The RF Link

Link analysis basically relates transmitted power and received power and shows in detail how the difference between these two is accounted for. To this end the fundamental elements of the communications satellite Radio Frequency (RF) or free space link are employed. Basic transmission parameters, such as antenna gain, beam width, free-space path loss, and the basic link power equation are exploited. The concept of system noise and how it is quantified on the RF link is then developed, and parameters such as noise power, noise temperature, noise figure, and figure of merit are defined. The carrier-to-noise ratio and related parameters used to define communications link design and performance are developed based on the basic link and system noise parameters. Hence, a new system named as Link Budget Calculator is successfully designed to formulate the mathematical of link budget. The system analyzesand calculates basic formulas which are related to link budget by using the MATLAB software.

On the maximal-ratio combining of FSO and RF links over generalized distributions and its applications in hybrid FSO/RF systems

Free-space optical (FSO) communication offers advantages such as high bandwidth and high transmission rates, making it suitable for 6G backhaul networks. However, FSO systems have drawbacks like sensitivity to fog, atmospheric turbulence, and pointing errors. Radio frequency (RF) links are incorporated to achieve more reliable and wider-range communication. RF links are susceptible to rain and have limited bandwidth but are not sensitive to fog or atmospheric turbulence. The complementary characteristics of RF and FSO systems make hybrid RF-FSO systems highly effective in mitigating environmental impairments. Reconfigurable intelligent surfaces (RISs) can also further extend the coverage and enhance the system bit error rate (BER) performance of RF-FSO. The state-of-the-art link selection schemes in RIS-assisted hybrid RF-FSO are normally based on channel state information (CSI). However, there is a chance that the CSI is inaccurate or even unavailable to the receiver. This paper investigates a hybrid RF-FSO communication system where CSI is unknown. In our system, the RF link is modeled using the Nakagami-m distribution, and the FSO link accounts for attenuation, Gamma-Gamma distributed turbulence, and pointing errors. The key concept of our system is the adoption of index modulation (IM) for link switching so that CSI is not needed at both transmitter and receiver, and we call it RIS-IM. Exact closed-form expressions for the signal-to-noise ratio (SNR) of the RIS-RF and RIS-FSO links are derived using multivariate Fox’s H-function without using central limit theorem, including the probability density function (PDF) and cumulative distribution function (CDF). We derive the closed-form expressions for the average BER (ABER) for RIS-IM using the SNR statistics. Furthermore, numerical analyses of RIS-IM are conducted. The numerical results of RIS-IM are given comparing with pure-IM, RIS-FSO, and hard switching as benchmarks. Mont-Carlo simulation results demonstrate the superiority of RIS-IM in terms of ABER. In the future, more analysis including outage probability (OP) and ergodic capacity (EC) await further explorations.

SummaryDue to the rapid development of satellite laser communication technology, free‐space optical (FSO) links present a promising alternative to traditional radio frequency (RF) links. In this paper, taking the influence of weather factors into consideration, we investigate the performance of the hybrid FSO/RF links where the feeder link operates in the FSO band and the user link operates in the hybrid FSO/RF band. Specifically, the FSO feeder link is modeled by the gamma–gamma distribution in the presence of beam wander and pointing error, and the detection method adopts either the intensity modulation with direct intensity (IM/DD) or heterodyne detection. The RF user link is assumed to follow the shadowed Rician model. In addition, in order to improve the transmission rate of the link under the time‐varying satellite–terrestrial channel, a rate adaptation scheme is proposed. The performance of the system under study is evaluated in terms of the outage probability, average bit error rate (BER), and average transmission rate. Our results provide some important insights, for example, (1) due to the constraints of the feeder link and weather factors, there is an upper limit on the outage performance and bit error rate of the hybrid link; (2) the adaptive transmission strategy can significantly improve the transmission rate of the link compared with traditional design.

