Radio Frequency Link Research Articles

Impact of Atmospheric Turbulence, Pointing Error, and Traffic Pattern on the Performance of Cognitive Hybrid FSO/RF System

Atmospheric turbulence conditions and mis-alignment in the free space optical (FSO) link are prime factors which necessitate the requirement of a back up radio frequency (RF) link to improve the link availability of the FSO systems. However, the utilization of this back-up RF link is limited. The RF channel which is meant chiefly as a back-up resource in case of FSO link failure, can be utilized more efficiently. In this work, it is attempted to improve the utilization of this RF resource. A second FSO link is converted to a Cognitive Hybrid FSO/RF link by accessing the already existing RF in the cognitive interweave mode. RF utilization factor is a measure of the efficiency with which the two FSO links utilize the same RF resource. The system performance is evaluated on the basis of RF utilization, outage probability and bit error rate (BER) under atmospheric turbulence and mis-alignment. Further, the impact of data traffic characteristics on system performance has been investigated. Closed form expressions for RF utilization, outage probability and BER have been derived for various traffic conditions. The analytical results have been verified through simulations. Results indicate improved system performance and better overall utilization of the RF resource.

Amplifier-Aware Content-Based Precoder Design for Hierarchical Image Transmission over a Realistic MIMO-OFDM Channel

This paper proposes the design of a precoder in MIMO systems dedicated to JPEG Wireless (JPWL) (ISO/IEC 15444-11) image transmission which jointly takes into account the channel noise and the nonlinear distortions due to the high power amplifier (HPA) used in the transmitter. MIMO-OFDM (Orthogonal Frequency Division Multiplexing) systems dedicated to image transmission rely only on the Channel State Information (CSI) and the image content to design their precoding solutions. However, the nonlinear behavior of the HPA used at the front-end is not taken into account when designing the current precoders. In order to ensure the QoS of the transmitted image, a large level back-off is usually applied to avoid the nonlinear distortions generated by the HPA. In addition, the Input Back-off (IBO) value is statically chosen and does not depend on the other parameters of the link such as the instantaneous MIMO channel status, the modulation order and the scalability of the JPWL content. This may reduce both the global energy efficiency and the transmission quality of the Radio-Frequency (RF) link. Therefore, this work proposes a new precoding scheme to allocate the total available power while dynamically adjusting the optimal power IBO value according to the HPA parameters, the instantaneous channel status and the image content to be transmitted. Numerical results using a commercial HPA model under the IEEE 802.11n standard show that the proposed power allocation algorithm improves the visual quality of the received JPWL images compared to the state-of-the-art over a realistic radio channel model.

Beamforming Galvanic Coupling Signals for IoMT Implant-to-Relay Communication

Implants are poised to revolutionize personalized healthcare by monitoring and actuating physiological functions. Such implants operate under challenging constraints of the limited battery energy, heterogeneous tissue-dependent channel conditions, and human-safety regulations. To address these issues, we propose a new cross-layer protocol for galvanic coupled implants, wherein the weak electrical currents are used in place of classical radio frequency links. As the first step, we devise a method that allows multiple implants to communicate individual sensed data to each other through code division multiple access, combined with compressive sensing method to lower the transmission time and save energy, as well as delegates the computational burden of dispreading and decoding only to the on-body surface relays. Then, we devise a distributed beamforming approach that allows coordinated transmissions from the implants to the relays by considering the specific tissue path chosen and tissue heating-related safety constraints. We then proceed to implement distributed beamforming on a phantom of human tissue and prove an increase in received signal strength and a decrease in BER due to constructive interference of the signals of each implant. Our contributions are twofold: First, we devise a collision-free protocol that prevents undue interference at neighboring implants, especially for multiple deployments. Second, this is the first application of near-field distributed beamforming in human tissue. Simulation results reveal significant improvement in the network lifetime for implants of up to 79% compared to the galvanic coupled links without beamforming. In addition, the implementation of phantom tissue proves improved communication metrics when beamforming is used.

