This work presents the design, simulation, and experimental validation of a 1-bit Reconfigurable Intelligent Surface (RIS) based on slot elements controlled by PIN diodes. It also extends the concept to 2-bit by introducing an asymmetric slot. In both cases, the phase of the reflected wave is digitally controlled through the diode switching states. Unlike solutions based on MEMS, varactors, or other devices, the proposed approach offers a simpler and easily scalable architecture, that enables increasing in the number of bits without major structural modifications. The fabricated 1-bit prototype demonstrates the RIS capability to control beamforming and signal steering. In the 2-bit version, validated through simulations, a significant reduction in secondary lobes is also observed.

This contribution presents a flexible radiofrequency (RFID) wristband tag which can be fabricated using low-cost substrates such as polyethylene (PET), Teflon (PTFE) or other low-loss polymers. The tag wristband consists of a multi-layer design that doesn’t require interconnecting metallic layers. The wristband design is constrained to a thickness of 1 mm and a width of 3 cm, respectively, to produce a light and comfortable wearable device that could be used in most applications, including activity monitoring, but with enough durability to be used for a prolonged period without significant antenna de-tuning. The proposed wristband is designed to operate in the European band (865-868 MHz) and its measured read range is over 2.5 meters.

In a practical wireless power transfer (WPT) system with multiple nonlinear receivers and a power-limited transmitter, rectification efficiency may be low because the received radio frequency (RF) power of the receiver does not match the optimal rectified power. To tackle the challenge that a power-limited transmitter actively transmits RF power to satisfy different power demands with high efficiency and large average output direct current (dc) power, this article proposes a novel high-efficiency multiuser WPT. In this scheme, the transmission signals are optimized based on the received RF signals with power equal to the optimal rectified power from the selected receivers, so the selected receivers can track the optimal rectified power within a time slot. Then, we formulate a bilevel optimization problem under the constraint of transmit power to group all receivers and optimize the transmission signals of each group. Multiple groups sequentially track the optimal rectified power across multiple time slots with a power-limited transmitter, thereby tackling the challenge. In order to solve the nonconvex bilevel optimization problem, a bilevel genetic algorithm is adopted to obtain the optimal solution. Numerical and experimental results show that the RF-RF-dc efficiency and output dc power of the proposed scheme are up to 2.6 and 3.7 times higher than those of the traditional schemes. Additionally, only our scheme meets the different power demands of multiple receivers, ensuring the same survival time. Furthermore, our scheme can also provide guidance for practical WPT system design and has the potential of compatibility with other existing systems.

In actual microwave power transmission (MPT) scenarios such as smart factory, the system efficiency and output dc power will decrease when the receiving antenna array of the multi-posture receiver does not face the transmitting antenna. To address the challenge of efficiently transmitting energy from the transmitter to a multi-posture receiver, a multi-posture MPT scheme based on forward and backward time reversal (TR) is proposed. This letter introduces phase shifters in the multi-posture receiver, enabling the beam of receiver to have steering capabilities. Importantly, inspired by the adaptive beam alignment characteristic of TR, the phase shifters of receiver are configured to align the beam of the offset receiver with the transmitter using the backward TR method, while the phase shifters of transmitter are configured to align the beam of the transmitter with the receiver using the forward TR method. So, dual alignment of the transmitting and receiving beams is achieved. A prototype of the multi-posture MPT system is established. The experiment results show that the output dc power of the multi-posture MPT scheme is 30.6 dB higher than the traditional MPT scheme when the multi-posture receiver changes its orientation, demonstrating the effectiveness of the multi-posture MPT scheme.

This letter proposes a 360(degrees)-beam-steering low-sidelobe time reversal microwave power transfer (TR-MPT) method. The beam steering is implemented by only one circular patch antenna fabricated with multiple feeding ports. The multiple feeding ports are designed to stimulate different radiation modes and their combinations to steer the beam within 360(degrees) in the azimuth plane. To depress the sidelobe levels and enhance the beam gain, a disc-top-loaded monopole antenna is added at the center of the patch antenna, which is used to excite an additional mode out of phase with the maximal sidelobe but in phase with the main lobe. Finally, time reversal (TR) is adopted for optimally weighting the stimulated modes and focusing the power beam adaptively to the desired receiver. A prototype of the TR-MPT system based on the proposed method is established. The experiments demonstrate that the new method can not only adaptively focus the power at the receiver placed at any azimuth angle within 360(degrees), but also achieve a low sidelobe level of smaller than -8.57 dB under a 0.7-wavelength aperture. Additionally, the power transferred to the receiver at different angles has a small fluctuation of less than 0.7 dB.

Directional modulation (DM) using multiport com-pact antennas shows some limitations when transmitting multiple independent beams. In this communication, we propose adding a monopole to a multiport compact stacked patch antenna in order to gain an extra degree of freedom in the transmission of DM. The improvement in security that can be achieved with the resulting antenna will be evaluated through bit error rate (BER) simulations for different configurations with two independent beams.

Directional modulation (DM) has been proposed as a technique to enhance physical layer security of wireless transmissions. In DM, the improvement of security is achieved by increasing the transmitted power in such a way that the bit error rate (BER) is degraded in the observation angles out of the desired secure direction. The performance of DM in terms of BER is typically evaluated by transmitting a stream of symbols for every observation angle, but this approach can be time consuming. In this communication, we propose an approach to evaluate, accurately and efficiently, the BER of dynamic DM (DDM) for standard modulation schemes. Several DDM configurations will be tested to illustrate the benefits and limitations of the evaluation method. The proposed approach is also used to present a non-iterative DDM synthesis with restrictions in the BER response.

