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MONDO ARC

Visual Light Communication

Issue 61 Jun / Jul 2011


Dr Geoff Archenhold delivers an update on the latest LED developments and explains why Visual Light Communication shows how we should think differently about LEDs and lighting.

One of the most frustrating things I see today with the general lighting industry is that it has somehow lost its ability to be truly creative, but it is really difficult to pinpoint the root cause. I have just returned from the 2011 LED Lighting Summit held in Berlin, which had an excellent range of speakers covering manufacturers, LED suppliers, lighting designers, construction and engineering companies, as well as end users such as Tesco’s. However, I felt the industry is still missing the point of new light sources such as LEDs. It was clear from several speakers that LEDs and OLEDs will become the dominant light source in the marketplace over the next five to eight years but the focus was still dominated by when conventional light sources such as halogen or metal halide will be displaced by LEDs and the huge potential of the retrofit market. Of course the retrofit market is a huge revenue opportunity for LEDs and quite rightly the large lighting companies will focus on them, but why should we just replace old sources with new ones in the same old packaging when there is a major opportunity to be much more creative with (O)LEDs in terms of design, human interaction, health and well-being and lamp formats.

This leads me to a new potentially exciting area, Visual Lighting Communications (VLC), for lighting that really shows what we could do as an industry with a pinch of innovation and new light sources such as LEDs and OLEDs. Admittedly, VLC is not widely used today in general lighting but in the future lighting could be used more as a means of sending data to your smart phone or connecting wirelessly to the Internet without the need for RF radiation. VLC is a beacon of hope that the lighting industry could start to look at LEDs as more than just a replacement light source for the CFL, Halogen, incandescent, metal halide and other plasma sources!

Visual Lighting Communications (VLC)
We have all heard about LED lighting but how many people have heard about LED communications?

Most of us are familiar with remote controls that used infra-red LEDs to transmit data to consumer devices such as TVs. However, these systems only transmit very low rate data which identifies the button which is being pressed. We are perhaps less familiar with the idea of VLC that actually uses the lighting infrastructure to transmit very high speed data. It will soon be possible to install smart lighting that will also provide all of the communications and control functions required within the building.

Data transmissions rates can achieve 10 Mbit/s at distances (from lamp to receiver) of over one mile. However Siemens, in collaboration with the Heinrich Hertz Institute in Berlin, has achieved a data transfer rate of up to 500 megabits per second (Mbit/s) using white LEDs at distances up to 5m. In the home or office, for example, VLC could represent a valuable addition to established Wireless LAN technology as shown in figure 1.



Figure 1: Typical VLC applications using LED lighting in the home

Increasingly, RF wireless networks are compromised by the fact that in many buildings the three independent WLAN frequency bands are multiply occupied, which leads to collisions among the data packets. A recent study showed the Internet connection speed within the home can be reduced by more than 30% when using a wireless broadband router due to interference and advice by equipment manufacturers is to reduce the number of other wireless devices in the home, such as baby monitors, TV remotes and cordless phones that also cause wireless interference. In a situation like this, visible light, as a currently unused and license-free medium, offers a suitable alternative.

Further VLC data transfer requirements exist within factory and medical environments such as hospitals (see figure 2), where in certain areas radio-borne transmission is either impossible or only a limited option. Additionally, applications exist in the field of transportation, where LED stoplights or railroad signals could be used to transmit information to cars or trains remotely.


Figure 2: VLC applications for LED lighting in the Hospital environment


Visual Light Communication offers many benefits over RF wireless communications including:
• Unlicensed spectrum, six orders of magnitude larger than RF, free and unregulated wavelengths;
• Short wavelength, harmless to humans and aesthetically pleasing;
• Little harm as visible and eye safe, little EMI compared to RF equipment;
•High gain antenna and high quality links.

Table 1 shows how RF wireless networks compare to LED lighting with optical communications. A key aspects of the LED method is the bandwidth speed and extra level of security, making it a much more secure means of transferring information between devices.
So how exactly does VLC work and what are the capabilities of this emerging technology?



Table 1: Comparisons between VLC and RF wireless communication methods. (Source: Boston University)

Solid state lighting based on LED technology enables an information signal to be superimposed onto an LED driver signal in order to carry data. This is a bit like the method used in radio broadcasts where an audio signal is superimposed onto the radio frequency carrier in order to transmit the information. In the case of the LED it is the wavelength of the light emitted that automatically provides the signal carrier and the intensity of the light is varied at high speed (modulated) to superimpose the data. At the receiver a photo-detector is used to remove the data from the optical signal.

Incandescent and fluorescent lamps cannot be modulated at high speeds. However an LED can be modulated at extreme rates, which cannot be detected by the human eye, but allows data to be transmitted at speeds well in excess of the data rates achievable using wireless LAN. It is therefore possible to turn every light bulb into a very high speed WiFi point.

This technology is being researched in a number of laboratories around the world and a communications standard (IEEE 802.15.7) is being developed. In the UK, Professor Harald Haas and fellow researchers at the University of Edinburgh’s Institute for Digital Communications have been working on the D-Light Project. This project has developed new modulation techniques to ensure the fastest data rates will be achieved from this technology in the future. The D-Light team are also looking to spin-out a company that will provide VLC products for use by the lighting industry.

Professor Haas believes the VLC technology not only provides speeds in excess of WiFi but the technology can be built at a much lower cost too. VLC can be created by just adding a digital processor circuit to make the technology work and it is entirely feasible to put a chip into every bulb in the future.

Dr Gordon Povey who is working with the University on creating spin-out activities has confirmed they have identified eleven distinct application areas for VLC technology. One of the big opportunities for VLC is in radio spectrum relief. The radio spectrum is already full but demand for wireless data doubles every year. The visible spectrum has massive amounts of capacity so it is logical to use this free and safe spectrum for communications as well as illumination.

How data is transmitted by VLC
There are a number of different methods that can be used to send data over the visible light spectrum and we discuss the main methods here.
On-off keying (OOK): As the name suggests the data is conveyed by turning the LED off and on. In its simplest form a digital ‘1’ is represented by the light ‘on’ state and a digital ‘0’ is represented by the light ‘off’ state. The beauty of this method is that it is really simple to generate and decode. However, this method is not optimal in terms of illumination control and data throughput.

Pulse width modulation (PWM): This method conveys information encoded into the duration of pulses. More than one bit of data can be conveyed within each pulse, but they may have to be longer pulses than for OOK, so there is no great advantage with this scheme. It is also possible to transmit data in an analogue format using this scheme which is also relatively simple to implement. 

Pulse position modulation (PPM): For PPM the data is encoded using the position of the pulse within a frame. Again more than one bit can be transmitted in each pulse, however the duration of the frame must be longer than for a single OOK bit, so again it is not necessarily more efficient. It does have the advantage of containing the same amount of optical energy within each frame.

Variable Pulse Position Modulation (VPPM):  This is similar to PPM but allows the pulse width to be controlled for light dimming support.
Pulse amplitude modulation (PAM): As the name suggests, the information is carried by the amplitude of the pulse. A number of data bits could be conveyed in a single pulse. e.g. off =00, 1/3 amplitude =01, 2/3 amplitude =10, full amplitude =11. In this example four different amplitude levels are used to carry two bits of information. PAM can carry more data in each pulse than OOK, but it is more complex and more susceptible to noise on the optical channel.

Colour shift keying (CSK): This can be used if the illumination system uses RGB type LEDs. By combining the different colours of light, the output data can be carried by the colour itself and so the intensity of the output can be constant. The disadvantage of this system is the complexity of both the transmitter and receiver.

Orthogonal frequency division multiplex (OFDM): This modulation scheme has been widely used for digital TV and radio and also for WiFi. It can be modified for use in optical communications. OFDM uses a set of sub-carriers each at different but harmonically related frequencies. There are a number of advantages including good spectral efficiency but this method is quite complex to implement. 

Spatial Modulation (SM): There are a number of techniques that allow one to determine the source of an optical signal. If one can determine its source one can either use the multiple sources of information to convey multiple stream of independent data (one from each source), or one can use the source of the signal as part of the information encoding itself. The multiple sources could be multiple LEDs within a single fixture.

VLC Standard (IEEE 802.15.7): Draft 4 of this standard was recently published. Three different operating modes were introduced; PHY I – up to 267kbit/s (OOK/VPPM), PHY II – up to 96 Mbit/s (OOK/VPPM) and PHY III – up to 96Mbit/s (CSK). As yet there are no products implementing this standard but there have been a number of large companies contributing towards this standard including Intel, Samsung and Siemens.

