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.
Highlights
A crucial challenge in the current and the coming wireless networks lies with the improvement of energy efficiency (EE)
For the demonstration of the performance that is achieved by the considered protocols, an ultra-dense wireless network has been considered with the positions of the remote radio heads (RRHs) being generated according to a homogeneous Poisson point process (PPP) of density λ (RRHss/m2)
This is because when the average transmit power is relatively low, mitigating the path-loss is more important than increasing the total time that is used for both information transmission and energy harvesting
Summary
A crucial challenge in the current and the coming wireless networks lies with the improvement of energy efficiency (EE). The most important challenge of WPNs is the double near-far problem, according to which the users located near the cell edge, receive a lower amount of wireless energy compared to users located near the base station, while they would require higher transmit power in order to achieve comparable quality-of-service (QoS) during uplink [9,10,11]. This challenge can be mitigated by using decentralized approaches that reduce the distance between the remote radio heads (RRHs) and the wireless powered device [12]. This leads to ultra-dense architectures, the performance of which can be modeled and further investigated by using tools from stochastic geometry [13,14]
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