Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

The solid-state lighting is revolutionizing the indoor illumination. Current incandescent and fluorescent lamps are being replaced by the LEDs at a rapid pace. Apart from extremely high energy efficiency, the LEDs have other advantages such as longer lifespan, lower heat generation, and improved color rendering without using harmful chemicals. One additional benefit of LEDs is that they are capable of switching to different light intensity at a very fast rate. This functionality has given rise to a novel communication technology (known as visible light communication—VLC) where LED luminaires can be used for high speed data transfer. This survey provides a technology overview and review of existing literature of visible light communication and sensing. This paper provides a detailed survey of 1) visible light communication system and characteristics of its various components such as transmitter and receiver; 2) physical layer properties of visible light communication channel, modulation methods, and MIMO techniques; 3) medium access techniques; 4) system design and programmable platforms; and 5) visible light sensing and application such as indoor localization, gesture recognition, screen-camera communication, and vehicular networking. We also outline important challenges that need to be addressed in order to design high-speed mobile networks using visible light communication.

Visible Light Communication, Networking, and Sensing: A Survey, Potential and Challenges

Underwater wireless information transfer is of great interest to the military, industry, and the scientific community, as it plays an important role in tactical surveillance, pollution monitoring, oil control and maintenance, offshore explorations, climate change monitoring, and oceanography research. In order to facilitate all these activities, there is an increase in the number of unmanned vehicles or devices deployed underwater, which require high bandwidth and high capacity for information transfer underwater. Although tremendous progress has been made in the field of acoustic communication underwater, however, it is limited by bandwidth. All this has led to the proliferation of underwater optical wireless communication (UOWC), as it provides higher data rates than the traditional acoustic communication systems with significantly lower power consumption and simpler computational complexities for short-range wireless links. UOWC has many potential applications ranging from deep oceans to coastal waters. However, the biggest challenge for underwater wireless communication originates from the fundamental characteristics of ocean or sea water; addressing these challenges requires a thorough understanding of complex physio-chemical biological systems. In this paper, the main focus is to understand the feasibility and the reliability of high data rate underwater optical links due to various propagation phenomena that impact the performance of the system. This paper provides an exhaustive overview of recent advances in UOWC. Channel characterization, modulation schemes, coding techniques, and various sources of noise which are specific to UOWC are discussed. This paper not only provides exhaustive research in underwater optical communication but also aims to provide the development of new ideas that would help in the growth of future underwater communication. A hybrid approach to an acousto-optic communication system is presented that complements the existing acoustic system, resulting in high data rates, low latency, and an energy-efficient system.

Underwater Optical Wireless Communication

Underwater wireless communications refer to data transmission in unguided water environment through wireless carriers, i.e., radio-frequency (RF) wave, acoustic wave, and optical wave. In comparison to RF and acoustic counterparts, underwater optical wireless communication (UOWC) can provide a much higher transmission bandwidth and much higher data rate. Therefore, we focus, in this paper, on the UOWC that employs optical wave as the transmission carrier. In recent years, many potential applications of UOWC systems have been proposed for environmental monitoring, offshore exploration, disaster precaution, and military operations. However, UOWC systems also suffer from severe absorption and scattering introduced by underwater channels. In order to overcome these technical barriers, several new system design approaches, which are different from the conventional terrestrial free-space optical communication, have been explored in recent years. We provide a comprehensive and exhaustive survey of the state-of-the-art UOWC research in three aspects: 1) channel characterization; 2) modulation; and 3) coding techniques, together with the practical implementations of UOWC.

A Survey of Underwater Optical Wireless Communications

Visible Light Communication (VLC) is an emerging field in Optical Wireless Communication (OWC) which utilizes the superior modulation bandwidth of Light Emitting Diodes (LEDs) to transmit data. In modern day communication systems, the most popular frequency band is Radio Frequency (RF) mainly due to little interference and good coverage. However, the rapidly dwindling RF spectrum along with increasing wireless network traffic has substantiated the need for greater bandwidth and spectral relief. By combining illumination and communication, VLC provides ubiquitous communication while addressing the shortfalls and limitations of RF communication. This paper provides a comprehensive survey on VLC with an emphasis on challenges faced in indoor applications over the period 1979–2014. VLC is compared with infrared (IR) and RF systems and the necessity for using this beneficial technology in communication systems is justified. The advantages of LEDs compared to traditional lighting technologies are discussed and comparison is done between different types of LEDs currently available. Modulation schemes and dimming techniques for indoor VLC are discussed in detail. Methods needed to improve VLC system performance such as filtering, equalization, compensation, and beamforming are also presented. The recent progress made by various research groups in this field is discussed along with the possible applications of this technology. Finally, the limitations of VLC as well as the probable future directions are presented.

