ICTON 2011 Tu.D5.2

978-1-4577-0882-4/11/$26.00 ©2011 IEEE 1

Exploring the Potentials of Optical-Wireless Communication using White LEDs

Klaus-Dieter Langer, Jelena Vučić, Christoph Kottke, Luz Fernández, Kai Habel, Anagnostis Paraskevopoulos, Michael Wendl and Veselin Markov

Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany Tel: (4930) 31002 457, Fax: (4930) 31002 250, e-mail: klaus-dieter.langer@hhi.fraunhofer.de

ABSTRACT This paper reviews the main experimental results achieved by our research group in the area of high-speed wire-less visible light communication (VLC). Depending on the modulation complexity and receiver sensitivity, various links operating at rates in the range of 10 – 800 Mbit/s have been demonstrated using off-the-shelf optical components. Keywords: Visible light communications, white LED, discrete-multitone modulation

1. INTRODUCTION In the past several years, communication via visible light (VLC) has been gaining attention in R&D, driven by progress on LED technology for solid-state lighting (380 – 750 nm). Novel high-power white LEDs are not only the most promising candidate for the future lighting market, but also offer potential for simultaneous illumina-tion and data transmission.

Due to reuse of the ubiquitous lighting (and signalling) infrastructure, VLC can provide an additional service at comparably low extra cost. Moreover, creating and perceiving a roughly directed VLC link between transmitter (Tx) and receiver (Rx) is easily achieved. Other features worth mentioning are the worldwide available and unlicensed spectrum, lack of interference with radio bands, and the potential for spatial reuse of the communication bandwidth in adjacent communication cells. Undoubtedly, since data transmission is tightly related to lighting, VLC is most attractive in permanently illuminated settings – like large offices, industrial or medical areas, public transport, but also museums, supermarkets, etc. It is widely believed that many applications for this type of optical communications are yet to be discovered.

In this paper, we review the main experimental results that our research group has achieved by extensive work on the VLC topic during the past several years. By considering off-the-shelf optical components, we have investigated the potentials of high-speed VLC. In various experimental efforts, analogue Ethernet-signal transmission as well as simple on-off keying (OOK) and advanced spectrally efficient discrete multitone (DMT) modulation (with off-line digital signal processing) have been applied in links according to Fig. 1 (a-c), respectively. Using these configurations, data rates in the range of 10 – 800 Mbit/s have been achieved. Moreover, a real-time DMT-based VLC demonstrator, broadcasting at 100 Mbit/s has been fully implemented within the FP7 Omega Project [1]. In order to achieve transmission of Ethernet frames, the system included customized MAC protocol functionalities, as outlined in Fig. 1(d).

As a principle, in all our experiments we assumed illuminance levels at the Rx, which are compliant to the lighting standard for working environment [2] and BER below the FEC limit [3]. Particular illuminance levels were obtained by setting appropriate wireless link lengths, depending on the optical source radiation characteris-tic. Note that a link length is scalable with the number of optical sources (LED luminaries) used.

LED driver

DMT

Lighting/Powersupply

Rx AMP

LEDLuminary

Photo-detector

b

AWG

MAC

PRBS

VLC channel

Ethernet Bitstream Electrical Signals

10BaseT

100BaseT

a

c

d

EthernetBitstreamElectrical Signals

10BaseT

100BaseT

a

dDMT

Scope c

b

MAC

BERT

Figure 1. VLC system configurations for transmission of (a) analogue signals, (b) OOK signals, (c) DMT

signals with off-line processing, (d) real-time DMT signals including a customized MAC layer.

ICTON 2011 Tu.D5.2

2

2. ANALOGUE ETHERNET-SIGNAL VLC DEMONSTRATOR We have developed a full duplex analogue transmission system, to demonstrate communication between a ceiling lamp and a mobile terminal placed at an office desk over a distance of about 1.5 m. Both communication directions were implemented according to Fig. 1(a), with a white LED serving as an optical source for downstream, and an infrared LED for upstream transmission. The light intensity of LEDs was directly modulated by an analogue 10BaseT Ethernet signal. The transmitter, consisting of driver and the LED, had a 10 dB bandwidth of ~54 MHz for white and ~77 MHz for infrared LED which was sufficient for the 10BaseT Ethernet signals. The transmitter bandwidth was mainly limited by the LED chips.

