Free Space Environment Research Articles

Full-duplex wireless light communication using green laser diodes

The integration of wireless light communication into a wireless fidelity (Wi-Fi) module and gateway enables real-time integrated communication networks that satisfy practical application demands. In particular, wireless green light communication tools can operate underwater and in free-space environments. Here, we design, fabricate, and characterize a full-duplex light communication system using green laser diodes (LDs). Operating within the transmission control protocol/internet protocol (TCP/IP), full-duplex wireless data transmission is confirmed in underwater and free-space environments at a communication rate of 10 Mbps. Through connections to a Wi-Fi module and gateway, the system is accessed by the network via the TCP/IPv4 internet scheme, and real-time video transmission is demonstrated.

Energy‐efficient selection of relay for UWSNs in the Internet of underwater things

SummaryInternet of underwater things (IoUT) for underwater monitoring is known worldwide for smart interlinked underwater things that exhibit the capacity to monitor the vast unexplored waters of the oceans. Concept of IoUT has been derived from Internet of Things (IoT) in order to acquire the exquisite benefits of networking in underwater environment. IOT standards and technologies do not work well in underwater environment, such as infrared, Wi‐Fi, and radio frequency (RF) due to high channel errors and limited range up to few meters. Acoustic waves, however, can be used to communicate both in shallow and deep oceans due to their low frequency (kHZ) signal. In context of IoUT, communication based on acoustic links enables different applications such as underwater exploration, environmental monitoring, and disaster prevention even without availability of GPS facility like free space environment. In unpredictable and changing underwater environment, energy efficiency becomes a major challenge during data routing along multiple devices. Batteries of the sensor nodes, autonomous underwater vehicle (AUV), and remotely operated vehicle (ROV) cannot be removed with easiness and difficult to recharge, and the only way out is efficient sensor node selection for relaying to save massive amount of energy. Energy aware channel routing protocol (ECARP) does not consider the depth of the node while selecting the relay nodes to forward the data. Relay node selection in underwater Internet of things (IOUT) is a primary problem addressed in this research based on channel state information (CSI) for establishing best path to relay information among IOUT devices. Our major focus was to develop better technique for the relay node selection using a CSI and select relay node by looking at its depth from ocean surface and residual energy in the proposed ED‐CARP. Simulation results validate that proposed ED‐CARP can decrease the communication cost and increase the network lifetime.

Detecting Angle of Arrival on a Hybrid RIS Using Intensity-Only Data

Reconfigurable reflective metasurfaces—or reconfigurable intelligent surfaces (RISs)—can redirect incident signals toward desired directions, an ability that allows for sculpting wireless communication channels with desired characteristics. This smart radio environment, however, necessitates the information about the transmitter(s) and the receiver(s) to be available at the RIS. One possible solution to this need is to add sensing capability to the RIS. However, sensing complex wireless signals (i.e. both amplitude and phase) often requires complicated setups. In this paper, we propose and numerically demonstrate a reconfigurable reflective metasurface that uses intensity-only samples of the incident signal to retrieve desired information about the propagation environment. To do that, we will randomly tune the metasurface elements to multiplex information incident on all of them. The phaseless multiplexed data is then processed using computational ghost imaging algorithm to retrieve the desired information. As a demonstrative example, we present the detection of the incident angle from a user in a free-space environment. This simplified sensing process can pave the way for incorporation of RISs with integrated sensing capabilities in future wireless communication or sensing systems.

An equivalent source method for holographic visualization of acoustic sources in an apertured enclosure based on exterior measurements

Most acoustical holography techniques are developed to visualize sound sources or reconstruct sound field in a free-space environment, where no boundary reflected or scattered sound are present. However, in some engineering applications, a sound scatter is an inseparable part of source mechanism. For example, for a centrifugal fan, the aerodynamic noise sources are the turbulent pressure inside the fan shell, but the source distribution is significantly altered if the fan shell is removed. Thus, in order to visualize the sources inside the fan shell by holography methods based on exterior sound measurements, the measurements need to be performed with the presence of the shell (which is a sound scatter). This obviously violates the free-space assumption for most acoustical holography methods. In this work, traditional free-space equivalent source methods are extended to visualize acoustic sources situated in an enclosure with openings based on sound measurements exterior to the enclosure. The key step is to replace the free-space Green’s function used in traditional equivalent source methods by a Green’s function that satisfies the boundary condition of the apertured enclosure. Simulations are performed to prove the concept of the proposed acoustical holography method.

