Article
Optimization of Ultra-Dense Wireless Powered Networks
Panagiotis D. Diamantoulakis , Vasilis K. Papanikolaou and George K. Karagiannidis *
Citation: Diamantoulakis, P.D.;
Papanikolaou, V.K.; Karagiannidis, G.K.
Optimization of Ultra-Dense Wireless
Powered Networks. Sensors 2021, 21,
2390. https://doi.org/10.3390/
s21072390
Academic Editor: Matteo Anedda
Received: 4 March 2021
Accepted: 23 March 2021
Published: 30 March 2021
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4.0/).
Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki,
54 124 Thessaloniki, Greece; padiaman@auth.gr (P.D.D.); vpapanikk@auth.gr (V.K.P.)
* Correspondence: geokarag@auth.gr
Abstract:
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.
Keywords:
wireless power transfer; internet-of-things; ultra-dense; remote radio heads; optimization
1. Introduction
A crucial challenge in the current and the coming wireless networks lies with the
improvement of energy efficiency (EE). Specifically, the reduction of energy consumption is
particularly important in the case of internet-of-things (IoT) devices, the number of which
continues to soar, reaching billions of connected devices in the coming decade [
1
]. A pivotal
issue, from which IoT devices suffer, is their limited battery lifetime, which is primarily
used for communications. At the same time, their deployment makes it difficult or even im-
possible to replace the batteries in each device. As such, the prolongation of battery lifetime
of mobile devices, i.e., sensors and actuators, has been identified as an important priority
in the next-generation IoT (NGIoT) [
2
,
3
], being of paramount importance in improving
autonomy, scalability, and intelligence of smart grids, manufacturing, transportation, smart
cities and communities, smart food and farming, and healthcare applications. To achieve
the aforementioned objectives, energy harvesting has been recognized as a promising
approach for the NGIoT, which creates several non-trivial challenges at the design and
optimization of appropriate communication protocols [4,5].
Sensors 2021, 21, 2390. https://doi.org/10.3390/s21072390 https://www.mdpi.com/journal/sensors