A wideband PIN photodiode was used as photodetector in combination with a plastic lens in order to enhance link robustness. One of the major challenges was to amplify the received signal to the level defined by the Ethernet standard, while keeping the noise sufficiently low [4]. The receiver had a 3-dB bandwidth of 37 MHz.

The demonstrated transmission system represents a simple wireless extension of a standard 10BaseT Ethernet link. Transmission of Fast Ethernet signals (125 Mbit/s), extending over a 42 MHz bandwidth, could be possible using the same electrical circuitry, but faster LEDs needed in this case, are currently not commercially available.

3. OOK-BASED EXPERIMENTS A block scheme encompassing the setups used in OOK experiments is given in Fig. 2. The same figure shows the experimental setups for the DMT-based experiments, which will be discussed in the following section. In all OOK-based experiments, a pattern generator was used to generate the OOK signal, and after detection at the receiver, BER was determined by an error counter.

First experimental demonstrations with high-speed OOK modulation were published in [5]. A low-power, phosphorescent white LED was used to achieve 40 Mbit/s. After being established as a promising technique in [6], blue filtering was applied at the Rx, in order to increase the modulation bandwidth. A PIN diode with an integrated transimpedance amplifier (TIA) served as detector.

Experimental demonstrations that followed used two types of commercially available high-power LEDs. In [7], an LED device consisting of 6 chips was used together with a custom-designed driver. The analogue receiver consisted of a blue filter, a large silicon PIN diode with a polymer lens and a low noise TIA. A data rate of about 125 Mbit/s was achieved.

In [8], we improved the achievable data rate to 230 Mbit/s by replacing the LED with a 4-chip device of a higher bandwidth and replacing the PIN diode with an avalanche photodiode (APD). Figure 3 presents the BER measurement results at two different illuminance levels. As a reference, plots for the same 4-chip device and a PIN diode are also shown.

AMP

AWG

out 2 out 1

dc

PC

opticalfilter

OSC

AMP coupler

RGB luminary

VLC channelR

chip

Gchip

Bchip

PD

LPF

AMPdc

dc

lens

dc

Tx (WDM + DMT)

AMPwhiteLEDTx

Rx

PC

PRBSgenerator

Error counter

Figure 2. Setups for both OOK- and DMT-based experiments.

AWG: arbitrary wave generator, LPF: low pass filter, OSC: oscilloscope

4. DMT-BASED EXPERIMENTS Due to a rather modest system bandwidth, transmission at high data rates requires a spectrally efficient type of modulation. In several of our experiments, we have deployed quadrature-amplitude-modulation (QAM) on dis-crete multitones (DMT). DMT technique is known from DSL; in radio systems it is known as orthogonal fre-quency division multiplex (OFDM). A number of orthogonal electrical subcarriers carrying QAM data are placed in the spectrum quite close to each other without interference, so that more information can be transmitted over a limited bandwidth. Apart from high spectral efficiency, DMT inherently deals with inter symbol interference (in dispersive channels), allows for simple equalization at the receiver, and can be realized entirely with digital signal processing.

Another DMT property, which proved as very valuable for VLC channels, is the possibility for bit- and/or power-loading. Constellation size can be chosen flexible on individual subcarriers, depending on the channel

ICTON 2011 Tu.D5.2

3

characteristic at particular frequencies. Similarly, the signal power can be distributed over the subcarriers as convenient, all leading to efficient use of the channel capacity.

50 75 100 125 150 175 200 225 25010

-8

10-6

10-4

10-2

100

Bit rate [Mbit/s]

BER

BER = 2·10-3

FEC limit

APD-based RxPIN-based Rx

~1000 lx illuminance @ Rx

~400 lx illuminance @ Rx

time-axis scaling 1ns/div

Figure 3. BER measurement with OOK, for two Rx structures and two illuminance levels at Rx.

