The automated electronic systems of the 'Smart Society' are delivering more comfort, convenience and security benefits wherever we go. Progress is astounding. A major challenge, though, is finding inexpensive ways to network the rapidly increasing number of remote sensors and other electronic system components that surround us. Connecting network nodes with wires is often too expensive, and creating networks by connecting devices wirelessly via radio wave links is becoming more difficult due to crowded spectrums and signal interference.
This fourth and final story in the EDGE series on wireless sensor networks explores a very promising alternative for wireless networking: visible light communication (VLC) technology. It describes and explains key points about this technology and then discusses two products developed by Renesas R&D engineers: a VLC receiver module and a VLC Evaluation Kit.
Communicating digital data reliably using visible light
Finding practical, inexpensive new ways to connect wireless sensors for implementing the Smart Society is essential, given the massive quantities of sensors and other control system elements needed in the near future. Today, wireless networking technologies mostly use electromagnetic (EM) waves. Radio waves are perhaps the most familiar type — they are used for Wi-Fi, for instance. But the EM spectrum is broad, so alternatives exist within government-specified spectrum allotments. Visible light is one of the most promising of these alternatives. It's the EM radiation we see with our eyes — electromagnetic waves whose wavelengths are within the visible range.
Figure 1, below, shows that the wavelengths of visible light extend from 380 to 780 nm (nanometers; 1 nm =10-9 m). The color of visible light varies according to its wavelength: the longest visible wavelengths are seen as red light, and successively shorter wavelengths are seen as orange, yellow, yellow-green, green, blue and violet, in that order; i.e., the seven colors of the rainbow. Visible lightwave communication (VLC) is point-to-point transmissions through the air and operates in the visible region of the spectrum generally unhampered by government restrictions.
As Figure 1 indicates, non-visible EM radiation just outside that visible range is called infrared (IR) light for wavelengths too long to see (above 740nm), and ultraviolet (UV) light for wavelengths too short to see (below 380nm). Infrared light is widely used in familiar devices such as TV remote controls because IR semiconductor lasers and light-emitting diodes (LEDs) are inexpensive and easy to work with. Ultraviolet light is not in general use, though, due to concerns that UV radiation might cause adverse health effects.
The terms 'VLC' and 'optical communication' should not be confused. VLC is wireless communication carried by visible light moving through space, as mentioned previously. By contrast, optical communication describes both wireless communication carried out by infrared transmission and wired communication implemented by visible light traveling through glass or plastic optical fibers.
Becoming a practical networking alternative offering multiple advantages
The VLC concept has been around for quite some time, but the recent spread of energy-saving LED lighting has brought new attention to the technology, encouraging investments in research and development. VLC offers a number of advantages relative to conventional electromagnetic communication:
- Existing LED lights can be used as transmitters. Digital signage, room lighting, traffic signals, and other existing illumination and information infrastructure elements can be used for implementing VLC.
- Transmission is quite secure because the communication range is very limited. Communication must always be carried out "visibly" and in the open.
- LED light sources are easy to acquire and require little maintenance.
- Communication functionality can be added to existing lighting equipment without comprising the aesthetic appearance of that equipment.
- Visible light does not pose a health threat.
- VLC is generally free from governmental restrictions. Japan's Radio Law does not apply to the wavelengths of visible light, and the same is generally true in most countries. Thus, companies building VLC systems don't need to obtain official approval for such products.
It must be acknowledged, however, that VLC technology presents multiple design challenges. Its range is limited (although this is a plus for security) and its performance can be degraded by dust or smoke in the air and reflections from shiny surfaces and objects. Problems caused by these issues can be minimized by careful designs and optimized networking installations.
Transmitting data with no noticeable changes in perceived illumination
VLC data transmitters turn a light source on and off at high speed — so fast that the eye cannot sense the switching, nor can it detect any change in color or intensity. The differences in illumination between the ON and OFF states of the light source correspond to the two binary values: Zero and One (see Figure 2), and data and messages are encoded digitally according to specific protocols. Transmission is serial and one-way only; i.e., it is simplex. A transmitter at one end of the communication link sends a modulated signal. Then a receiver at the other end detects the modulated light, converts it back to Zeros and Ones, and decodes the digital messages and data. To implement two-way (duplex) communication, devices at both ends of the network link must have both transmitters and receivers.
A VLC transmitter typically uses an LED that can quickly switch On and Off and vice versa. (Other types of light sources can also be used.) The transmitter also contains a power circuit, a drive circuit that turns the light source on and off, and a microcontroller (MCU) that controls the signal modulation. The main components of a VLC receiver are a sensor (a photo diode, etc.) that converts the incoming light into an electronic signal, an amplifier that boosts the sensor's output, and an MCU that detects and decodes the Zeros and Ones of the digital signal.
VLC transmitters and receivers can be standalone devices. However, as this technology gains wider use, it's likely that most often they will be built into products used every day. For instance, as Figure 2 shows, the transmitter can be an integral part of a light fixture or it can be built into the control circuit for the backlight of an LCD TV set. VLC receivers can be incorporated readily into smartphones or tablet computers. If duplex communication links are needed, the transmitter and receiver can be paired together in a transceiver module.