Exploration of Hybrid FSO/RF Availabilities in Optical Wireless System

In this paper a more practical solution has investigate by extend the range of availability through FSO back-up link with a radio frequency (RF) link of lower data rate. This paper investigate the technical advantages of integrating FSO and RF communication systems to get availability of 99.999% to expand a new system called hybrid/dual RF/FSOF system. Analysis shows that a consequences of rain and fog have an effect on the RF and FSO links severally however not at the same time and barely occur at the same time to grant the choice to build up new hybrid FSO/RF system

Physical-Layer Security Analysis of Mixed SIMO SWIPT RF and FSO Fixed-Gain Relaying Systems

This paper studies the physical-layer security problem for mixed single-input multiple-output (SIMO) simultaneous wireless information and power transfer (SWIPT) based radio frequency (RF) and free-space optical (FSO) communication systems. The FSO link experiences Malaga turbulence and each RF link suffers from Nakagami- $m$ fading and path loss. We consider one energy harvesting receiver in our system model that may act as a potential eavesdropper. More precisely, to investigate the secrecy performance of considered mixed SIMO SWIPT based RF and FSO communication system, we derive closed-form expressions for the average secrecy capacity and the lower bound of secrecy outage probability by considering the fixed-gain relaying scheme, the multiple-antenna technique, the energy harvesting, the intensity modulation with direct detection, and the heterodyne detection techniques in the presence of pointing error.

Hybrid RF-FSO System Cascaded with UWOC Link

This work presents outage analysis of a hybrid communication system, where radio frequency (RF) and free space optical (FSO) communication links transmit in parallel to the then cascaded underwater wireless optical communication (UWOC) link forming a RF/FSO-UWOC cooperative system. Optical wireless communication (OWC) is being used as a viable communication technology by researchers due to its advantages such as fast speed, cost effectiveness, and highly secure transmission. The proposed architecture connects the terrestrial and the underwater regions to form an end-to-end cooperative system. This hybrid RF/FSO back-haul link makes the system more reliable as the optical link is highly susceptible to weather conditions. The channel models considered are Nakagami-m distributed fading, Double Generalized Gamma (DGG) distributed atmospheric turbulence along with Rayleigh distributed misalignment losses and mixture of Exponential Generalized Gamma (EGG) distributed oceanic turbulence for RF, FSO, and UWOC links, respectively. Results show that outage performance is severely affected due to RF fading, DGG distributed irradiance, air bubble concentration, temperature gradient, and saltiness of water source.

Underwater Wireless Communications for Cooperative Robotics with UWSim-NET

The increasing number of autonomous underwater vehicles (AUVs) cooperating in underwater operations has motivated the use of wireless communications. Their modeling can minimize the impact of their limited performance in real-time robotic interventions. However, robotic frameworks hardly ever consider the communications, and network simulators are not suitable for HIL experiments. In this work, the UWSim-NET is presented, an open source tool to simulate the impact of communications in underwater robotics. It gathers the benefits of NS3 in modeling communication networks with those of the underwater robot simulator (UWSim) and the robot operating system (ROS) in modeling robotic systems. This article also shows the results of three experiments that demonstrate the capabilities of UWSim-NET in modeling radio frequency (RF) and acoustic links in underwater scenarios. It also permits evaluating several MAC protocols such as additive links online Hawaii area (ALOHA), slotted floor acquisition multiple access (S-FAMA) and user defined protocols. A third experiment demonstrated the excellent capabilities of UWSim-NET in conducting hardware in the loop (HIL) experiments.

A new relay based architecture in hybrid RF/FSO system

In this paper, a novel architecture in mixed Radio-Frequency (RF)/Free-Space-Optical (FSO) communication systems is presented. RF and FSO links based on Amplify-and-Forward (AF) relay with variable gain, are serially connected. In the proposed method, RF-FSO or FSO-RF paths are used for data transmission. The fading of RF and FSO systems are modeled by Rayleigh and Gamma–Gamma distributions, respectively. It is assumed that the Intensity Modulation with Direct Detection (IM/DD) and the heterodyne detection techniques are used in FSO links. New closed-form expressions for Outage Probability (OP) and Bit Error Rate (BER) are presented in this article. The performance of the proposed system is compared with the single RF-FSO system in different atmospheric disturbance conditions. Numerical results show that the new RF-FSO/FSO-RF system has a significant improvement over the single RF-FSO system. For SNR=40 dB, the outage probability for our proposed system and RF-FSO are obtained, 2.3×10−6, and 3×10−3, respectively.

Performance analysis of mixed RF/FSO system with CCI

In this study, the performance of the mixed radio-frequency (RF)/free-space optical (FSO) system with co-channel interference (CCI) is investigated. The RF links are modelled as Rayleigh fading, whereas FSO links follow M-distribution with pointing errors. Assume the amplify-and-forward relay is corrupted by multiple CCIs, and both fixed and variable gain relay schemes are considered. Novel expressions for the end-to-end outage probability (OP), symbol error rate (SER) and outage capacity in different cases are presented and analysed. Numerical results demonstrate the existence of OP and SER floors in the presence of CCIs, which implies system performance is dominant by RF links, and interference is the main factor affecting the system performance. Additionally, the fixed gain scheme provides a performance enhancement compared with the variable gain scheme in the authors' system.

