Visible Light Communication- An Introduction

Wireless Communication has gone through several paradigm shifts and the idea of Visible Light Communication is nothing new. Fire and smoke signaling were used as a means of long-distance communications in ancient civilizations like the ancient Greeks, the Romans, Chinese and American Indians.

In the early 1800s, the US military used a wireless solar telegraph called “Heliograph” that signals using Morse code flashes of sunlight reflected by a mirror. The navy often uses blinking lights, i.e. Aldis lamps, to send messages also using Morse code from one ship to another. In 1880, the first example of VLC technology was demonstrated by Alexander Graham Bell with his “photophone” (shown in Fig. 1) that used sunlight reflected off a vibrating mirror and a selenium photocell to send voice on a light beam.

Fig. 1: Demonstration of Photophone

During the last ten years, the emergence of visible light communications (VLC) is witnessed. Along the EM spectrum, as the wavelength decreases, the frequency as well as the energy of the waves increases. The visible light band occupies the frequency range from 400 THz to 800 THz and the radio wave occupies the band from 3 kHz to 300 GHz. Radio Frequency (RF) has been the most widely used portion of the EM spectrum for communication purposes, mainly due to little interference in the frequency band and wide area coverage. With a rapidly decreasing RF spectrum and fueled by solid-state lighting (SSL) technology, VLC is emerging as analternative technology and a solution to overcome the overcrowded RF wireless communication technology. These SSL sources, being semiconductor devices, come with an additional feature. Their light intensity can be varied at very high speeds, and so their functionality can be extended utilizing intensity modulation (IM) to also become a wireless communication device.

In VLC, information is transmitted by modulating the intensity of an optical source operating in the visible range of the EM spectrum at a rate much faster than the response time of the human eye, which is effectively perceived as a steady glow. Typical VLC links use LOS configuration, due to its illumination purpose. Furthermore, lower path loss and dispersion over short distances gives way to higher bandwidth. LEDs emit incoherentlight, hence Intensity Modulation (IM) is done where the transmitted signal is modulated into the instantaneous optical power of the LED. Since IM changes instantaneous power of the LED, Direct Detection (DD) is the only feasible down conversion method. DD uses a photodiode to convert the incident optical signal power into a proportional current. The setup is far simpler than coherent detection used in RF, where a local oscillator is used to extract the baseband signal from the carrier.

Visible Light Communication (VLC) is still in the early stage. Some challenges and limitations needed to be solved. Modulation techniques for different data rates are standardized by IEEE 802.15.7. For low data rates of 11.67 kbps to 266.6 kbps PHY I type of VLC is offered. PHY II operates from 1.25 Mbps to 96 Mbps and PHY III operates between 12 Mbps to 96 Mbps. PHY I and PHY II are defined for the single light source and supports on-off keying (OOK) and variable pulse position modulation (VPPM). PHY III uses multiple optical sources with different frequencies (colors) and uses a particular modulation format known as color shift keying. Most indoor VLC so far uses PHY I type of VLC with OOK modulation scheme. Unfortunately, this modulation scheme has a lower data rate as stated earlier, and is susceptible to Intersymbol Interference (ISI). To reduce the effects of ISI, forward error correction methods are suggested and another approach to the solution of ISI and lower data rate problem is optical orthogonal frequency division multiplexing (OFDM). OFDM is resilient to the effect of ISI and provides a higher data rate. But, this higher data rate can be achieved at very high signal to noise ratio (SNR) values and the problem of high peak average power ratio (PAPR) prevails.

Apart from the challenges faced in modulation techniques required in VLC to maintain higher data rates, two more important challenges are there for communication using VLC. They are flicker mitigation and dimming support. Flicker refers to the fluctuation of the brightness of the light. Any potential flicker resulting from modulating the light sources for communication must be mitigated because flicker can cause noticeable, negative or harmful physical/psychological impact on humans. Thus, modulation process involved in VLC should not introduce any noticeable flicker.

 

Dimming support is another important consideration for VLC and was identified as one of the key challenges in VLC by IEEE 802.15.7 task group. Dimming methods can be of two types: analog and digital dimming. Depending on the methods it can be classified into two categories: modulation based dimming methods and coding based dimming methods. One of the most popular modulation schemes used in VLC is on-off keying (OOK), where binary bit ‘1’ or ‘0’ are represented by ‘ON’ or ‘OFF’ pulses respectively. OOK dimming is achieved by altering the ‘ON’ and ‘OFF’ levels of the OOK symbol. But, this scheme lowers the achievable bit rate as the light dims. Similarly, in pulse position modulation scheme dimming was implemented by designing variable-PPM (VPPM), which combines PPM and PWM. Though VPPM provides full dimming range to the VLC link the communication range is reduced due to low energy per bit low-intensity levels. Similarly, in coding dimming methods, coding schemes like inverse source coding (ISC), Reed-Muller (RM) code, and forward error coding (FEC). Although this coding technique can provide stable brightness it can only support specific dimming levels.

 

Fig. 2 Application of Indoor VLC in an office.

Prof. Sandip Das,
Head, Department of Electronics Engineering,
University of Engineering & Management (UEM), Jaipur