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

Weather effects on FSO network connectivity

The use of relays is one of the most promising methods for mitigating impairments of the performance of free-space optical (FSO) systems and extending their limited transmission range. However, several factors contribute to significant link performance degradation. Most severe is the influence of the adverse atmospheric conditions that frequently appear, thus making the design of strongly connected networks a demanding issue. In this paper, we consider a multiple-hop FSO network, where the nodes are distributed at fixed positions on a given path-link. We take account of the most critical weather phenomena, i.e., fog, rain, and snow, and derive analytical expressions for the node isolation probability, assuming a suitable path loss model. Next, we find the number of transceivers for a given path-link in order to achieve reliable performance. We also examine the reverse case; i.e., we find the total service length for a known number of FSO transceivers. The effect of the prime FSO modulation formats is also considered. The addressed analytical framework offers significant insights into the main factors that degrade the performance of FSO networks. It constitutes a valuable tool for telecom researchers to design such networks in practice. © 2009-2012 OSA

Connectivity issues for ultraviolet UV-C networks

This paper focuses on the connectivity issues of a non-line-of-sight (NLOS) optical wireless network operating in the ultraviolet UV-C spectral region. NLOS UV-C transmitters have a limited effective coverage and, hence, a dense node distribution is required in order to efficiently cover a large geographical area. Under this assumption, the concept of connectivity is more than important since it provides a strong indication of the network reliability and robustness. In the present study, we consider transmission with onoff keying and pulse position modulation schemes assuming both Gaussian and Poisson noise and adopt an effective experimental path loss model. Then, we evaluate the k-connectivity properties in terms of several network parameters. More precisely, we present and analyze the trade-off between node density and the degree of k-connectivity against other parameters (i.e.,transmitted power, supported data rate, and error probability). The derived results are depicted using appropriate figures and tables and constitute the theoretical basis for the design and implementation of a reliable UV-C network in practice. © 2011 Optical Society of America

Optimal Trade-Off between Depth and Coverage for a Vertical UOWC Link

The optimal trade-off between coverage and depth for a vertical underwater optical wireless communication (UOWC) link is thoroughly investigated in this letter. In the scenario under consideration, the optical transmitter is mounted at a fixed position on the sea surface and illuminates a subsea area through a Lambertian radiation pattern. A generic path loss expression is considered with a simplified extinction coefficient, and the most critical parameter values are taken into account. Then, a tractable mathematical formula relating the coverage radius with depth is readily extracted. The analysis of the coverage issues discussed herein takes on a particular research interest due to the rapid growth of UOWC networks nowadays. © 1989-2012 IEEE

Node Isolation Probability for Serial Ultraviolet UV-C Multi-hop Networks

Non-line-of-sight optical wireless transmission, operated in the unlicensed ultraviolet UV-C band, has been recently suggested as an alternative means of communication. However, due to limited coverage, relayed UV-C networks need to be deployed in order to supply communication services at large distances. In this paper, we consider a serial multi-hop UV-C network where the nodes are distributed at fixed positions on a given service interval. We adopt a suitable path loss model and derive analytical expressions for the node isolation probability assuming on-off keying and pulse position modulation formats. Moreover, we investigate the node density required to achieve connectivity for several geometrical transceiver configurations. The numerical results of this paper are of significant value for telecom researchers working toward a flexible UV-C network deployment in practice

Impact of a Randomly Placed Terminal on LiFi Performance

The ambitious goal of light-fidelity (LiFi) technology to provide seamless user mobility in indoor environments requires strong link connections to confront any stochasticity that possibly comes out. In this context, a robust LiFi network should effectively address the spatial random terminal locations inside its coverage volume. In contrast with the previously published works dealing mainly with randomness in two dimensions, the present study aims to provide a more realistic framework by treating the problem in the three-dimensional space. In view of the foregoing, a LiFi configuration is adopted where a terminal lies at a random position inside its truncated conic coverage volume at a distance from the transmitter. Some key statistics for a distance-related metric and the channel gain, including the probability density function, are derived in the first place. The investigation is then headed on the performance analysis of the LiFi link by providing closed-form expressions of the outage probability and the average bit error rate for on-off keying assuming indirect modulation/direct detection at the receiver. A multiuser scenario with several users placed inside the coverage volume is examined as well. A series of numerical results is also depicted to provide valuable insights into how the random terminal placement affects the link performance. © 1972-2012 IEEE

Exploring the Random Location Problem Inside a Truncated Conic Shape: Application in UAV Communications

More often than not, transmitters (Txs) in wireless networks operate using directional radiation patterns where the main radiation lobe creates a three-dimensional conic-shaped volume. In this context, any receiver (Rx) placed inside it can establish a strong communication link. Since the Rx location is not always known, it is desirable to investigate its random position effect to describe the link performance in a reliable manner. As in most cases, Rxs lie far away from the apex where the Tx is placed, we investigate the random location problem inside a truncated conic shape where the cone arises as a limiting case. To this end, we deduce a set of statistical metrics, including the random distance from the apex to a random point position. As an application, we focus on the outage performance of an unmanned aerial vehicle (UAV) communications scenario susceptible to Nakagami- m fading and derive novel closed-form expressions for the composite model. © 1972-2012 IEEE