1. Organic-based Visible Light Communications
Dr Hoa Le Minh, Prof. Zabih Ghassemlooy
Optical Communications Research Group
(School of CEIS, Northumbria University, UK)
TU Graz, Austria – 11th May 2012

2. Outline Organic light emitting diode (OLED) OLED-based VLC
Visible light communications
Organic light emitting diode (OLED)
OLED-based VLC
Challenges and possibilities discussion

3. Visible Light Communications
Ancient time: light was used for medium-range signalling

4. Visible Light Communications
Modern time: light is used for high speed communications

5. General Light Sources Incandescent bulb First industrial light source
5% warm light, 95% heat
Few thousand hours of life
Fluorescent lamp
White light, cheap
25% light
Lifetime ~10,000s hours
Solid-state light emitting diode (LED)
Compact, cheap, powerful
50% light
More than 50,000 hours lifespan
Organic light emitting diode (OLED)
Flexible and bendable panel
Extensively used in high-end display products,
HDTV and Smartphone

6. LED / OLED Devices RGB Blue chip + Phosphor OLED Emerging technology
Early stage of development
High potentials
Well-known technology
Limited use due to difficulties in RGB balancing
Phasing out in lighting industry
Popular for today general lighting industry
Standardised for illumination and communications

7. OLED Current State-of-Art
Efficiency
100% internal quantum efficiency (Fraunhofer IPMS – COMEDD, 2012)
Brightness cd/m², 5mm thickness (Verbatim Velve, 2012)
120 lumen (~table lamp) (Philip Lumiblade GL350, 2012)
80 lumen/watt with hours of lifetime (LG, 2012)

8. OLED Current State-of-Art
Applications
Dominant in high end Smartphone display products: Super-AMOLED)
(Samsung Galaxy S3 phone, 2012)
55 inch OLED HDTV (Samsung Electronics, 2012)
6 inch E-paper on plastic (XGA, 14 gram, 0.7mm thickness), (LG, 2012)
Solar OLED car (BASF, 2012)
Flexible AMOLED display (Samsung patent, 2012)
None of the commercial applications is for communications!

9. Communications Modulation bandwidth?
Measured frequency response of
(Philips) Luxeon-star white LED
Measured frequency response of
(Philips) Lumiblade white OLED
1. Why OLED modulation bandwidth is too narrow?
2. How to improve the OLED bandwidth?

10. OLED Structure Typical structure:
Glass (filled with inert gas to protect other layers)
Anode/Hole transport layer (HTL)
Organic emitting layers (to control emissive colours)
Including organic compounds
4. Electron transport layer (ETL)
5. Cathode (typically indium tin oxide (ITO))
Thin film technology:
OLED layers ~1-200 nm

11. Electrical Characterisation
For lighting
Large panel  better for illumination
 larger capacitor value
For communications
Larger capacitor value  slow response
Rp - electrode contact resistance
Rd - diode resistance
C - diode capacitance

12. Bandwidth Improvement
Bandwidth equalisation (Analogue)
Digital filtering
Complex modulation

13. Equalisation (First order).
Equalisation (First order)
First order equaliser experimental test-bed
𝐻 𝜔 = 1 𝑘 × 1+𝑗𝜔𝑇 1+𝑗𝜔 𝑇 𝑘
Simple implementation
1st order response (linear)
Cost effective
T=𝑅𝑒𝑞𝐶𝑒𝑞
𝑯 𝝎 = 𝟏 𝒌 × 𝟏+ 𝝎 𝟐 𝑻 𝟐 𝟏+ 𝝎 𝟐 𝑻 𝒌 𝟐
1 𝑘 = 𝑅 𝐿 𝑅 𝑒𝑞 +𝑅 𝐿
H. Le-Minh, D. C. O'Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y. Oh, "100-Mbit/s NRZ Visible Light Communications Using a Post-Equalized White LED", IEEE Photonics Technology Letters, vol. 21, no. 15, pp , 2009

14. Equalisation – BW Improvement.
Equalisation – BW Improvement
Philip Lumiblade OLED
~£70
Map of frequency response corresponding to different equalisers
H. Le Minh, Z. Ghassemlooy, A. Burton and P. A. Haigh, "Equalization for Organic Light Emitting Diodes in Visible Light Communications" IEEE GLOBECOM, Workshop on Optical Wireless Communications in Houston, USA, 5-9 December, 2011
H. Le Minh, Z. Ghassemlooy, A. Burton, P. A. Haigh, and S.-K. Liaw, "Bandwidth Improvement for Organic Light Emitting Diodes Based Visible Light Communications", IEEE Communications Letters, 2012 (submit)

15. Equalisation – BER performance.
Equalisation – BER performance
Measurement/Simulation – 2Mbps NRZ at 400 lux
Improved modulation bandwidth
(experiment)
Experiment – Issue with baseline wandering
 Need to optimise Tx/Rx circuitry
Impulse response

