Radio Frequency Signal Research Articles

Physical Layer Security Analysis of Multi-Hop Hybrid RF/FSO System in Presence of Multiple Eavesdroppers

In this paper, the physical layer security (PLS) analysis for a decode-and-forward (DF) protocol based multi-hop hybrid radio frequency (RF)/ free space optics (FSO) is presented. Herein, two different scenarios are considered for detecting the secured information at the destination. In the first scenario, at each hop, the received signals with higher secrecy capacity are selected. However, in the second scenario, the RF and FSO signals are simply decoded and forwarded to the next hop and selection is done at the last hop only. Each node in the system is connected to its subsequent node through parallel RF and FSO links. The FSO links and RF links are characterized by Málaga ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {M}$</tex-math></inline-formula> ) and Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> composite distributions respectively. For both scenarios, the secrecy outage probability (SOP), strictly positive secrecy capacity (SPSC), intercept probability (IP), and effective secrecy throughput (EST) are obtained as performance metrics. These performance metrics are obtained by considering pointing error, different optical signal detection methods, turbulence effect, and the RF link fading parameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> ). Using the results, the security and reliability trade-off analysis is discussed for two scenarios. In addition, the asymptotic results are also obtained to study the system in depth. Further, the secrecy diversity order (SDO) and secrecy coding gain (SCG) are obtained to analyse the system in depth. Finally, Monte-Carlo Simulation is performed to verify the obtained results. The results manifest that the scenario with selection at each hop provides better secrecy performance than the scenario with selection at the last hop.

Silicon Photonic Radar Receiver IC for mm-Wave Large Aperture MIMO Radar Using Optical Clock Distribution

A silicon photonics millimeter-wave (mm-wave) radar receiver (RX) integrated circuit (IC) for a multiple-input–multiple-output (MIMO) imaging radar is presented. Optical clock distributed over fiber enables coherent operation of several transceiver frontends for large apertures and finer angular resolution in the MIMO imaging radars. All electronic and photonic components needed to receive the optical clock signal, the electronic radio frequency (RF) signal, and the in-phase quadrature (IQ) downconvertion to the baseband is integrated in the RX frontend IC. The RX chip supports optical clock signals at 1310- and 1550-nm optical wavelengths. The chip is implemented in IHPs pre-production photonic 250-nm SiGe BiCMOS technology. At an intermediate frequency (IF) of 1 MHz, the measured conversion gain (CG) is 6.95 dB at an RF of 66 GHz and −11.18 dB at an RF of 77 GHz from −5-dBm optical modulation amplitude (OMA) and −13-dBm RF power. The RX IC dissipates 770 mW and is operated with a single 3.6-V power supply. For the realization of an optical receive path, an off-chip lithium niobate (LiNbO3) Mach–Zehnder modulator (MZM) was used. The optical receive path achieves a saturated OMA of −6 dBm at an RF of 77 GHz and at an IF of 1 MHz.

Robust Transceiver Design for SWIPT DF MIMO Relay Systems With Time-Switching Protocol

In this article, we investigate a dual-hop simultaneous wireless information and power transfer decode-and-forward multiple-input and multiple-output relay communication system, in which the relay node harvests energy based on the radio frequency (RF) signal transmitted from the source node through the time-switching (TS) protocol to decode and forward the re-encoded information to the destination node. With the consideration of the channel estimation error, the joint optimization of the TS factor and source and relay precoding matrices is proposed with robustness against the channel state information mismatch to maximize the mutual information (MI) between the source and destination nodes. We derive the optimal structure of the source and relay precoding matrices to simplify the transceiver optimization problem under fixed and flexible power constraints. Numerical examples demonstrate that the proposed algorithms with robustness provide better MI performance compared to the nonrobust algorithm.