Implementing fast and slow data rates, sight unseen
VLC technology is mainly suitable for wireless networking in two types of applications: those that use slow data transmission (below 100kbps), and those that use much faster data rates (over 1Mbps). These are shown in Figure 3. That illustration also shows a third category: simultaneous multiple data communication, which can make good use of augmented reality(AR) technology.
Slow-transmission applications use a dedicated or general-purpose receiving terminal that takes in a fixed quantity of data. An example would be a system that receives location information and then indicates that location on a map displayed on the receiving terminal. Other examples include receivers that get coupons and promotional sales information from retail-store transmitters, and portable guide devices that give museum visitors audio and video information about the exhibit near where they are standing.
Fast-transmission VLC is considered a viable alternative for conventional wireless networking. Visible light technology will become increasingly attractive for this task as the radio wave and infrared portions of the wireless spectrum fill up, making available wireless bands scarce. VLC will be useful in factories where electrical interference renders conventional wireless networks unreliable, as well as in hospitals where radio wave links can disrupt life-support equipment. Additionally, because conventional wireless signals do not pass easily through water, VLC technology is a good candidate for short-range underwater voice and data communication.
Providing accurate location information for navigating within indoor spaces
One very promising VLC application area currently being studied by Renesas R&D engineers is semiconductors for navigation-type services that receive low-speed position data from VLC transmitters installed within buildings. Indoor VLC signals — from room lights and emergency lamps, for example — can be used together with outdoor GPS signals to provide seamless indoor/outdoor navigation. Accurate indoor position locating systems can help people better navigate office buildings and large industrial facilities, eliminating the need to stop and look at floor plans or maps. They can also help consumers find specific merchandise within grocery or department stores. These types of advanced navigation aids would be welcomed because they save time and reduce frustration.
Admittedly alternative indoor-navigation approaches currently exist. However, they are not without difficulties. To apply Wi-Fi technology, a dense arrangement of access points is needed, yet the location information provided isn't very precise. Indoor GPS-based navigation systems are also possible, but they require the costly installation of indoor base stations. VLC navigation systems, though, deliver very accurate location information. Locations can be pinpointed within a circle about 50cm in diameter, an accuracy of better than ±1 foot. Moreover, they can be implemented using existing lighting equipment. For indoor navigation, then, the visible light approach offers compelling advantages.
Offering proven solutions for VLC data transmission and reception
Renesas R&D engineers are creating optimized solutions for VLC implementations. In fact, they have recently produced two support products — a VLC receiver module and a VLC Evaluation Kit — that customers can use to research and analyze application opportunities and jump-start product designs.
Figure 4 shows the VLC receiver module. It incorporates a light sensor, a smart analog chip with automatic gain adjustment capability, and a low-power MCU. All of these components have been optimized for VLC use, and they apply techniques for reducing optical interference. Notably, the MCU runs special signal-recognition software that enables reception over a wide dynamic range.
Primary advantages of this VLC receiver module include the following:
- Largely impervious to optical noise — Communicates normally even when exposed to sunlight
- Wide dynamic range — Works with VLC luminance that ranges from several tens of lux up to over 10,000 lux
- Mostly unaffected by handshake — Readily handles sudden changes in luminance
- Low power consumption — Operates for over a year (several hours each day) on a single battery.
The photograph in Figure 5 shows how the receiver module is built into the USB device ('dongle') featured in the VLC Evaluation Kit that Renesas now offers. Besides the receiver, our EZ-0012 LED control board (containing an RL78/I1A MCU) that uses visible-light LEDs transmits ID information.
The VLC Evaluation Kit is designed to be very easy to use. After the included communication evaluation software is installed on a PC and the USB device is plugged in, customers can measure and analyze reception rates, error rates, and other evaluation parameters. The kit supports data transmission speeds up to 4.8 kbps.
Delivering CP-1223 compliant semiconductors for VLC applications
The VLC Evaluation Kit is built with high-performance Renesas products that incorporate our latest advancements in design, process and production technologies. Here are more details on the products incorporated into the USB transceiver board:
- RL78/I1A — a low-power MCU that implements precise LED lighting control
- EZ-0012 — a DC/DC LED evaluation board containing the RL78/I1A MCU
- SAIC500 — a smart analog IC whose circuit structure and performance characteristics can be set and changed by software
- RL78/G13 — a general-purpose MCU in the low-power, standard-performance RL78 family
All of these semiconductor products conform to the CP-1223 (VLC beacon system) technical specifications, which Renesas helped to establish under the auspices of Japan Electronics and Information Technology Industries Association (JEITA). Renesas was the first company capable of producing semiconductor implementations of CP-1223 compliant VLC solutions. (When this article was written, we were the only supplier of such components.)
Renesas continues to apply its powerful technological capabilities and experience to help customers take advantage of the tremendous global markets being opened by VLC technology. Industry interest in our development work and solutions is keen and rising. Enabling customers to develop innovative VLC products for consumer, business and industrial applications is an important corporate objective.
We welcome comments, questions and suggestions relating to VLC technology and its uses. To submit them, please contact us via the link below.