4.1 500 Mbit/s transmission using an APD Concerning the practical data rates achievable in DMT-based VLC links, some time ago we demonstrated 230 Mbit/s with a phosphorescent white LED luminary, a low-cost PIN photodiode and off-line DSP [9]. In our most recent effort towards capacity increase, we achieved a link operating at 513 Mbit/s by deploying an APD instead [10]. Moreover, this unprecedented high data rate resulted from the efficient exploitation of the analogue bandwidth available, by a combination of DMT, multi-level modulation (QAM), bit- and power-loading, and symmetrical clipping. As in [9], digital signal processing was performed off-line with an arbitrary wave genera-tor and a storage oscilloscope. Optical front-ends were the same as in [8] with an APD at Rx.

In the measurements, we applied a bit- and power-loading algorithm proposed for DSL [11]. Based on the knowledge of the previously measured E-O-E channel, the algorithm determined the QAM-orders and powers of individual subcarriers, leading to a maximized link transmission rate. Hereby, it complied with the constraints 1) on the overall DMT signal power and, 2) on the maximum BER of 10-3. Such BER target was chosen in order to allow a favourable margin in the design for the clipping induced errors before the BER limit of 2⋅10-3 was reached (FEC limit).

Figure 4 depicts the optimal bit-loading mask applied in the measurements. It shows N = 128 subcarriers within the baseband bandwidth of B = 100 MHz (much larger than the analogue system bandwidth, which was about 35 MHz). The subcarrier n is carrying nR bits per DMT symbol, resulting in an overall gross data rate

( ) 1

1

Nnn

R B N R−

== ∑ .

The LED nonlinearity was denoted as one of the main factors limiting further data rate enhancement. Assum-ing the same noise level at the receiver, additional simulations were performed for a linear system model. Bit distribution over one DMT symbol for this case is included in Fig. 4. Simulations predicted that compensating for the LED nonlinearities would enhance the data rate to 600+ Mbit/s. Going even further, a theoretical maxi-mum throughput (capacity) of 750+ Mbit/s was derived for the same linear system by application of the well-known water-filling method. Note that water-filling distributes the information continuously over the subcarriers, opposed to practical systems, which handle with bits.

0 20 40 60 80 100 1200

5

10

Subcarrier index

Info

rmat

ion

(bit

)di

stri

buti

on [b

it/s

/Hz]

Measurements(R=513 Mbit/s)Simulations(R=604 Mbit/s)Capacity upper bound(C=757 Mbit/s)

Figure 4. Performance comparison of the experimental system (measurements) and its linear system model

(simulations), on condition that BER ≤ 2⋅10-3. Optimal loading masks applied in measurements and simulations.

4.2 800 Mbit/s WDM transmission using an APD Whereas the phosphorescent white LEDs (a blue chip plus phosphor layer) are the simpler variant, white LEDs consisting of red, green and blue chips (i.e. RGB LEDs) allow the possibility for wavelength division multiplex-ing (WDM) in VLC links. An RGB LED emits at peak wavelengths of 470, 530, and 700 nm for blue, green, and

ICTON 2011 Tu.D5.2

4

red respectively. WDM feature offers service or user separation as well as potential increase in overall data rate. In the most recent publication [12], we have investigated the potential of this LED type, demonstrating a rate of 803 Mbit/s.

The experimental setup is also included in Fig. 2. Off-line pre-calculated DMT signals for the three WDM channels were loaded on an AWG. Due to a limitation of two AWG outputs, the WDM colour under test was driven by the first while the remaining two colours were driven by the second output. The optical frontend of the receiver contained optical filters to separate the WDM channels and a glass lens to focus the light onto the large-area APD. Electrical amplifiers have been used to bring the signal to an appropriate level for the storage oscillo-scope. The recorded time traces of the received signal have been processed off-line.

