Project: IEEE P802.15 Working Group for Wireless Personal Area Networks N (WPANs( WPANs) Submission Title: [VLC with white-light LEDs: strategies to increase data rate] Date Submitted: [10 May 2008] Source: [D C O Brien] Company [University of Oxford] Address [Department of Engineering Science, Parks Road, Oxford, OX1 3PJ,UK] Voice:[+441865273916], FAX: [+441865273906], E-Mail:[dominic.obrien@eng.ox.ac.uk] Abstract: [Presentation on techniques to improve transmission data-rate for VLC systems that use whitelight LEDs] Purpose: [Information] 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. Slide 1
VLC with white-light LEDs: strategies to increase data rate Dominic O Brien oa Le Minh Lubin Zeng Grahame Faulkner University of Oxford Slide 2
Contents The VLC link Sources Propagation Receiver Strategies to increase data rate Pre-equalisation Post-equalisation Complex modulation Parallel transmission (optical MIMO) Conclusions Slide 3
Sources Blue LED & Phosphor Low cost Phosphor limits bandwidth RGB triplet igher cost Potentially higher bandwidth Potential for WDM Slide 4
Sources: Phosphor-based LED Emitter R I s Spice model L V R s = 0.9727 Ω, L = 33.342 n C s = 2.8 nf, C d = 2.567 nf, tt = 1.09 ns V d (1) Intrinsic LED modulation bandwidth is narrow C d C s (2) Blue component offers wider bandwidth Normalized response (db) 5 0-5 -10-15 -20-25 LED frequency response Blue response White response -30 0 10 20 30 40 50 60 Frequency (Mz) LED temporal impulse response 100ns/div 50ns/div Blue filtering Slide 5
Sources: typical bandwidths Available bandwidth LED modulation bandwidth is narrow ~3 Mz Blue-part has wider bandwidth ~12-20 Mz (dependent on devices) Slide 6
Propagation: modelling - Transmitter: LEDs, lens and driver - Channel: LOS and diffuse paths - Receiver: Optics, PD and amplifiers A typical geometry for indoor VLC Slide 7
Propagation: summary Power Illumination levels ensure strong communications signal Typical signal to noise ratio of >~40dB Bandwidth Channel bandwidth potentially affected by Inter-symbol interference from multiple line of sight paths Diffuse reflections from surfaces Modelling indicates bandwidth >~90Mz within typical room (results from einrich erz Institute) Slide 8
Propagation: conclusions Very high SNR available Bandwidth of channel >~90Mz Slide 9
Receiver Bandwidth set by photo-detector and preamplifier combination Capacitance and transit time of photo-detector Impedance of front end of amplifier Constraints Increasing area increases collected power Increased capacitance therefore reduced bandwidth Examples 20mm 2 bootstrapped APD receiver (155Mb/s -40dBm OOK 1E- 9)[1] 14.4mm 2 PIN diode receiver using commercial transimpedance amplifer- bandwidth of 77Mz (100Mb/s -27dBm OOK 1E-9 BER)[2] Conclusion Receiver bandwidths of up to 100Mz available with reasonable collection areas Greater bandwidths more challenging [1] McCullagh-Mj and Wisely-Dr, "155 Mbit/s optical wireless link using a bootstrapped silicon APD receiver," Electronics Letters, vol. 30, pp. 430-2, 3 March 1994. [2] Khoo-S (DPhil Thesis, University of Oxford) Slide 10
Summary of VLC link properties If required bandwidth<~100mz LED provides constraints Channel and receiver constraints need consideration if required bandwidth >~100Mz Slide 11
Strategies for igh-speed VLC Equalization Transmitter (pre-) equalization Receiver (post-) equalization Complex modulation Multiple-Input-Multiple-Output (MIMO) Slide 12
(Pre-) Equalization: Multiple Resonant LEDs Combination of the responses from multiple LED devices being driven at different resonant frequencies larger VLC bandwidth Input data signal DC Transmitter Modulators LED array Concentrator L Pre- PD amplifier R Optical receiver LPF Recovered data signal Normalized response (db) 5 0-5 -10-15 -20-25 -30-35 -40 G 1 (ω) G 2 (ω) Resonant responses Measured resonant LED responses G 3 (ω) G 4 (ω) G 5 (ω) G 6 (ω) G 7 (ω) Raw LED BW Measured resonant peak curve Fitted resonant peak curve Resonant peaks G 8 (ω) Signal A igh-speed buffer Z Resonant Capacitor (C) Bias Tee DC arm DC bias current from Laser driver Resonant modulation circuit Inductance (L series) Luxeon LED, R Normalized response (db) -45 0 5 10 15 20 25 30 35 40 Frequency (Mz) 5 0-5 -10-15 -20-25 Equalized bandwidth N ( ω ) = ( ω ) F 0a i G i i = (1) (2) (3) Multiple-resonant 16-LED VLC demonstration system -30 0 5 10 15 20 25 30 35 40 Frequency (Mz) Slide 13
