Line Of Sight Connection Research Articles

HAPS-Assisted Hybrid RF-FSO Multicast Communications: Error and Outage Analysis

In this article, we study the performance of multiple-hop mixed radio frequency (RF)/free-space optical (FSO) communication-based decode-and-forward protocol for multicast networks. So far, serving a large number of users is considered a promising approach for real-time applications to address the massive data traffic demands. In this regard, we propose two practical use cases. In the former model, we propose a high altitude platform station (HAPS)-aided mixed RF/FSO/RF communication scheme where a terrestrial ground station intends to communicate with a cluster of nodes through two stratospheric HAPS systems. In the latter model, we assume that the line of sight connectivity is inaccessible between the two HAPS systems due to high attenuation caused by large propagation distances. Thereby, we propose a low Earth orbit satellite-aided mixed RF/FSO/FSO/RF communication. For the proposed scenarios, closed-form expressions of outage probability (OP) and bit error rate are derived. In addition, to illustrate the asymptotic behavior of the proposed models, diversity gains are obtained. Furthermore, ergodic capacity and energy efficiency (EE) are provided for both scenarios. Finally, the simulation results are provided to validate the theoretical derivations. The results show that a satellite-aided mixed RF/FSO/FSO/RF scenario achieves better OP, whereas an HAPS-aided mixed RF/FSO/RF scenario can achieve a higher EE.

Performance analysis of multiple-beam WDM free space laser-communication system using homodyne detection approach

A free space optical module is used in laser communication to transport a signal from the transmitter to the receiver. Free Space Optical Communication (FSOC) is a Line of Sight connectivity that sends a highly narrow beamwidth. FSOC provides high bandwidth and data rates greater than 10 Gbps. Although FSOC technology has several advantages, it is inefficient for long-distance transmission because of many constraints caused by atmospheric variables. In FSOC connections, turbulence-induced scintillation is a severe problem that significantly reduces link performance. Keeping this problem in mind, the objective of this study is to enhance FSOC performance in terms of energy efficiency, spectral efficiency and long-distance transmission. To achieve this, a study is employed using a hybrid combination of Higher-order Gaussian filter (HGF), post-amplification and a homodyne detection method. Precisely, the simulative study of 32-channel wavelength division multiplexing (WDM) FSOC has used channel model Gamma-Gamma with single-beam (SB), dual-beam (DB), four multiple-beam (MB4) and eight multiple-beam (MB8) techniques. The proposed framework has achieved a Channel capacity of more than 320 Gbps. The transmission range enhancement of 112% and reduction in transmitted power of 100% are achieved, which are considerably more significant compared with state-of-the-art literature studies. The OptiSystem platform is used to gather the outcomes. The performance is based on parametric analysis of bit error rate (BER), Quality (Q) factor and eye height.

Hybrid RF/VLC Systems: A Comprehensive Survey on Network Topologies, Performance Analyses, Applications, and Future Directions

Wireless communications refer to data transmissions in unguided propagation media through the use of wireless carriers such as radio frequency (RF) and visible light (VL) waves. The rising demand for high data rates, especially, in indoor scenarios, overloads conventional RF technologies. Therefore, technologies such as millimeter waves (mmWave) and cognitive radios have been adopted as possible solutions to overcome the spectrum scarcity and capacity limitations of the conventional RF systems. In parallel, visible light communication (VLC) has been proposed as an alternative solution, where a light source is used for both illumination and data transmission. In comparison to RF links, VLC links present a very high bandwidth that allows much higher data rates. VLC exhibits also immunity to interference from electromagnetic sources, has unlicensed channels, is a very low power consumption system, and has no health hazard. VLC is appealing for a wide range of applications including reliable communications with low latency such as vehicle safety communication. Despite the major advantages of VLC technology and a variety of its applications, its use has been hampered by its cons such as its dependence on a line of sight connectivity. Recently, hybrid RF/VLC systems were proposed to take advantage of the high capacity of VLC links and better connectivity of RF links. Thus, hybrid RF/VLC systems are envisioned as a key enabler to improve the user rates and mobility on one hand and to optimize the capacity, interference and power consumption of the overall network on the other hand. This paper seeks to provide a detailed survey of hybrid RF/VLC systems. This paper represents an overview of the current developments in the hybrid RF/VLC systems, their benefits and limitations for both newcomers and expert researchers.