This letter investigates the possibility of using deliberated geometries for rotating targets, which may improve the detectability and classification by means of its micro-Doppler signature. Beyond the signature provided by the rotation rate of the target, every particular geometry leads to specific and unique modulation waveforms that increase the information related to this target. Different star-shaped wheels with particular geometries have been designed demonstrating different reflection characteristics and modulation waveforms. The signature of these rotating star-shaped wheels has been measured in an anechoic chamber using a general-purpose 24 GHz frequency-modulated continuous wave (FMCW) radar platform and a detection algorithm based on the derivative of the cross-correlation is proposed to validate the proposed approach.

In this contribution, we investigate a multiport compact stacked patch antenna with 360° beam steering when transmitting multichannel dynamic directional modulation. Two secure observation angles that can be chosen freely in XY plane will be considered. The antenna behavior in the main beam and the side lobes is assessed through bit error rate simulations for different observation angle pairs. Trade-offs between security and power efficiency are also discussed.

Dynamic Directional Modulation (DDM) has become an attractive option to achieve physical layer security. In this contribution, we evaluate the generation of DDM with software defined radio for transmitting simultaneously two uncorrelated signals along two different observation angles. The generation of DDM relies on the knowledge of the channel vector and an accurate adjustment of the weights that feed the phased array, for that reason, the components of the transmitter need to be characterized accurately. Experimental results that assess the performance of the system for the observation angles under consideration are shown.

In this contribution, we propose a multiport compact antenna that supports four radiating modes operating at the same frequency thus making it a suitable candidate to substitute a linear array of four antennas. It will be also shown as this antenna produces dynamic directional modulation (DDM) with low bit error rate (BER) in a unique, unambiguous (secure) angular region that can be chosen freely in the XY plane. Tradeoffs between the BER beamwidth of the secure angular region, the BER at side lobes, and the extra power required in the generation of DDM are assessed through measurements with real-time data transmission.

Dynamic directional modulation (DDM) has already proven to be an efficient technique to achieve physical layer security in wireless communications. System architectures based on vector modulators provide a flexible framework to implement synthesis methods that allow us to obtain increased security and/or independent multichannel transmissions. However, the implementation of DDM with vector modulators requires an accurate calibration (amplitude and phase) of every component in the RF path. In this contribution, we study the sensitivity of the response of a DDM system based on commercial vector modulators showing how to correct the nonideal behavior of all the components thanks to the flexibility provided by the vector modulator.

Dynamic Directional Modulation (DDM) is a technique for phased arrays that can increase security in the physical layer. In this work, we present a setup for a DDM system where the array feeding network is fully implemented with Software Defined Radio (SDR). The calibration of the system is discussed, and some experiments are carried out to demonstrate the tradeoff between security and transmitted power.

This paper reports on a new method for characterizing intermodal isolation, power penalty, and reception zone area of helical beam (HB) antennas. As an example, an eight-element circular patch array is fully characterized and its performance is critically assessed. Validation through beam measurements is accompanied with precise electromagnetic modeling using conformal finite-difference time domain computation on a graphics processing unit.

The performance of a directional modulation system based on continuous phase control of a 2- and 4-element array is presented. Measured results obtained outside an anechoic chamber are provided to demonstrate the transmission of secure and multichannel communications using quadrature amplitude modulation.

This contribution proposes a novel radio frequency identification tag antenna operating in the EU UHF band (865-868 MHz). Miniaturization techniques have been used to achieve a reduced volume (20 2.56 3 mm(3)) in order to make it suitable for labeling metallic tools. Simulations are used to assess the tag performance when operating on tools of different sizes. These simulations are validated through measurements in a reference scenario (anechoic chamber). Finally, a real scenario (tool hanging board) is considered in order to evaluate tag performance when interacting with other tags and tools.

This contribution addresses the electromagnetic feasibility of the wireless temperature monitoring inside a coaxial cavity resembling a portion of a high-power high-frequency cyclotron auto-resonance maser for plasma heating in the new generation of DEMO TOKAMAK machines. The scenario is investigated as a potential communication channel for a ultrahigh-frequency radiofrequency identification (RFID) sensor network where cavity probes are used to both excite the coaxial cavity and to collect the temperature data scattered back by sensor antennas. By using a theoretical near-field analysis of a simplified model of the cavity and of the reader/sensor devices it is demonstrated that a two-probes architecture is suitable to interact with more than N = 16 equally spaced RFID temperature sensors (having power sensitivity of -8.3 dB mW) over the surface of a 0.5 m tube by using less than 20 dB mW power emitted by the reader. The theoretical results are corroborated by experimental data with a mock-up of the cavity and realistic prototypes of miniaturized RFID radio-sensors and excitation probes.

In this contribution a Dynamic Directional Modulation (DDM) system is modelled including mutual coupling between antennas and real non-ideal components. The performance of the system will be evaluated via the constellation patterns transmitted in the desired secure observation angle. The trade-off between security and transmitted power will be discussed

Emerging RFID applications in the UHF band in some cases require very specific antenna solutions. This letter considers the identification and tracking of patients inside hospital facilities using wristbands, which impose strict tag size and cost limitations. We propose a solution for passive tags based on a normal-mode helical antenna as an alternative to bulky and/or expensive solutions that can be found in the literature. The robustness of the resulting wristband will be assessed in a particular implementation by measuring the maximum read range for a variety of subjects with different physical constitutions.