Complete Targets Coverage in Energy Harvesting Internet of Things With Ambient Backscatter

This article considers deriving set covers or the set of active nodes that are responsible for monitoring all targets in an Internet of Things (IoT) network. A key distinction to prior works is that this article considers sensor nodes aided by backscatter communications, which allow them to communicate with negligible energy cost using ambient radio frequency (RF) signals. The article contains three main novelties. First, it presents a mixed integer linear program (MILP) that can be used to compute the global optimal solution. Second, it also outlines a centralized greedy scheduling (CGS) algorithm that selects nodes based on their energy and the number of covered targets. Third, it presents a distributed greedy scheduling (DGS) algorithm that selects nodes according to their energy level. The simulation results show that equipping nodes with ambient backscattering capability increases the ratio of complete targets coverage by more than 100% as compared to existing techniques.

Adaptive Detector for FDA-Based Ambient Backscatter Communications

Ambient backscatter communication (AmBC), whose passive tags transmit information to readers through radio frequency (RF) signals in the air, has attracted much attention due to its promising prospects in green internet of things (IoTs). However, AmBC detection has faced a new challenge at the reader because the received signal is mainly a hybrid signal from direct link and backscatter link, which makes it difficult to detect symbols of the backscatter tag. To address this problem, in this paper, frequency diverse array (FDA) is utilized to transmit time-variant ambient signal, which can separate the two link signals received by the reader. We propose an adaptive dual-threshold detector by employing the time-variant characteristic of FDA RF signal, which is based on the position of the boundary backscatter tag defined. Specifically, the closed form expressions for maximum likelihood (ML) threshold and adaptive thresholds are derived, respectively. Furthermore, the computational complexities are analyzed. Finally, numerical results are presented to show that the proposed adaptive dual-threshold detector achieves better bit error rate (BER) performance than the conventional ML detector.

DRL-Based Partial Offloading for Maximizing Sum Computation Rate of Wireless Powered Mobile Edge Computing Network

The advanced Internet of Things (IoT) enables more and more interactions between people and machines in the emerging applications, which rely on real-time communication and computing. However, the limited battery capacity and low computing capacity of IoT nodes can hardly support high-performance computing applications. The integration of wireless power transmission (WPT) and mobile edge computing (MEC) is a feasible and promising solution to address the energy shortage and computing capacity limitation of IoT nodes by harvesting radio frequency signal’s energy and offloading the nodes’ computation tasks to edge computing servers (ECSs). In this work, we focus on the wireless powered MEC network with an ECS and multiple edge devices (EDs), and study the joint optimization of WPT duration, transmission time allocation of each ED and partial offloading decision to maximize the sum computation rate. First, we formulate this as a non-convex problem which is hard to solve. Second, to conquer this problem, we decompose the original offloading problem into the sub-problem of optimizing the offloading time allocation among EDs and the proportion of harvested energy allocated for offloading at each ED under a given WPT duration and the top-problem of optimizing the WPT duration. Finally, we design an online DRL-based framework where one DNN together with its exploration strategy and training strategy is adopted to learn the near-optimal WPT duration and an efficient optimal algorithm is designed to solve the sub-problem. Numerical results show that the DRL-based offloading algorithm achieves the near-maximal sum computation rate while greatly reducing the processing time by at least three orders of magnitude compared with using the solver CVX for the sub-problem and the DNN for the top-problem.

Multicast Secrecy Rate Maximization for Reconfigurable Intelligent Surface Backscatter Communication

This letter innovatively studies a reconfigurable intelligent surface (RIS) backscatter multicast secrecy communication system. In this system, a RIS acts as a transmitter to multicast confidential information for multiple single-antenna legitimate users by the backscatter of the radio frequency signal from a power beacon (PB), while multiple passive multi-antenna eavesdroppers lurking nearby attempt to intercept and recognize the confidential information with linear decoders. To maximize the multicast secrecy rate, we develop an alternative algorithm to optimize the reflection matrix at the RIS and the beamforming vector at the PB under the constraints of total transmit power and unit modulus. Simulation results verify the feasibility of the considered system and show the achievable secrecy performance of the proposed optimization scheme.

