Dual-hop System Research Articles

Enhancing Underwater Wireless Sensor Networks With Flexible Communication and Positioning

An innovative approach centred around underwater wireless communication and positioning in Underwater Wireless Sensor Networks, addressing limited coverage by proposing a dual-hop system combining optical fiber and wireless links. Time frame design and Code Division Multiplexing Access enhance multi-user signal transmission efficiency. Proof-of-concept experiments validate feasibility, with the introduction of the hybrid fish eye routing protocol further enhancing performance by intelligently combining fish-eye routing and traditional protocols for improved coverage and reliability. Moreover, the hybrid fish eye protocol incorporates adaptive routing algorithms that dynamically adjust to changes in underwater conditions, ensuring robustness and adaptability in challenging environments. Additionally, the hybrid fish eye protocol optimizes energy consumption by minimizing unnecessary transmissions, thereby prolonging the lifespan of underwater sensor networks. With full-duplex communication, precise positioning, and an extended transmission range, convergent system demonstrates significant potential for supporting high-capacity transmission among mobile nodes in forthcoming underwater scenarios

Optical Communication and Positioning Convergence for Flexible Underwater Wireless Sensor Network

This paper presents a novel scheme based on underwater optical wireless communication and positioning (UOWC-UOWP) for an underwater wireless sensor network (UWSN). A dual-hop system is adopted to extend the limited coverage area of the UWSN with mobile nodes, consisting of a grade-index plastic optical fiber (GI-POF) link and a wireless link. Specifically, the mini-LED offers high-speed and flexible uplink transmission and the high-power LED enables both downlink transmission and positioning. The time frame design and code division multiplexing access (CDMA) support multi-user signal transmission. Proof-of-concept experiments with GI-POF and wireless link demonstrate the feasibility of the proposed scheme, which offers full-duplex communication, high-precise positioning and extended distance. To the best of our knowledge, the UOWC-UOWP convergent system is proposed for the first time. It has great potential to support high-capacity transmission between mobile nodes for future underwater transmission scenarios.

Outage Probability and Average BER of UAV-Assisted Dual-Hop FSO Communication With Amplify-and-Forward Relaying

Unmanned aerial vehicles (UAVs) can be harnessed as relay nodes in free-space optical (FSO) communication systems to realize a flexible and cost-effective approach for delivering on-demand communication in next-generation wireless communications. However, fading channels can adversely affect the performance of UAV-assisted dual-hop FSO systems owing to various factors. This paper presents a unified system performance analysis of UAV-assisted dual-hop FSO/FSO systems with the amplify-and-forward relaying protocol and intensity modulation/direct detection technique. For general applicability, the influence of the attenuation loss, atmospheric turbulence, pointing error impairments, and angle-of-arrival (AOA) fluctuations on the FSO link between the source and the UAV is considered. Meanwhile, the FSO link between the UAV and the destination is modeled using Málaga distribution channels, which are affected by atmospheric turbulence. The tight closed-form expressions for the outage probability and the average bit error rate of the dual-hop system are derived, followed with the diversity order of the system in the high signal-to-noise ratio range. Furthermore, the impact of various system parameters and channel parameters correlated with the attenuation loss, atmospheric turbulence, pointing error impairments, and AOA fluctuations on the system performance is studied, and a detailed comparative investigation of the various modulation schemes is conducted. Finally, simulation results are presented to validate the analytical results.

Performance of Hybrid THz and Multiantenna RF System With Diversity Combining

Recent studies investigate single-antenna radio frequency (RF) systems mixed with terahertz (THz) wireless communications. This article considers a two-tier system of THz for backhaul and multiple-antenna-assisted RF for the access network. We analyze the system performance by employing both the selection combining and maximal ratio combining receivers for the RF link integrated with the THz link using the fixed-gain amplify-and-forward protocol. We develop the probability density function and the cumulative distribution function of the end-to-end signal-to-noise ratio (SNR) of the dual-hop system over independent and nonidentically distributed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> fading channels with a statistical model for misalignment errors in the THz wireless link. We use the derived statistical results to develop analytical expressions of the outage probability, average bit error rate, and ergodic capacity for the performance assessment of the considered system. We develop a diversity order of the system using asymptotic analysis in the high-SNR region, demonstrating the scaling of system performance with the number of antennas. We use computer simulations to show the effect of system and channel parameters on the performance of the hybrid THz-RF link with multiantenna diversity schemes.

