Alírio Soares Boaventura

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.

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.

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.

Spatial Power Combining of Multi-Sine Signals for Wireless Power Transmission Applications

This paper presents two transmitter architectures for wireless power transmission (WPT) applications. These architectures aim at transmitting high peak-to-average power ratio (PAPR) multi-sine signals, which have demonstrated to improve the RF-dc conversion efficiency of rectifier circuits, such as the ones in the receiving end of a WPT system. In order to overcome the challenges associated to the amplification of high PAPR signals, the proposed schemes make use of the spatial power combining concept, in which the individual signal components are amplified, radiated, and then combined in free space. In order to achieve the high PAPR signals, proper synchronization of the individual tones is required. Two architectures are proposed. The first one is based on the transmission of single-tone signals that are externally locked to a common reference signal that establishes the necessary phase reference. The second architecture is based on a mode-locked oscillator scheme that requires no external reference signal. Instead, this scheme takes advantage of the synchronization phenomena in oscillator circuits to establish the phase reference. Measurements are presented to validate both schemes and to show their effectiveness in improving the RF-dc conversion efficiency in rectifier circuits.

Extending Reading Range of Commercial RFID Readers

In this paper, multisine excitation signals are used to extend the reading range of commercial RF identification readers. To do so, a commercial reader is equipped with an external multisine front-end that implements previous mathematical proposals. A mathematical description is presented in order to show the ability of multisine signals to communicate data, with minimal changes in the downlink path, while no changes are required in the conventional tag architecture. Moreover, and most important, if a proper multisine design is performed, a conventional reader receiver is still able to demodulate and decode the backscattered multisine signal from the tag, without any hardware change. Thus, guidelines are presented for multisine design, including multisine nature, central tone positioning, tone separation, and bandwidth requirements. In order to evaluate the reading range improvement, when compared with the conventional single carrier approach with the same average power, two experiments are conducted: in the first one, an oscilloscope is used to measure the tag response and to decide whether the tag does or does not respond. In the second measurement scenario, the downlink path is implemented by the reader combined with the front-end and the uplink is implemented solely by the reader. In this case, the decision on successful tag response is taken when the reader reads the tag identification. The first measurement scenario has pointed out for a maximum reading range improvement of near 43% for an eight-tone multisine signal with 2-MHz tone separation. In the second scenario, a more realistic one, a reading range improvement of almost 25% has been obtained for a 8 + 1 tones multisine.

A Batteryless RFID Remote Control System

This paper presents a novel eco-friendly batteryless remote control (RC) system based on a multi-RF identification (RFID) scheme. The proposed RC device does not require the use of batteries or other installed power source. Instead, it relies on passive RFID chips that are remotely powered by an RFID reader. The controlled device (e.g., a TV) incorporates an RFID reader to power up and communicate with the RC. The proposed batteryless RC is composed of an antenna, a plurality of N passive RFID chips and N switches, and a multi-port microstrip network that interconnects the various RFID chips, allowing them to share a common antenna. Each key of the RC is associated to an RFID with a unique identifier, which allows the device to be controlled to identify the key pressed by the user. The proposed arrangement ensures that only the chip associated to the pressed key is read by the RFID reader, while the other chips remain inactive. First of all, the system is described including the multi-RFID scheme and the proposed multi-port network. Afterword, a characterization of the contact switches and RFID chips is performed. This is followed by RFID chip impedance matching and switch tuning. A multi-port network is fabricated (in low-cost FR4) and measured in order to access networks behavior depending on some parameters such as the number of ports and distance from the active port to the antenna. Finally, a four-key RC unit is prototyped, and an RFID reader system is integrated in a TV by using an external RFID-to-infrared interface. Four control functionalities are implemented and tested (CH+, CH-, Vol+, and Vol-). Measurements are also conducted to evaluate the system coverage range and line-of-sight capability.

Increasing the range of wireless passive sensor nodes using multisines

Wireless Sensor Networks (WSNs) usually consist of battery-powered nodes. Therefore, once the batteries deplete, the networks collapse. Passive sensor nodes are immune to this kind of problem because they do not have batteries, but on the other hand, their range is significantly shorter. This paper shows that this range can be enhanced (in a noticeable manner) if the Radio Frequency (RF) source that powers the nodes is set up to radiate a multisine waveform instead of a pure sinusoid, considering the same average power. The passive sensor used to demonstrate the usefulness of combining multiple sinusoidal waveforms is based on a low-power 16-bit microcontroller, and includes circuits for bi-directional wireless binary communication (envelope detection for downlink, backscatter for uplink). The sensor also features a 50Ω antenna port and an interface for debugging and expansion composed of 26 pins. Considering a power source of 2Werp, the maximum range of the wireless sensor (together with a half-wave dipole antenna) is 5.3 meters.

Enhanced front-end to extend reading range of commercial RFID readers using efficient multisine signals

In this paper, multisine excitation signals are used to extend the reading range of commercial RFID readers. To do so, a commercial reader is equipped with an external multisine front-end that implements previous mathematical proposals. A reading range improvement is achieved when compared with conventional single carrier approaches, even with the same average power being transmitted. A reading range improvement of near 43% is obtained for a 8-tones multisine signal with 2MHz tone separation.

Amplitude and frequency analysis of multi-sine wireless power transfer

Transmitting multi-sine signals with high peak-to-average power ratio (PAPR) has emerged as an efficient solution to improve the power conversion efficiency (PCE) of wireless power transfer (WPT). This paper analyzes the radio frequency (RF) PCE as a function of the number of tones (Nt) and signal bandwidth (BW) in multi-sine based WPT networks. We show how the energy harvesting efficiency depends on the BW and the non-linear relationship between the instantaneous input amplitude and output voltage. As a result of these, the PCE first increases with increasing Nt and then decreases. We confirm this analysis using measurements. Depending on the circuit chosen for energy harvesting, we can improve PCE of the multi-sine WPT with 25.1% compared to a CW excitation WPT for -5 dBm input power.