LED Based Indoor Visible Light Communications: State of the Art

Light-emitting diodes (LEDs), which will be increasingly used in lighting technology, will also allow for distribution of broadband optical wireless signals. Visible-light communication (VLC) using white LEDs offers several advantages over the RF-based wireless systems, i.e., license-free spectrum, low power consumption, and higher privacy. Mostly, optical wireless can provide much higher data rates. In this paper, we demonstrate a VLC system based on a white LED for indoor broadband wireless access. After investigating the nonlinear effects of the LED and the power amplifier, a data rate of 1 Gb/s has been achieved at the standard illuminance level, by using an optimized discrete multitone modulation technique and adaptive bit- and power-loading algorithms. The bit-error ratio of the received data was $1.5\cdot 10^{-3}$ , which is within the limit of common forward error correction (FEC) coding. These results twice the highest capacity that had been previously obtained.

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1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation

In this paper, we experimentally realized a gigabit-class indoor visible light communication system using commercially available RGB White LED and exploiting an optimized DMT modulation. We achieved data rate of 1.5 Gbit/s with single channel and 3.4 Gbit/s by implementing WDM transmission at standard illumination levels. In both experiments, the resulting bit error ratios were below the FEC limit. To the best of our knowledge, these values are the highest ever achieved in VLC systems.

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3.4 Gbit/s visible optical wireless transmission based on RGB LED

The state-of-the-art of Er-doped integrated optical devices in LiNbO/sub 3/ is reviewed starting with a brief discussion of the technology of Er-indiffusion. This technique yields high-quality waveguides and allows a selective surface doping necessary to develop optical circuits of higher complexity. Doped waveguides have been used as single- and double-pass optical amplifiers for the wavelength range 1530 nm</spl lambda/<1610 nm. If incorporated in conventional, lossy devices loss-compensating or even amplifying devices can be fabricated. Examples are an electrooptically scanned Ti:Er:LiNbO/sub 3/ waveguide resonator used as an optical spectrum analyzer and an acoustooptically tunable filter used as a tunable narrowband amplifier. Different types of Ti:Er:LiNbO/sub 3/ waveguide lasers are presented. Among them are free running Fabry-Perot lasers for six different wavelengths with a continuous-wave (CW)-output power up to 63 mW. Tunable lasers could be demonstrated by the intracavity integration of an acoustooptical amplifying wavelength filter yielding a tuning range up to 31 nm. With intracavity electrooptic phase modulation modelocked laser operation has been obtained with pulse repetition frequencies up to 10 GHz; pulses of only a few ps width could be generated. With intracavity amplitude modulation Q-switched laser operation has been achieved leading to the emission of pulses of up to 2.4 W peak power (0.18 /spl mu/J) at 2 kHz repetition frequency. Distributed Bragg reflector (DBR) lasers of emission linewidth /spl les/8 kHz have been developed using a dry-etched surface grating as one of the mirrors of the laser resonator. Finally, as an example for a monolithic integration of lasers and extracavity devices on the same substrate, a DBR-laser/modulator combination is presented.

Er-doped integrated optical devices in LiNbO/sub 3/

We experimentally prove high-speed underwater optical wireless transmission over 2.5 m distance, using different bit rates and modulation schemes. The system uses two low-cost Light Emitting Diodes (LEDs) arrays as optical transmitter and an avalanche photodiode module as receiver. The measurements are taken using an outdoor water tank having 3.3 m diameter, where two waterproof boxes containing the transmitter and the receiver are fixed underwater at the inner borders. We test 6.25 Mbit/s with Manchester coding, 12.5 Mbit/s with NRZ 8b/10b coding and 58 Mbit/s with Discrete Multitone modulation. Bit Error Rate measurements are collected over several hours under typical summer sunlight illumination conditions. In all the experimental conditions we achieve error free transmission.

Experimental demonstration of high speed underwater visible light communications

In this paper we experimentally realized bidirectional optical wireless communication (OWC) link using four channel visible LED board exploiting wavelength division multiplexing (WDM) for the downlink and infrared LED for uplink. We achieved greater than 5 Gbit/s data rate at common indoor distance (1.5 to 4 m) for downlink and 1.5 Gbit/s for uplink using commercially available LEDs. We achieved these results after a careful choice of the LED emission wavelengths and the optical filter spectra. Moreover, we investigate the optimal LED working current and the optimal modulation depth. The bit error ratios of all the channels were maintained lower than the FEC limit (3.8·10−3).

Raffaele Corsini

Papers

1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation

3.4 Gbit/s visible optical wireless transmission based on RGB LED

Er-doped integrated optical devices in LiNbO/sub 3/

Experimental demonstration of high speed underwater visible light communications

Gigabit-class optical wireless communication system at indoor distances (1.5 – 4 m)