A rapid growth of the volume of low-power wireless communication terminals is anticipated in the foreseeable future. This trend is attributed to the emergence of the 5G and Internet-of-Things (IoT)/Machine-to-Machine (M2M) communication paradigms. The considered energy-constrained devices are often installed at hard-to-reach locations or they correspond to mobile nodes. Smartphones, wireless sensors, and IoT nodes constitute representative examples of user equipment (UE) devices with limited energy storage capacity.

The explosive number, inaccessibility, and mobility of such UEs render the frequent wired recharging or replacement of their batteries a challenging and costly task. Wireless power transfer (WPT) represents a paradigm that addresses this issue via wireless replenish of the UEs’ energy through the transmission of radio frequency (RF) signals. WPT through radio waves achieves long ranges, as opposed to corresponding techniques based on electric or magnetic coupling. Furthermore, the use of dedicated RF energy sources provides stable and controllable wireless power supply, thus allowing for quality-of-service (QoS) provisioning. This is in contrast to energy harvesting (EH) based on renewable energy sources such as solar and wind or on ambient RF signals. In addition, WPT inherently supports one-to-many wireless charging and user mobility. These attributes make WPT a promising energy solution for the highly dense heterogeneous network setups envisioned in 5G and beyond (B5G), thus paving the way for the next-generation energy-autonomous wireless communication systems.

The integration of RF-WPT technology into communication networks introduces a fundamental co-existence of information and energy flows, wherein RF signals are used in order to convey information and/or energy. The efficient management of these two flows through sophisticated networking protocols, signal processing and communication techniques, and network architectures gives rise to a new communication paradigm called wireless powered communications (WPC).

APOLLO targets the emerging frontier research field of WPC technology. The realization of the WPC vision is a demanding endeavour that requires the establishment of the fundamental performance limits and trade-offs; the development of novel physical layer (PHY) and resource allocation techniques, including concepts like waveform design, precoding design, and information/energy transfer scheduling, or/and the adaption of the relevant state-of-the-art (SoTA) schemes; the design of new network architectures and communication protocols or/and the adaptation of existing ones; the design of innovative receiver architectures; the evaluation of the derived WPC methods both analytically and via link-level and system-level numerical simulations; the design, implementation, and testing of relevant prototypes; their integration into testbeds; and the conduction of proof-of-concept (PoC) experimental demonstrations in realistic environments.

APOLLO considers all the aspects mentioned above, but it deviates from the vast majority of relevant studies in that it focuses mainly on multi-cell, multi-user systems with multi-antenna nodes as well as on the integration of WPC with novel relevant technologies for M2M communications and 5G/B5G systems, such as small cells and ultra-dense networks, massive multiple-input multiple-output (MIMO) and cell-free massive MIMO, and millimetre-wave (mmWave). The increased degrees-of-freedom (DoF) and advanced features of these setups and technologies can benefit both information and energy transmission and significantly affect all the considered areas of research on WPC.
By bringing together a team of experts on complementary disciplines such as information theory, signal processing, stochastic geometry, probability theory, optimization theory, antenna design, and electronics/microwave engineering, APOLLO aspires to achieve a holistic study of all WPC aspects under the aforementioned context and, eventually, to: 1) push the boundaries of WPC well beyond what is currently possible; 2) shed light on the performance, design, testing, and development of contemporary and future WPC systems; and 3) facilitate the transition of this radically new approach in wireless communications from its conceptual phase to practical implementation.

The duration of APOLLO will be 5 years (60 months). The various task are grouped into Work Package (WP).

• WP1: Project management. The objective of WP1 is to guarantee the success of the project by ensuring efficient operation of all project components, including proper decision-making, conflict resolution and risk management at all levels, as well as tight coordination among researchers with different backgrounds involved in the project. The project will be implemented based on the following Gantt chart.
• WP2: Fundamental theoretical studies of WPC. This WP discusses the required theoretical framework to target the objectives of this proposal. It includes the considered application scenarios and fundamental concepts associated with information theoretic limits and waveform design.
• WP3: 5G/B5G for WPC networks. This WP discusses the impact of small-cell architectures as well as of millimetre-wave (mmWave) and multi-antenna communication technologies on WPC systems.
• WP4: WPC-based M2M communications. This WP studies the integration of WPC technology in machine-type communications. Battery-powered/battery-less wireless sensor networks, event monitoring sensors, and mobility issues represent relevant research areas.
• WP5: Hardware implementations and testbed. This WP deals with the implementation of a proof-of-concept (PoC) demonstration for SWIPT. It also includes the implementation of some promising schemes that will be developed in WP3 and WP4.

The concept proposed by APOLLO constitutes a paradigm shift for wireless networks and will radically change the way these networks are designed, organized, and operated. WPC systems require a fundamental redesign in all the layers of the protocol stack in order to handle the conflicting interaction between information and energy transfer. APOLLO targets fundamental issues regarding modeling, analysis, design, and implementation of WPC systems, considering all architectures mentioned in the literature and focusing on applications in two areas of particular high potential: 1) M2M communications, and 2) 5G/B5G mobile communication networks.