16. Decision Feedback Equalisation.
Decision Feedback Equalisation
Osram Orbeos OLED
£85
DFE: widely used in digital systems transmitting through BW-limited AWGN channels
Better performance than ZF and MMSE-based filter
𝒛 𝒎 = 𝒏=𝟎 𝑵 𝟏 𝒄 𝒏 𝒚 𝝁𝑻−𝒏𝝉 − 𝒏=𝟏 𝑵 𝟐 𝒃 𝒏 𝒂 𝒎−𝒏
𝑦 𝜇𝑇−𝑛𝜏 is the sampled incoming signal
μT is the μth sample of the bit period T
zm is the estimated output signal

17. Least Mean Square (LMS).
DFE - Setup
Parameter
Value
Data format
OOK-NRZ
PRBS length
2^10 - 1
Number of feed-forward taps 18
Number of feed-back taps 9
Algorithm
Least Mean Square (LMS)
Algorithm step size
0.03

18. .
DFE - Results
Unequalised and baseline-wandered RC equaliser’s BER performance
DFE’s BER performance
Measured BER vs. Bandwidth at different illumination level (lux)
A. Burton, P. A. Haigh, H. Le Minh, Z. Ghassemlooy, S. Rajbhandari and S. K. Liaw, "A Comparative Investigation Study of Modulation and Equalization Techniques for White-Light Emitting Organic Light Emitting Diodes Using in Visible Light Communications", IEEE Communications Magazine, 2012 (submitted)

19. .
Complex Modulation
Multiple carrier modulation: Orthogonal Frequency Division Multiplexing
Carriers are orthogonal to each others
Each carrier is modulated by QAM, PSK etc.
Equalisation in small band of modulation bandwidth is feasible

20. OFDM Carrier is orthogonal.
OFDM
OLED OFDM/QAM-based VLC system
Carrier is orthogonal
1 𝑇 𝑠𝑦𝑚 0 𝑇 𝑠𝑦𝑚 𝑒 𝑗2𝜋 𝑓 𝑘 𝑡 𝑒 −2𝜋 𝑓 𝑖 𝑡 𝑑𝑡 = 1, &∀ 𝑘=𝑖 0, otherwise
Data is mapped to QAM signal
A cyclic prefix (CP ) is added to protect from multipath effects  
A. Burton, P. A. Haigh, H. Le Minh, Z. Ghassemlooy, S. Rajbhandari and S. K. Liaw, "A Comparative Investigation Study of Modulation and Equalization Techniques for White-Light Emitting Organic Light Emitting Diodes Using in Visible Light Communications", IEEE Communications Magazine, 2012 (submitted)
P. A. Haigh, et. al., "Exploiting Equalization Techniques for Improving Data Rates in Organic Devices for Visible Light Communications", IEEE Journal of Lightwave Technology, 2012 (submitted)

21. OFDM @ ~400 lux 64 OFDM carriers, 16-QAM 1 Mbit/s 3 Mbit/s 5 Mbit/s.
OFDM
64 OFDM carriers, 16-QAM
1 Mbit/s
3 Mbit/s
5 Mbit/s
@ ~400 lux
Osram Orbeos OLED

22. OFDM 64 OFDM carriers, 16-QAM, raw OLED bandwidth 93 kHz.
OFDM
64 OFDM carriers, 16-QAM, raw OLED bandwidth 93 kHz
Measured BER performance (no FEC) of OLED at ~400 lux

23. .
Challenges
OLED is under development, therefore challenges are widely expected from
Materials and device structures are being evolved and varied from different manufacturers
Heavily calibrated for display purpose (unlike LED used for signalling and illumination)
In the early stage of lighting and decoration utilisation
Expensive (~10/20 times costlier than the same performing LED)
Lack of wide range of commercially available products
Communications aspects
Light efficiency is low  large illumination panels are typically fabricated  high capacitance thus limiting the device modulation bandwidth (100’s kHz)
Limited researches in data communications
Not yet being standardised

24. Possibilities and Potentials.
Possibilities and Potentials
Possibilities and Future Work
Achieving higher data rate, such as Mbit/s, so that OLED can be adopted in standard 10BASE-T Ethernet communications
Working with the manufacturers to improve the device response time (newer display has faster response and wider dynamic contrast range)
Device modelling and characterisation to optimise the performance
Possible to adopt the existing VLC standard (IEEE /16)
FEC inclusion
Potentials and Opportunity
OLED is more available in many displays, tablets and phones  new areas of short-range and personal VLC applications and researches
Toward mobile and flexible VLC
Environmental friendly  potentially to be adopted in wide range of VLC

25. Acknowledgement OCRG’s OLED / VLC team Paul A. Haigh Andrew Burton
Dr. Sujan Rajbhandari
Prof. Erich Leitgeb
Institute Hochfrequenztechnik, TU Graz

26. Thank you