Patients who are suffering from conductive hearing loss and single sided deafness cannot sometimes be rehabilitated by conventional air conduction hearing aids. Today, percutaneous Bone Anchored Hearing Aid (BAHA) is an important alternative for such patients. BAHA uses a titanium implant which penetrates the skin and can cause skin infection, skin redness and requires life-long commitment of care every day. A novel Bone Conduction Implant (BCI) is designed as an alternative to the percutaneous BAHA, because it leaves the skin intact. It comprises an external audio processor and an implanted unit called the BCI Bone Bridge. Sound is transmitted to the implant via an inductive radio frequency (RF) link through the intact skin using amplitude modulation. This paper presents an analog RF data and power link design as a first implementation for the BCI. The RF link is designed to operate in critical coupling to transmit maximum power to the implant. Maximum Power Output of the BCI was measured at 2 mm skin thickness and was found to be 105 dB relative to 1 µN at the transducer resonance frequency. This output force is fairly robust in 2 mm to 6 mm skin thickness range.

With the advent of the new decade, unmanned aerial vehicles (UAVs) will increasingly play an active role mainly as broadband transmission relays for broadcast applications. In the present study, we consider a feasible network where a transmitter, mounted on a hill, is connected through an optical wireless link with a UAV, hovering at a certain height above the earth. The UAV provides coverage over a circular geographic area and communicates through a radio frequency (RF) link with a ground user who is randomly located inside the circular region. In this context, a mixed optical/RF link is established where the performance is thoroughly investigated in terms of the outage probability. We assume that the optical wireless link is affected by a plethora of deteriorating factors, including the presence of fog, or the atmospheric scintillation, while the RF link is subject to small‐scale fading. Depending on the appearance of fog or turbulence, two different problems occur with a different mathematical formulation. For both cases, we extract closed‐form expressions for the signal‐to‐noise ratio statistics, which are the critical component towards determining the outage probability. Then, the impact of the impairments for some typical parameter values is properly depicted in appropriate graphs.

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.

Space–air–ground integrated network (SAGIN) is considered an enabler for sixth-generation (6G) networks. By integrating terrestrial and non-terrestrial (satellite, aerial) networks, SAGIN seems to be a quite promising solution to provide reliable connectivity everywhere and all the time. Its availability can be further enhanced if hybrid free space optical (FSO)/radio frequency (RF) links are adopted. In this paper, the performance of a hybrid FSO/RF communication system operating in SAGIN has been analytically evaluated. In the considered system, a high-altitude platform station (HAPS) is used to forward the satellite signal to the ground station. Moreover, the FSO channel model assumed takes into account the turbulence, pointing errors, and path losses, while for the RF links, a relatively new composite fading model has been considered. In this context, a new link selection scheme has been proposed that is designed to reduced the signaling overhead required for the switching operations between the RF and FSO links. The analytical framework that has been developed is based on the Markov chain theory. Capitalizing on this framework, the performance of the system has been investigated using the criteria of outage probability and the average number of link estimations. The numerical results presented reveal that the new selection scheme offers a good compromise between performance and complexity.

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.

A three-hop long range communication system with mixed fading for smart grid (SG) wide area network (WAN) is proposed. The three hops proposed here are power line communication (PLC), radio frequency (RF) and free space optical (FSO) in cascade. The smart meter (SM) transfers the data to the access point (AP) through a PLC link. The AP is acting as a decode-and-forward (DF) relay. It retransmits the data to data aggregator unit (DAU) through RF link. The DAU also acts as a DF relay and retransmits the information to meter data management system (MDMS) through FSO link. The PLC, RF, and FSO links are distributed with Log-Normal, Nakagami-m, and Gamma-Gamma distribution, respectively. The modulation scheme considered here is binary phase shift keying (BPSK). A closed form expression for average bit error probability (ABEP) is obtained. Numerical and Monte-Carlo simulation results demonstrate the effect of impulsive noise scenario in the PLC channel, fading severity in Nakagami-m channel, and different FSO parameters in SG communication system.

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.

