We asked doctoral candidate Onel Alcaraz López a few questions to explore resource allocation for machine-type communications and its immense potential for massive IoT and wireless energy transfer (WET). These are his thoughts on his research outcomes and their significance.
How would you describe yourself as a doctoral candidate?
I’m an enthousiastic researcher, passionate about math and technology, and whose mind never truly rests as long there is a defiant problem at hand. I do my best to help others, meet the goals and deadlines set (some of which are established by myself), and enjoy different points of views and opinions.
What is the link between Machine-type Communications and IoT in your thesis?
The Internet of Things (IoT) is a revolutionary paradigm that promises to bring wireless connectivity to anything, ranging from tiny static zero-power sensors to vehicles and drones, to communicate and collaborate over the Internet by circulating information directly among themselves and/or from the surrounding environment. Enabled by efficient Machine-type Communications (MTC), the IoT is key to realize the vision of a data-driven society relentlessly pursued by industry and academia, in special by 6G Flagship.
Since when has the research community explored possibilities for wirelessly powered IoT?
It’s been more than a century since Nicola Tesla conceived the idea of wireless energy transfer (WET) using radio frequency waves. However, the technological advances did not allow much progress at that time and the following decades. Just recently, the latest breakthroughs in electronics, antenna design, and densification via small cells, are allowing completely safe and practical WET implementations, especially for power energy-limited devices.
Since the IoT is (at the core) conformed of such low-power devices, it is expected that WET becomes an important green enabler of many challenging IoT deployments. This is mainly because it allows wireless battery-charging, which simplifies the servicing and maintenance, form factor reduction of the IoT devices, increase of durability and reliability of such devices thanks to their contact-free design; and waste-free (ecological-friendly) IoT deployments. Also, it enables the operation in difficult or hazardous environments building structures and/or inside the human body.
What triggered your own interest in the topic?
The issue of (as many researchers say) “cutting the last wires”, which refers to eliminate the need of the traditional wired battery-charging, while making the new wireless solutions more attractive in the sense of allowing relative long distance operations and ubiquity, significantly attracted my interest. The widely and efficient adoption of WET in practical scenarios really seemed very challenging and futuristic at the time I was starting my research, which made grow even more my passion for this research area. As I advanced in the studies, I discovered many possible research directions with lots of room for advances and improvements, some of which I decided to delve into.
What is lacking from current solutions?
There have being significant progresses on WET technology and its integration to wireless communications in the last decade. Still, there are some challenges that are not completely resolved such that the need of increasing the end-to-end WET efficiency (how to make the IoT devices to benefit the most from the powering station transmissions), the need of supporting low-speed mobility, and the need of seamless network-wide integration of wireless communication and energy transfer. In addition, providing wide coverage with quality-of-service (QoS) guarantees is critical since the number of low-power IoT devices with challenging requirements will dramatically increase in the coming years. These are open problems for which both, academy and industry, are constantly looking at solutions in many possible scenarios.
What was most challenging in the research work that led to this thesis?
The most challenging part was (and it still is) to keep up to date with the continuous advances in the field. One realizes that the field moves fast by just performing a simple search in google scholar with the key words: RF Wireless Energy Transfer, which provides 25 400 fast results just in 2019. By considering that 10% of those are publications in high-standard journals, the resulting figure is still quite significative, and one needs to go over many of them to identify innovative results, key findings and specific open problems. The same phenomenon occurs with the other topics addressed in the thesis, such as Ultra-reliable Low-latency Communications, Traffic Aggregation, Non-Orthogonal Multiple-Access and Resource Allocation for MTC. Luckily, the assistance of my colleagues in this regard, some of which have also co-authored publications with me in this field, has been extremely valuable.
In your solution, how are battery-constrained devices powered wirelessly?
Basically, there are two possible scenarios: 1) non-dedicated WET, where there is no dedicated energy transmitter, and the IoT devices harvest energy from readily available RF signals, such as the ones coming from TV, WiFi or cellular transmissions; and 2) dedicated WET, where dedicated power transmitters, named power beacons (PBs), are deployed to power specific IoT deployments. The latter is mandatory for realizing efficient IoT deployments with QoS requirements. In general, we look at such dedicated WET architectures, and investigate ways to provide efficient energy supply and/or the best way for the devices to utilize such harvested energy for reliable information transmission and always-alive functionality.
