro / W) DC power to a load at a vibration frequency of only 2 Hz and 72 mg peak acceleration. The maximum overall power density of the system operating at 2 Hz is 6.1 &micro / W/cm3, which is the highest reported value in the literature at this operation frequency. Also, the operation of a commercially available temperature sensor using the provided power of the energy harvester has been shown. Another system utilizing the comparator-based active rectifier implemented with a volume of 16 cm3, has a dual rail output and is able to drive a 1.46 V, 37 &micro / A load with a maximum power density of 6.03 &micro / W/cm3, operating at 8 Hz. Furthermore, a signal conditioning system for EM energy harvesting has also been designed and simulated in TSMC 90 nm CMOS process. The proposed ASIC includes a highly efficient AC-DC converter as well as a power processing unit which steps up and regulates the converted DC voltages using an on-chip DC/DC converter and a sub-threshold voltage regulator with an ultra low power management unit. The total power consumption on the totally passive IC is less than 5 &micro / W, which makes it suitable for next generation MEMS-based EM energy harvesters. In the frame of this study, high efficiency CMOS rectifier ICs have been designed and tested together with several vibration based EM energy harvester modules. The results show that the best efficiency and power density values have been achieved with the proposed energy harvesting systems, within the low frequency range, to the best of our knowledge. It is also shown that further improvement of the results is possible with the utilization of a more advanced CMOS technology.