Comprehensive performance analysis of hybrid FSO/RF space–air–ground integrated network

Free space optics (FSO) communication due to its large optical carrier bandwidth offers better data rates close to gigabits per second (GBPS) compared to radio frequency (RF) communication and can support seamless ubiquitous high-speed broadband connectivity, which is a significant requirement for sixth-generation (6G) communication. It is also envisioned that the data transmission in 6G communication will be over space–air–ground integrated network (SAGIN) to provide broadband global mobile connectivity. Thus, our main aim in this paper is to investigate the performance of a dual-hop (DH) system model utilizing high-altitude platform station (HAPS) as a relay station with FSO communication for SAGIN scenario, which is expected to be featured in 6G communication. Despite various advantages, HAPS-based FSO satellite communication (SATCOM) is undermined by several factors such as atmospheric turbulence, pointing errors, and atmospheric attenuation. To improve the system performance in terms of capacity and reliability, hybrid FSO/RF communication with RF link as a backup is utilized between ground station (GS) and HAPS. Further, comprehensive end-to-end system performance is investigated by deriving unified closed-form expressions for outage probability and average symbol error rate (SER). We also obtain approximate upper bound expressions for ergodic capacity and outage capacity. The asymptotic expressions are also derived to obtain the diversity order. Here, FSO and RF links are modelled using generalized Malaga distribution with non-zero boresight pointing errors and shadowed κ−μ distribution, respectively. Further, the closed-form expressions for optimum switching threshold signal-to-noise ratio (SNR) are derived and the obtained optimal SNR values are verified using a numerical optimization technique. Finally, the results show that the proposed DH SAGIN-based hybrid FSO/RF SATCOM system is more reliable as well as achieves higher capacity compared to the FSO-based SATCOM systems.

Packet Routing in Dynamic Multi-Hop UAV Relay Network: A Multi-Agent Learning Approach

The multi-hop unmanned aerial vehicle (UAV) network can serve as data relays where ground users (GUs) do not have reliable direct connections to the base station (BS). Existing works mainly focus on simple dual-hop system. In this paper, we investigate the packet routing problem in a multi-hop UAV relay network to minimize the data transmission time and enhance the network throughput. However, the dynamic network topology due to UAV mobility makes the packet routing challenging since the limited communication range of each UAV leads to volatile wireless connection. Moreover, the line-of-sight communication links may cause strong interference among UAVs. Towards this end, we propose a novel multi-agent deep reinforcement learning based algorithm, named as multi-agent QMIX (MAQMIX) to: 1) design proper UAVs’ trajectories to serve the moving GUs while maintaining the network connection; 2) allocate frequency resource properly among UAVs to alleviate the impact of interference; and 3) choose a proper next hop UAV for each data packet to reduce the transmission time and probability of network congestion. The proposed MAQMIX has two novel training mechanisms, i.e., intra-UAV and inter-UAV training mechanisms, which can tackle the large action space issue and coordinate the training among UAVs in the multi-hop UAV relay network. Simulation results demonstrate that the MAQMIX outperforms baseline schemes in terms of the network congestion avoidance, throughput, and transmission time.

Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes

A significant challenge of visible-light communication systems (VLC) is to overcome their limited converge area in non-line-of-sight (NLOS) transmission. To tackle this problem, for the first time, a real-time high-speed dual-hop VLC system based on blue micro-light-emitting diodes (micro-LED) is proposed and experimentally demonstrated. Benefiting from the advantage of high electrical-to-optical (E-O) bandwidth of the micro-LED, the frequency-response measurements show that the 3-dB modulation bandwidth of 2 m free-space single-hop link is 880 MHz, and the dual-hop system can reach to 715 MHz over a 4 m communication distance. We then investigated the communication performance of our proposed single-hop and dual-hop systems. The real-time waveforms are analyzed at different positions of the dual-hop link and eye diagrams at the receiving terminal are captured for evaluation. Furthermore, the bit error rate (BER) at the target node is measured. The results demonstrate that a 1.1 Gbps on-off keying (OOK) signal with a BER less than the forward-error-correction (FEC) limit could be achieved over a 4 m NLOS free-space link. This work shows that the proposed dual-hop system based on a micro-LED can meet the requirements for most indoor NLOS-transmission scenarios.

Cooperative Terrestrial-Underwater Wireless Optical Links by Using an Amplify-and-Forward Strategy.

In this paper, we analyze a combined terrestrial-underwater optical communication link for providing high-speed optical connectivity between onshore and submerge systems. For this purpose, different transmission signaling schemes were employed to obtain performance results in terms of average bit error rate (ABER). In this sense, from the starting point of a known conditional bit-error-rate (CBER) in the absence of turbulence, the behavior of the entire system is obtained by applying an amplify-and-forward (AF) based dual-hop system: The first link is a terrestrial free-space optical (FSO) system assuming a Málaga distributed turbulence and, the second one, is an underwater FSO system with a Weibull channel model. To obtain performance results, a semi-analytical simulation procedure is applied, using a hyper-exponential fitting technique previously proposed by the authors and leading to BER closed-form expressions and high-accuracy numerical results.

Performance Analysis of RIS-Assisted Mixed Dual-Hop FSO-RF Communication Systems over Exponentiated Weibull Channels

Information security is the basis of economic development, information construction, and the protection of people’s and national interest. In this paper, the performance of reconfigurable intelligent surface (RIS)-assisted mixed dual-hop free-space optical-radio frequency (FSO-RF) communication systems is studied and analyzed to ensure the safe transmission of information. In this scheme, it is assumed that the atmospheric turbulence in the fading of the FSO channel follows the exponentiated Weibull distribution model and the fading of the RF channel follows the Rayleigh distribution model. Based on the mathematical function, the probability density function and cumulative distribution function of RIS-assisted mixed dual-hop FSO-RF communication systems are derived. Then, the expression of outage probability and the bit error rate of the system are derived from the channel statistics. Finally, the analysis results are verified by Monte Carlo simulation, and the effects of different parameters on the system performance are analyzed. The simulation results show that the atmospheric turbulence parameters, channel distance of FSO, intensity of atmospheric turbulence, and number of reflectors all affect the system performance.

HAPS-Based Relaying for Integrated Space–Air–Ground Networks With Hybrid FSO/RF Communication: A Performance Analysis

Optical wireless communication (OWC) has garnered significant importance to provide gigabit capacity links owing to its unique features. Free-space optics (FSO) communication, which is the term used for OWC in an outdoor scenario, is a good candidate to establish ground-to-satellite/satellite-to-ground links. Despite of many advantages, FSO communication in uplink is limited by the adverse effects of beam-wander, beam scintillation, and pointing errors. Therefore, to counteract the limitations, it is wise to backup FSO links with reliable radio frequency (RF) links. In addition, the complementary characteristics exhibited by FSO and RF links to atmospheric effects have paved the way for hybrid FSO/RF communication. In recent years, a growing interest is also witnessed in the research and development of space-air-ground integrated network (SAGIN). This article aims to fuse FSO and RF technologies for integrated space-air-ground uplink networks utilizing unmanned aerial vehicle (UAV), such as high-altitude platform station (HAPS) as a relay station for achieving better reliability. Thus, we propose single-hop and SAGIN-based dual-hop system models for uplink satellite communication with hybrid FSO/RF links and the performance is investigated using analytical and simulation results. In summary, the results show that hybrid FSO/RF systems perform better than the FSO systems in an uplink scenario due to the backup RF link.