Performance analysis for a mixed RF and multihop FSO communication system in 5G C-RAN

In this paper, we investigate the performance of a mixed radio frequency (RF) and multihop free space optical (FSO) system in a fifth-generation cloud radio access network (C-RAN) architecture. To cope with the practical channel characteristics, the RF link is assumed to experience the $ \kappa - \mu $ shadowed fading that has recently been proposed to model the small-scale multipath fading as well as line-of-sight shadowing. For FSO links, we adopt the exponentiated Weibull distribution, which can represent irradiance fluctuations at the finite receiving aperture over a wide range of turbulence conditions. Furthermore, we consider the pointing error between the FSO end devices. We first derive a novel probability density function (PDF) expression for the $ \kappa - \mu $ shadowed fading so that the $ \kappa $, $ m $, and $ \mu $ can be equal to the arbitrary positive real value. Then, we study the outage probability and the average symbol error rate (ASER) of the considered system. Our analysis is based on a decode-and-forward (DF) relaying scheme. We choose M-ary quadrature amplitude modulation (M-QAM) for the RF link and binary pulse position modulation (BPPM) for the FSO links. Finally, to provide more insights, we present the asymptotic approximate expressions in high signal-to-noise ratio regimes. Our research shows that the performance of the mixed RF and multihop FSO system can be improved by the large receiving aperture and the increasing number of hops.

A Framework on the Performance Analysis of Dual-Hop Mixed FSO-RF Cooperative Systems

Free space optical (FSO) communication systems provide line-of-sight wireless connectivity in the unlicensed optical spectrum and achieve higher data rates in comparison to their radio frequency (RF) counterparts. FSO systems are particularly attractive for the last mile access problem by establishing a connectivity bridge between optical fiber-based backbone and RF access networks. To address this practical deployment scenario, there has been an increasing attention on the so-called mixed (dual-hop) FSO-RF systems where FSO transmission is utilized at one hop followed by RF transmission at the other one. In this paper, we investigate the performance of mixed FSO-RF transmission systems where the FSO link is modeled by the double generalized Gamma distribution with generalized pointing error impairments and the RF link experiences the extended generalized-K shadowed fading. For both amplify-and-forward and decode-and-forward relaying, we derive closed-form expressions for outage probability, bit error probability, and ergodic capacity in terms of bivariate Fox-H function. Monte Carlo simulation results are provided to verify the accuracy of derived expressions. We further provide an asymptotic analysis and discuss the achievable diversity orders of mixed FSO-RF systems.

A load-balanced cross-layer design for energy-harvesting sensor networks

To extend the lifetime of the individual nodes in a wireless sensor network (WSN), energy conservation has been a prominent design goal. Among the most popular energy optimization schemes is the pursuance of a hierarchical topology, e.g., forming clusters, and energy aware routing. Recent advances in the area of energy harvesting from ambient sources have enabled the development of rechargeable devices that can be leveraged in developing a long-lasting WSN. Furthermore, since nodes may be scavenging energy at a varying rate, energy transfer over radio frequency (RF) links has been adopted as a means for the network to perpetually remain operational. Thus, an energy-harvesting WSN (EHWSN) introduces new challenges and warrants unconventional management strategies. In this paper, we propose a novel CRoss-layer design for WSNs with Energy Scavenging and Transfer capabilities (CREST). CREST is a distributed approach that forms a two-tier routing topology by grouping nodes into clusters. The network is divided into virtual tracks, whose width is based on proximity to the base-station (BS) and node density. The rate of energy gain from the environment and from other nodes is the criterion for selecting a cluster-head (CH). The cluster size is determined in order to even the load among all CHs while observing that the traffic load increases when getting closer to the BS. Finally, the sensor data are disseminated to the BS over inter-CH paths determined by a novel energy-transferring-based routing algorithm. The performance of CREST is validated through analysis and simulation.

Performance Analysis of Hard-Switching Based Hybrid FSO/RF System over Turbulence Channels

The hybrid system of free space optic (FSO) and radio frequency (RF) has come forth as alternative good solution for increasing demand for high data rates in wireless communication networks. In this paper, wireless networks with hard-switching between FSO and RF link are analyzed, assuming that at a certain time point either one of the two links are active, with FSO link having higher priority. As the signal-to-noise ratio (SNR) of FSO link falls below a certain selected threshold, the RF link is activated. In this work, it is assumed that the FSO link follows Gamma-Gamma fading due to the atmospheric turbulence effect whereas RF link experiences Rayleigh fading. To analyze the proposed hybrid model, analytical expressions are derived for the outage probability, bit error rate and ergodic capacity. A numerical comparison is also done between the performances of the proposed hybrid FSO/RF model and the single FSO model.

