60 research outputs found
Polarity Information Coded Flip-OFDM for Intensity Modulated Systems
A polarity-information-coded flip orthogonal frequency division multiplexing (PIC-flip-OFDM) is proposed for intensity modulation/direct detection (IM/DD) optical communications in this letter. In the proposed scheme, the modulated signals in the frequency domain are not constrained to have Hermitian symmetry. The real and imaginary parts of the timedomain complex signals are separated, and the polarities of the real and imaginary parts are jointly encoded and modulated. The transmit strategy and the receive algorithm of the proposed scheme are analyzed in detail. The major advantage of the proposed scheme is that its spectral and optical power efficiencies are higher than existing schemes, which is validated in simulation
Coded Index Modulation for Non-DC-Biased OFDM in Multiple LED Visible Light Communication
Use of multiple light emitting diodes (LED) is an attractive way to increase
spectral efficiency in visible light communications (VLC). A non-DC-biased OFDM
(NDC OFDM) scheme that uses two LEDs has been proposed in the literature
recently. NDC OFDM has been shown to perform better than other OFDM schemes for
VLC like DC-biased OFDM (DCO OFDM) and asymmetrically clipped OFDM (ACO OFDM)
in multiple LEDs settings. In this paper, we propose an efficient multiple LED
OFDM scheme for VLC which uses {\em coded index modulation}. The proposed
scheme uses two transmitter blocks, each having a pair of LEDs. Within each
block, NDC OFDM signaling is done. The selection of which block is activated in
a signaling interval is decided by information bits (i.e., index bits). In
order to improve the reliability of the index bits at the receiver (which is
critical because of high channel correlation in multiple LEDs settings), we
propose to use coding on the index bits alone. We call the proposed scheme as
CI-NDC OFDM (coded index NDC OFDM) scheme. Simulation results show that, for
the same spectral efficiency, CI-NDC OFDM that uses LDPC coding on the index
bits performs better than NDC OFDM
A novel unipolar transmission scheme for visible light communication
This paper proposes a novel unipolar transceiver for visible light communication (VLC) by using orthogonal waveforms. The main advantage of our proposed scheme over most of the existing unipolar schemes in the literature is that the polarity of the real-valued orthogonal frequency division multiplexing (OFDM) sample determines the pulse shape of the continuous-time signal and thus, the unipolar conversion is performed directly in the analog instead of the digital domain. Therefore, our proposed scheme does not require any direct current (DC) biasing or clipping as it is the case with existing schemes in the literature. The bit error rate (BER) performance of our proposed scheme is analytically derived and its accuracy is verified by using Matlab simulations. Simulation results also substantiate the potential performance gains of our proposed scheme against the state-of-the-art OFDM-based systems in VLC; it indicates that the absence of DC shift and clipping in our scheme supports more reliable communication and outperforms the asymmetrically clipped optical-OFDM (ACO-OFDM), DC optical-OFDM (DCO-OFDM) and unipolar-OFDM (U-OFDM) schemes. For instance, our scheme outperforms ACO-OFDM by at least 3 dB (in terms of signal to noise ratio) at a target BER of 10 −4 , when considering the same spectral efficiency for both schemes
Estudio y simulación de un sistema Flip-OFDM para sistemas de comunicación por luz visible (VLC)
El objetivo de este Trabajo Fin de Grado es implementar y simular dos técnicas de modulación para sistemas
Li-Fi mediante la herramienta Matlab. Posteriormente, los resultados de las simulaciones serán analizados,
comparados y corroborados con los artículos de investigación en los que se basa este proyecto. Además,
también se hará una investigación y estudio del Estado del Arte de los distintos esquemas de modulación que
existen para desarrollar un sistema Li-Fi.The aim of this project is to implement and simulate two modulation techniques for Li-Fi systems using
Matlab. Later, the results of the simulations will be analyzed, compared and corroborated with the research
articles on which this project is based. In addition, there will also be a research and a study of the State of the
Art of various modulation schemes that exist to develop a Li-Fi system.Universidad de Sevilla. Grado en Ingeniería de las Tecnologías de Telecomunicació
Physical Layer Techniques for Indoor Wireless Visible Light Communications
The growing demand for bandwidth-hungry applications and increasing number of smart interconnected devices has increased the data traffic on radio access networks. Subsequently, the saturating spectral efficiencies in crowded radio frequency spectrum has impelled the researchers to exploit the optical spectrum for communications. In particular, many developments in the visible light communication (VLC) as a combined lighting and communications system have taken place.
Despite abundant optical bandwidth, the data transmission rates and power efficiencies in VLC are partly limited by the electrical channel bandwidth and the type of signalling sets which can be used in this intensity modulated, direct detected system. In order to improve the power and spectral efficiencies, this thesis focuses on physical layer (PHY) techniques. The state-of-the-art single channel modulations (SCM) based on M-PAM, multi-channel modulations (MCM) based on OFDM, and IEEE standardised multi-colour modulations are investigated comprehensively through simulations and theoretical analysis, over representative VLC channels considering the optical properties of front-end devices.
The bit error performances and spectral efficiencies of DC-biased and non DC-biased MCM systems are compared. A new vector coding based MCM is proposed to optimally utilise the channel state information at the transmitter as an alternative to optical OFDM. The throughputs, peak-to-average power ratios and DC-bias requirements of SCM and MCM systems are investigated which show that the lower DC-bias requirements reduce power consumed for the same throughput in SCM systems when compared to MCM systems. A new quad-chromatic colour shift keying (CSK) system is proposed which reduces power requirements and complexity, enhances throughput and realises a four-dimensional signalling to outperform the IEEE standardised tri-chromatic CSK system.
For improved power efficiency and throughput of VLC PHY, use of rate-adaptive binary convolutional coding and Viterbi decoding is proposed along with frequency domain channel equalisation to mitigate temporal dispersion over representative VLC channels
Spectrum and energy efficient digital modulation techniques for practical visible light communication systems
The growth in mobile data traffic is rapidly increasing in an unsustainable direction
given the radio frequency (RF) spectrum limits. Visible light communication (VLC)
offers a lucrative solution based on an alternative license-free frequency band that is safe
to use and inexpensive to utilize. Improving the spectral and energy efficiency of intensity
modulation and direct detection (IM/DD) systems is still an on-going challenge in
VLC. The energy efficiency of inherently unipolar modulation techniques such as pulse-amplitude
modulation discrete multitone modulation (PAM-DMT) and asymmetrically
clipped optical orthogonal frequency division multiplexing (ACO-OFDM) degrades at
high spectral efficiency. Two novel superposition modulation techniques are proposed
in this thesis based on PAM-DMT and ACO-OFDM. In addition, a practical solution
based on the computationally efficient augmented spectral efficiency discrete multi-tone
(ASE-DMT) is proposed. The system performance of the proposed superposition
modulation techniques offers significant electrical and optical power savings with up
to 8 dB in the electrical signal-to-noise ratio (SNR) when compared with DC-biased
optical orthogonal frequency division multiplexing (DCO-OFDM). The theoretical bit
error ratio (BER) performance bounds for all of the proposed modulation techniques
are in agreement with the Monte-Carlo simulation results. The proposed superposition
modulation techniques are promising candidates for spectrum and energy efficient
IM/DD systems.
Two experimental studies are presented for a VLC system based on DCO-OFDM with
adaptive bit and energy loading. Micrometer-sized Gallium Nitride light emitting
diode (m-LED) and light amplification by stimulated emission of radiation diode (LD)
are used in these studies due to their high modulation bandwidth. Record data rates are
achieved with a BER below the forward error correction (FEC) threshold at 7.91 Gb/s
using the violet m-LED and at 15 Gb/s using the blue LD. These results highlight
the potential of VLC systems in practical high speed communication solutions. An
additional experimental study is demonstrated for the proposed superposition modulation
techniques based on ASE-DMT. The experimentally achieved results confirm the
theoretical and simulation based performance predictions of ASE-DMT. A significant
gain of up to 17.33 dB in SNR is demonstrated at a low direct current (DC) bias.
Finally, the perception that VLC systems cannot work under the presence of sunlight is
addressed in this thesis. A complete framework is presented to evaluate the performance
of VLC systems in the presence of solar irradiance at any given location and time. The
effect of sunlight is investigated in terms of the degradations in SNR, data rate and
BER. A reliable high speed communication system is achieved under the sunlight
effect. An optical bandpass blue filter is shown to compensate for half of the reduced
data rate in the presence of sunlight. This thesis demonstrates data rates above 1 Gb/s
for a practical VLC link under strong solar illuminance measured at 50350 lux in clear
weather conditions
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
- …