Worldwide interest in VLC development has attracted both academic and industrial interest as shown in Table 2.


Table 2: Main Visual Lighting Communication activities to date


VLC data rates
The implementation of the PHY II 96 Mbit/s standard would require an LED with 120 MHz bandwidth, which is currently unrealistic. Siemens have achieved 500Mbit/s using a specialist RCLEDs in laboratory conditions. The University of Edinburgh D-Light project uses OFDM with a standard single OSRAM Ostar phosphor coated LED with 17 MHz bandwidth and claim they can achieve 100 Mbit/s.

Implementing VLC within the Lighting Industry
The VLC advantages depend on the application, but considering VLC within lighting infrastructure is going to be very interesting. It has been suggested that fixtures would contain VLC technology enabling them to transmit and receive data at high data rates. If the fixtures are connected to the IP network via Ethernet cables or by power line communications, then the entire lighting infrastructure can communicate with any other IP device and VLC devices indoor or outdoor. For example, smart LED lighting at bus stops could be used to light up the bus shelter at night for safety reasons, while also delivering information to a mobile device, allowing commuters to obtain updates such as times for the next bus to arrive, detailed bus timetable and discount ticket vouchers.

Critical to commercial adoption of smart visual communication lighting is integration, miniaturisation, and packaging with conventional lighting standards, and the achievement of higher data rates and lower integration costs.

There are many questions to be asked about this emerging technology but if, as is claimed, it is cheaper to implement than equivalent radio communications technology, the lighting infrastructure might also become the communications infrastructure within a building which would represent an exciting opportunity for the lighting industry and create a completely new dimension to lighting.

Lighting companies could create a new business model that delivers increased revenues but more importantly offers an operational expenditure revenue model similar to utility companies based on communications revenues.

VLC is a new sub-sector within lighting enabled solely by the introduction of solid-state lighting technologies such as LEDs and OLEDs and this provides the lighting industry with an opportunity to think outside the normal space-time continuum.

I would like to thank both Professor Harald Haas and Dr Gordon Povey for their contribution, explanations and images of VLC applications.



LEDS FINALLY GO GREEN WITH ENVY

Green LED efficiency has long lagged behind the capabilities of red and blue devices, but researchers at the Rensselaer Polytechnic Institute (RPI) have reported significant improvements in their laboratories. The RPI team formed nanoscale patterns on a sapphire substrate that boosted light extraction, internal efficiency, and light output of green LEDs as shown.

More efficient green LEDs would benefit solid-state lighting (SSL) in general-illumination applications as equally efficient red, green, and blue (RGB) LEDs could yield very high efficiency white light for illumination. Indeed the DOE roadmaps predicts that RGB based white LED lighting will offer significantly higher efficiencies than blue LEDs coated with Phosphors but the high inefficiencies in green LEDs limit the efficacy of such systems currently.

Green LED inefficiency stems from a semiconductor physics phenomenon called the charge separation effect. In green LEDs, electrons and electron holes are separated in the quantum-well region of the device. Light is generated when electrons combine with electron holes, but the separation results in fewer such combinations in green LEDs. Researchers have struggled for years with the problem. The RPI team reported a doubling in the internal quantum efficiency of the green LED. Moreover, the design enhances light extraction by a factor of 58%. The result could be brighter green LEDs although it is unclear how soon the research could be applied in LED production but successful implementation could enable white LED lighting based on multiple colours to reach over 240 lm/W.

The rapid pace of LED research demonstrates there is still a huge amount of further performance to be delivered over the next five years which can only be good news for the lighting industry as energy costs spiral to unforeseen levels.


LED DROOP PROBLEM SOLVED
A really important technical breakthrough has happened for LEDs with researchers at the University of California, Santa Barbara, announcing they have figured out the cause of the problem known as LED droop which meant that as an LED’s forward current was increased the LED became less efficient and generated more heat. The LED droop phenomenon has typically meant that LEDs couldn’t operate over a wide range of forward currents. However the discussion on what physical phenomenon would cause the inefficiencies have been widely debated for many years with various research groups offering different perspectives.