Modern observation systems can be composed by heterogeneous entities (e.g., buoys, USVs, UAVs, on-shore sensors, etc.) that rely on dependable communications for coordination and data collection, often provided by over-water radio-frequency (RF) links. In tide-affected water bodies, RF links at a fixed height from the shore can experience the so-called tidal fading, a cyclic time-varying tide-induced interference. To mitigate it, the classical space-diversity reception technique (i.e., the use of two or more receiver antennas positioned at different heights) is often applied, commonly combined with the consideration of having one of the antennas at the largest possible height. Yet, this approach does not always ensure the best performance. In this work, we focus on static over-water links of short-to-medium-range distances that use antennas installed at a few meters above surface. We leverage the geometrical basis of the two-ray propagation model to investigate the optimal single-antenna height design that minimizes overall average path losses over a given tidal range. We then extend this analysis to incorporate a second receiver antenna and identify its optimal antenna height. Analytical results show that our method considerably outperforms the more classical approach, thus enabling superior (average) link capacities.

In this paper, the error performance analysis is performed for the decode-and-forward (DF) based dual hop mixed radio frequency-free space optical (RF-FSO) communication system with a direct RF link. The direct link between the source-destination (S-D) and the link between source-relay (S-R) are Nakagami-m distributed asymmetric radio frequency (RF) links and the R-D link is a free space optical (FSO) link characterized by the path loss, Gamma-Gamma distributed turbulence, and pointing errors. Selection combining diversity scheme is used to evaluate the performance of this dual branch cooperative system. For the considered system, novel closed-form mathematical expressions of cumulative distribution function and probability density function of instantaneous signal-to-noise ratio are derived. Using these, new mathematical expressions are obtained for the outage probability and the average bit error rate (BER) for various modulation techniques. The effects of fading, turbulence, and pointing errors are observed on the outage probability and the BER of the considered system. These performance metrics are also compared with those of a DF based dual hop mixed RF-FSO system without direct link. The improvement in the performance is observed due to the diversity introduced by addition of RF link.

The quest to meet diverse services and applications that are emanating from the increasing inter-connected devices including machine-to-human communication has brought about consideration of massive deployment of small cells for fifth generation (5G) and Beyond network. With densification of small cells, mm-wave and THz frequencies play significant role in delivering mobile services. Consequently, supporting radio frequency (RF) communication with visible light communications (VLC) link exhibits potentials to ameliorate the challenges of RF communication especially bandwidth limitations and interference. Capitalising on the strength of VLC, we propose a basic end-to-end system to experimentally evaluate the suitability of VLC as fronthaul using real-time mobile traffic. Successful downlink (DL) transmission was achieved via VLC. To investigate the performance of the VLC link, transmission signal-to-noise ratio (SNR) and throughput of the VLC and RF Links are measured and compared. The light emitting diodes (LEDs) and Photo-diodes (PD) have shown to have significant effects on the performance of the VLC link. However, the results establish a fundamental premise for the suitability of VLC as fronthaul for small cell network.

The generalized order user scheduling is among the efficient user selection schemes which can be used in conditions where the opportunistic scheduling could fail. The opportunistic scheduler could erroneously select a user other than the best user for data transmission or reception due to imperfect channel estimation or outdated channel information issues. In this paper, we study the performance of multiuser mixed radio frequency (RF)/free-space optical (FSO) relay network with generalized order user scheduling. Due to its importance on the system performance, the outdated channel information problem is also addressed in this paper. The considered system includes K sources or users, one decode-and-forward relay and one destination. The users are connected with the relay node through RF links, and the relay is connected with the destination through an FSO link. In the analysis, the first-hop channels are assumed to follow Rayleigh fading model and the second-hop channel is assumed to follow Gamma–Gamma fading model including the effect of pointing errors. Closed-form expressions are derived for the outage probability, average symbol error probability and ergodic channel capacity. Moreover, in order to get more insights about the system behavior, the system is studied at the high signal-to-noise ratio regime where the diversity order and coding gain are provided and studied. Monte Carlo simulations are given to validate the achieved exact and asymptotic results. Main findings illustrate that under weak atmospheric turbulence conditions, the system performance is dominated by the RF links and a diversity order of K − N + 1 is achieved by the system, whereas under sever atmospheric turbulence conditions, the system is dominated by the FSO link and the diversity order is related to the minimum value of the turbulence fading and pointing error parameters.