Nuno Borges Carvalho

Wireless Power Transmission: R&D Activities Within Europe

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.

Two-tone IMD asymmetry in microwave power amplifiers

This paper presents the first study of the relation between the IMD asymmetries, often observed in almost all power amplifiers subject to a two-tone test, and the nonlinear characteristics of their active devices. First, the reasons for the different amplitudes of the two adjacent tones are investigated using a general circuit with frequency dependent embedding impedances, and resistive and reactive nonlinearities. Those theoretical conclusions are then extrapolated for real circuits, and validated by comparing results obtained from nonlinear simulation to laboratory measurements of a microwave power amplifier.

Designing multisine excitations for nonlinear model testing

This paper deals with the design of bandpass multisine excitations appropriate for microwave nonlinear model testing. A formal analysis showed that if the output power spectral density of a general nonlinear dynamic system is used as a metric for signal similarity, then it is possible to derive a set of statistical characteristics to be approximated. Using these nth-order extensions of the autocorrelation or the power spectral-density functions as the approximation goals, the multisine design problem is formulated as an optimization scheme where the various tone amplitudes and phases become the target design variables.

Designing and Testing Software-Defined Radios

This article reviews the main parts of a software-defined radio (SDR) to emphasize several possible implementations of both receivers and transmitters. We describe solutions for testing and characterizing these types of devices. SDRs typically operate in both the analog and the digital domains, thus mixed-domain instrumentation is necessary to carry out testing. We first give a short overview of several architectures for SDR receiver front ends. Then, several possible architectures for transmitter front ends are described. We discuss methods that can be used to improve amplifier efficiency. Instrumentation currently available in the commercial market that allows the characterization of such types of transceivers is presented.

Modeling nonlinear behavior of band-pass memoryless and dynamic systems

This paper addresses nonlinear distortion arising in microwave band-pass memoryless and dynamic systems, focusing on wideband power amplifiers (PA). It first identifies the minimum requirements for their correct representation. Then, it shows that the complex behavior of long term memory effects does not allow a unique characterization procedure, but requires various nonlinear distortion figures according to the type of nonlinear RF impairments the actual system is sensitive to.

Optimum behavior: Wireless power transmission system design through behavioral models and efficient synthesis techniques

In this article, the different challenges and open issues regarding WPT systems have been presented considering both the transmitting and receiving ends. The challenges in the receiving side include the accurate modeling of the rectifying elements, the adequate selection of the rectifier topology, and the joint optimization of antenna and rectifier circuit. The challenges in the transmitting side are mainly in the improvement of the dc-to-RF conversion efficiency and in the selection of the optimum transmitted signal waveform. The final goal of most of these challenges is to take advantage of the nonlinear nature of the rectifying elements in order to maximize the RF-to-dc conversion efficiency in the receiving end of WPT systems.

Boosting the Efficiency: Unconventional Waveform Design for Efficient Wireless Power Transfer

Traditionally, wireless power is delivered through single-carrier, continuous-wave (CW) signals. Most research efforts to enhance the efficiency of wireless power transfer systems have been confined to the circuit-level design. However, in recent years, attention has been paid to the waveform design for wireless power transmission. It has been found that signals featuring a high peak-to-average power ratio (PAPR) can provide efficiency improvement when compared with CW signals. A number of approaches have been proposed, such as multisines/multicarrier orthogonal frequency division multiplex (OFDM) signals, chaotic signals, harmonicsignals, ultrawideband (UWB) signals, intermittent CW (ICW) signals, or white-noise signals. This article reviews these techniques with a focus on multisines/multicarrier signals, harmonic signals, and chaotic signals. A theoretical explanation for efficiency improvement is provided and accompanied by experimental results. Circuit design considerations are presented for the receiver side, and efficient transmission architectures are also described with an emphasis on spatial power combining.

Simulation of RF circuits driven by modulated signals without bandwidth constraints

A new nonlinear analysis technique of RF circuits subject to modulated signals is discussed. As the envelope and carrier are uncorrelated, the circuit is modeled by a multi-rate partial differential equation. The carrier response is then simulated with harmonic-balance for each envelope time-step, without imposing any restrictions to signal bandwidth.

Maximizing DC power in energy harvesting circuits using multi-sine excitation

This paper presents an approach to signal excitation specially designed to improve the DC power obtained in a RF to DC converter and consequently its RF-DC efficiency conversion. In this sense a multisine signal is used as the excitation, and it is proved either theoretically, by simulations and by measurements, that a multisine signal with 0° phase relationship between the tones can give better DC values in an energy harvester, when compared with a single tone excitation with the same input average power.

Multisine signals for wireless system test and design [Application Notes]

Multisine signals are used in the laboratory and in the field to provide a periodic, well-characterized waveform that can simulate complex modulated radio frequency (RF) signals. In the field of wireless telecommunications, multisines are often used to provide realistic test signals that have statistics similar to various types of digitally modulated signals. It is concluded that the multisine approach is well suited for evaluating a nonlinear system excited by real communication signals. Multisine signals enable to gather important information about the in-band distortion and to excite long-term memory effects by generating baseband components. They are periodic and straightforward to characterize, making them ideal for identifying both magnitude and phase distortion during test and verification. Using multisines to simulate standard wireless system excitations and figures of merit allows for very good laboratory nonlinear distortion measurement setups.