The current state of the multi-antenna technologies ensures PBs equipped with multi-element antenna arrays. Consequently, energy beamforming based on the Channel State Information (CSI) of the links towards the IoT devices allows the energy signals at different antennas to be carefully weighted to achieve constructive superposition at the intended IoT receivers, hence providing significant gains in the energy they can harvest. However, CSI is difficult to acquire in practical WET systems, because it demands costly procedures in terms of energy expenditure from the IoT devices, specially as the number such powered devices increases. We propose and delve into efficient methods that avoid the strict need of such information and can still benefit from the PB’s capabilities. We explore CSI-free/limited schemes such that the PB can be used to power massive deployments of power-limited IoT devices, while relying on statistical knowledge (on average sense) of the channels, devices’ position information and antenna array architectures.
What is the essence of the suggested distributed architecture?
The novel solutions introduced in this thesis alleviate the immense and costly overhead from traditional CSI-based solutions, while they profit even more from a distributed architecture, say, an architecture where multiple PBs are deployed and no strict coordination between them exist. The proposed solutions are local by nature, hence, easily adapted to multi-PB setups. However, new and complementary solutions benefiting from certain low-coordination mechanisms from the PBs are also attractive and worth to deeply investigate.
How and for whom can your results be of benefit?
Students, practitioners, engineers and researchers working on massive IoT can benefit from the results of the thesis. The results are not restricted to WET, with proper adaptations the proposed solutions can be applied to other areas in wireless communications. From an antenna design perspective, our analysis highlights the difference in performance for different antenna/array configurations, which in turn implicate the need for novel transceiver and antenna designs that are more energy-efficient, which in a more practical sense is of interest for the industry as well as other communities such as signal processing and antenna and radio frequency designers.
What is the potential in selected scenarios in number of battery-constrained devices?
Industry and academy are foreseeing an astonishing increase in the number of devices in the coming decade. Some keynote speakers in the recently concluded 6G Wireless Summit where referring to 1-10 devices per square-meter in the 6G or beyond 6G era. This would correspond to hundreds to thousands of devices in a 10 meter-radius circular coverage area, which is quite challenging to serve/power. In such scenarios, CSI-based solutions may be completely undesirable, not only because of the required energy expenditure from the IoT devices but also because the gains from CSI decrease quickly as the number for devices increases with respect to the number of transmit antennas. In such scenarios, devices’ positioning and statistical channel information could provide significant gains, as evidenced by our earlier results.
What would the proposed solution, once implemented, mean for device lifecycles and sustainability?
Our solutions are just the initial spark for what we think will be a wide area of research, at which we are in fact already planning more contributions.
At the end, the final solutions, once implemented, would mean having “always-alive” devices with ubiquitous QoS guarantees and zero battery wastage, which is of paramount importance for green sustainability as demanded by the United Nations’ 2030 Sustainable Development Goals. It is our mission to contribute towards those goals by pushing the limits of energy efficiency and intelligent resource usage so to build green sustainable networks.
What inspiration are you hoping this thesis brings to developers?
In our research team, we believe that researchers on the area of MTC, as well as in wireless communications in general, will benefit most of the analysis carried out in this thesis. In the publications that led to this thesis, we delve into several key concepts that are relevant moving forward to the 6G-era. For instance, we propose and evaluate the performance of novel data aggregation and scheduling mechanisms for massive IoT. We have also proposed the use of non-orthogonal solutions that allow a larger number of devices to be connected while improving spectral efficiency. In addition, we have provided solutions for distributed radio resource management in Ultra-Reliable Low-Latency Communication (URLLC) networks, which enables devices to attain strict latency and reliability constraints that are often imposed by industrial applications. Besides those, we have the novel CSI-free schemes for massive WET, of course.
How will your work on this topic continue?
We are planning to improve the proposed solutions to make them exploit clustering information of the devices and their corresponding energy demands. We will investigate the most appropriate antenna array architectures at the PB(s) and will evaluate collaborative schemes, while designing new CSI-free/limited solutions exploiting all this information. We intent to exploit machine-learning/artificial intelligence to track devices’ moving and clustering patterns, such that the proposed schemes update and optimize themselves on the fly. But first, please be a participant and enjoy the defense of my doctoral thesis!
Last updated: 14.4.2020