High Bandwidth Series-Form Switch-Linear Hybrid Envelope Tracking Power Supply With Reduced Bandwidth Envelope and Step-Wave Edge Adjustment Methods

The series-form switch-linear hybrid envelope tracking (ET) power supply is constituted by a multilevel converter and a linear amplifier connected in series, featuring high efficiency and wide tracking bandwidth. With the development of wireless communication, the envelope bandwidth of the radio-frequency signal reaches tens of megahertz and even hundreds of megahertz, leading to a high switching loss in the multilevel converter and thus degraded overall efficiency of the ET power supply. To reduce the switching loss and ensure high tracking bandwidth, this article proposes to let the multilevel converter track the reduced bandwidth envelope signal and the linear amplifier track the original one. Then, a step-wave edge adjustment method is proposed to adjust or eliminate the rising and falling edges of the step-wave voltage for further reducing the equivalent switching frequency of the multilevel converter and the voltage across the power device of the linear amplifier. As a result, the switching loss in the multilevel converter and the power loss in the linear amplifier could be reduced, leading to a notably higher overall efficiency. A prototype with 40 MHz tracking bandwidth, 7–27 V output voltage, and 15 W average output power is fabricated and tested in the lab, and the experimental results are provided to verify the effectiveness of the proposed method.

Quantum positioning and ranging via a distributed sensor network

A quantum sensor network with multipartite entanglement offers a sensitivity advantage in optical phase estimation over the classical scheme. To tackle richer sensing problems, we construct a distributed sensor network with four nodes via four partite entanglements, unveil the estimation of the higher order derivative of radio-frequency signal phase, and unlock the potential of quantum target ranging and space positioning. Taking phased-array radar as an example, we demonstrate the optimal quantum advantages for space positioning and target ranging missions. Without doubt, the demonstration that endows innovative physical conception opens up widespread application of quantum sensor networks.

Electromagnetic Fields as a Health Risk Factor

This study is a collection of findings of published articles from the Department of Medical Biophysics and Department of Public Health at the Jessenius Faculty of Medicine in Martin, Comenius University Bratislava (JFM CU) with the topic on epidemiology of electromagnetic fields. During 14 years, 13 principal publications were created, of which 11 studies deal with radio frequency signals from mobile phones (mainly GSM900 and GSM1800 standard); one study describes the construction of a new exposure system and the other deals with the effects of low frequency electromagnetic fields. The results showed that the electromagnetic fields can affect heart rate variability both in humans and animals; increasing the activity of the parasympathetic or sympathetic nerve systems depending on place of body exposure (head or thorax, respectively); may cause disorders of heart rhythm; loss of concentration; headaches; and/or burning sensations in the ear area. Parameters of exposition were compared with the values issued by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). They had not exceeded the permitted limits. However, compared to data of the Bio Initiative Report, in some cases, our measured values exceeded their limits.

A high-speed multi input multi output Ro-IsOWC system incorporating Manchester coding and E-LP, O-LP modes

Abstract Aiming at the link length enhancement in intersatellite optical wireless communication (IsOWC) system, considering the hybrid radiofrequency (RF)-enabled multi input multi output (MIMO) system, we accentuate the realization of medium earth orbit (MEO)-based IsOWC system by incorporating different modulation formats such as return to zero (RZ), non-RZ (NRZ), and Manchester coding. A 40 Gbps and 16 × 8 MIMO system enabling 40 GHz RF signal is proposed by using even linear polarized (E-LP) mode, and odd linear polarized (O-LP) mode over a 22,000 km IsOWC channel. Further, a performance comparison of NRZ, RZ, and Manchester coding is performed at different IsOWC distances in terms of the Q factor. Results revealed that the proposed system with Manchester coding and O-LP mode can successfully cover the 22,000 km IsOWC link length by exhibiting a 6.35 Q factor value.

