This paper presents a novel approach to address the challenges in backscatter communication for the Internet of Things. The traditional use of I/Q load modulator based on parametric transistor models often suffers from performance degradation due to discrepancies with actual transistor models caused by thermal and environmental noise sources. To overcome this issue, the paper proposes an active circuit modeling technique based on artificial neural network (ANN)-based transfer learning, which utilizes actual measurement data to model the I/Q load modulator accurately. Furthermore, an optimization algorithm is applied to achieve an optimal high-order modulation scheme, leading to improved energy efficiency by 40%. By leveraging machine learning-based modeled I/Q modulators, the proposed approach enables high-speed wireless data communication in a dual-channel configuration. The paper also conducts theoretical analysis to define the required performance of a dual-polarized Vivaldi antenna for implementing polarization diversity in backscatter communication. This analysis provides guidelines for achieving optimal performance in terms of spectral efficiency and error vector magnitude (EVM). The experimental results demonstrate that the proposed approach achieves a spectral efficiency of 2.0 bps/Hz based on 4-QAM modulation within a 150 MHz bandwidth. The measured EVM is 9.35%, indicating the effectiveness of the proposed technique in achieving reliable and efficient wireless data communication in backscatter systems. This paper presents a comprehensive approach combining accurate circuit modeling, optimization algorithms, and theoretical analysis to enable high-speed, ultra-low-power wireless data communication in backscatter communication systems.

Backscatter communication is an emerging paradigm for pervasive connectivity of low-power communication devices. Wirelessly powered backscattering wireless sensor networks (WSNs) become particularly important to meet the upcoming era of the Internet of Things (IoT), which requires the massive deployment of self-sustainable and maintenance-free low-cost sensing and communication devices. This article will introduce the state-of-the-art antenna design and radio frequency (RF) system integration for wirelessly powered backscatter communications, covering both the node and the base unit. We capture the latest development in ultralow-power RF front ends and coding schemes for$\mu \text{W}$-level backscatter modulators, as well as the latest progress in wireless power transfer (WPT) and energy harvesting (EH) techniques. Newly emerged rectenna system, waveform design, and channel optimization are reviewed in light of the opportunities for adaptively optimizing the WPT/EH efficiency for low-power signals with varying conditions. In addition, advanced device packaging and integration technologies in, e.g., additively manufactured RF components and modules for microwave and millimeter-wave ubiquitous sensing and backscattering energy-autonomous RF structures are reported. Inkjet printing for the sustainable and ultralow-cost fabrication of flexible RF devices and sensors will be reviewed to provide a prospective insight into the future packaging of backscatter communications from the chip-level design to complete system integration. Finally, this article will also address the challenges in fully wireless powered backscatter radio networks and discuss the future directions of backscatter communication in terms of “Green IoT” and “Low Carbon” smart home, smart city, smart skin, and machine-to-machine (M2M) applications.

This paper demonstrates the application of the Finite-Difference Time-Domain method for dispersive media to indoor ultra-wideband channel modeling. A new description of the frequency dispersion of building materials, based on a partial-fraction approach, is proposed, utilizing experimentally measured data on complex permittivity values reported in the literature. The analytical dispersion model for a series of building materials is estimated through the Vector Fitting technique and the through-the-wall penetration is calculated for indicative cases. Finally, a small two-dimensional office environment is studied and several channel characteristics are calculated demonstrating the flexibility and robustness of the proposed formulation in communication modeling. The proposed FDTD implementation covers all the bandwidth in a single run instead of running simulations for every frequency or subband.

In this paper, various ambient energy-harvesting technologies (solar, thermal, wireless, and piezoelectric) are reviewed in detail and their applicability in the development of self-sustaining wireless platforms is discussed. Specifically, far-field low-power-density energy-harvesting technology is thoroughly investigated and a benchmarking prototype of an embedded microcontroller-enabled sensor platform has been successfully powered by an ambient ultrahigh-frequency (UHF) digital TV signal (512-566 MHz) where a broadcasting antenna is 6.3 km away from the proposed wireless energy-harvesting device. A high-efficiency dual-band ambient energy harvester at 915 MHz and 2.45 GHz and an energy harvester for on-body application at 460 MHz are also presented to verify the capabilities of ambient UHF/RF energy harvesting as an enabling technology for Internet of Things and smart skins applications.

This paper presents a review of existing works and solutions in the field of solar/electromagnetic energy harvesting and wireless power transmission. More specifically, the paper covers: solar/electromagnetic harvesters where solar antenna structures are used to obtain a compact implementation, direct current (dc) combining circuits necessary to combine the outputs of the solar and the electromagnetic harvesters, and efficient solar-to-electromagnetic (EM) converters that can be used to synthesize autonomous wireless power transmission radio-frequency (RF) signal generators. Finally, novel topologies to minimize the sensitivity of rectifier circuits to variations in the received RF power levels are presented.

The articles in this special issue cover recent advances in energy-harvesting and energy-scavenging systems with a focus on numerous "renewable" transducer technologies as well as emerging applications. energy-harvesting technologies are fundamental in enabling the realization of "zero-power" wireless sensors and implementing the Internet-of-Things (IoT) and machine-to-machine (M2M) communication. Their increasing utilization in low-power and power-efficient sensors and electronics could potentially find application in numerous critical areas ranging from health, agricultural, structural health monitoring to logistics, localization, and security. Energy-harvesting devices, including solar panels, piezoelectric devices, thermocouples, and RF energy scavengers, can dramatically extend the operating lifetime of nodes in wireless sensor networks (WSNs). Furthermore, this technology enables a completely battery-less operation and reduces the operation cost of WSNs, which is mainly due to battery replacement, thus making it very important for a sustainable "near-perpetual" WSN operability.

The main goal of the SWAP project is that of designing, implementing and ultimately testing a new breed of wireless sensor nodes with energy scavenging capabilities. Our design will include novel energy scavenging hardware as well as network protocols and algorithms. In this paper, we summarize the outcomes of the first year of the project as well as the way forward to the further phases. In particular, we analyze the state of the art in the main research areas: energy efficient communication protocol design, ultra-low-power hardware design and most advanced harvesting techniques. For what concerns the future phases of the project we elaborate on the adoption of statistical predictive models for the energy description, we account for game theoretic approaches for distributed optimization and we apply our considerations on the most modern standards for wireless sensor networks communication. We review the state of the art on hardware components, to provide a shortlist of the most efficient building blocks of the SWAP platforms as well as a draft version of the schematics of the module. Finally, we provide a brief overview on the latest energy harvester for miniature scale devices and we argue on the feasibility of a hybrid solar-electromagnetic harvesting module.