Adaptive Continuous-Variable Quantum Key Distribution with Discrete Modulation Regulative in Free Space

The finite sampling bandwidth of an analog-to-digital converter has a negative effect on the continuous-variable quantum key distribution (CVQKD), which leaves a potential loophole for an eavesdropper and weakens the practical security of the system. To compensate for the loss in free space, we deploy an adaptive optics (AO) unit in the detector of the CVQKD system with discrete modulations. Since the closed-loop control bandwidth of the embedded AO unit can be optimized with the sampling frequency, the practical security of the system can be enhanced in terms of the secret key rate. The security analysis is demonstrated on the basis of the derived secret key rate with numerical simulations, providing a feasible implementation of CVQKD in realistic free-space environments.

High-Stiffness Control of Series Elastic Actuators Using a Noise Reduction Disturbance Observer

Rendering a high impedance on a series elastic actuator (SEA) can improve motion control performance while retaining collision safety by a lower physical stiffness. The safe impedance range can be increased with high-gain velocity feedback control, but, in practice, this is limited by noise. This article applies the noise reduction disturbance observer to inner-loop velocity control of an SEA, attenuating noise and allowing higher safe rendered stiffness compared with standard torque and torque/velocity hierarchical control. Closed-form expressions for maximum passive stiffness and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Z$</tex-math></inline-formula> -region are found, shown to depend strongly on the high-frequency noise gain, and used to optimize the control gains. Performance is experimentally verified on a reaction-force SEA, validating the passivity of the high-stiffness control in impact (free space and stiff environment) while rendering a safe stiffness 3.0 times the intrinsic stiffness.

Reconfigurable intelligent surface-assisted free-space optical communication system under the influence of signal blockage for smart-city applications.

Atmospheric turbulence and pointing errors represent substantial hurdles to free-space optical communications (FSOs), impeding their practical design. The reconfigurable intelligent surface (RIS) is an emerging technology that enables reflective radio transmission conditions for next-generation 5G/6G wireless frameworks by intelligently adjusting the beam in the desired direction using low-cost inactive reflecting elements. In this paper, we proposed an RIS-assisted FSO system for mitigating the effects of atmospheric turbulence, pointing errors, and communication system signal blockage. The probability density function and cumulative distribution functions of an FSO system composed of N-RIS elements are evaluated in a free-space environment that contains obstructions. We derived closed-form expressions for the proposed system's bit error rate (BER), outage probability, and channel capacity. The proposed system's performance is analyzed in terms of BER, outage probability, and channel capacity under various weather conditions, pointing errors, and signal blockage. The results are plotted as a function of number of RIS elements and average signal-to-noise ratio. The proposed system will be beneficial in smart-city applications since it will provide reliable connectivity in urban environments with a high population density and high-rise buildings.

A novel high gain dual band ear bud shaped patch antenna for under water communications

AbstractThis article elucidates a high gain dual band ear bud shaped patch antenna with mm wave technology. The pivotal characteristic of this design is to provide higher gain and less interference at higher frequencies. It operates at a frequency of 40.59 and 52.002 GHz. The major advantage of this design is that it produces the circular polarization in entire band of frequency; moreover this antenna is useful for under water communication. The proposed design introduces two splits, imprinted at the walls of patch to get a wide band. For an optimal design, a future antenna parametric analysis was carried out. The proposed design is developed with the aid of HFSS software; we measured major parameters S11, VSWR, gain and radiation patterns in free space and underground environment. In the results, the predicted antenna radiation pattern is seen for E‐plane and H‐plane at various frequency bands. The gain 3D polar plots and current distribution plots are presented, including a comparison of previous work to the current work. We assure that the suggested design is applicable for under water communication.