Trajectory Optimization of Laser-Charged UAVs for Charging Wireless Rechargeable Sensor Networks

This paper considers a laser-powered unmanned aerial vehicle (UAV)-enabled wireless power transfer (WPT) system. In the system, a UAV is dispatched as an energy transmitter to replenish energy for battery-limited sensors in a wireless rechargeable sensor network (WRSN) by transferring radio frequency (RF) signals, and a mobile unmanned vehicle (MUV)-loaded laser transmitter travels on a fixed path to charge the on-board energy-limited UAV when it arrives just below the UAV. Based on the system, we investigate the trajectory optimization of laser-charged UAVs for charging WRSNs (TOLC problem), which aims to optimize the flight trajectories of a UAV and the travel plans of an MUV cooperatively to minimize the total working time of the UAV so that the energy of every sensor is greater than or equal to the threshold. Then, we prove that the problem is NP-hard. To solve the TOLC problem, we first propose the weighted centered minimum coverage (WCMC) algorithm to cluster the sensors and compute the weighted center of each cluster. Based on the WCMC algorithm, we propose the TOLC algorithm (TOLCA) to design the detailed flight trajectory of a UAV and the travel plans of an MUV, which consists of the flight trajectory of a UAV, the hovering points of a UAV with the corresponding hovering times used for the charging sensors, the hovering points of a UAV with the corresponding hovering times used for replenishing energy itself, and the hovering times of a UAV waiting for an MUV. Numerical results are provided to verify that the suggested strategy provides an effective method for supplying wireless rechargeable sensor networks with sustainable energy.

Very Long-Length FFT Using Multi-Resolution Piecewise-Constant Windows for Hardware-Accelerated Time-Frequency Distribution Calculations in an Ultra-Wideband Digital Receiver.

The hardware-accelerated time-frequency distribution calculation is one of the commonly used methods to analyze and present the information from intercepted radio frequency signals in modern ultra-wideband digital receiver (DRX) designs. In this paper, we introduce the piecewise constant window blocking FFT (PCW-BFFT) method. The purpose of this work is to show the generation of spectrograms (formed by a number of spectrum lines) using a very large number of samples (N) in an FFT frame for each spectrum line calculation. In the PCW-BFFT, the N samples are grouped into K consecutive time slots, and each slot has M number of samples. As soon as the M samples in the current time slot are available from a high-speed analog-to-digital convertor (ADC), the frequency information will be obtained using K M-point FFTs. Since each time the FFT frame hops one time slot for the next spectrum line calculation, the frequency information obtained from a time slot will be reused in many spectrum line calculations, as long as these spectrum lines share those samples in the time slot. Although the use of the time domain PCW introduces spikes in the frequency spectrum of the window, the levels of those spikes are still much lower than the first side lobe level of a rectangular window. Using a Gaussian window as an example, the highest spike level can be lower than the main lobe level by at least 38 dB. The PCW-BFFT method allows a DRX to produce multiple spectrograms concurrently with different analysis window widths when the time domain samples become available continuously from the ADC. This paper presents the detailed derivation process of the PCW-BFFT method and demonstrates the use of the method with simulation results. The hardware implementation process will be reported in another paper. The computer simulation results show that long signals with slowly changing frequencies over time can be depicted on the spectrograms with wide analysis windows, and short pulses and signals with rapidly changing instantaneous frequencies can be captured in the narrow analysis window spectrograms.

A Wideband and Low-Power Distributed Cascode Mixer Using Inductive Feedback.

A wideband and low-power distributed cascode mixer is implemented for future mobile communications. The distributed design inspired by the distributed amplifier (DA) enables a mixer to operate in a wide band. In addition, the cascode structure and inductive positive feedback design allow high conversion gain with low-power consumption. The proposed mixer is fabricated using a 130 nm commercial complementary metal-oxide-semiconductor (CMOS) process. It consists of three cascode gain cells and operates with a drain voltage of 1.5 V and a gate voltage of 0.5 to 0.7 V. The fabricated mixer exhibits conversion gain of -2.9 to 3.1 dB at the radio frequencies (RFs) of 4 to 30 GHz and -1.9 to 0.4 dB at RFs of 54 to 66 GHz under the conditions of 8 to 10 dBm of local oscillator (LO) power and 650 MHz of intermediate frequency (IF). The LO-RF isolation is more than 15 dB over the entire measurement band (0.2 to 67 GHz) as the RF and LO signals are applied to different transistors owing to the cascode structure. The total power consumption is only within 12 mW, and the chip size is 0.056 mm2, making it possible to implement a compact mixer. The proposed mixer shows broadband characteristics covering from ultra-wideband (UWB) and the 28 GHz fifth-generation (5G) communication band to the 60 GHz wireless gigabit alliance (WiGig) band.

