Mitigation of Side-Effect Modulation in Optical OFDM VLC Systems

Side-effect modulation (SEM) has the potential to be a significant source of interference in future visible light communication (VLC) systems. SEM is a variation in the intensity of the light emitted by a luminaire and is usually a side-effect caused by the power supply used to drive the luminaires. For LED luminaires powered by a switched mode power supply, the SEM can be at much higher frequencies than that emitted by conventional incandescent or fluorescent lighting. It has been shown that the SEM caused by commercially available LED luminaires is often periodic and of low power. In this paper, we investigate the impact of typical forms of SEM on the performance of optical OFDM VLC systems; both ACO-OFDM and DCO-OFDM are considered. Our results show that even low levels of SEM power can significantly degrade the bit-error-rate performance. To solve this problem, an SEM mitigation scheme is described. The mitigation scheme is decision-directed and is based on estimating and subtracting the fundamental component of the SEM from the received signal. We describe two forms of the algorithm; one uses blind estimation while the other uses pilot-assisted estimation based on a training sequence. Decision errors, resulting in decision noise, limit the performance of the blind estimator even when estimation is based on very long signals. However, the pilot system can achieve more accurate estimations, thus better performance. Results are first presented for typical SEM waveforms for the case where the fundamental frequency of the SEM is known. The algorithms are then extended to include a frequency estimation step and the mitigation algorithm is shown also to be effective in this case

DC-Informative Joint Color-Frequency Modulation for Visible Light Communications

In this paper, we consider the problem of constellation design for a visible light communication (VLC) system using red/green/blue light-emitting diodes (RGB LED), and propose a method termed DC-informative joint color-frequency modulation (DCI-JCFM). This method jointly utilizes available diversity resources including different optical wavelengths, multiple baseband subcarriers, and adaptive DC-bias. Constellation is designed in a high dimensional space, where the compact sphere packing advantage over lower dimensional counterparts is utilized. Taking into account multiple practical illumination constraints, a non-convex optimization problem is formulated, seeking the least error rate with a fixed spectral efficiency. The proposed scheme is compared with a decoupled scheme, where constellation is designed separately for each LED. Notable gains for DCI-JCFM are observed through simulations where balanced, unbalanced and very unbalanced color illuminations are considered.Comment: submitted to Journal of Lightwave Technology, Aug. 5th 201

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

Optics for AI and AI for Optics

Artificial intelligence is deeply involved in our daily lives via reinforcing the digital transformation of modern economies and infrastructure. It relies on powerful computing clusters, which face bottlenecks of power consumption for both data transmission and intensive computing. Meanwhile, optics (especially optical communications, which underpin today’s telecommunications) is penetrating short-reach connections down to the chip level, thus meeting with AI technology and creating numerous opportunities. This book is about the marriage of optics and AI and how each part can benefit from the other. Optics facilitates on-chip neural networks based on fast optical computing and energy-efficient interconnects and communications. On the other hand, AI enables efficient tools to address the challenges of today’s optical communication networks, which behave in an increasingly complex manner. The book collects contributions from pioneering researchers from both academy and industry to discuss the challenges and solutions in each of the respective fields

Améliorations des transmissions VLC (Visible Light Communication) sous contrainte d'éclairage : études théoriques et expérimentations

Abstract : Indoor visible light communication (VLC) networks based on light-emitting diodes (LEDs) currently enjoy growing interest thanks in part to their robustness against interference, wide license-free available bandwidth, low cost, good energy efficiency and compatibility with existing lighting infrastructure. In this thesis, we investigate spectral-efficient modulation techniques for the physical layer of VLC to increase throughput while considering the quality of illumination as well as implementation costs. Numerical and experimental studies are performed employing pulse amplitude modulation (PAM) and carrierless amplitude and phase (CAP) modulation under illumination constraints and for high modulation orders. Furthermore, the impact of LED nonlinearity is investigated and a postdistortion technique is evaluated to compensate these nonlinear effects. Within this framework, transmission rates in the order of a few hundred Mb/s are achieved using a test bench made of low-cost components. In addition, an imaging multiple input multiple-output (MIMO) system is developed and the impact on performance of imaging lens misalignment is theoretically and numerically assessed. Finally, a polynomial matrix decomposition technique based on the classical LU factorization method is studied and applied for the first time to MIMO VLC systems in large space indoor environments.Les réseaux de communication en lumière visible (VLC) s’appuyant sur l’utilisation de diodes électroluminescentes (LED) bénéficient actuellement d’un intérêt grandissant, en partie grâce à leur robustesse face aux interférences électromagnétiques, leur large bande disponible non-régulée, leur faible coût, leur bonne efficacité énergétique, ainsi que leur compatibilité avec les infrastructures d’éclairage déjà existantes. Dans cette thèse, nous étudions des techniques de modulation à haute efficacité spectrale pour la couche physique des VLC pour augmenter les débits tout en considérant la qualité de l’éclairage ainsi que les coûts d’implémentation. Des études numériques et expérimentales sont réalisées sur la modulation d’impulsion d’amplitude (PAM) et sur la modulation d’amplitude et de phase sans porteuse (CAP) sous des contraintes d’éclairage et pour des grands ordres de modulation. De plus, l’impact des non-linéarités de la LED est étudié et une technique de post-distorsion est évaluée pour corriger ces effets non-linéaires. Dans ce cadre, des débits de plusieurs centaines de Mb/s sont atteints en utilisant un banc de test réalisé à partir de composants à bas coûts. Par ailleurs, un système multi-entrées multi-sorties (MIMO) imageant est également développé et l’impact du désaxage de l’imageur sur les performances est étudié. Finalement, une technique de décomposition polynomiale basée sur la méthode de factorisation classique LU est étudiée et appliquée aux systèmes MIMO VLC dans des grands espaces intérieurs