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.

Cluster-Based Resource Allocation and User Association in mmWave Femtocell Networks

In millimeter wave (mmW) dense femto-networks, major challenges are overcoming the performance loss imposed by the channel and managing the co-channel interference. The former is due to the mmW susceptibility to pathloss and shadowing and the latter is due to density of the network. We cope with both challenges by a clustering method designed for mmW environment. In our approach, the femto access points (FAP) and femto users (FU) are clustered based on having the most line of sight connectivity. We modify this binary optimization problem into a continuous problem using deductive penalty functions and solve it by difference of two convex functions (D.C.) programming. Our clustering algorithm achieves higher data rate compared to the foremost clustering method. We also propose a technique to assign FUs to FAPs in each cluster which has near-optimal performance and polynomial time complexity. We solve mixed integer nonlinear programming of power and sub-channel allocation by D.C. programming. Instead of using the deductive penalty terms in D.C. programming, we penalize the objective function in a multiplicative manner. Thus, the penalty term depends on both constraint violation and objective function. Our scheme achieves around 10% higher data rate compared to the method using deductive penalty terms.

An Internet of Things (IoT) Application on Volcano Monitoring.

In the last few years, there has been a huge interest in the Internet of Things (hereinafter IoT) field. Among the large number of IoT technologies, the low-power wide-area network (hereinafter LPWAN) has emerged providing low power, low data-rate communication over long distances, enabling battery-operated devices to operate for long time periods. This paper introduces an application of long-range (hereinafter LoRa) technology, one of the most popular LPWANs, to volcanic surveillance. The first low-power and low-cost wireless network based on LoRa to monitor the soil temperature in thermal anomaly zones in volcanic areas has been developed. A total of eight thermometers (end devices) have been deployed on a Teide volcano in Tenerife (Canary Islands). In addition, a repeater device was developed to extend the network range when the gateway did not have a line of sight connection with the thermometers. Combining LoRa communication capabilities with microchip microcontrollers (end devices and repeater) and a Raspberry Pi board (gateway), three main milestones have been achieved: (i) extreme low-power consumption, (ii) real-time and proper temperature acquisition, and (iii) a reliable network operation. The first results are shown. These results provide enough quality for a proper volcanic surveillance.

Flying Drone Base Stations for Macro Hotspots

We study a scenario where multiple drone-mounted base stations cruise freely over a macro hotspot to serve mobile users on the ground. The drone base stations move constantly and update their moving directions following our proposed mobility control algorithm. The constant movement of drones reduces the distance between the base stations and users, which in turn improves the probability of having a line of sight connection. We consider a practical user association scheme for the moving base stations, which enables user equipments to switch their serving base stations based only on the received signal strength. via extensive simulations, we demonstrate that the drone base stations moving according to our proposed algorithms can improve the average packet throughput by 82% and the 5th-percentile packet throughput by 430% compared to a baseline scenario, where drones hover over fixed locations. These improvements can be realized regardless of users’ and base stations’ density. The constant movement of the drones also helps reduce the total number of drones required to cover the macro hotspot.

COORDINATED MOTION CONTROL OF SWARMS WITH DYNAMIC CONNECTIVITY IN POTENTIAL FLOWS

This paper presents a general framework for the coordinated motion control of autonomous swarms in the presence of obstacles. The proposed framework judiciously combines concepts and techniques from potential flows, artificial potentials and dynamic connectivity to realize complex swarm behaviors. To begin with, existing concepts from potential flows in fluid mechanics are used to solve the single-agent navigation problem. As an extension, an analytical solution to the stagnation point problem is provided. The potential flow based framework is then modified significantly to facilitate the coordinated control of swarms navigating through multiple obstacles. Artificial potentials are employed for swarming as well as enhanced obstacle avoidance. A novel concept of dynamic connectivity is utilized to improve the performance of obstacle avoidance (Line of Sight Connectivity) and to organize diverse swarm behaviors (Probabilistic Connectivity). Simulation results with a set of developed algorithms are included to illustrate the viability of the proposed framework.