Lights and Shadows: A Comprehensive Survey on Cooperative and Precoding Schemes to Overcome LOS Blockage and Interference in Indoor VLC

Visible light communications (VLC) have received significant attention as a way of moving part of the saturated indoor wireless traffic to the wide and unregulated visible optical spectrum. Nowadays, VLC are considered as a suitable technology, for several applications such as high-rate data transmission, supporting internet of things communications or positioning. The signal processing originally derived from radio-frequency (RF) systems such as cooperative or precoding schemes can be applied to VLC. However, its implementation is not straightforward. Furthermore, unlike RF transmission, VLC present a predominant line-of-sight link, although a weak non-LoS component may appear due to the reflection of the light on walls, floor, ceiling and nearby objects. Blocking effects may compromise the performance of the aforementioned transmission schemes. There exist several surveys in the literature focused on VLC and its applications, but the management of the shadowing and interference in VLC requires a comprehensive study. To fill this gap, this work introduces the implementation of cooperative and precoding schemes to VLC, while remarking their benefits and drawbacks for overcoming the shadowing effects. After that, the combination of both cooperative and precoding schemes is analyzed as a way of providing resilient VLC networks. Finally, we propose several open issues that the cooperative and precoding schemes must face in order to provide satisfactory VLC performance in indoor scenarios.This work has been supported partially by Spanish National Project TERESA-ADA(TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE), the research project GEOVEOLUZ-CM-UC3Mfunded by the call “Programa de apoyo a la realización de proyectos interdisciplinares de I+D parajóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020” under the frame ofthe Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid and projectMadrid Flight on Chip (Innovation Cooperative Projects Comunidad of Madrid - HUBS 2018/MadridFlightOnChip). Additionally, it has been supported partially by the Juan de la CiervaIncorporación grant IJC2019-040317-I and Juan de la Cierva Formación grant (FJC2019-039541-I/AEI/10.13039/501100011033)

Interference mitigation in LiFi networks

Due to the increasing demand for wireless data, the radio frequency (RF) spectrum has become a very limited resource. Alternative approaches are under investigation to support the future growth in data traffic and next-generation high-speed wireless communication systems. Techniques such as massive multiple-input multiple-output (MIMO), millimeter wave (mmWave) communications and light-fidelity (LiFi) are being explored. Among these technologies, LiFi is a novel bi-directional, high-speed and fully networked wireless communication technology. However, inter-cell interference (ICI) can significantly restrict the system performance of LiFi attocell networks. This thesis focuses on interference mitigation in LiFi attocell networks. The angle diversity receiver (ADR) is one solution to address the issue of ICI as well as frequency reuse in LiFi attocell networks. With the property of high concentration gain and narrow field of view (FOV), the ADR is very beneficial for interference mitigation. However, the optimum structure of the ADR has not been investigated. This motivates us to propose the optimum structures for the ADRs in order to fully exploit the performance gain. The impact of random device orientation and diffuse link signal propagation are taken into consideration. The performance comparison between the select best combining (SBC) and maximum ratio combining (MRC) is carried out under different noise levels. In addition, the double source (DS) system, where each LiFi access point (AP) consists of two sources transmitting the same information signals but with opposite polarity, is proven to outperform the single source (SS) system under certain conditions. Then, to overcome issues around ICI, random device orientation and link blockage, hybrid LiFi/WiFi networks (HLWNs) are considered. In this thesis, dynamic load balancing (LB) considering handover in HLWNs is studied. The orientation-based random waypoint (ORWP) mobility model is considered to provide a more realistic framework to evaluate the performance of HLWNs. Based on the low-pass filtering effect of the LiFi channel, we firstly propose an orthogonal frequency division multiple access (OFDMA)-based resource allocation (RA) method in LiFi systems. Also, an enhanced evolutionary game theory (EGT)-based LB scheme with handover in HLWNs is proposed. Finally, due to the characteristic of high directivity and narrow beams, a vertical-cavity surface-emitting laser (VCSEL) array transmission system has been proposed to mitigate ICI. In order to support mobile users, two beam activation methods are proposed. The beam activation based on the corner-cube retroreflector (CCR) can achieve low power consumption and almost-zero delay, allowing real-time beam activation for high-speed users. The mechanism based on the omnidirectional transmitter (ODTx) is suitable for low-speed users and very robust to random orientation

Coexistence of directional and non-directional technologies in 6G wireless dense networks

Dense networks are characterized by the prevalence of wireless access points (APs) in close proximity to a population of user devices on a similar scale. By increasing AP density, the aggregate data consumption of a system can be dramatically increased. In this dissertation we consider dense deployment of directional visible light APs. Firstly, we analyze the performance of a visible light communication (VLC) link and propose algorithmic methods as well as novel receiver structures to enhance its quality. Secondly, we study handover algorithms and investigate an AP placement strategy that ties to the system outage probability. Thirdly, we use a geometric model for an indoor space and a reference optical channel model to formulate an optimization problem that proposes a dynamic field of view (FOV) receiver with a goal of optimizing receiver FOV for maximum signal to noise ratio (SNR). From the promising results we get, we then propose the dynamic FOV technique with receiver tracking capability. Its results show an average SNR increase of up to 40% when compared to a fixed FOV receiver. These results motivate the adoption of dynamic pointing and adaptive FOV at the receiver in order to realize improved performance for mobile devices in an optical wireless dense network. This opts us to study interference in VLC systems and how to mitigate it using our proposed receivers. In the context of multi-user networks, we formulate two main novel optimization problems i) a joint optimization of transmit emission pattern and transmit power while satisfying illumination requirements and ii) an optimization to allocate users, balance the network load and optimize device FOV for best performance. We then evaluate the effect of self-blockage as well as random human blockers on our proposed receivers. Finally, we propose to deploy the VLC system in a hybrid setting of other technologies to evaluate the overall system performance for future 6G networks.2022-01-17T00:00:00

