1 Massive Wireless Energy Transfer: Enabling Sustainable IoT Towards 6G Era Onel L. A. L´opez, Member, IEEE, Hirley Alves, Member, IEEE, Richard D. Souza, Senior Member, IEEE, Samuel Montejo-S´anchez, Member, IEEE, Evelio M. G. Fern´andez, Member, IEEE, and Matti Latva-aho, Senior Member, IEEE Abstract—Recent advances on wireless energy transfer (WET) major concern due to the lack of mature solutions for powering make it a promising solution for powering future Internet of and keeping uninterrupted operation of the massive number of Things (IoT) devices enabled by the upcoming sixth generation devices. (6G) era. The main architectures, challenges and techniques for efficient and scalable wireless powering are overviewed in Technological advances on artificial intelligence (AI)/ma- this paper. Candidates enablers such as energy beamforming chine learning (ML), molecular, backscatter and visible (EB), distributed antenna systems (DAS), advances on devices’ light communications, fog/edge computing, and metamateri- hardware and programmable medium, new spectrum opportu- als/metasurfaces will certainly facilitate sustainability [3]–[5]. nities, resource scheduling and distributed ledger technology are In addition to these, research community and industry are outlined. Special emphasis is placed on discussing the suitability of channel state information (CSI)-limited/free strategies when considering energy harvesting (EH) techniques an attractive powering simultaneously a massive number of devices. The solution to externally recharge batteries or avoid replacement benefits from combining DAS and EB, and from using average [6]–[9], which may be not only costly but also impossible CSI whenever available, are numerically illustrated. The pros in hazardous environments, building structures or the human and cons of the state-of-the-art CSI-free WET techniques in body. Therefore, EH is foreseen as a key component of future ultra-low power setups are thoroughly revised, and some possible future enhancements are outlined. Finally, key research directions IoT networks since it allows i) wireless charging, which towards realizing WET-enabled massive IoT networks in the 6G significantly simplifies the servicing and maintenance of IoT era are identified and discussed in detail. devices, while increasing their durability thanks to contact- Index Terms—massive wireless energy transfer, channel state free feature; and ii) enhanced energy efficiency and network- information, sixth generation, Internet of Things, distributed wide reduction of emissions footprint. Notice that the battery antenna systems, distributed ledger technology, energy beam- recharging and waste processing is already a critical problem forming, intelligent reflective surfaces, millimeter wave, ultra-low for which EH is an attractive clean solution [8], [10]. power A. EH Technologies I. INTRODUCTION EH technologies can be classified into the following two The sixth generation (6G) of wireless systems targets a data- categories [6]: driven sustainable society, enabled by near-instant, secure, un- limited and green connectivity [1]–[3]. Stringent performance • ambient EH, which relies on energy resources that are requirements in terms of security and trust, throughput, sensing readily available in the environment and that can be capabilities, dependability, scalability and energy efficiency, as sensed by EH receivers; illustrated in Fig. 1, have been set by industry and academy to • dedicated EH, which are characterized by on-purpose fulfill such a vision. Specifically, the ultimate vision in terms energy transmissions from dedicated energy sources to of energy efficiency is that of a green society assisted by 6G EH devices. networks, specially by zero-energy/cost/emission Internet of Different from dedicated EH setups, ambient EH does not arXiv:1912.05322v2 [cs.NI] 2 Jan 2021 Things (IoT) deployments [3], [4]. However, this is still a require additional resource/power consumption from the sur- rounded (sometimes newly-deployed) energy network infras- Onel L. A. L´opez, Hirley Alves and Matti Latva-aho are with the tructure. However, temporal/geographical/environmental cir- Centre for Wireless Communications (CWC), University of Oulu, Finland. {onel.alcarazlopez,hirley.alves,matti.latva-aho}@oulu.fi cumstances may limit their service guarantee making them Richard D. Souza is with Federal University of Santa Catarina (UFSC), inappropriate (at least as standalone) for many use cases Florian´opolis, Brazil. {[email protected]}. with quality of service (QoS) requirements. The main energy S. Montejo-S´anchez is with Programa Institucional de Fomento a la I+D+i, Universidad Tecnol´ogica Metropolitana, Santiago, Chile. sources within the above categories and their associated char- {[email