On the performance of a multi-user multi-hop hybrid FSO/RF communication system

One of the main problems in mobile communication systems is long-range impassable links or some specific atmospheric conditions under which Radio Frequency (RF) connection becomes easily disrupted. One way to solve this problem is by consuming more power or adding processing complexity, but a small user mobile phone cannot deserve more complexity or power supply. Another way is to use an access point that amplifies received signal via short-range RF link and forwards it to via long-range Free Space Optical (FSO) link. In this paper, a novel multi-user multi-hop hybrid FSO / RF communication system is presented. This structure consists of two main parts. The first part establishes the connection between the mobile users and the source base station, and the second part establishes the connection between the source and the destination base stations. In the first part, mobile users want to connect to the source base station via a long-range link; therefore, an amplify and forward relay is used for communication establishment. In the second part, the source and the destination base stations are connected via a multi-hop hybrid parallel FSO / RF link with demodulating and forward relaying. The FSO link in moderate to strong and in saturated regimes is assumed at Gamma–Gamma and Negative Exponential atmospheric turbulence, respectively, also the effect of pointing error is considered, and RF link has Rayleigh fading. New closed-form exact and asymptotic expressions are derived for the Outage Probability and Bit Error Rate (BER) of the proposed structure. Derived expressions are verified with MATLAB simulations. The proposed structure has advantages of FSO, RF, relay-assisted, and multi-user systems at the same time. Results indicate that it has low dependence on the number of users and the number of relays. Therefore, it is suitable for areas with varying population and long-range links. This structure offers independent performance without additional power consumption, processing latency, and complexity. The innovations and contributions of this paper, which are first introduced in the multi-hop hybrid FSO / RF hybrid structure, include the novelty of the proposed structure, the presentation of new exact and asymptotic mathematical solutions, use of multi-user scheme in a multi-hop FSO / RF structure, BER analysis, using amplify and forward as well as decode and forward protocols, taking into account a wide range of atmospheric turbulence with the effect of pointing error, using opportunistic selection schemes at each relay.

Performance Analysis and Power Optimization for Spectrum-Sharing Mixed RF/FSO Relay Networks With Energy Harvesting

In this paper, the energy harvesting in mixed multiple-input-single-output radio frequency (RF)/free space optical (FSO) networks is studied. Assuming underlay mode, the secondary user (SU) source with limited battery communicates with its destination via a hybrid SU relay. The SU source harvests energy from both the hybrid relay and the primary network. The hybrid SU relay node is equipped with two antennas; one for energy transmission to the secondary user and the other for data reception. The SU source transmits its data over an RF link to the relay. Then, the relay decodes the SU data before retransmitting it to the SU destination over an optical link. The RF/FSO channels are assumed to follow Nakagami-m/Malaga-M fading models with pointing errors on the FSO link. Closed-form expressions for the exact outage probability, average bit error rate, and ergodic capacity are derived. For high signal-to-noise ratio values, closed-form expressions for the asymptotic outage probability and average bit error rate are derived. Based on the asymptotic results, a power allocation model is proposed to enhance the system outage performance. Some simulation and numerical results are employed to validate the derived expressions.

Performance analysis of mixed RF/FSO system with spatial diversity

In this paper, we consider a dual-branch system where parallel radio-frequency (RF) link and mixed RF/free space optical (FSO) link are deployed between the source and destination. More specifically, multiple antennas are employed at both transmitter and receiver of mixed RF/FSO link to migrate channel fading. We assume RF links are subjected to Nakagami-m distribution, while FSO links follow exponentiated Weibull (EW) fading channel. Novel expressions for the end-to-end outage probability (OP), symbol error rate (SER) and ergodic capacity with different relay schemes are presented and analyzed. Moreover, the effect of pointing errors on the OP performance is further investigated. Numerical results demonstrate that due to the superiority of spatial diversity, the system performance can be enhanced significantly.

Efficient Recognition of Informative Measurement in the RF-Based Device-Free Localization.

