Optimization of Ultra-Dense Wireless Powered Networks.

The internet-of-things (IoT) is expected to have a transformative impact in several different domains, including energy management in smart grids, manufacturing, transportation, smart cities and communities, smart food and farming, and healthcare. To this direction, the maintenance cost of IoT deployments has been identified as one of the main challenges, which is directly related to energy efficiency and autonomy of IoT solutions. In order to increase the energy sustainability of next-generation IoT, wireless power transfer (WPT) emerged as a promising technology; however, its effectiveness is hindered as the distance between the base station and the wireless powered IoT devices increases. To counter this effect, decentralized approaches based on the use of distributed densely deployed remote radio heads (RRHs) can be utilized to diminish the distance between the transmitting and the receiving nodes. A trade-off ensues from the use of RRHs as power beacons (PBs) or access points (APs) that enable either energy transfer during downlink or information reception during uplink, respectively. To balance this trade-off, in this work, the maximization of the ergodic rate in ultra-dense wireless powered networks is investigated. In more detail, three different protocols are introduced, optimized, and compared to each other: density splitting, time splitting, and hybrid time and density splitting, which are based on the optimization of the portion of the number of RRHs that are employed as PBs or APs at a specific time instance. Additionally, two different policies are taken into account regarding the PBs' power constraint. The formulated problems that correspond to the combination of the proposed protocols with each of the two considered power constraint policies are optimally solved by using convex optimization tools and closed-form solutions are derived that result to useful insights. Finally, numerical results are provided, which illustrate the ergodic rate achieved by each of the proposed protocols and offer interesting conclusions regarding their comparison, which are directly linked to design guidelines and the required capital and operational expenses.

Ground-to-air FSO communications: when high data rate communication meets efficient energy harvesting with simple designs.

The capability of free-space optical (FSO) communications in delivering very high data rates and the agility of unmanned aerial vehicle (UAV) flying platforms render FSO-UAV-based solutions attractive for delivering 5G wireless communication services. In parallel, research on simultaneous information and power transfer, whether in the context of radio frequency (RF) networks or indoor optical wireless communication (OWC) networks, is on the rise. Even though the operation of a UAV is limited by its battery lifetime, the concept of energy harvesting (EH) from the information-carrying FSO signals was not deeply investigated in the literature. This paper highlights the inherent EH capabilities in FSO transmissions and investigates novel signal design methodologies for boosting the EH efficiency. We focus on the ground-to-air communications where the ground-based FSO transmitter is connected to the power grid and, hence, is governed by a peak-power constraint and not an average-power constraint. For this setup, simulations carried out under different weather conditions demonstrate that high data rates can be associated with significant amounts of harvested energy using simple transceiver architectures.

Optical wireless cochlear implants.

In the present contribution, we introduce a wireless optical communication-based system architecture which is shown to significantly improve the reliability and the spectral and power efficiency of the transcutaneous link in cochlear implants (CIs). We refer to the proposed system as optical wireless cochlear implant (OWCI). In order to provide a quantified understanding of its design parameters, we establish a theoretical framework that takes into account the channel particularities, the integration area of the internal unit, the transceivers misalignment, and the characteristics of the optical units. To this end, we derive explicit expressions for the corresponding average signal-to-noise-ratio, outage probability, ergodic spectral efficiency and capacity of the transcutaneous optical link (TOL). These expressions are subsequently used to assess the dependence of the TOL's communication quality on the transceivers design parameters and the corresponding channels characteristics. The offered analytic results are corroborated with respective results from Monte Carlo simulations. Our findings reveal that the OWCI is a particularly promising architecture that drastically increases the reliability and effectiveness of the CI TOL, whilst it requires considerably lower transmit power when compared to the corresponding widely-used radio frequency (RF) solution.

Mean level signal crossing rate for an arbitrary stochastic process: comment.

In J. Opt. Soc. Am. A27, 797 (2010), Yura and Hanson derived what they claim to be "a general expression for the mean level crossing rate for an arbitrary" random process following any desired probability distribution function. On the other hand, the authors themselves assert that in some cases their results "differ somewhat from the result reported in the literature." This discrepancy "remains unexplained" by the authors "and is laid open for future discussion." In this note, we explain the reason for such discrepancy and show that the Yura-Hanson formula is indeed a special-case solution. A more general solution is then given that is applicable to arbitrary random processes and that is fully consistent with the Rice mean level crossing rate formula.