Analysis of random orientation and user mobility in LiFi networks

Mobile data traffic is anticipated to surpass 49 exabyte per month by 2021. Smartphones, as the main factor of generating this huge data traffic (86%), are expected to require average speed connection of 20 Mbps by 2021. Light-fidelity (LiFi) is a novel bidirectional, high-speed and fully networked optical wireless communication and it is a promising solution to undertake this huge data traffic. However, to support seamless connectivity in LiFi networks, real-time knowledge of channel state information (CSI) from each user is required at the LiFi access point (AP). The CSI availability enables us to achieve optimal resource allocation and throughput maximization but it requires feedback transmitted through the uplink channel. Furthermore, the important aspects of the indoor LiFi channel such as the random orientation of user device, user mobility and link blockage need to be carefully analysed and effective solutions should be developed. In contrast to radio frequency (RF) channels, the LiFi channel is relatively less random. This feature of LiFi channel enables a potential reduction in the amount of feedback required to achieve high throughputs in a dynamic LiFi network. Based on this feature, two techniques for reducing the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback scheme based on reducing the content of feedback information and 2) limited-frequency feedback scheme based on the update interval. It is shown that these limited-feedback schemes can provide almost the same downlink performance as full feedback scheme. Furthermore, an optimum update interval which provides maximum bidirectional user equipment (UE) throughput, has been derived. Device orientation and its statistics is an important determinant factor that can affect the users throughput remarkably in LiFi networks. However, device orientation has been ignored in many previous performance studies of LiFi networks due to the lack of a proper statistical model. In this thesis, a novel model for the orientation of user device are proposed based on experimental measurements. The statistics of the device orientation for both sitting and walking activities are presented. Moreover, the statistics of the line-of-sight (LOS) channel gain are calculated. The influence of random device orientation on the received signal-to-noise-ratio (SNR) and bit-error ratio (BER) performance of LiFi systems has been also evaluated. To support the seamless connectivity of future LiFi-enabled devices in the presence of random device orientation, mobility and blockage, efficient handover between APs are required. In this thesis, an orientation-based random waypoint (ORWP) mobility model is proposed to analyze the performance of mobile users considering the effect of random device orientation. Based on this model, an analysis of handover due to random orientation and user mobility is presented. Finally, in order to improve seamless connectivity, a multi-directional receiver (MDR) configuration is proposed. The MDR configuration shows a robust performance in the presence of user mobility, random device orientation and blockage

Internet of Things and Sensors Networks in 5G Wireless Communications

The Internet of Things (IoT) has attracted much attention from society, industry and academia as a promising technology that can enhance day to day activities, and the creation of new business models, products and services, and serve as a broad source of research topics and ideas. A future digital society is envisioned, composed of numerous wireless connected sensors and devices. Driven by huge demand, the massive IoT (mIoT) or massive machine type communication (mMTC) has been identified as one of the three main communication scenarios for 5G. In addition to connectivity, computing and storage and data management are also long-standing issues for low-cost devices and sensors. The book is a collection of outstanding technical research and industrial papers covering new research results, with a wide range of features within the 5G-and-beyond framework. It provides a range of discussions of the major research challenges and achievements within this topic

Internet of Things and Sensors Networks in 5G Wireless Communications

This book is a printed edition of the Special Issue Internet of Things and Sensors Networks in 5G Wireless Communications that was published in Sensors

Advanced Trends in Wireless Communications

Physical limitations on wireless communication channels impose huge challenges to reliable communication. Bandwidth limitations, propagation loss, noise and interference make the wireless channel a narrow pipe that does not readily accommodate rapid flow of data. Thus, researches aim to design systems that are suitable to operate in such channels, in order to have high performance quality of service. Also, the mobility of the communication systems requires further investigations to reduce the complexity and the power consumption of the receiver. This book aims to provide highlights of the current research in the field of wireless communications. The subjects discussed are very valuable to communication researchers rather than researchers in the wireless related areas. The book chapters cover a wide range of wireless communication topics

Internet of Things and Sensors Networks in 5G Wireless Communications

This book is a printed edition of the Special Issue Internet of Things and Sensors Networks in 5G Wireless Communications that was published in Sensors

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM

Visible Light Communications for Indoor Applications

The field of visible light communications (VLC) has undergone a rapid development in recent years. The increased utilization of light emitting diodes (LEDs) has opened new possibilities for especially indoor services such as broadband internet connection and po- sitioning. Thus, a research within VLC is the main focus of the thesis and is divided into two main parts. At rst, the multiband carrier-less amplitude and phase (m-CAP) mod- ulation, introducing a newly adopted format for spectrally e cient VLC links, is under investigation using both theoretical and experimental approaches. The recommendations for m-CAP transmitter site design are proposed. Next, the channel modeling of indoor VLC is investigated with emphasis on the dynamically changing environments caused by moving people and non-line of sight (NLOS) propagation and new statistical models are derived.Katedra elektromagnetického pol