On the Terminal Location Uncertainty in Elliptical Footprints: Application in Air-to-Ground Links

Wireless transmitters (Txs) radiating directionally downwards often generate circular footprints on the ground. In certain scenarios, using elliptical cells can offer increased flexibility for providing user coverage, owing to the unique network characteristics. For instance, an elliptical footprint can be produced when a practical directional antenna with unequal azimuth and elevation half-power beamwidths is used in high-speed railway networks. Another common scenario involves the production of an elliptical footprint when an airborne Tx radiates at an angle by tilting its directional antenna by a few degrees. This paper aims to investigate, for the first time, the association between the random user location within an elliptical coverage area and the performance of a wireless communication link by considering these scenarios. We assume an unmanned aerial vehicle (UAV) as a Tx, although a tall cellular base station tower could also be employed without losing generality. To better understand the impact of random location, we derive relevant distance metrics and investigate the outage probability of the link for the two scenarios, taking both random terminal location and fading impairments into account. The findings may provide valuable insights into the performance of similar wireless systems.Comment: 23 pages, 11 figure

Underwater Optical Wireless Communication Systems: A Concise Review

Underwater optical wireless communications (UOWC) have gained a considerable interest during the last years as an alternative means for broadband inexpensive submarine communications. UOWC present numerous similarities compared to free space optical (FSO) communications or laser satellite links mainly due to the fact that they employ optical wavelengths to transfer secure information between dedicated point‐to‐point links. By using suitable wavelengths, high data rates can be attained. Some recent works showed that broadband links can be achieved over moderate ranges. Transmissions of several Mbps have been realized in laboratory experiments by employing a simulated aquatic medium with scattering characteristics similar to oceanic waters. It was also demonstrated that UOWC networks are feasible to operate at high data rates for medium distances up to a hundred meters. However, it is not currently available as an industrial product and mainly test‐bed measurements in water test tanks have been reported so far. Therefore, extensive research is expected in the near future, which is necessary in order to further reveal the “hidden” abilities of optical spectrum to transfer broadband signals at higher distances. The present work summarizes the recent advances in channel modeling and system analysis and design in the area of UOWC

Connectivity issues for optical wireless networks

Optical Wireless Communications (OWC) networks are becoming more and more popular for delivering broadband traffic since they are introducing significant advantages against the other alternative technologies. Operating wavelengths range from ultraviolet (UV) to the infrared (IR) portion of the electromagnetic spectrum and present significant attenuation from channel impairments. As a result, the transmission range is significantly reduced when a single hop is used. Therefore, multi-hop operation, which is a common technique in wireless RF communication systems, is adopted in order to increase the effective distance between transmitter and receiver. To improve their reliability connectivity issues need to be investigated. Connectivity has been investigated in RF ad hoc networks (either one or two dimension) in contrast with OWC networks. The present dissertation aims to examine this research area by connecting the minimum transmission range ensuring connectivity with a plethora of parameters such as the adopted modulation and/or coding format, the transmitted power, the supported data rate and the error probability. Analytical expressions are extracted and the derived results are depicted using appropriate figures. The outcomes constitute a valuable tool to design such networks in practice.Οι ασύρματες οπτικές επικοινωνίες (Optical Wireless Communications – OWC) γίνονται ολοένα και πιο δημοφιλείς καθώς παρουσιάζουν σημαντικά πλεονεκτήματα για τη μεταφορά ευρυζωνικής κίνησης ως προς τις άλλες εναλλακτικές τεχνολογίες. Τα μήκη κύματος λειτουργίας αυτής της τεχνολογίας εκτείνονται από το υπεριώδες (Ultraviolet – UV) έως το υπέρυθρο (infrared – IR) τμήμα του ηλεκτρομαγνητικού φάσματος και παρουσιάζουν σημαντική εξασθένιση από τις αλλοιώσεις που εισάγει το κανάλι διάδοσης. Ως αποτέλεσμα, η εμβέλεια εκπομπής περιορίζεται σημαντικά με χρήση απλής ζεύξης.Για το λόγο αυτό, υιοθετείται η χρήση πολυαλματικών ζεύξεων με χρήση ενδιάμεσων πομποδεκτών (κόμβων), όπως και στα ασύρματα δίκτυα ραδιοσυχνοτήτων, προκειμένου να αυξηθεί η ενεργός απόσταση μεταξύ πομπού και δέκτη. Σημαντική παράμετρος για την αξιοπιστία τέτοιων δικτύων αποτελεί η επίτευξη κατάλληλων συνθηκών που να εγγυώνται έναν ικανοποιητικό βαθμό συνδεσιμότητας των κόμβων τους. Η συνδεσιμότητα έχει μελετηθεί εκτενώς σε ad-hoc δίκτυα ραδιοσυχνοτήτων, είτε μιας είτε δυο διαστάσεων, σε αντίθεση με τα ασύρματα οπτικά δίκτυα όπου υπάρχει ένα κενό στη σχετική βιβλιογραφία. Η παρούσα διατριβή συμβάλει στη μελέτη της περιοχής αυτής συνδέοντας την ελάχιστη εμβέλεια που διασφαλίζει τη συνδεσιμότητα με μια πληθώρα παραγόντων όπως το σχήμα διαμόρφωσης ή/και κωδικοποίησης που έχει υιοθετηθεί, η ισχύς εκπομπής, ο ρυθμός δεδομένων και η πιθανότητα σφάλματος. Η μελέτη της απόδοσης γίνεται με την εξαγωγή κατάλληλων αναλυτικών εκφράσεων των βασικών δεικτών που μετρούν το βαθμό συνδεσιμότητας και τα αποτελέσματα παρουσιάζονται με τη βοήθεια κατάλληλων σχημάτων και πινάκων. Τα συμπεράσματα της εργασίας αποτελούν χρήσιμο θεωρητικό εφόδιο για την πρακτική υλοποίηση και εφαρμογή εύρωστων ασύρματων οπτικών συστημάτων

Using a modified 3D-printer for mapping the magnetic field of RF coils designed for fetal and neonatal imaging

An experimental setup for characterizing the magnetic field of MRI RF coils was proposed and tested. The setup consisted of a specially configured 3D-printer, a network analyzer and a mid-performance desktop PC. The setup was tested on a single loop RF coil, part of a phased array for fetal imaging. Then, the setup was used for determining the magnetic field characteristics of a high-pass birdcage coil used for neonatal MR imaging with a vertical static field. The scattering parameter S21, converted into power ratio, was used for mapping the B1 magnetic field. The experimental measurements from the loop coil were close to the theoretical results (R = 0.924). A high degree of homogeneity was measured for the neonatal birdcage RF coil. The development of MR RF coils is time consuming and resource intensive. The proposed experimental setup provides an alternative method for magnetic field characterization of RF coils used in MRI