Overcoming the challenge of battery recharging and replacement in industry Internet-of-Things (IoT) applications is considered by proposing the design of a triple-band high-gain multibeam ambient radio frequency (RF) energy harvesting system utilizing hybrid combining. The novelty of the design is that it simultaneously exploits frequency, space, and polarization to maximize the harvested RF energy. Wideband hybrid combining is proposed, which enables the harvesting of energy at low RF power densities while maintaining the wide frequency and space coverage necessary for ambient RF energy harvesting. The antenna design consisting of 16-ports has an average area per port of 0.3λ × 0.3λ (where λ is the freespace wavelength at 1.8 GHz) and is demonstrated to achieve a wide relative bandwidth of 38.5% covering the GSM-1800, UMTS-2100, and WiFi frequency bands. Hybrid combining of the 16-port antenna provides multiple antenna beams each with up to 11 dBi antenna gain and these provide broad beam coverage. The design for the rectifiers, using multistub impedance matching, is also provided and these are shown to be efficient over the frequency bands of interest. Measurements in an anechoic chamber demonstrate that the proposed energy harvesting system can provide an output dc voltage of more than 755 mV, an output dc power of more than -6.4 dBm and RF-to-dc efficiencies greater than 40% when the power density is more than 1400 μW/m ² . It is also shown that the proposed system can provide output dc power of 80 μW and 7.3 μW in real outdoor and indoor ambient environments, respectively.

Active reconfigurable intelligent surfaces (RISs) have recently been proposed to complement and generalize passive RIS. In this work, the design for an active RIS based on phase-reconfigurable reflection amplifiers is proposed. The proposed active RIS elements’ design consists of a two-layer patch antenna and a phase-reconfigurable reflection amplifier that is realized by a cascade of a phase shifter and a reflection amplifier. Theoretical and numerical analyses are used to quantify key parameters that need to be met in the design of the reflection amplifier and phase shifters. These show that a tradeoff between the reflection amplifier’s gain and the phase shifter’s return and insertion loss is required to be balanced to obtain a stable and effective overall design. In particular, a reflection amplifier with about 13-dB gain and a 2-bit phase shifter with only about 1.9-dB insertion loss and $-$ 25-dB return loss are designed. The final RIS element obtains 8.5-dB gain with four reconfigurable phases ranging from 0 $^{\circ}$ to 360 $^{\circ}$ with small root mean square (rms) gain and phase errors. An active RIS can be constructed by concatenating these elements together, and an efficient analytical method to calculate the scattered pattern by the active RIS is also proposed. Experimental results for a $2$ $\times$ $2$ element active RIS prototype are also provided. These show that the active RIS provides received powers around 8.5 dB higher than that from using an equivalent passive RIS. The results demonstrate the feasibility of achieving gain in active RISs and help demonstrate their promise as a technology for future high-capacity wireless communication systems.

We describe the design of a multiport rectenna system for ambient radio frequency (RF) energy harvesting where the number of ports utilized is dependent on frequency. A unique aspect of the design is the use of different numbers of antenna ports for harvesting RF energy at different frequencies. This allows the available area for the rectenna to be fully utilized at all frequencies. In particular, the proposed antenna is designed to have four ports for harvesting energy from the GSM-900 frequency band and 12 ports for the GSM-1800 frequency band in the same area. The design for the rectifiers for the GSM-900 and GSM-1800 frequency bands with direct current (dc) combining is provided and prototypes are demonstrated. Field-test measurements show that the proposed rectenna can provide an output dc voltage of more than 3.2 V, an output dc power of more than -10 dBm, and an RF-to-dc efficiency of greater than 42% when the power density is greater than 1400 μW/m ² . Measurements in an ambient RF environment at a university campus with commercial GSM-900 and GSM-1800 systems in operation show that the proposed rectenna can achieve output dc voltages of up to 2.2 V and dc power up to -13.6 dBm.

A design for a highly reconfigurable intelligent surface (RIS) that can provide 3-D passive reflective beamforming suitable for sub-6-GHz frequency bands is proposed. The RIS is based on a novel double-layer structure consisting of independently controllable and highly reconfigurable elements on top of a ground plane. The novelty of the structure is the use of reconfigurable RIS elements, whose size is of the order of a wavelength, which are optimized to function over wide incident and reflected angles. In addition, the individual elements each have four RF switches, in a $5$ $\times$ $5$ subelement structure, providing 16 different reflective phases, from $0^{\circ}$ to $360^{\circ}$ relative to the incident waves. To quantify the performance of RIS, the performance metric of phase entropy is defined and it is also used as an objective function for optimizing the design. An efficient analytical method for predicting the reflected waves from the RIS elements, as well as phase entropy, is also provided. For verification, a prototype of a $4$ $\times$ $4$ -element RIS, with a total size of $0.32~\textrm{m}$ $\times$ $0.34~\textrm{m}$ , is fabricated and an experimental setup for measuring the scattered pattern is described. It is shown that waves incident from various angles can be passively beam steered to any angle within $0^{\circ}$ – $360^{\circ}$ azimuth and $\pm$ $50^{\circ}$ elevation angles. The high reconfigurability to manipulate incident waves makes the proposed RIS a promising surface for use in future high-capacity communication systems.

The design of a hybrid radio frequency (RF) and solar energy harvesting (EH) system utilizing a transparent multiport antenna for indoor applications is described. The system incorporates an eight-port transparent antenna, operating in the unlicensed 2.4-GHz band, and is constructed from transparent copper micromeshed planar conductor. The antenna is integrated on the top surface of a solar cell with rectifiers positioned underneath to form a hybrid RF-solar indoor EH system. A key advantage of this approach is that the surface area of the solar cell is reused for the antenna saving space. Another novelty is the use of a multiport antenna for increasing the RF harvested energy. It is demonstrated that with a light intensity of 360 lux, the solar cell can obtain 1.68-mW power while the rectenna can achieve an additional 4.8%-45.8% harvested power when the incident RF input power density is varied from 13.30 to 52.96 mW/m². The transparent antenna achieves 72.4% efficiency and is one of the highest reported results. In addition, the rectifiers obtain 53.2% RF-to-dc conversion efficiency for an RF input power of -10 dBm. These results demonstrate that the proposed RF-solar energy harvester can increase harvested energy and provide energy diversity.

A novel single-fed horizontally polarized pattern-reconfigurable antenna is proposed with the key feature that is highly pattern-reconfigurable. The antenna is able to generate a large variety of radiation patterns, including omnidirectional, single-directive, and multi-beam patterns without using phase shifters. The antenna is based on a planar geometry, which consists of an Alford loop and four compact parasitic pixel rings with 60 integrated controlling units utilizing p-i-n diodes. Experimental and simulation results are provided for the proposed antenna design operating at 5 GHz. The results show that the single-beam steering with 7.3 dBi gain through 360° in the azimuth plane is achieved. Moreover, flexible dual-beam steering with one beam fixed and the other beam scanning independently among 360° in the azimuth plane is also demonstrated where each beam has 5 dBi gain. Other radiation modes, such as the omnidirectional mode, the tri-beam-steering mode, and the elevation plane steering mode, are also verified. The features of high pattern reconfigurability, 360° single-beam- and multi-beam-steering ability, full 3-D space scanning, planar geometry, and compatibility with a digital controller make it useful in various sub-6 GHz wireless applications, such as wireless power transfer, RF sensing, and analog precoding.