Performance of Hybrid RF/FSO Cooperative Systems Based on Quasicyclic LDPC Codes and Space-Coupled LDPC Codes

Free space optical (FSO) communication systems provide wireless line of sight connectivity in the unlicensed spectrum, and wireless optical communication achieves higher data rates compared to their radio frequency (RF) counterparts. FSO systems are particularly attractive for last mile access problem by bridging fiber optic backbone connectivity to RF access networks. To cope with this practical deployment scenario, there has been increasing attention to the so-called dual-hop (RF/FSO) systems where RF transmission is used at a hop followed by FSO transmission to another. In this article, we study the performance of cooperative transmission systems using a mixed RF-FSO DF (decode and forward) relay using error-correcting codes including QC-LDPC codes at the relay level. The FSO link is modeled by the gamma-gamma distribution, and the RF link is modeled by the Additive White Gaussian Noise (AWGN) model. Another innovation in this article is the use of cooperative systems using a mixed FSO/RF DF relay using quasicyclic low-density parity check (QC-LDPC) codes at the relay level. We also use the space-coupled low-density parity check (SC-LDPC) codes on the same scheme to show its importance in cooperative optical transmission but also in hybrid RF/FSO transmission. The latter will be compared with QC-LDPC codes. The use of mixed RF/FSO cooperative transmission systems can improve the reliability and transmission of information in networks. The results demonstrate an improvement in the performance of the cooperative RF/FSO DF system based on QC-LDPC and SC-LDPC codes compared to RF/FSO systems without the use of codes, but also to the DF systems proposed in the existing literature.

Outage probability of dual-hop cooperative communication networks over the Nakagami-m fading channel with RF energy harvesting

Cooperative communication systems with wireless power transfer are being investigated for future advanced wireless networks. In this research, we propose applying a wireless power transfer to dual-hop cooperative communication systems, in which a relay node harvests the energy from the radiofrequency in order to forward the received signal, and a source node can communicate with a destination node directly or through the selected relay nodes. The system performance is evaluated by an outage probability that is calculated over independent and identically distributed (i.i.d) Nakagami-m distributions in two scenarios, i.e., integer m and arbitrary m. Furthermore, the closed forms of the outage probability expressions are derived in the case of both the amplify-and-forward (AF) and decode-and-forward (DF) protocols. The Monte Carlo method is utilized to simulate the system with the aims of evaluating the system performance and verifying the theoretical analysis. The numerical results highlight that the proposed calculation method and the closed form of the outage probability are accurate. We also compare the system performance of both the AF and DF protocols and show the effect of parameter m on the performance of the system.

Performance analysis for mixed [formula omitted] shadowed and exponentiated Weibull distributed dual-hop system with multiuser diversity in C-RAN

This paper investigates the performance of mixed radio-frequency/free-space optical (RF/FSO) system with multiuser diversity over mixed fading channels suitable for the Cloud Radio Access Network (C-RAN). Specifically, the considered system consists of K users, a decode-and-forward (DF) relay and a destination. The relay receives the data from the user with best channel conditions and forwards it to the destination. Furthermore, supposing that the RF link experiences κ−μ shadowed fading which combines small-scale multipath fading with line-of-sight shadowing, whereas the FSO link is subject to Exponentiated Weibull (EW) distributed turbulence. We first derived a novel probability density function (PDF) expression for the κ−μ shadowed fading so that the κ, m and can be equal to the arbitrary positive real value. Then, analytical expressions for the end-to-end outage probability (OP) and average bit error rate (ABER) of the overall network are derived assuming an intensity modulation with direct detection (IM/DD) scheme. The closed-form asymptotic expressions at high signal-to-noise (SNR) regimes are also presented. Capitalizing on the asymptotic analysis, we can obtain the diversity gain and discuss the bottlenecks in the mixed RF/FSO systems. Additionally, the impact of several parameters, including the number of users, atmospheric turbulence conditions and pointing errors on the overall system are studied. The simulation results are provided to prove the feasibility of deployment of the system in real-life scenarios.