Performance Enhancement of Wearable Antenna Using High Impedance Surfaces

This paper presents a novel textile wearable antenna that has been designed to operate at Wi-Fi bands of 2.4GHz &amp; 5.8GHz. Antenna performance in free space environment showed Gain of 1.8dBi at 2.4GHz and 4.5dBi at 5.8GHz. However, performance deteriorated when antenna was operated near human body which is lossy and complex in nature. For mitigating the human body effect on antenna performance, high impedance surface (HIS) was designed and integrated with this textile antenna. Due to shielding effect of HIS, antenna Gain increased to 8dBi at 2.4GHz and 9dBi at 5.8GHz. The SAR values were also reduced to 0.682W/Kg at 2.4GHz and 0.0692W/Kg at 5.8GHz for 10g tissue. The proposed antenna was also tested under bending and crumpling conditions. It was observed that antenna performance was not significantly deteriorated. The proposed textile antenna can have exciting applications in emerging wearable technologies.

IR Attenuation Estimation in a Free-Space Environment with Sand and Dust Particles

An approach for estimating the attenuation constant of propagation in a sandstorm at a wavelength of 1550 nm with various prevailing visibility is described in this study. An adequate dust particle size, distribution statistics, and generalized Mie scattering theory were used in the estimation. The visibility is calculated using the computed attenuation at a wavelength of 550 nm. The findings are compared to the measured data found in the literature. To regale, a detailed sensitivity analysis for various sandstorm structures is presented. This analysis is used to signalize a dusty channel in order to simulate a single input-output 1550 nm free space optical link. By computing the bit error rate of the system, the performance of the optical link in flurry dust with attenuation smaller than 10 dB/km is investigated.

Wide-Angle Ceramic Retroreflective Luneburg Lens Based on Quasi-Conformal Transformation Optics for Mm-Wave Indoor Localization

This paper presents a quasi-conformal transformation optics (QCTO) based three-dimensional (3D) retroreflective flattened Luneburg lens for wide-angle millimeter-wave radio-frequency indoor localization. The maximum detection angle and radar cross-section (RCS) are investigated, including an impedance matching layer (IML) between the lens antenna and the free-space environment. The 3D QCTO Luneburg lenses are fabricated in alumina by lithography-based ceramic manufacturing, a 3D printing process. The manufactured structures have a diameter of 29.9 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \lambda _{0}$ </tex-math></inline-formula> ), showing a maximum realized gain of 16.51 dBi and beam steering angle of ±70° at 40 GHz. The proposed QCTO Luneburg lens with a metallic reflective layer achieves a maximum RCS of −20.05 dBsqm at 40 GHz with a wide-angle response over ±37°, while the structure with an IML between the lens and air improves these values to a maximum RCS of −15.78 dBsqm and operating angular response between ±50°.

Wearable Pattern-Diversity Dual-Polarized Button Antenna for Versatile On-/Off-Body Communications

A wearable dual-port button antenna that excites pattern-diversity dual-polarized waves is proposed for on-/off-body applications. Its simple structure comprises of two radiators and a common ground plane designed into two substrates. The crossed-dipole radiator is made by two symmetric bowtie dipoles printed on a circular-shaped semi-rigid substrate, which covers an ultra-wideband (UWB) application with circular polarization and directional radiation patterns, high gain, and high efficiency suitable for off-body communication. The annular-ring radiator and common ground plane are made by a conductive textile on the square-shaped flexible-felt substrate. The annular-ring radiator is shorted to the ground plane using four vias, which generates triple TM modes covering the 2.45/5.85 GHz wireless body area networks (WBAN) with omni-directional radiation patterns and a 3.8 GHz C-band with a directional radiation pattern suitable for on-/off-body communications. The fabricated antenna is verified by measurement in both free space and phantom body environment. Measurements agreed well with simulations. Simulated specific absorption rates (SARs) under US and EU standards are below the safe level, making the proposed antenna suitable for on-/off-body communications.