(Pre-) Equalization: Multiple Resonant LEDs Link performance 4 3.5 Received power in dbm/cm 2 10-2 10-3 L = 2m L = 2.5m 3 10-4 y coordinate(m) 2.5 2 1.5 1 0.5-25 0.5 1 1.5 2 2.5 3 3.5 4 x coordinate(m) Receiving power plane-distribution -20-25 -30-35 -40 BER 10-5 10-6 10-7 10-8 10-9 30 35 40 45 50 Data rate (Mbit/s) BER performance 40 Mbit/s OOK-NRZ in standard room lighting condition [3] [3]Le-Minh,., O'Brien-Dc, Faulkner, G., Zeng, L., and Lee, K.: igh-speed Visible Light Communications Using Multiple-Resonant Equalization, Photonics Technology Letters, 2008, 20, (15), pp. 1243-1245 Slide 14
(Pre-) Equalization: Single LED Link Single LED is driven by multiple resonant driver branches + bluefiltering at receiver driver 1 driver 2 Data driver 3 C 1 R 1 C 2 R 2 Pre- Equalizer Bias- Tee DC current source LED Beamshaping lens White light Blue filter VLC link configuration Blue light PIN Concentrator Oscilloscope Slide 15 Amplifier 45 Mz equalized bandwidth achieved (3 drivers) 80 Mbit/s OOK-NRZ transmission [4] [4]. Le-Minh, D. C. O Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y. Oh, 80 Mbit/s Visible Light Communications Using Pre-Equalized White LED, accepted for poster presentation at European Conference on Optical Communications (ECOC 2008) Response (db) BER -40-45 -50-55 -60 Driver 1 Driver 2-65 Driver 3 LED bandwidth -70 0 10 20 30 40 50 60 Frequency (Mz) 10-1 10-2 10-3 10-4 10-5 Equalization BER performance Blue-filtering Pre-Equalization 10-6 10 20 30 40 50 60 70 80 90 100 Data rate (Mbit/s)
Complex Modulation igh optical SNR (OSNR) Potential for complex modulation But Driving devices potentially challenging DMT/OFDM Link of (equivalent data-rate) 101-Mbit/s is demonstrated using 20-Mz bandwidth [5] M-PAM Potential (OSNR is high) Tx Rx 50 Mbit/s 4-PAM VLC link (from [4]) (100 Mbit/s equivalent NRZ rate) [5] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. European Conference on Optical Communications (ECOC 2007) (PDS 3.6), pp. 1-2. ECO [06.11], Sep. 2007, Berlin, Germany Slide 16
(Post-) Equalization: LED Impulse Response Fall time of devices >> Rise time Equalization of exponential decay Fitted response Equalization Equalization process Bandwidth improvement Slide 17
Post-equalisation Equalisation Simulation of 1 st order equaliser OOK-NRZ data rate is increased from 16 Mbit/s to 32 Mbit/s [6] Pre and post equalisation Resonant LED array+1 st order equaliser (simulation) 42Mb/s to 73 Mb/s (using 25Mz bandwidth) [6] L. Zeng, D. C. O Brien,. Le-Minh, K. Lee, D. Jung and Y. Oh, Improvement of Data Rate by Using Equalization in an Indoor VLC System, IEEE International Conference on Circuits and Systems for Communications 2008 (IEEE ICCSC 2008), Shanghai, China, May 2008 Slide 18
Equalisation summary Pre-equalisation Possible with single or multiple LEDs Substantial bandwidth improvement Issues Energy efficiency Driver complexity Effect of device variation Post-equalisation Simulations indicate substantial improvement Preliminary experimental results promising Attractive as no complex LED drive circuitry Post-equalisation preferable from complexity point of view Combination of pre-and post offers substantial improvements (in simulation) Slide 19
MIMO using VLC Many sources offers the potential for parallel data transmission 1Gb/s parallel proof-of concept by VLCC Would normally require careful alignment of sources and detectors MIMO processing allows signals to be recovered without precise alignment Slide 20
Multiple-Input-Multiple-Output System Tx1 Tx2 Tx3 Tx4 = 11 21 31 41 12 22 32 42 13 23 33 43 14 24 34 44 4 Rx Channel matrix needs to be estimated at different receiver positions Simulation shows that data rate is linearly increased if is full rank Geometric symmetry can reduce rank- MIMO does not work Slide 21
MIMO System: Room Test Performance Room eight LED Array 1 LED Array 3 LED Array32 Ceiling LED Array 2 Room Width 2.15m Receiver Plane Receiver 0.85m Floor 5m Room Width Room Length (a) 5m 4 x 20 Mb/s channels Aggregate 80Mb/s transmission Low BER except lines of symmetry (b) Slide 22
MIMO summary Initial results show linear capacity growth Possibility of increasing capacity by transmitting data Not possible at all locations due to symmetry of -matrix Work to develop a receiver optical system that addresses this issue underway Slide 23
Conclusions VLC has the potential to offer high data rates 100Mb/s either demonstrated or simulated using a number of different techniques Data rates of Gbit/s possible with more advanced techniques Further work required on Development Comparison of alternatives Slide 24