Closed Form Solution for Outage and Average Bit Error Rate of Asymmetric Relaying System

In this paper, closed form solution of outage and bit error rate (BER) is evaluated for the purpose of performance analysis of the amplify and forward relaying scheme under the asymmetric fading environment. In this dual hop system, we have used Rayleigh fading along source S to relay R and Mixture Gamma fading along relay R to destination D. First, we have derived closed form solution of outage and then we have used this derived expression for getting closed form solution of bit error rate (BER) for different kinds of modulation. Since Mixture Gamma fading channel represents many fading channels as its special case, so proposed closed form solution of outage and bit error rate may be used for analysis of outage and bit error rate under various fading scenarios. Specifically, in this paper, for analysis of outage and bit error rate (BER), we have taken Nakagami-m fading as a particular case of Mixture Gamma fading and analyzed the performance of proposed system after observing effect of fading severity factor and signal to noise ratio (SNR) on outage probability and bit error rate (BER).

On the performance of transmit antenna selection in multiuser asymmetric RF/FSO system under generalized order user scheduling

In this paper, the performance analysis of transmit antenna selection on a multiuser asymmetric RF/FSO system under generalized order user scheduling is investigated. A dual-hop system with one source, one relay and multiple FSO users at destination is considered without direct link between the source and destination. The RF link is subjected to Nakagami-m fading and FSO links follow Exponential Weibull (EW) distribution. Using the amplify-and-forward (AF) channel-state-information (CSI) based gain relay protocol, the equivalent cumulative distribution function (CDF) end-to-end SNR at the destination is derived and used to obtain the analytical expression for the system outage probability. By utilizing the derived statistics, the closed-form expression for the system average bit error rate (ABER) is then derived through the Gauss–Laguerre quadrature rule and second kind Stirling number. Moreover, the results demonstrate that the atmospheric turbulence significantly degrades the system performance and increase in the number of source transmit antenna offers the system better performance. Also, the system performance is deteriorated by increasing the orders N or decreasing the number of user K. The accuracy of the derived expressions is validated through Montel-Carlo Simulations.

Enabling Non-Linear Energy Harvesting in Power Domain Based Multiple Access in Relaying Networks: Outage and Ergodic Capacity Performance Analysis

The Power Domain-based Multiple Access (PDMA) scheme is considered as one kind of Non-Orthogonal Multiple Access (NOMA) in green communications and can support energy-limited devices by employing wireless power transfer. Such a technique is known as a lifetime-expanding solution for operations in future access policy, especially in the deployment of power-constrained relays for a three-node dual-hop system. In particular, PDMA and energy harvesting are considered as two communication concepts, which are jointly investigated in this paper. However, the dual-hop relaying network system is a popular model assuming an ideal linear energy harvesting circuit, as in recent works, while the practical system situation motivates us to concentrate on another protocol, namely non-linear energy harvesting. As important results, a closed-form formula of outage probability and ergodic capacity is studied under a practical non-linear energy harvesting model. To explore the optimal system performance in terms of outage probability and ergodic capacity, several main parameters including the energy harvesting coefficients, position allocation of each node, power allocation factors, and transmit signal-to-noise ratio (SNR) are jointly considered. To provide insights into the performance, the approximate expressions for the ergodic capacity are given. By matching analytical and Monte Carlo simulations, the correctness of this framework can be examined. With the observation of the simulation results, the figures also show that the performance of energy harvesting-aware PDMA systems under the proposed model can satisfy the requirements in real PDMA applications.