protected]}. acteristics are summarized in Fig. 1. E.M.G. Fern´andez is with Federal University of Paran´a(UFPR), Curitiba, The ambient EH methods based on light intensity, thermal Brazil. {[email protected]}. This work is partially supported by Academy of Finland (Aka) (Grants energy or even wind, are either highly sensitive to blocking n.319008, n.307492, n.318927 (6Genesis Flagship)), as well as FONDE- or perform with low conversion efficiency [11]. But maybe CYT Iniciaci´on No. 11200659, FONDECYT Regular No. 1201893, and more importantly, they demand an add-on EH material & FONDEQUIP EQM180180, in Brazil by the National Council for Scientific and Technological Development (CNPq), and project Print CAPES-UFSC circuit, which in practice limits the form factor reduction to the “Automation 4.0”. desired levels for many use cases. The same strong limitation 2 Fig. 1. Radio frequency (RF)-WET as key enabler of energy-efficient and scalable 6G networks. Specifically, the figure summarizes 6G performance requirements [1]–[3], technological enablers of energy efficiency in 6G [3]–[5], EH techniques [11]–[13], characteristics and advantages of RF-WET [6], [14]. Regarding the latter, a comparison (in terms of coverage, form factor, harvestable energy, and multi-user and mobility native support) between RF-WET and main competitors (electrostatic or capacitive coupling, inductive coupling and magnetic coupling) is depicted [6], [14]. All in all, RF-WET constitutes the most prominent technology for massively and wirelessly powering low-energy IoT deployments. is characteristic of the induction, magnetic resonance coupling worth pointing to laser power beaming as another potential and many piezoelectric-based dedicated EH methods. The EH technology, which uses highly concentrated laser light induction method is based on the inductive coupling effect of aiming at the EH receiver to achieve efficient power delivery non-radiative electromagnetic fields, including the inductive over long distances [15]. However, this technology may be and capacitive mechanisms, and is subject to coupling mis- just suitable for powering complex devices with high power alignment impairments that limit the range and scalability [12]. consumption demands, e.g., smart phones, while it requires The magnetic resonance coupling exploits the fact that two ob- accurate pointing towards the receiver. jects resonating at the same frequency tend to couple with each other most efficiently. In fact, by carefully tuning the transmit- B. Scope and Contributions of this Work ter and receiver circuits, magnetic resonant coupling is able to In contrast to the above discussed EH methods, RF-based achieve higher power transfer efficiency over longer distances than inductive coupling [12]. Meanwhile, piezoelectric-based EH inherently allows: EH relies on the energy coming from a mechanical strain • small-form factor implementation. Devices’ dimensions captured by a, usually fragile, piezoelectric material layer on have been determined by the size of traditional bat- top of the wireless device [12]. In general, electrostatic and teries, while RF-EH batteries (if needed) are smaller acoustic methods overcome the devices’ size limitation, but [8]. Additionally, the same RF circuitry for wireless they are either limited to very short distance operations or communications can be re-utilized totally or partially for to very specific applications. Specifically, in the electrostatic RF EH; method, a mechanical motion or vibration is used to change • native multi-user support since the same RF signals can the distance between two electrodes of a capacitor against an be harvested simultaneously by several devices. electric field, thus, transforming the vibration or motion into The above key features, when combined, make RF EH a electricity due to the capacitance change [12]. Meanwhile, strong candidate, much more suitable than the EH technologies acoustic energy transfer, which is usually in the range of based on other energy sources, for powering many low-power ultra-sound, is quite efficient but mostly for transferring the IoT use cases. When the number of devices increases, RF- energy over non-aerial media such as water, tissue or metals, EH technologies become even more appealing. Fig. 1 also and it is more appropriate for medical applications [13]. It is illustrates a comparison between radiative RF wireless energy transfer (WET) and the main non-radiative competitors, and 3 TABLE I BRIEF SUMMARY ON EXISTING SURVEYS AND OVERVIEWS OF RF-EH/WET (2015-2020) Year Ref. Focused issues Main contents 2015 [6] wireless powered commu- ⊲RF-enabled WET technologies & their applications to wireless communications, –network models, –signal nications processing methods to enhance WET efficiency,