Escape Path Obstacle-Based Mobility Model (EPOM) for Campus Delay-Tolerant Network

In Delay-Tolerant Networks (DTNs), humans are the main carriers of mobile devices, signifying that human mobility can be exploited by extracting nodes’ interests, social behavior, and spatiotemporal features for the performance evaluation of DTNs protocols. This paper presents a new mobility model that describes students’ daily activities in a campus environment. Unlike the conventional random walk models, which use a free space environment, our model includes a collision-avoidance technique that generates an escape path upon encountering obstacles of different shapes and sizes that obstruct pedestrian movement. We evaluate the model’s usefulness by comparing the distributions of its synthetic traces with realistic traces in terms of spatial, temporal, and connectivity features of human mobility. Similarly, we analyze the concept of dynamic movement clusters observed on the location-based trajectories of the studied real traces. The model synthetically generates traces with the distribution of the intercluster travel distance, intracluster travel distance, direction of movement, contact duration, intercontact time, and pause time similar to the distribution of real traces.

Assessment of Indoor-Outdoor Wi-Fi Radiation on Human Body and its Precise SAR Measurement

There has been a lot of awareness concern about the health hazards of RF electromagnetic radiation on the human body, the radiofrequency electromagnetic field (RF-EMF) exposure assessment needs attention. This paper reports experimentally measured electromagnetic radiation emission from public place Wi-Fi devices. Indoor and outdoor measurements are done at different geographic locations in India. For the indoor assessment, the experiment was carried out in the national physical laboratory (NPL) New Delhi for 1-4 antennas Wi-Fi routers at a distance of 0-10-meter range are taken into consideration. The power density and electrical intensity were measured using a spectrum analyzer with isotropic E-field probe TSEMF-B1. For the outdoor measurements, four different Delhi metro stations Wi-fi devices at a distance of 20-meter range towards the train coach and below the Wi-fi routers have been considered. In this experiment, we used an instrument is Narda NBM-550 E-field probe for electric field and radiated power density estimation. In addition, the human whole body 3D model has been simulated in a free space environment, the measured outcome of indoor-outdoor electromagnetic radiation and specific absorption rate (SAR) is compared with the ICNIRP, FCC guideline limits at 2.45-5.87 GHz and precise SAR has been evaluated.

Implementation of the Friis Free Space Propagation Model in the Dynamic Source Routing (DSR) Routing Protocol in the Vehicular Ad-hoc Network (VANET) with Variations of Road Models

The use of VANET technology is to create better traffic management. Different road conditions and high mobility of vehicles cause problems in VANET such as efficient search processes, network routes that need to be maintained, and data transmission problems. DSR (Dynamic Source Routing) predefines and generates routes on demand and is designed to work on multihop ad-hoc networks without centralized control. The Friis Free Space propagation model is used to model the Line of Sight (LOS) that occurs in a free space environment. The variation of road models in Indonesia is grouped into four groups, namely express roads, intercity roads, urban roads, and residential roads. In each of these road models, simulation testing is carried out based on different vehicle number scenarios. Network Simulator v3.27 (NS-3.27) and SUMO as a mobility generator were used for the simulation. The parameters of average throughput, packet delivery ratio and MAC/PHY overhead sequentially measure the DSR performance. The results on the express roads with Friis Free Space propagation produce values of 6.52 KBps, 7.71%, 39.59 KBytes. Test results on intercity roads 5.72 KBps, 11.17%, 5.48 KBytes. Test results on urban roads 16.50 KBps, 32.22%, 0.0016 KBytes. While the test results on residential roads were 12.66 KBps, 24.72%, 0.779 KBytes. The test results show that the road model in Indonesia with the performance of the DSR routing protocol with Friis Free Space propagation on the VANET network from the best to the worst, respectively, is the urban road model, residential road model, intercity road model, and express road model.

Observing quantum coherence from photons scattered in free-space

Quantum channels in free-space, an essential prerequisite for fundamental tests of quantum mechanics and quantum technologies in open space, have so far been based on direct line-of-sight because the predominant approaches for photon-encoding, including polarization and spatial modes, are not compatible with randomly scattered photons. Here we demonstrate a novel approach to transfer and recover quantum coherence from scattered, non-line-of-sight photons analyzed in a multimode and imaging interferometer for time-bins, combined with photon detection based on a 8 × 8 single-photon-detector-array. The observed time-bin visibility for scattered photons remained at a high 95% over a wide scattering angle range of −450 to +450, while the individual pixels in the detector array resolve or track an image in its field of view of ca. 0.5°. Using our method, we demonstrate the viability of two novel applications. Firstly, using scattered photons as an indirect channel for quantum communication thereby enabling non-line-of-sight quantum communication with background suppression, and secondly, using the combined arrival time and quantum coherence to enhance the contrast of low-light imaging and laser ranging under high background light. We believe our method will instigate new lines for research and development on applying photon coherence from scattered signals to quantum sensing, imaging, and communication in free-space environments.

