Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

Recently, ambient backscatter communication has been introduced as a cutting-edge technology which enables smart devices to communicate by utilizing ambient radio frequency (RF) signals without requiring active RF transmission. This technology is especially effective in addressing communication and energy efficiency problems for low-power communications systems such as sensor networks, and thus it is expected to realize numerous Internet-of-Things applications. Therefore, this paper aims to provide a contemporary and comprehensive literature review on fundamentals, applications, challenges, and research efforts/progress of ambient backscatter communications. In particular, we first present fundamentals of backscatter communications and briefly review bistatic backscatter communications systems. Then, the general architecture, advantages, and solutions to address existing issues and limitations of ambient backscatter communications systems are discussed. Additionally, emerging applications of ambient backscatter communications are highlighted. Finally, we outline some open issues and future research directions.

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Ambient Backscatter Communications: A Contemporary Survey

Radio waves carry both energy and information simultaneously. Nevertheless, radio-frequency (RF) transmissions of these quantities have traditionally been treated separately. Currently, the community is experiencing a paradigm shift in wireless network design, namely, unifying wireless transmission of information and power so as to make the best use of the RF spectrum and radiation as well as the network infrastructure for the dual purpose of communicating and energizing. In this paper, we review and discuss recent progress in laying the foundations of the envisioned dual purpose networks by establishing a signal theory and design for wireless information and power transmission (WIPT) and identifying the fundamental tradeoff between conveying information and power wirelessly. We start with an overview of WIPT challenges and technologies, namely, simultaneous WIPT (SWIPT), wirelessly powered communication networks (WPCNs), and wirelessly powered backscatter communication (WPBC). We then characterize energy harvesters and show how WIPT signal and system designs crucially revolve around the underlying energy harvester model. To that end, we highlight three different energy harvester models, namely, one linear model and two nonlinear models, and show how WIPT designs differ for each of them in single-user and multi-user deployments. Topics discussed include rate-energy region characterization, transmitter and receiver architectures, waveform design, modulation, beamforming and input distribution optimizations, resource allocation, and RF spectrum use. We discuss and check the validity of the different energy harvester models and the resulting signal theory and design based on circuit simulations, prototyping, and experimentation. We also point out numerous directions that are promising for future research.

Fundamentals of Wireless Information and Power Transfer: From RF Energy Harvester Models to Signal and System Designs

The fifth generation (5G) mobile networks are envisaged to enable a plethora of breakthrough advancements in wireless technologies, providing support of a diverse set of services over a single platform. While the deployment of 5G systems is scaling up globally, it is time to look ahead for beyond 5G systems. This is mainly driven by the emerging societal trends, calling for fully automated systems and intelligent services supported by extended reality and haptics communications. To accommodate the stringent requirements of their prospective applications, which are data-driven and defined by extremely low-latency, ultra-reliable, fast and seamless wireless connectivity, research initiatives are currently focusing on a progressive roadmap towards the sixth generation (6G) networks, which are expected to bring transformative changes to this premise. In this article, we shed light on some of the major enabling technologies for 6G, which are expected to revolutionize the fundamental architectures of cellular networks and provide multiple homogeneous artificial intelligence-empowered services, including distributed communications, control, computing, sensing, and energy, from its core to its end nodes. In particular, the present paper aims to answer several 6G framework related questions: What are the driving forces for the development of 6G? How will the enabling technologies of 6G differ from those in 5G? What kind of applications and interactions will they support which would not be supported by 5G? We address these questions by presenting a comprehensive study of the 6G vision and outlining seven of its disruptive technologies, i.e., mmWave communications, terahertz communications, optical wireless communications, programmable metasurfaces, drone-based communications, backscatter communications and tactile internet, as well as their potential applications. Then, by leveraging the state-of-the-art literature surveyed for each technology, we discuss the associated requirements, key challenges, and open research problems. These discussions are thereafter used to open up the horizon for future research directions.

A Prospective Look: Key Enabling Technologies, Applications and Open Research Topics in 6G Networks

In this paper, we propose hybrid backscatter communication for wireless-powered communication networks (WPCNs) to increase transmission range and provide uniform rate distribution in the heterogeneous network (HetNet) environment. In such HetNet, where the TV tower or high-power base station (macrocell) coexists with densely deployed small-power access points (e.g., small-cells or WiFi), users can operate in either bistatic scatter or ambient backscatter, or a hybrid of them, given that the harvested energy from the dedicated or ambient RF signals may not be sufficient enough to support the existing harvest-then-transmit protocol for WPCN, which is extended to the wireless-powered heterogeneous network (WPHetNet). Considering the hybrid and dual mode operation, we formulate a throughput maximization problem depending on the user location, namely Macro-zone or WiFi-zone. After performing the optimal time allocation for the above operation, we show that the proposed hybrid backscatter communication can increase the transmission range of WPHetNet, while achieving uniform rate distribution.