Optically Powered and Controlled Drones Using Optical Fibers for Airborne Base Stations

Mobile communication services are crucial during emergency disasters and temporary events, and future mobile communication systems should be able to provide such services. Airborne base stations using drones are highly effective as stand-in base stations in areas where the ground base stations are inoperable or at temporary event sites. However, it is difficult for conventional drones to provide mobile communication services without interruption due to flight time limitations caused by their limited battery capacity. Thus, a drone drive with a non-interrupted power supply is urgently needed. In this study, we developed an airborne base station that enables drones to be driven and maneuvered by optical fibers. We simultaneously transmitted radio frequency (RF) data signals for the airborne base station and control signals for the drone and evaluated the transmission performances of the RF signals and the controllability of the drone. Furthermore, we conducted a flight experiment on a medium-sized drone powered by optical fibers.

UW-GRE: Underwater Greedy Geographic Routing by Network Embedding

This paper presents the novel routing protocol called Underwater Greedy Geographic Routing by Network Embedding (UW-GRE) for underwater wireless networks. Geographic routing is a promising network routing protocol strategy for wireless networks because it is stateless. However, this routing strategy faces various problems in the underwater environment. This is caused by some challenging characteristics of this environment, such as its 3D nature and the no propagation of GPS signal due to high radio frequency signal attenuation. Accurate localization, void regions, and guaranteed delivery are then some of the problems faced by this routing strategy in this environment. In this paper, we solve those problems efficiently by using geographic routing over virtual coordinates obtained through a network embedding in an n-dimensional (n≥ 2) Euclidean virtual space. By properly embedding the network in this virtual Euclidean space, no geographic information is needed. We implemented the protocol on ns-3. Our results show that, when compared to a conventional geographic routing protocol with perfect localization information, our protocol improves the number of transmissions and energy consumption by up to or higher than 30%, end-to-end delay by up to 28%, while achieving similar network throughput.

Evaluation of Hepatic Fibrosis Using Ultrasound Backscattered Radiofrequency Signals and One-Dimensional Convolutional Neural Networks.

The early detection of hepatic fibrosis is of critical importance. Ultrasound backscattered radiofrequency signals from the liver contain abundant information about its microstructure. We proposed a method for characterizing human hepatic fibrosis using one-dimensional convolutional neural networks (CNNs) based on ultrasound backscattered signals. The proposed CNN model was composed of four one-dimensional convolutional layers, four one-dimensional max-pooling layers, and four fully connected layers. Ultrasound radiofrequency signals collected from 230 participants (F0: 23; F1: 46; F2: 51; F3: 49; F4: 61) with a 3-MHz transducer were analyzed. Liver regions of interest (ROIs) that contained most of the liver ultrasound backscattered signals were manually delineated using B-mode images reconstructed from the backscattered signals. ROI signals were normalized and augmented by using a sliding window technique. After data augmentation, the radiofrequency signal segments were divided into training sets, validation sets and test sets at a ratio of 80%:10%:10%. In the test sets, the proposed algorithm produced an area under the receive operating characteristic curve of 0.933 (accuracy: 91.30%; sensitivity: 92.00%; specificity: 90.48%), 0.997 (accuracy: 94.29%; sensitivity: 94.74%; specificity: 93.75%), 0.818 (accuracy: 75.00%; sensitivity: 69.23%; specificity: 81.82%), and 0.934 (accuracy: 91.67%; sensitivity: 88.89%; specificity: 94.44%) for diagnosis liver fibrosis stage ≥F1, ≥F2, ≥F3, and ≥F4, respectively. Experimental results indicated that the proposed deep learning algorithm based on ultrasound backscattered signals yields a satisfying performance when diagnosing hepatic fibrosis stages. The proposed method may be used as a new quantitative ultrasound approach to characterizing hepatic fibrosis.

MONITORING OF PULMONARY EDEMA USING ULTRASOUND RADIOFREQUENCY SIGNAL.