Unified Ergodic Capacity Expressions for AF Dual-Hop Systems With Hardware Impairments

This letter presents novel analytical expressions for the ergodic capacity (EC) of dual-hop amplify-and-forward (AF) relaying systems subject to hardware impairments operating over generalized fading channels. Our expressions are quite generic, providing that the moment-generating function (MGF) of the signal-to-noise ratio (SNR) or the MGF of the inverse SNR of each hop is readily available. The proposed analytical framework is further applied to assess the EC performance of two system setups: 1) a variable-gain relaying system equipped with multiple antennas at the relay and at the destination, assuming an interference-limited relay and a noise-limited destination and 2) a multi-source multi-destination AF system equipped with variable- or fixed-gain relays. The correctness of the proposed mathematical analysis is verified by Monte Carlo simulations results.

Partial relay selection in mixed RF/FSO dual-hop system over unified M-distributed fading channel with non-zero boresight pointing errors

In this paper, the performance of a partial relay selection-based amplify-and-forward mixed radio frequency/free space optical (RF/FSO) system with a direct RF link and outdated channel state information (CSI) is presented. The source-to-relay and source-to-destination RF links are subjected to Rayleigh distributions while the relay-to-destination FSO link experiences Malaga (M)-distribution with generalized non-zero boresight pointing errors. It is also assumed that selection combining diversity with heterodyne detection is considered for the signal reception at the destination. Moreover, the cumulative distribution functions of the end-to-end signal-to-noise under the influence of with/or without a direct link are then derived for the concerned system. Through this, the closed-form expressions for the system outage probability and average bit error rate are obtained. Thus, the influence of atmospheric turbulence, outdated CSI and relay selection are evaluated for the system under zero boresight and non-zero boresight displacement pointing errors scenarios. The results illustrated that the mixed RF/FSO system with a direct link offers better performance than the one without a direct link. The accuracy of the presented results is validated by Monte-Carlo simulation.

New Analytical Approach in the SER Evaluation of CSIN-Assisted AF Dual-Hop Wireless Systems

Among the different relaying modes employed in amplify-and-forward (AF) dual-hop wireless cooperative systems, the channel-state-information (CSI)-and-noise (CSIN)-assisted mode generates the optimal amplification-gain that limits the relay's retransmitted power. Despite that, and up to this moment, its symbol-error-rate (SER) performance has been evaluated in literature only through approximate analyses. In this letter, we introduce a new analytical approach to analyze its SER exactly. More specifically, we consider a CSIN-assisted AF dual-hop system operating in Rayleigh fading environment with high nodes' mobility and incorrect CSI estimates, and derive novel, generic, and tractable exact expression for its per-frame-average SER performance. The derived expression is also valid for non-moving nodes and perfect CSI estimation scenarios. We also derive the irreducible error floors that appear due to nodes' motion and incorrect CSI estimation.

OFDM-IM Based Dual-Hop System Using Fixed-Gain Amplify-and-Forward Relay With Pre-Processing Capability

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has recently attracted many researchers’ attention due to its superior spectrum efficiency and reliability compared to the traditional OFDM. Cooperative decode-and-forward relaying has been incorporated with OFDM-IM, which provides a higher energy efficiency and better network coverage. However, it might not be feasible in realistic applications, owing to the high system complexity and transmission delay rendered by complex decoding and channel estimation procedures. Therefore, in this paper, we propose a fixed-gain (FG) amplify-and-forward (AF) relay-assisted OFDM-IM system, which does not need to perform complex decoding and channel estimation at the relay, but only requires a pre-processing capability at the relay, e.g., cyclic prefix removal and re-insertion. Therefore, the system complexity can be reduced and the forwarding delay as well as power consumption caused by processing at the relay also decline. We analyze the average outage probability, block error rate, and achievable rate of the proposed system and verify all analysis by numerical results. The proposed FG AF relay-assisted OFDM-IM provides a simple solution to the implementation of OFDM-IM in new network paradigms, where nodes are simple, power limited, and/or complexity limited.