Device-Free Localization (DFL) based on the Radio Frequency (RF) is an emerging wireless sensing technology to perceive the position information of the target. To realize the real-time DFL with lower power, Back-projection Radio Tomographic Imaging (BRTI) has been used as a lightweight method to achieve the goal. However, the multipath noise in the RF sensing network may interfere with the measurement and the BRTI reconstruction performance. To resist the multipath interference in the observed data, it is necessary to recognize the informative RF link measurements that are truly affected by the target appearance. However, the existing methods based on the RF link state analysis are limited by the complex distribution of the RF link state and the high time complexity. In this paper, to enhance the performance of RF link state analysis, the RF link state analysis is transformed into a decomposition problem of the RF link state matrix, and an efficient RF link recognition method based on the low-rank and sparse decomposition is proposed to sense the spatiotemporal variation of the RF link state and accurately figure out the target-affected RF links. From the experimental results, the RF links recognized by the proposed method effectively reflect the target-induced RSS measurement variation with less time. Besides, the proposed method by recognizing the informative measurement is helpful to improve the accuracy of BRTI and enhance the efficiency in actual DFL applications.

Assessment of fog and rain induced-attenuation on terrestrial FSO links

Launching into the next generation of wireless communication network (5G network) requires secure high data rate, high speed and huge bandwidth links. With the tremendous increase in broadband users, the existing communication systems such as radio frequency (RF) and microwave links cannot meet up with the challenges due to their link interference and low bandwidth. A current technology that promises such requirements and more is Free Space Optical (FSO) communication. The FSO basically involves the transmission of signal-modulated optical radiation from a transmitter to a receiver through the atmosphere or outer space. It is designed to complement the traditional fibre optical communication links. However, location-variant atmospheric channel degrades the quality and performance of an FSO system under severe atmospheric conditions. This paper attempts to assess both fog- and rain-induced attenuation on the performance of FSO link in a terrestrial terrain using measured visibility and rain rate data at Akure, Nigeria. 5-year (2012-2016) archived visibility data and measured rain rate data of 1-minute integration time obtained from Nigerian Meteorological Agency (NIMET) and the Department of Physics, Federal University of Technology, Akure respectively, were used to compute the fog- and rain-induced specific attenuations using Kruse and Carboneur models. The performance of the FSO system was analyzed through link margin by using the parameters of a commercial optical transceiver, Terescope 5000. Findings from this work will be useful for FSO system design in the area.

Assessment of FOG and Rain Induced-Attenuation on Terrestrial FSO Links

Notice of Retraction-----------------------------------------------------------------------After careful and considered review of the content of this paper by a duly constituted expert committee, this paper has been found to be in violation of APTIKOM's Publication Principles.We hereby retract the content of this paper. Reasonable effort should be made to remove all past references to this paper.The presenting author of this paper has the option to appeal this decision by contacting ij.aptikom@gmail.com.----------------------------------------------------------------------- Launching into the next generation of wireless communication network (5G network) requires secure high data rate, high speed and huge bandwidth links. With the tremendous increase in broadband users, the existing communication systems such as radio frequency (RF) and microwave links cannot meet up with the challenges due to their link interference and low bandwidth. A current technology that promises such requirements and more is Free Space Optical (FSO) communication. The FSO basically involves the transmission of signal-modulated optical radiation from a transmitter to a receiver through the atmosphere or outer space. It is designed to complement the traditional fibre optical communication links. However, location-variant atmospheric channel degrades the quality and performance of an FSO system under severe atmospheric conditions. This paper attempts to assess both fog- and rain-induced attenuation on the performance of FSO link in a terrestrial terrain using measured visibility and rain rate data at Akure, Nigeria. 5-year (2012-2016) archived visibility data and measured rain rate data of 1-minute integration time obtained from Nigerian Meteorological Agency (NIMET) and the Department of Physics, Federal University of Technology, Akure respectively, were used to compute the fog- and rain-induced specific attenuations using Kruse and Carboneur models. The performance of the FSO system was analyzed through link margin by using the parameters of a commercial optical transceiver, Terescope 5000. Findings from this work will be useful for FSO system design in the area.

A Flexible Telecommunication Architecture for Human Planetary Exploration Based on the BASALT Science-Driven Mars Analog.

There is a synergistic relationship between analog field testing and the deep space telecommunication capabilities necessary for future human exploration. The BASALT (Biologic Analog Science Associated with Lava Terrains) research project developed and implemented a telecommunications architecture that serves as a high-fidelity analog of future telecommunication capabilities for Mars. This paper presents the architecture and its constituent elements. The rationale for the various protocols and radio frequency (RF) link types required to enable an interdisciplinary field mission are discussed, and the performance results from the BASALT field tests are provided. Extravehicular Informatics Backpacks (EVIB) designed for BASALT and tested by human subjects are also discussed, and the proceeding sections show how these prototype extravehicular activity (EVA) information systems can augment future human exploration. The paper concludes with an aggregate analysis of the data product types and data volumes generated, transferred, and utilized by the ground team and explorers over the course of the field deployments.