28-07-2014, 11:32 AM
VISIBLE LIGHT COMMUNICATION
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INTRODUCTION
The visible light communication (VLC) refers to the communication technology which utilizes the visible light source as a signal transmitter, the air as a transmission medium, and the appropriate photodiode as a signal receiving component. The visible light communication technology has a short history compared with other communication technology, for example, public old telephone service, Ethernet, high-speed optical communication, wireless cellular communication, IrDA, etc. It is due to that the development and commercialization of light emitting diodes (LEDs) which emits the light in visible wavelength range have been successful for illumination in recent decade.
It is said that the illumination LEDs will replace the conventional illumination lightings such as incandescent bulbs and fluorescent lamps since they have the characteristics of long lifetime, mercury free, color mixing, fast switching, etc. By utilizing the advantage of fast switching characteristic of the LEDs compared with the Conventional lightings, i.e., modulating the LED light with the data signal, the LED Illumination can be used as a communication source. Since the illumination exists everywhere, it is expected that the LED illumination device will act as a lighting device and a communication transmitter simultaneously everywhere in a near future.
There have been researches on application of visible LEDs. The audio system using visible
Light LEDs was reported in Hong Kong by G. Pang et al. (Pang, 1999) and the visible light Communication with the power line communication was reported in Japan by Komine et al. It can be considered that the active research has been started since 2005. Still the VLC system is not close to commercialization, but in the basic research.
SYSTEM DESCRIPTION 2.1 CHANNEL CONFIGURATION
The optical wireless communication (OWC) is a general term for explaining wireless communication with optical technology. Usually, OWC includes infrared (IR) communication for short range and free-space optics (FSO) communication (FSO website) for longer range.
The visible light communication (VLC) denotes a communication technology which uses
visible light as optical carrier for data transmission and illumination. Nowadays, light emitting diode (LED) at visible wavelengths (380 nm ~ 780 nm) has been actively developed and can be used as a communication source and, naturally, the silicon photodiode which shows good
COMPARISON WITH IR COMMUNICATION
To have a clear notion about VLC, it is needed to compare it with the infrared communication technology. The differences between VLC and infrared communication are listed in Table 1.The infrared communication is standardized by the IrDA (Infrared Data Association) and
the IrDA is still developing advanced application of infrared communication. The data rate for infrared communication includes 4 Mb/s (FIR), 16 Mb/s (VFIR), and etc. On the other hand, the VLC data rate is dependent on the LED’s modulation bandwidth and the standardization on physical layer specifications has not yet been published. Some of researches have reached around 20 Mb/s. Since the resonant-cavity LEDs shows the modulation bandwidth > 100 Mb/s, it is expected that the VLC system with > 100 Mb/s data rate is possible by using the high-speed LEDs and appropriate multiplexing techniques.
The transmission distance for VLC is possible up to several meters due to its illumination
requirement. Since the infrared communication is used for a remote controller, the maximum distance is ~ 3 meters. The VLC transmitter emits multiple-wavelength light from red to violet and the exact analysis will become more complex than infrared communication. Due to the
VLC TRANSMITTER
The technical considerations for VLC transmitter are mentioned. The main components of
VLC transmitter are visible LEDs. Fig. 2 shows a configuration of a VLC link and an VLC transmitter .The VLC transmitter is different from conventional communication transmitter in viewpoint that it must act as a communication transmitter and an illumination device simultaneously. Therefore, we must consider the following two requirements simultaneously.
LED CHARACTERISTICS
For appropriate VLC transmitter design, the LED characteristics needs to be understood.
The general characteristics of LED are well described in. Here, we focus on the high-brightness LED for visible wavelength range. There are two types of visible wavlength LEDs. One category is single color LED, for example, red ®, green (G), blue (B) LEDs. The other category is white LED, which uses phosphors for converting the emission wavelength from the original active area. Typically, red, green, and blue LEDs emits a band of spectrum, depending on the material system. Red LEDs emits the wavelength around 625 nm, green LEDs around 525 nm, and blue
OPTICAL INTERFERENCE NOISE
The noise sources in VLC system include the sunlight, the incandescent light and the fluorescent light. Moreira et al. measured the average background current for a couple of typical optical interferences .The background current was detected with a 0.85 cm2 silicon PIN photodiode in a differential structure.
Table 1 shows the measured background currents from 60 Watt incandescent bulb at 1 m
distance and from eight 36 Watt fluorescent lamps at 2.2 meters distance in a 5 m × 6 m room. From the Table, the background current of the sunlight is the largest one. Also, the background
STANDARDIZATION ACTIVITIES
In Japan, the visible light communication consortium (VLCC) is organized for collaboration
between industrial companies, universities, and research institutes (VLCC website). The VLCC member includes NEC corporation, Panasonic Electric Works, Nippon Signal, Toshiba corporation, Samsung Electronics, NTT DoCoMo, Casio Computer, Nakagawa Laboratories, Sumitomo Mitsui Construction, Sharp corporation, etc. The VLCC concentrates on activating technology exchange, system development, demonstration, and standardization of VLC inside Japan.
In Europe, the working group 5 of the wireless world research forum (WWRF) deals with
VLC technology as one of next-generation wireless access technology (WWRF website). The
WWRF has published a white paper on killer application of VLC, market forecast, and
technology roadmap.
In IEEE, 802.15 in IEEE 802 LMSC (LAN/MAN Standards Committee) has organized the
study group on VLC and the group is now the task group 7 (TG7) (TGVLC website).
CONCLUSION
In this chapter, the key ideas on visible light communication (VLC) have been reviewed in
relationship with optical wireless communication and infrared communication. The channel characteristics for VLC system were mentioned comparing it with infrared communication and the VLC transmitter and receiver are described including the basic characteristics of LED. Also, the considerable topics have been described including LED dimming, optical devices, and the effect of multiple wavelengths. The recent research results and standardization activities are summarized