Visible Light Communication

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October 9, 2012 by Nakul

Author: Swetha Jose

 

Wireless technologies such as Wi-fi, Bluetooth, RFID and other standards opened up a range of new applications and have developed at phenomenal rate. We have become very much dependant on these technologies. In this era of information technology, the demand for high-speed wireless data is doubling every year, but the available RF spectrum is insufficient to keep up with the growing demand. The markets are flooded with mobile devices such as ipad, tablets, smartphones, etc that crave for more and more spectrum. Truth be told, when spectrum run short, mobile services can degrade and data speeds will slow down. So even with 3G on your smartphone, you could still be browsing at 2G speeds!

 

In a quest for better ways to transmit data, researchers have turned to a novel idea of using spectrum in the visible region for wireless communication. Using light to transmit data wirelessly?!

 

Using light to communicate is nothing new. Throughout history, there are reference to the use of fire signal, torches and beacons for signalling and relaying messages. The last few decades has seen the growth of optical fibre communication. But that’s a wired technology. The demand is for wireless data. We love everything that is wireless!  In the near future, we will see visible light communications systems deployed in our homes, colleges, offices, hospitals, airplanes, etc, providing us access to high speed wireless data access.

 

Visible light communication, in short, VLC is a technology that uses visible light, frequencies in the range 400THz to 800THz, to transmit data signals wirelessly. This new technology will not only solve the spectrum crisis, but will also open up a whole new range of exciting applications.  At present, VLC is limited to short and medium range point-to-point communication applications.

 

The driving force behind this technology is the progress of LED technology for lighting solutions. LED lamps are highly energy efficient, have long life of 100,000+ hours (35 years with 8 hours of usage per day!!), good colour quality, low maintenance and replacement costs. The last few years has seen remarkable improvements in the light output of LED bulbs and also in driver design and packaging. LED bulbs are certainly lighting up a much greener future!!

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Indoor-based VLC system (courtesy: http://www.wikid.eu/index.php/Visible_Light_Communication)

A basic indoor-based VLC system consists of a light source, preferably a LED bulb, free space as the propagation medium, and a light detector. Information, in the form of digital or analog signals, is input to electronic circuitry that modulates the light intensity at high speeds.  The variations in light intensity are so fast and subtle that our eyes cannot detect such changes and the room will still seem constantly lit. So the LED light source, whose primary function being illumination, could also transmit information simultaneously.

Modulation schemes such as on-off keying (OOK) for short range point-to-point communication or schemes such as orthogonal frequency-division multiplexing (OFDM) for high data rate transmission are preferred according to researchers. A photo detector is used at the receiving end to demodulate the information into electrical signals for further signal processing. So the data undergoes an electrical-to-optical-to electrical process, much like in optic fibre communications. An algorithm at the receiving end processes the data stream and converts it into the ones and zeroes of digital data.

Power over Ethernet (PoE), which is now growing in popularity, is ideal for VLC since power for lighting and data signal are all carried over a single cable. The power that can be provided by PoE has increased over the years. The PoE infrastructure could be utilised for lighting, control, wired and wireless communications within a building in the near future.

VLC throughput is increasing. The long term goal of researchers is to achieve a data rate of 1Gbps in the near future. That sounds exciting, doesn’t it??  This speed would mean downloading a full HD movie within a matter of few seconds.

In Japan, where this technology has its roots, the VLC consortium is organized for collaboration between industrial companies, universities, and research institutes.  IEEE 802.15.7 standard for short range wireless optical communication using visible light was published in 2010. There is growing interest not only in the academic circles. The industry has taken notice of this technology as they see in it enormous  potential. Seimens, a German company is primarily developing high-speed VLC links based on commercial LEDs, focusing on modulation techniques, as well as LED-driver circuitry and analog receivers. Siemens has been working with one of the brightest LEDs available commercially, made by its Osram subsidiary.

At the 2012 Consumer Electronics Show in Las Vegas, Casio demonstrated a pair of smartphones able to exchange data using light. These phones, as yet only prototypes, transmit digital signals by varying the intensity of the light given off from their screens.

Potential applications are numerous. In an airliner, internet service can be provided with integration of both wired, wireless and satellite technologies. An LED lamp above every passenger could not only allow internet access but could also transmit films on demand to individual seats, removing the need for lots of expensive and heavy cabling, thus saving airlines fuel.  In places where RF signals and radio communication systems are a threat due to interference problem, sparking, etc. such as in hospitals, oil factories, VLC systems can be employed for communication purposes.  New and better user-interfaces for controlling robots, machines and video games can emerge.

Using light to communicate has several advantages. Let me list a few. The visible spectrum has 10,000 times more bandwidth than RF spectrum. So we have huge unregulated (THz) bandwidth with license-free operation. The entire infrastructure being centred on light sources, that are highly energy efficient, makes VLC one the most economical ways of communication. Modulation and demodulation are direct and there are no radios or antennae. Most important of all, light waves are completely harmless unlike radio and microwaves.

If you’re wondering, why anyone didn’t think of this before…well actually Alexander Graham Bell had experimented with visible light communication way back in 1880. In fact, he used sunlight. He sent a voice message by modulating this light and used parabolic mirrors to capture and project his vocal vibrations! Now after 130 years, there is a promising future for visible light communications.

 

At TED Global 2011, Dr Harald Haas of Edinburgh University, fascinated the audience by transmitting high definition video from a standard LED lamp. The video is a must watch!

 

Some interesting websites are listed below for further reading.

http://visiblelightcomm.com

http://www.lificonsortium.org/

 

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