Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery-free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections. WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe.

Over at least the last two decades, digital predistortion (DPD) has become the most common and widespread solution to cope with the power amplifier's (PA's) inherent linearity-versus-efficiency tradeoff. When compared with other linearization techniques, such as Cartesian feedback or feedforward, DPD has proven able to adapt to the always-growing demands of technology: wider bandwidths, stringent spectrum masks, and reconfigurability. The principles of predistortion linearization (in its analog or digital forms) are straightforward, and the linearization subsystem precedes the PA (a nonlinear function in a digital signal processor in the case of DPD or nonlinear device in the case of analog predistortion and counteracts the nonlinear characteristic of the PA. Some excellent overviews on DPD can be found in [1]-[4]. Let us now look at the challenges that DPD linearization has faced and will continue to face in the near future with 5G new radio (5G-NR).

In this paper, the design and performance of class E² resonant topologies for dc/dc power conversion at UHFs are considered. Combining the use of RF gallium-nitride HEMT devices, both for the inverter and synchronous rectifier, with high-Q lumped-element terminating networks, peak efficiency values over 70% may be obtained. Control strategies based on carrier bursting, switching frequency modulation, or outphasing are also shown to be feasible. Taking advantage of their improved dynamic response, when compared to low-frequency more traditional switched-mode converters, a class E³ polar transmitter for the EDGE standard has been designed and tested at 770 MHz, offering an average global efficiency over 46% at 4.3 W of output power, through RF-based amplitude and phase constituting branches. Finally, the potential of such a high frequency of operation in terms of power density is explored, absorbing undesired coil parasitics for the original LC series interconnecting network in a 1-GHz design methodology.

Since Nathan Sokal's invention of the class-E power amplifier (PA), the vast majority of class-E results have been reported at kilohertz and millihertz frequencies, but the concept is increasingly applied in the ultrahigh-frequency (UHF) [1]-[13], microwave [14]-[20], and even millimeter-wave range [21]. The goal of this article is to briefly review some interesting concepts concerning high-frequency class-E PAs and related circuits. (The article on page 26 of this issue, "A History of Switching-Mode Class-E Techniques" by Andrei Grebennikov and Frederick H. Raab, provides a historical overview of class-E amplifier development.).

This paper presents the design and characterization of dc-dc converters operating at microwave frequencies. The converters are based on GaN transistor class-E power amplifiers (PAs) and rectifiers. Three topologies are presented, which are: 1) a PA and synchronous rectifier, requiring two RF inputs; 2) a PA and self-synchronous rectifier with a single RF input; and 3) a power oscillator with a self-synchronous rectifier with no required RF inputs. The synchronous 1.2-GHz class- E ² converter reaches a maximum efficiency of 72% at 4.6 W. By replacing the RF input at the rectifier gate with a specific termination, a self-synchronous circuit demonstrates 75% efficiency at 4.6 W, with a maximum output power of 13 W at 58% efficiency. In the third topology, the PA is replaced by a power oscillator by providing correct feedback for class-E operation, resulting in a circuit requiring no RF inputs. This oscillating self-synchronous dc-dc converter is demonstrated at 900 MHz with an efficiency of 79% at 28 V and 12.8-W output power. Self-synchronous class-E transistor rectifier operation is analyzed theoretically in the time domain and validated with harmonic-balance simulations using an improved nonlinear model for a GaN HEMT. The simplified theoretical analysis provides a useful starting point for high-efficiency self-synchronous power rectifier design, which can, in turn, be extended to high-efficiency oscillating power inverter design.