DMT subcarrier index, n

Red LED chip

Blue LED chipGreen LED chip

1 5 10 15 20 25 31

Bit-

load

ing

mas

k, R

n[b

it / s

ubca

rrie

r]

0

2

4

6

8

10

Figure 5. Bit-loading mask for each WDM channel.

Each DMT signal consisted of N = 32 subcarriers within a baseband bandwidth of 50 MHz. The reduction to 50 Hz compared to 100 MHz for the phosphorescent LED (from the previous section) is mainly caused by the reduced analogue bandwidth. Each channel with the RGB LED had a bandwidth of about 15 MHz, whereas the use phosphorescent LED resulted in 35 MHz. The bit-loading and the power levels for individual subcarriers have been adapted to the channel quality, similarly as in Section 4.1. The optimal loading masks are given in Fig. 5. These loading masks resulted in bit-rates of ~294, ~223 and ~286 Mbit/s for the red, green, and blue channel, respectively, giving an aggregate rate of ~803 Mbit/s. The total BER for each WDM channel was below the FEC-threshold of 2⋅10-3.

To reach 1 Gbit/s, the WDM transmission system could be further improved by selecting a RGB-LED with higher 3-dB-bandwidth and by adapting the optical filter to the respective WDM source.

5. REAL-TIME DMT-BASED VLC DEMONSTRATOR Within the FP7 OMEGA project, a real-time DMT-based VLC system running at 100 Mbit/s was implemented. The system broadcasts up to 4 HD video streams (about 20 Mbit/s each) from 16 LED ceiling lamps to a photo detector placed within the lit area of ~10 m2 [13]. An illustrative photo is presented in Fig. 6.

A MAC protocol (Optical Wireless MAC, OWMAC) developed especially for this purpose, and digital signal processing functionalities for the PHY layer, were implemented on FPGA boards. The PHY processing encom-passed a typical DMT modulator/demodulator including a scrambler and an FEC encoder/decoder with parame-ters summarized in Table 1. The physical layer also featured full synchronization on bit and frame levels.

Table 1. Digital PHY parameters.

Parameter Value Signal bandwidth (MHz) 30.5947 Number of subcarriers (incl. DC) 32

Length of cyclic prefix in samples 4

QAM order 16 FEC Reed Solomon (187,207)

Figure 6. OMEGA VLC demonstrator.

After passing through a DAC and an amplifier, the DMT signal intensity-modulated the LED-based luminaries. Synchronous modulation of the luminaries was achieved by means of cabling, i.e. an electrical signal distribution network. The Rx analogue front-end consisted of imaging optics, a colour filter, a photodiode and

Ceiling lighting (Tx)

analogue Rx unit

Received video streams

ICTON 2011 Tu.D5.2

5

a transimpedance amplifier. The analogue signal was then converted to digital by an ADC, and further processed in the PHY FPGA-board, and consequently in the OWMAC board.

The overhead on the physical layer, introduced by cyclic prefix, FEC, training sequence and synchronization, was ~15%.

6. CONCLUSIONS AND OUTLOOK In this paper, we reviewed our main experimental results achieved over the past years in the area of high-speed visible-light communications. Depending on the hardware and modulation complexity, various links providing transmission rates in the range of 10 – 800 Mbit/s have been demonstrated using off-the-shelf optical components. Depending on targeted bit rate, the system design was strongly influenced by the available modulation bandwidth. Bit rates of up to some Mbit/s were achieved with rather simple and low-cost designs. Implementation of a specialized PHY or MAC layer (including FEC) provides significant increase in robustness and system margin.

For data rates up to 100 Mbit/s, standard OOK is quite sufficient. Advanced modulation schemes such as DMT are spectrally efficient and allow modulation far beyond the analogue system 3-dB-point by application of bit- and power-loading. Using these techniques in lab experiments, we have shown data rates of 500 Mbit/s, or rather 800 Mbit/s when the WDM property of the RGB LED is exploited.