Investigation of male searching for female silkworm using an artificial pheromone in free-space environment

A male insect can exhibit interesting motions and behaviours when it detects the female pheromone. The behaviour shown by a male insect is frequently used for nature-inspired robotics research especially for various applications such as teleoperated and autonomous source locating robots. In this research, we investigate the male searching for female behaviour by changing the source of pheromone using an artificial pheromone. This research is conducted by using three male insects, where, each insect performs three trials and the chosen environment is a free-space environment. The three male insects perform satisfactorily without having any hurdle in a free-space scenario using artificial pheromone. It is concluded that the motion behaviour of insects can be applied in the real robot for searching mission.

Radio Channel Modeling in a Ship Hull: Path Loss at 868 MHz and 2.4, 5.25, and 60 GHz

This letter presents the analysis of a channel sounding campaign in the hull of a bulk carrier vessel at sub-6 GHz frequencies 868 MHz and 2.4 and 5.25 GHz as well as millimeter-wave (mmWave) frequency 60.48 GHz. For the sub-6 GHz frequencies, path loss (PL) is measured using omnidirectional antennas. The measured PL is fitted to a one-slope PL model that takes into account attenuation due to engine room floor crossings, obstructions, and walls. The fitted model has a reference PL lower than free space and a PL exponent around 1.2. For mmWave wireless communication, high gain directive antennas are typically used. Therefore, the directive radio channel for line-of-sight (LOS) communication is measured using the Terragraph channel sounder and an antenna beamwidth dependent PL model is created. At mmWave frequencies, LOS PL in the vessel is close to PL in a free-space environment, but angular spread values are lower compared to other indoor scenarios.

Feasibility analysis for active near/far field acoustic pattern synthesis in free space and shallow water environments

In this paper, a detailed sensitivity and feasibility analysis of the active manipulation scheme for scalar Helmholtz fields proposed in our previous works, in both free space and constant-depth homogeneous ocean environments, is presented. We apply the method of moments (MoM) together with Tikhonov regularization with the Morozov discrepancy principle to investigate the effects of varying the problem parameters to the accuracy and feasibility of the proposed active field control strategy. We discuss the feasibility of the active scheme (with respect to power budget, control accuracy and process error) as a function of the frequency, the distance between the control region and the source, the mutual distance between the control regions, and the size of the control region. Process error is considered as well to investigate the possibility of an accurate active control in the presence of manufacturing or feeding noise. The numerical simulations show the accuracy of the active field control scheme and indicate some challenges and limitations for its physical implementation.

Directional Antenna With Lightweight Metamaterial Reflector for UAV-Based Networks

The wireless relay network utilizing multiple Unmanned Aerial Vehicles (UAVs) has now been widely studied. Since the radio propagation in the UAV network is close to free space environment, co-channel interference to other UAVs and the ground station (GS) becomes a serious problem. Designing a directional and multiband antenna is quite effective against this issue; multiple metamaterial reflectors are our key solution. Meanwhile, the flight time of the UAV imposes a limitation on the payload weight including communication equipment. This paper proposes a directional antenna employing a lightweight metamaterial reflector to reduce the antenna weight. It is realized by the woodpile structure which consists of layers of dielectric rods with a stacking sequence. Making the hole in the center of each dielectric rod can reduce the woodpile reflector's weight with maintaining the antenna radiation performance. The electromagnetic field simulation clarified the reflection characteristics by making the hole. Both simulative and experimental evaluations clarified that the proposed antenna has almost the same radiation characteristics as the conventional one while reducing its weight by 25%, which saves the power consumption by 12.5%.