Project: IEEE P802.15 Working Group for Wireless Personal Area Networks N (WPANs( WPANs) Title: [Some challenges for visible light communications] Date Submitted: [Revised version July 24 th 2008] Source: [Dominic O Brien] Company [University of Oxford] Address [Parks Road, Oxford, OX1 3PJ UK] Voice:[+441865273916], FAX: [+441865273906], E-Mail:[dominic.obrien@eng.ox.ac.uk] Abstract: [VLC has a number of technical challenges, which are discussed in the presentation] Purpose: [Informing those interested in VLC of some of the technical challenges faced Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.
Some Challenges for Visible Light Communications Dominic O Brien Lubin Zeng Hoa Le Minh Grahame Faulkner Department of Engineering Science, University of Oxford
Introduction Typical VLC link characteristics Challenges Technical Bandwidth limitations Providing an uplink Regulatory Compatibility with Lighting Control systems Illumination systems Conclusions
Typical link characteristics Source Channel Receiver
LED Modulation Opto-electronic response doc.: IEEE 802.15-15-08-0131-01-vlc R s L V Vd :KLWHUHVSRQVH %OXHUHVSRQVH I SPICE Model Luxeon LED C d C s R s = 0.9727 Ω L = 33.342 nh C s = 2.8 nf C d = 2.567 nf tt = 1.09 ns % G H V Q R S V UH H WLY OD H 5 IUHT 0+] Measured LED small-signal bandwidth 3
Improvement of LED Response Using blue-response only (blue filtering) ~130 ns G H OLV D U P R Q LW\ V Q WH,Q :DYHOHQJWK QP Blue filtering ~25 ns Measured optical spectrum Measured impulse response Issue: Only 10% of signal power is recovered Reducing SNR, link distance LEDs with more blue energy [1] could be used to gain more filtered power, however the balance of white colour is shifted [1] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. ECOC 07 (PDS 3.6), pp. 1-2. ECO [06.11], 16-20 Sep. 2007, Berlin, Germany 4
VLC Channel doc.: IEEE 802.15-15-08-0131-01-vlc 8
Room Power Distribution doc.: IEEE 802.15-15-08-0131-01-vlc Assume 1% modulation of typical illumination power Typical receiver performance Conclusions Very high SNR available SNRmin = 38.50dB SNRmax = 49.41dB Modulation limited by source bandwidth 9
Optical Receiver doc.: IEEE 802.15-15-08-0131-01-vlc Receiver consists of Optical filter Rejects out-of-band ambient illumination noise Lens system or concentrator Collects and focuses radiation Photodetector (or array of detectors) Converts optical power to photocurrent Incoherent detection Preamplifier (or number of preamplifiers) Determines system noise performance Post-amplifier and subsequent processing Input radiation Optical filter Optical system Photodetector Amplifier Output 11
Optical Receiver: Constant Radiance Theorem Optical gain of receiver limited by required field of view A i Ω i A i Ω i <=A o Ω ο A i Ω i <=A o 2π Ω ο A o 12
Receiver Performance: Figure of Merit Receiver Figure of Merit (FOM) Fibre systems Performance determined by sensitivity (given sufficient detector area) FOV usually not relevant Free space systems Etendue crucial determinant FOM = 2πR P b min A Detector Area A Receiver sensitivity Pmin Field of view 2π Sr Bit rate R b 13
Improving data rate: equalisation Transmitter equalisation High bandwidth Energy efficiency Blue filtering Lose low frequency energy from phosphor Receiver Simple analogue equalisation More complex also
Typical waveforms for RX equalisation 1.2 1 Recovered data transmitted data 1.2 1 Recovered data transmitted data 0.8 0.8 0.6 0.6 Signal 0.4 Signal 0.4 0.2 0.2 0 0-0.2 0 500 1000 1500 2000 2500 3000 3500 time(ns) -0.2 0 500 1000 1500 2000 2500 3000 3500 4000 time(ns) Data rate 33Mb/s Data rate 14Mb/s NRZ data Manchester data
Bandwidth Improvement: Post Equalisation Pre- and post-equalization: single LED link Pre-equalisation: experiment Post-equalisation: simulation 17
Improving data rate: complex modulation High SNR channel Complex modulation attractive OFDM 100Mb/s over 20MHz channel [1] PAM Simulations show LED characteristics not optimal [1] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. ECOC 07 (PDS 3.6), pp. 1-2. ECO [06.11], 16-20 Sep. 2007, Berlin, Germany
Improving data rate: PAM Simulation uses measured LED impulse response Simple 1 st order RX equaliser 4-PAM 24Mb/s (33Mb/s NRZ) Signal 1.2 1 0.8 0.6 0.4 Recovered data transmitted data 0.2 0 Further work required -0.2 0 2000 4000 6000 8000 10000 time(ns) Data rate 24Mb/s (4-PAM)
Improving data rate: MIMO Parallel alignment free data links Simulations show linear capacity growth Experimental results for a simple IR system Simulations of in-room VLC system
Simple IR system Normalised Amplitude 0.5 Channel 1 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 x 10-6 1x2 Laser array 3x3 photodiode array Experimental system Normalised Amplitude 1 0.5 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 x 10-6 Recovered data Transmitted data x Channel 2
MIMO VLC: Simulation System 24
MIMO VLC: Preliminary Results Position of the receiver Aggregate data rate is linearly proportional to the number of channels and channel rate 25
Providing an uplink VLC good at broadcast Uplink difficult to achieve Retro-reflectors Low speed Low cost IR uplink Separate system Infrastructure complex and expensive
Retro-Reflecting Link doc.: IEEE 802.15-15-08-0131-01-vlc Novel optical communications between reader and tag Low power (tag has no source) Long range (determined by illumination source ) Visibly secure (user can see beam of light) Illuminating Source Beamsplitter θ Tag Reader Receiver Retroreflecting Transceiver showing angle of rotation 18
Cooperative communications Base station RF transceiver VLC transmitter RF communications RF Optical communications RF transceiver 1 VLC receive r RF transceiver communications VLC receiver Terminal Terminal Terminal outside hotspot Terminal within hotspot O'Brien, D.C.: Cooperation and cognition in optical wireless communications, in Fitzek, M.K.a.F. (Ed.): Cognitive Wireless Networks: Concepts, Methodologies and Visions - Inspiring the Age of Enlightenment of Wireless Communications - (Springer, 2007)
Providing an uplink: Cooperative systems Combine VLC with RF Optical downlink only RF uplink/downlink 100Mb/s downlink/10mb/s RF LAN Fuzzy logic decision making Typical traffic asymmetry Significant performance benefits using combination Hou-J, and O'Brien-Dc: Vertical handover-decision-making algorithm using fuzzy logic for the integrated Radio-and-OW system, IEEE Transactions on Wireless Communications, 2006, 5, (1), pp. 176-185
Compatibility with lighting Most modern systems use PWM dimming Channel does not exist when light is dimmed Solutions Use modulation scheme that incorporates PWM dimming (PPM-like) Use sensing to only transmit in active regions But both reduce overall data rate Requirement for closer collaboration with lighting industry.
Conclusions VLC offers high SNR low bandwidth channel Naturally suited to broadcast Challenges Data rate Uplink Compatibility If overcome possibility of low cost method to augment wireless capacity