Interference Suppression in Massive MIMO VLC Systems

The focus of this dissertation is on the development and evaluation of methods and principles to mitigate interference in multiuser visible light communication (VLC) systems using several transmitters. All components of such a massive multiple-input multiple-output (MIMO) system are considered and transformed into a communication system model, while also paying particular attention to the hardware requirements of different modulation schemes. By analyzing all steps in the communication process, the inter-channel interference between users is identified as the most critical aspect. Several methods of suppressing this kind of interference, i.e. to split the MIMO channel into parallel single channels, are discussed, and a novel active LCD-based interference suppression principle at the receiver side is introduced as main aspect of this work. This technique enables a dynamic adaption of the physical channel: compared to solely software-based or static approaches, the LCD interference suppression filter achieves adaptive channel separation without altering the characteristics of the transmitter lights. This is especially advantageous in dual-use scenarios with illumination requirements. Additionally, external interferers, like natural light or transmitter light sources of neighboring cells in a multicell setting, can also be suppressed without requiring any control over them. Each user's LCD filter is placed in front of the corresponding photodetector and configured in such a way that only light from desired transmitters can reach the detector by setting only the appropriate pixels to transparent, while light from unwanted transmitters remains blocked. The effectiveness of this method is tested and benchmarked against zero-forcing (ZF) precoding in different scenarios and applications by numerical simulations and also verified experimentally in a large MIMO VLC testbed created specifically for this purpose

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)

High Speed Optical Wireless Communication Systems.

Visible light communication (VLC) systems have become promising candidates to complement conventional radio frequency (RF) systems due to the increasingly saturated RF spectrum and the potentially high data rates that can be achieved by VLC systems. Over the last decade, significant research efforts have been directed towards the development of VLC systems due to their numerous advantages over RF communication systems, such as the availability of simple transmitters (light emitting diodes, (LEDs)) and receivers (silicon photo detectors), better security at the physical layer, improved energy efficiency due to the dual functionally (i.e., illumination and communication) and hundreds of THz of license-free bandwidth. However, several challenges face VLC systems to achieve high data rates (multi gigabits per second). These challenges include the low modulation bandwidth of the LEDs, inter symbol interference (ISI) due to multipath propagation, co-channel interference (CCI) and the light unit (i.e., VLC transmitter) should be ‘‘ON’’ all the time to ensure continuous communication. This thesis investigates a number of techniques to design robust highspeed indoor VLC systems that support a single user and multi-users. Light engines composed of RYGB laser diodes (LDs) are used for communication and illumination. The main goal of using LDs is to enable the VLC system to achieve high data rates while using simple modulation techniques (such as on off keying (OOK)), which adds simplicity to VLC systems. Three VLC systems based on the computer generated holograms (CGHs) are proposed in this thesis, which are single beam static CGH-VLC system, static CGH-VLC system and adaptive CGH-VLC system. Whereas in the first and the second systems a single photodetector is used (added simplicity), an imaging receiver is used in the third one to obtain spatial multiplexing. We consider the lighting constraints where illumination should be at an acceptable level and should consider diffuse reflections (up to second order) to find the maximum data rate that can be offered by each system. In the first system, the CGH is used to produce one fixed broad beam from the best light unit and focus it to a specific area on the communication floor. In the second system, the CGH generates 100 beams (all these beams carry same data) from the best transmitter and directs these beams to an area of 2 m × 2 m on the communication floor. In the third system, the CGH is used to generate eight beams from the best transmitter and steer these beams to the receiver’s location. In addition, each one of these eight beams carries a different data stream. This thesis also presents an indoor VLC system in conjunction with an imaging receiver with parallel data transmission (spatial multiplexing) to reduce the effects of inter-symbol-interference (ISI). To distinguish between light units (transmitters) and to match the light units used (to convey the data) with the pixels of the imaging receiver, we proposed the use of subcarrier multiplexing (SCM) tones. Each light unit transmission is multiplexed with a unique tone. At the receiver, a SCM tone decision system is utilised to measure the power level of each SCM tone and consequently associate each pixel with a light unit. We proposed a high data rate single user VLC system based on wavelength division multiplexing (WDM) in this thesis. An imaging diversity receiver (IMDR) is used as an optical receiver. Based on the location of the IMDR, each colour of the RYGB LDs sends a different data stream at a different rate where the variable data rates are attributed mainly to the different power levels assigned to the colours to yield the desired white colour. Each pixel of the IMDR is covered by a specific colour optical filter. WDM in conjunction with the SCM tones are used to realise a high data rate multi-user VLC system in this thesis. The SCM tones are used to allocate an optimum transmitter to each user and to calculate the co-channel interference (CCI). Two novel optical receivers are used to evaluate the performance of the VLC systems: an array of non-imaging receivers (NI-R) and an array of non-imaging angle diversity receivers (NI-ADR). This thesis proposes a multi-branch transmitter (MBT) as a solution that can improve the VLC system performance over an indoor channel and support multi-user operation. The MBT has many transmitter branches (TBs) where each branch is directed to a specific area. By reducing the semi angle of each TB, the effect of multipath propagations is reduced and the received optical power is improved. The performance of the MBT is evaluated with a single user VLC system using a wide field of view (W-FOV) receiver and then with an angle diversity receiver (ADR). The results show that this system can provide a data rate of 4 Gb/s and 10 Gb/s when using wide FOV receiver and ADR, respectively. In addition, the performance of the MBT is evaluated in a multi-user scenario. We used the MBT with WDM and SCM tones to realise a high data rate multiuser indoor VLC system. Four colour laser diodes (RYGB LDs) are used as sources of illumination and data communication. One colour of these four colours is used to convey the SCM tones at the beginning of the connection to set up the connection. When the connection is set up, the data is transmitted in parallel through the RYGB LDs

Indoor Visible Light Communication:A Tutorial and Survey

Abstract With the advancement of solid-state devices for lighting, illumination is on the verge of being completely restructured. This revolution comes with numerous advantages and viable opportunities that can transform the world of wireless communications for the better. Solid-state LEDs are rapidly replacing the contemporary incandescent and fluorescent lamps. In addition to their high energy efficiency, LEDs are desirable for their low heat generation, long lifespan, and their capability to switch on and off at an extremely high rate. The ability of switching between different levels of luminous intensity at such a rate has enabled the inception of a new communication technology referred to as visible light communication (VLC). With this technology, the LED lamps are additionally being used for data transmission. This paper provides a tutorial and a survey of VLC in terms of the design, development, and evaluation techniques as well as current challenges and their envisioned solutions. The focus of this paper is mainly directed towards an indoor setup. An overview of VLC, theory of illumination, system receivers, system architecture, and ongoing developments are provided. We further provide some baseline simulation results to give a technical background on the performance of VLC systems. Moreover, we provide the potential of incorporating VLC techniques in the current and upcoming technologies such as fifth-generation (5G), beyond fifth-generation (B5G) wireless communication trends including sixth-generation (6G), and intelligent reflective surfaces (IRSs) among others

A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined