21 research outputs found
Visible Light Communication (VLC)
Visible light communication (VLC) using light-emitting diodes (LEDs) or laser diodes (LDs) has been envisioned as one of the key enabling technologies for 6G and Internet of Things (IoT) systems, owing to its appealing advantages, including abundant and unregulated spectrum resources, no electromagnetic interference (EMI) radiation and high security. However, despite its many advantages, VLC faces several technical challenges, such as the limited bandwidth and severe nonlinearity of opto-electronic devices, link blockage and user mobility. Therefore, significant efforts are needed from the global VLC community to develop VLC technology further. This Special Issue, âVisible Light Communication (VLC)â, provides an opportunity for global researchers to share their new ideas and cutting-edge techniques to address the above-mentioned challenges. The 16 papers published in this Special Issue represent the fascinating progress of VLC in various contexts, including general indoor and underwater scenarios, and the emerging application of machine learning/artificial intelligence (ML/AI) techniques in VLC
PM-DCO-OFDM for PAPR reduction in visible light communications
Phase modulation is used in DCO-OFDM for visible light communications, whereby around 10 dB PAPR reduction is achieved. Considering the nonlinearity of LED emitters, our proposed method achieves better BER performance than that of conventional DCO-OFDM
Ellipse-based DCO-OFDM for visible light communications
Ellipse-based DC-biased optical orthogonal frequency division multiplexing (E-DCO-OFDM) is proposed for visible light communications (VLC), which achieves a significant peak-to-average power ratio (PAPR) reduction, thus enhancing the overall performance when light-emitting diode (LED) nonlinearity is considered. In E-DCO-OFDM, the real-valued output of OFDM is modulated onto an ellipse, whereby only the imaginary part of the complex point on the ellipse is transmitted. Although the PAPR of E-DCO-OFDM decreases as the ratio of major radius to minor radius becomes larger, it may be more vulnerable to the effect of noise, leading to the performance loss. Therefore, the relationship between the system performance and the critical parameters in E-DCO-OFDM, such as the ratio between the major and minor radius of the ellipse, is investigated. Meanwhile, simulations demonstrate that E-DCO-OFDM adopting the optimal parameters achieves a considerable signal-to-noise ratio (SNR) gain over the conventional DCO-OFDM
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Channel Estimation, Modulation Techniques and Decoding Algorithms for Hybrid WiFi-FSO (WiFO) WLAN of Femtocells
Due to the limitation of the radio frequency (RF) spectrum, it is increasingly more difficult to support billions of wireless devices in the age of Internet-of-Things. Consequently, many recent wireless indoor communication systems have been developed using free space optical (FSO) communication technologies that exploit the extremely large light spectrum to transmit data. This dissertation contributes three physical layer techniques to an FSO system called WiFO, which is integrated with the existing WiFi transmission. (1) We proposed a Pulse Amplitude Modulation (PAM) decoding scheme that estimates the channel parameters and determine the optimal decoding thresholds to minimize the average bit error rate (BER). The conditions on channel parameters for which signal recovery is impossible, are also determined. (2) A pre-shaping technique called Adaptive Sending Duration Algorithm (ASDA), that modifies the durations of input bits dynamically, is proposed to be used at an LED-transmitter. Simulation results show that ASDA working with equalization successfully improves the performance of BERs. (3) We propose a decoding scheme called Memory Decoding Algorithm (MDA) used at a receiver and show that it effectively reduces the bit error rates via maximum likelihood decoding principle
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 rĂ©seaux de communication en lumiĂšre visible (VLC) sâappuyant sur lâutilisation de diodes Ă©lectroluminescentes (LED) bĂ©nĂ©ficient actuellement dâun intĂ©rĂȘt grandissant, en partie grĂące Ă leur robustesse face aux interfĂ©rences Ă©lectromagnĂ©tiques, leur large bande disponible non-rĂ©gulĂ©e, leur faible coĂ»t, leur bonne efficacitĂ© Ă©nergĂ©tique, ainsi que leur compatibilitĂ© avec les infrastructures dâĂ©clairage dĂ©jĂ existantes. Dans cette thĂšse, nous Ă©tudions des techniques de modulation Ă haute efficacitĂ© spectrale pour la couche physique des VLC pour augmenter les dĂ©bits tout en considĂ©rant la qualitĂ© de lâĂ©clairage ainsi que les coĂ»ts dâimplĂ©mentation. Des Ă©tudes numĂ©riques et expĂ©rimentales sont rĂ©alisĂ©es sur la modulation dâimpulsion dâamplitude (PAM) et sur la modulation dâamplitude et de phase sans porteuse (CAP) sous des contraintes dâĂ©clairage et pour des grands ordres de modulation. De plus, lâimpact des non-linĂ©aritĂ©s de la LED est Ă©tudiĂ© et une technique de post-distorsion est Ă©valuĂ©e pour corriger ces effets non-linĂ©aires. Dans ce cadre, des dĂ©bits de plusieurs centaines de Mb/s sont atteints en utilisant un banc de test rĂ©alisĂ© Ă partir de composants Ă bas coĂ»ts. Par ailleurs, un systĂšme multi-entrĂ©es multi-sorties (MIMO) imageant est Ă©galement dĂ©veloppĂ© et lâimpact du dĂ©saxage de lâimageur sur les performances est Ă©tudiĂ©. Finalement, une technique de dĂ©composition polynomiale basĂ©e sur la mĂ©thode de factorisation classique LU est Ă©tudiĂ©e et appliquĂ©e aux systĂšmes MIMO VLC dans des grands espaces intĂ©rieurs
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
Physical Layer Security for Visible Light Communication Systems:A Survey
Due to the dramatic increase in high data rate services and in order to meet
the demands of the fifth-generation (5G) networks, researchers from both
academia and industry are exploring advanced transmission techniques, new
network architectures and new frequency spectrum such as the visible light
spectra. Visible light communication (VLC) particularly is an emerging
technology that has been introduced as a promising solution for 5G and beyond.
Although VLC systems are more immune against interference and less susceptible
to security vulnerabilities since light does not penetrate through walls,
security issues arise naturally in VLC channels due to their open and
broadcasting nature, compared to fiber-optic systems. In addition, since VLC is
considered to be an enabling technology for 5G, and security is one of the 5G
fundamental requirements, security issues should be carefully addressed and
resolved in the VLC context. On the other hand, due to the success of physical
layer security (PLS) in improving the security of radio-frequency (RF) wireless
networks, extending such PLS techniques to VLC systems has been of great
interest. Only two survey papers on security in VLC have been published in the
literature. However, a comparative and unified survey on PLS for VLC from
information theoretic and signal processing point of views is still missing.
This paper covers almost all aspects of PLS for VLC, including different
channel models, input distributions, network configurations,
precoding/signaling strategies, and secrecy capacity and information rates.
Furthermore, we propose a number of timely and open research directions for
PLS-VLC systems, including the application of measurement-based indoor and
outdoor channel models, incorporating user mobility and device orientation into
the channel model, and combining VLC and RF systems to realize the potential of
such technologies
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