The research objectives include:

• Establishment of the fundamental theoretical studies on WPC. While potential applications of WPC are abundant, so far there has been no comprehensive theoretical study for the design of such systems. We will study the fundamental limits and provide a rigorous analysis of the trade-off between energy and information transfer, as well as the channel capacities of single-user and multi-user configurations with/without feedback channel. In addition, the impact of the waveform design on the efficiency of the WPC systems will be investigated. The goal is the coordination of information and energy transfer to achieve higher information throughput and/or power transfer, via network information theory and centralized/decentralized optimization techniques.
• PHY-layer design adaptation and development of resource allocation techniques for WPC. Traditionally, the purpose of PHY design and resource allocation is to ensure effective and reliable information transmission. With this goal in mind, many efficient techniques such as multiple antennas, cooperative communications, scheduling etc. have been proposed. In APOLLO, PHY design and resource allocation techniques need to integrate information and energy transmission. Hence, we will re-consider and re-think current design criteria and propose new algorithms to enhance the efficiency, reliability, and robustness of both information and energy transfer.
• Investigation of small-cell WPC network architectures. One of the main design tools for the next generation of communication systems is the heterogeneity and the ultra-densification of the network, in order to ensure high area spectral efficiency. This technology has a fundamental beneficial impact on the performance of WPC systems, since it can overcome the RF propagation losses, which is the main drawback of the current WPC deployments. We will study the interplay between heterogeneous small-cell networks with WPC by taking into account associated 5G developments, such as mmWave spectrum, antenna directionality, cache-enabled cells, and massive MIMO. The challenge of designing new WPC network architectures will be tackled using the powerful tool of stochastic geometry.
• Hardware implementation of SWIPT. Most current demonstrations of WPT focus on energy harvesting and ignore the information transmission dimension. In contrast, we will implement metamaterial energy harvesting sensors that are capable of harvesting ambient RF signals from indoor WiFi APs and to convert these to direct-current (DC) power, while allowing information to be sent through the same WiFi channels. The APOLLO project will develop PoC devices for illustrating the promising benefits of SWIPT in practical systems.

WPC technology will have a profound impact on future M2M communications and 5G/B5G mobile communication networks and may fundamentally reshape the landscape of related industries. These include a wide range of verticals, such as the automotive industry, the healthcare industry, the logistics industry, etc.

We envision that WPC will be an important building block in both areas for the control and the wireless charging of billions of devices in order to improve user experience and convenience. We believe that by addressing the fundamental WPC technical challenges in these two futuristic applications, fruitful cross-fertilization of results will occur. Specifically, the main applications of interest are summarized as follows:

1. M2M Communications: M2M communications aim to connect electronic devices through wired or wireless networks without human intervention, forming part of the IoT. M2M communications can support a variety of new applications such as smart metering, smart cities, smart homes, environmental monitoring, automated health management systems, automobile safety control, and traffic management. They are characterized by a massive number of low-power devices, which correspond to a high aggregate of data traffic, control and high total energy consumption. The integration of the WPC concept in M2M communication systems could be a valuable energy solution for the M2M nodes. It eliminates the need for battery replacement or even achieves wireless battery-less solutions with main objective to enable cost-efficient and sustainable operations. In contrast to human-to-human communications, M2M systems are mainly characterized by random medium access, bursty data traffic and low data rates. The communication profile of these devices and their low data rate/power requirements perfectly match with the harvesting behavior and efficiency of WPC systems, which is mainly a long-term process due to the RF propagation losses. Controlled WPT could charge a massive number of M2M devices and facilitate their integration in new applications and services.In APOLLO, we will address several key topics related to WPC in M2M communications, including fundamental theoretical limits, wavefom design, physical (PHY)/medium access control (MAC) layer schemes, resource allocation and network architectures.
2. 5G/B5G Mobile Networks: 5G mobile networks need to support unprecedented requirements for the wireless access connection, targeting cell throughput capacities 1000-fold higher than the ones offered by the current 4G technology and round-trip latency of about 1 msec. 5G mobile networks are introduced as an enabling umbrella technology for the interconnection of a massive number of systems, devices and services. Towards this direction, three major 5G technologies have attracted considerable attention in both industry and academia, namely ultra-densification, mmWave, and massive MIMO. The constructive combination of these three 5G technologies can overcome the current RF propagation weaknesses of WPC and consists of a fertile field for the deployment of future WPC systems. Current research results show that despite the poor penetration characteristics of the mmWave signals, their combination with small-cell architectures and high antenna directionality could achieve high WPT efficiency in comparison to conventional microwave signals. The integration of the WPC concept in future 5G networks is a promising research direction and can motivate the development of a plethora of new technological applications and services. Specifically, the 5G small-cell infrastructure can be considered as a distributed highly-dense network of energy sources to power independent wireless networks (i.e., WPCN architecture) and/or as a global network that simultaneously conveys information/energy to a huge number of inter-connected devices (SWIPT architecture).In APOLLO, we will study the integration of WPC in future 5G communication networks. The impact of basic 5G tools such as small cells, mmWaves, massive MIMO, and cache-enabled cells will be investigated through rigorous analytical studies.

Integration of WPC in future heterogeneous 5G networks.