Objectives: Excessive accumulation of extravascular lung water impairs respiratory gas exchange and results in respiratory distress. Real-time radiofrequency signals of ultrasound can continuously and quantitatively monitor excessive lung water. This study aims to evaluate the availability of continuous real-time quantitative pulmonary edema monitoring using ultrasound radiofrequency signals and compare it with Pa o2 (partial pressure of arterial oxygen)/F io2 (fraction of inspired oxygen) (PF) ratio, conventional lung ultrasound, and the Hounsfield unit of chest computed tomography. Methods: Male Yorkshire pigs (40.5 ± 0.5 kg) were anesthetized and mechanically ventilated. A balanced crystalloid was administered to induce hydrostatic pulmonary edema. Three different infusion rates of 2, 4, and 6 mL/kg per minute were tested to determine the infusion rate for the appropriate swine model. The chest computed tomography and ultrasonography with radiofrequency signals were taken every 5 min during the full inspiration. The ultrasonography scans with radiofrequency signals were measured at the intercostal space where the line crossing the two armpits and the right anterior axillary line intersected. Results: The infusion rate of fluid for the pulmonary edema model was determined to be 6 mL/kg per minute, and a total of four pigs were tested at an injection rate of 6 mL/kg. The adjusted R2 values of regression analysis between the radiofrequency signal and computer tomography Hounsfield score were 0.990, 0.993, 0.988, and 0.993 (all P values <0.05). All radiofrequency signal changes preceded changes in PF ratio or lung ultrasound changes. The area under the receiver operating characteristic curve of the radiofrequency signal for predicting PF ratio <300 was 0.88 (95% confidence interval, 0.82-0.93). Conclusion: We evaluated ultrasound radiofrequency signals to assess pulmonary edema in a swine model that can worsen gradually and showed that quantitative ultrasound radiofrequency signal analysis could assess pulmonary edema and its progression before PF ratio or lung ultrasound changes.

Low‐Loss Graded Dielectrics via Active Mixing of Nanocomposite Inks during 3D Printing

AbstractAdditive manufacturing has emerged as a promising approach for fabricating graded refractive index structures that control the electromagnetic response of radio frequency (RF) devices. However, current 3D printing methods cannot produce continuous gradients from multiple materials. Here, low‐loss graded dielectrics via active mixing of nanocomposite inks composed of block copolymers and oxide nanoparticles are designed and printed. By simultaneously tailoring their rheological, printing, and their local filler particle‐to‐polymer ratio using an active mixing printhead, a conductive microstrip‐graded substrate matching network with a gradually changing dielectric response, is created. In these printed devices, the impedance of the RF signal is controlled by the graded substrate rather than by varying the conductive microstrip geometry, enabling the fabrication of smaller RF devices. This approach enables the rapid design and fabrication of high‐performance RF devices with locally tunable dielectric properties.

High-performance electrically conductive adhesives with aluminum-doped zinc oxide (AZO) for various flexible electronic devices

Electrically conductive adhesives (ECAs) are a kind of attractive alternative to tin solders in microelectronics. However, fabricating high-performance, low-cost ECAs remains challenging due to the need for high-content metal fillers. This work reported a facile preparation of cost-effective ECAs composed of aluminum-doped zinc oxide (AZO) nanoparticles and silver (Ag) micro-flakes. The AZO nanoparticles could serve as a “bridge” to connect neighboring Ag micro-flakes to significantly improve the ECAs′ conductivity (from 1638.8 × 10-5 Ω⋅cm to 8.5 × 10-5 Ω⋅cm). After the aging and bonding performance tests, prototyped applications of the as-prepared ECAs for three flexible electronic devices were demonstrated sequentially: (Ⅰ) The resistance of flexible conductive film showed excellent dynamic stability after being folded for 1000 cycles, bent/released within 6 mm radius for more than 2000 cycles, or applied with 1000 kPa pressure; (Ⅱ) The coil antenna achieved fast transmission of radio frequency signal for near field communication (NFC) system; (III) The flexible heater performed efficient thermal management at 0.4–1.4 V input voltages. Therefore, the fabricated ECAs are expected to provide new opportunities for the development of novel flexible electronic devices for future applications.