Hybrid Backscatter Communication for Wireless-Powered Heterogeneous Networks

In this paper, we investigate a multi-node multi-antenna wireless-powered sensor network (WPSN) comprised of one power beacon and multiple sensor nodes. We have implemented a real-life multi-node multi-antenna WPSN testbed that operates in real time. We propose a beam-splitting beamforming technique that enables a power beacon to split microwave energy beams toward multiple nodes for simultaneous charging. We experimentally demonstrate that the beam-splitting beamforming technique achieves the Pareto optimality. For perpetual operation of the sensor nodes, we adapt an energy neutral control algorithm that keeps a sensor node alive by balancing the harvested and consumed power. The joint beam-splitting and energy neutral control algorithm is designed by means of the Lyapunov optimization technique. In our experiments, the proposed algorithm has successfully kept all sensor nodes alive by optimally splitting energy beams toward multiple sensor nodes.

Theory and Experiment for Wireless-Powered Sensor Networks: How to Keep Sensors Alive

With the emerging Internet-of-Things services, massive machine-to-machine (M2M) communication will be deployed on top of human-to-human (H2H) communication in the near future. Due to the coexistence of M2M and H2H communications, the performance of M2M (i.e., secondary) networks depends largely on the H2H (i.e., primary) network. In this paper, we propose ambient backscatter communication for the M2M networks which exploits the energy (signal) sources of the H2H network, referring to its traffic applications and popularity. In order to maximize the harvesting and transmission opportunities offered by varying traffic sources of the H2H network, we adopt a Bayesian nonparametric (BNP) learning algorithm to classify traffic applications (patterns) for secondary transmitters (STs). We then analyze the performance of STs using the stochastic geometric approach, based on a criterion for optimal traffic selection. Because of the mathematical intractability of the optimal criterion, we have attained a suboptimal traffic selection criterion which provides more tractable analysis. Results are presented to validate the performance of the proposed BNP classification algorithm and the criterion, as well as the impact of traffic sources and popularity.

Traffic-Aware Backscatter Communications in Wireless-Powered Heterogeneous Networks

We propose hybrid backscatter communication for wireless powered communication networks (WPCNs) to increase transmission range and provide uniform rate distribution in heterogeneous network (HetNet) environment. In such HetNet where the TV tower or high-power base station (macrocell) coexists with densely deployed small-power access points (e.g., small-cells or WiFi), users can operate in either dedicated bistatic scatter or ambient backscatter, given that the harvested energy from the dedicated or ambient RF signals may not be sufficient to support the existing harvest-then-transmit (HTT) protocol for WPCNs. Considering this dual mode operation, we formulate throughput maximization problem depending on the user location, namely indoor or outdoor zone. After showing the optimal time allocation for the dual mode operation, we show that the performance of the proposed hybrid backscatter communication is superior to the other schemes.

Hybrid backscatter communication for wireless powered communication networks

We propose backscatter-aided cooperative transmission for wireless-powered heterogeneous networks (WPHetNets). In WPHetNets, where various kinds of nodes such as high-power base station (e.g., TV tower and macro base station) and small-power access point (e.g., WiFi access point) coexist, we aim to increase transmission range and support fair communication through internet-of-things (IoT) device cooperation. For this, we first propose long-range bistatic backscatter (BB)-aided cooperative transmission with two-device cooperation where ambient backscatter (AB) enables short-range information exchange in sequential mode between devices nearby under energy neutral operation, termed ‘Cooperation mode’. To ensure fairness between devices, we formulate common-throughput maximization problem where an algorithm for time allocation is presented. Compared with ‘Non-cooperation mode’ (i.e., no information exchange via AB) and active RF based cooperation schemes, the proposed scheme is shown to increase both the coverage and fairness between devices for battery-less IoT networks. We then generalize the architecture of backscatter-aided cooperative transmission with multiple-device cooperation where the information exchange is performed in sequential mode and parallel (broadcasting) mode. In the sequential mode, a graph-matching based suboptimal pairing algorithm is proposed whose validity is corroborated through comparison with a heuristic search based optimal pairing, and then we compare the two modes for multiple-device cooperation.

Dong In Kim

Papers

Hybrid Backscatter Communication for Wireless-Powered Heterogeneous Networks

Theory and Experiment for Wireless-Powered Sensor Networks: How to Keep Sensors Alive

Traffic-Aware Backscatter Communications in Wireless-Powered Heterogeneous Networks

Hybrid backscatter communication for wireless powered communication networks

Backscatter-Aided Cooperative Transmission in Wireless-Powered Heterogeneous Networks