A complete system, broadcasting at 100 Mbit/s (net!) has been demonstrated in a real application environment using DMT. One of the challenges in the system was the design of the LED driving circuitry, which was to enable both decent exploitation of the LED modulation bandwidth and acceptable efficiency with respect to power dissipation. DSP for modulation and demodulation is similar to those used e.g. in DSL technology. Thus, low-cost single-chip solutions are feasible without any doubt.

The VLC technology is completely applicable to infrared LEDs, which exhibit quite similar modulation char-acteristics as colour LEDs (possibly of higher bandwidths). In most cases, the uplink of bidirectional systems with visible downlink is assumed to use infrared light.

Occasionally, depending on the application, the modulation must comply with standard dimming schemes. This aspect was not considered in our work. Regarding further potential for improvements, e.g. dealing with low illuminance levels and partial shading, it would be useful to apply dynamic data rate adaptation and thus bring in more robustness in the system.

Potential VLC applications range from low-speed broadcast (indoor and outdoor, for the purpose of position-ing, information, alerts, system control etc.), to high-speed such as video transmission in operation theatres or cellular communications for manufacturing (e.g. testing of products during assembly on a conveyor).

As far as standardisation of VLC is considered, it is worth mentioning that besides existing standards of IrDA and Visible Light Communications Consortium (VLCC), IEEE has finished a completely new draft standard [14].

ACKNOWLEDGEMENTS The work resulting in the ideas presented here received partial funding from the European Commission’s seventh Framework Programme FP7/2007-2013 under grant agreement № 213311, also referred to as OMEGA. The authors would like to thank their partners from the OMEGA VLC team for fruitful collaboration.

REFERENCES [1] http://www.ict-omega.eu/ [2] European Standard EN 12464-1: Lighting of indoor work places, 2003. [3] ITU-T Recommendation G.975.1 (02/2004). [4] R.G. Smith, S.D. Personick: Receiver design of optical fiber communications systems, in Semiconductor Device for Optical

Communication, H. Kressel, Ed., New York: Springer-Verlag, 1980. [5] J. Grubor, S.C.J. Lee, K.-D. Langer, T. Koonen, J. Walewski: Wireless high-speed data transmission with phosphorescent white-light

LEDs, in Proc. ECOC 2007, Vol. 6, post-deadline paper PD3.6. [6] J. Grubor, O.C. Gaete Jamett, J.W. Walewski, S. Randel, K.-D. Langer: High-speed wireless indoor communication via visible light,

in ITG Fachbericht 198, pp. 203-208, 2007. [7] J. Vučić, et al.: 125 Mbit/s over 5 m wireless distance by use of OOK-modulated phosphorescent white LEDs, in Proc. ECOC 2009,

paper 9.6.4. [8] J. Vučić, et al.: 230 Mbit/s via a wireless visible-light link based on OOK modulation of phosphorescent white LEDs, in Proc.

OFC/NFOEC, 2010, paper OThH3. [9] J. Vučić, et al.: White light wireless transmission at 200+ Mbit/s net data rate by use of discrete-multitone modulation, IEEE PTL,

vol. 21, pp. 1511-1513, 2009. [10] J. Vučić, C. Kottke, S. Nerreter, K.-D. Langer, J.W. Walewski: 513 Mbit/s visible light communications link based on DMT-

modulation of a white LED, J. Lightwave Technol., vol. 28, pp. 3512-3518, 2010. [11] B.S. Krongold, et al.: Computationally efficient optimal power allocation algorithms for multicarrier communication systems, IEEE

Trans. on Communications, vol. 48, pp. 23-27, 2000. [12] J. Vučić, C. Kottke, K. Habel, and K.-D. Langer: 803 Mbit/s visible light WDM link based on DMT modulation of a single RGB LED

luminary, in Proc. OFC 2011, paper OWB6. [13] http://www.youtube.com/watch?v=AqdARFZd_78 [14] http://standards.ieee.org/develop/project/802.15.7.html