LED droop, scientists conclude, can be attributed to Auger recombination, a process that occurs in semiconductors, in which three charge-carriers interact without giving off light. The researchers also discovered that indirect Auger effects, that involve a scattering mechanism, are significant—a finding that accounts for the discrepancy between the observed degree of droop and that predicted by other theoretical studies, which only accounted for direct Auger processes.

LED droop can’t be fully eliminated because Auger effects are intrinsic in LED material, but it could be minimised, the researchers say, by using thicker quantum wells in LEDs or growing devices along non-polar or semi-polar growth directions in order to keep carrier density low.

This development enables LED manufacturers to look for ways of minimising the droop effects and in recent years, Philips Lumileds have paved the way industrially by improving the forward current range of its LEDs without the need to increase the LED die size. This research will help engineers develop a new generation of high-performance, energy-efficient LED lighting products – so watch this space!


LED LIGHTING QUALITY DRASTICALLY IMPROVED IN 2011
Philips have recently launched a range of new LED emitters named the Luxeon S and A, both with precise correlated colour temperature control, so freeing the lighting fixture manufacturers from the need to bin LEDs as they deliver colour consistency with a single 3-step MacAdam ellipse. The Luxeon S emitter delivers uniform beam intensity and high flux density from a uniform source using 9 tightly packed LEDs (as shown in figure A) to enable a narrow beam control at the system level using secondary optics to provide a high centre beam intensity, uniform beam and crisp, single shadow required for high performance accent spotlighting applications.








Figure A: The 9 die LED Luxeon S package





The Luxeon S delivers a minimum of 1175 lumens at a CCT of 3000K with a typical CRI 85 at a forward current of 700mA and a junction temperature of 85C. The Luxeon S package has dimensions of only 14mm x 13mm x 6mm.

Philips also announced the Luxeon A LED, shown in figure B, that comes in 2700K and 3000K colour temperatures. Again all Luxeon A emitters fall within a single 3-step MacAdam ellipse on the black body curve as shown in figure C and the Luxeon A is the first single emitter that is part of the company’s Freedom From Binning programme. Luxeon A shares the LUXEON Rebel ES platform and footprint and incorporates a 2 square millimetre thin film flip chip and Lumiramic phosphor technology to deliver the highest quality of light with very high efficacy and light output. The Luxeon A will deliver more than 175 hot lumens at 700mA whilst exceeding a hot efficiency of 100 lm/W at 350mA with junction temperatures at 85ºC.



Figure B: The new Luxeon A LED package



Figure C: The three step MacAdam ellipse of the new Luxeon A and S LEDs


Cree’s new XM-L EasyWhite LEDs reduce the cost and complexity associated with binning and colour mixing, as well as using multiple discrete LEDs. This can enable customers to reduce the price and improve the performance of LED solutions for compact directional lighting. At just 4 watts of power, XM-L EasyWhite LEDs produce up to 340 lumens at an operating temperature of 85 C in warm white (3000K) in a single component. The LEDs are available in 2-step MacAdams Ellipse colour points. They are also available in either a 6V or a high voltage 12V configuration. XM-L EasyWhite LEDs have the same low profile dome and 5mm x 5mm footprint as the current XM-L LED family.
Cree has also decided to launch a range of LED arrays similar to Bridgelux, Sharp, and Citizen with the introduction of The Cree XLamp® CXA2011 LED array. The CXA2011 LED array delivers up to 4000 lumens at an operating temperature of 85 C (1A, 5000K) in a single component. It is available in 2 and 4 MacAdam step options ensuring colour consistency and does not reflow soldering as the arrays have top-side solder-pads, allowing for hand assembly or the use of modular holders. The CXA2011 also comes in multiple chromaticity choices of 2700 K, 3000 K, 3500 K, 4000 K and 5000K with up to 70 lm/watt system efficacy as shown in table A.



Table A: Typical applications and system efficacy of the CREE CXA2011

g.archenhold@mondiale.co.uk

 

Figure 1: Typical VLC applications using LED lighting in the home


  • Figure 2: VLC applications for LED lighting in the Hospital environment


  • Table 1: Comparisons between VLC and RF wireless communication methods. (Source: Boston University)


  • Table 2: Main Visual Lighting Communication activities to date

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