Opportunities and Challenges for High-Speed Optical-Wireless Links

Fraunhofer Networks Heinrich Hertz + Systems Institute Opportunities and Challenges for High-Speed Optical-Wireless Links Jelena Vučić and Klaus-Dieter Langer Fraunhofer Heinrich-Hertz-Institut Fraunhofer Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin www.hhi.fraunhofer.de

Outline Introduction to Optical Wireless Communications Motivation and Domains of Application Infrared Links Main Features and Recent Efforts Visible Light Links Main Features and Recent Efforts Summary and Conclusions J. Vučić and K.-D. Langer Fraunhofer HHI, Berlin 2

Domains of Wireless Communication via Light Satellite communications Inter-satellite, satellite-to-earth links, ~10 000 km P-t-P, line-of-sight (LOS), < 1 Gbit/s Terrestrial free-space optics (FSO) LOS city links (between building rooftops) P-t-P, relaying, ~ km range, ~ Gbit/s range Optical wireless (OW) communications Indoor applications in ~ m range P-t-P, P-t-MP links (LOS and/ or reflections) 10 1000 Mbit/s Satellite Communications Terrestrial FSO Communications Optical Wireless communications 3

Optical Wireless Communications No EMI with radio systems, no e-smog Available and unregulated spectrum Simple shielding by opaque surfaces easily obtainable privacy Complementary to radio for wireless access 4

Basic Types of OW Links directed non-directed Tx Tx LOS diffuse non-directed LOS Rx Tx Rx Rx LOS High data rates Sensitive to blocking Reflections High coverage High path loss LOS + diffuse High coverage + potentially high data rates 5

Outline Introduction to Optical Wireless Communications Motivation and Domains of Application Infrared Links Main Features and Recent Efforts Visible Light Links Main Features and Recent Efforts Summary and Conclusions J. Vučić and K.-D. Langer Fraunhofer HHI, Berlin 6

Main Framework for IR Links Tradition: PtP links, e.g. IrDA: 16 Mbit/s up to Giga-IR short-range, low coverage Eye and skin safety Available Rx components (850 or 1550 nm range) mainly developed for fibre optics small-area PDs, OW needs large-area ones Background light noise, multipath dispersion High-speed indoor use coverage and mobility wanted Multibeam-forming Tx and angle diversity Rx (complex) Simple optics with advanced signal processing 7

Gbit/s Cellular LOS Communication Universities of Oxford & Ilmenau, 2010 7-cell design Tx laser 14 dbm @ 825 nm, APD-Rx 1.25 Gbit/s (gross) 4 m reach, ~1.5 m diameter (3 m below Tx) HD video transmission demonstrated Bidirectional Base station Terminal 8

Rate-Adaptive Signal Processing Tx Channel information diffuse LOS Data in Tx IFFT DMT IR channel Ambient light Rx DMT FFT Data out Rx Single-element optical front-ends Channel varies from low-pass to flat (LOS / diffuse signal ratio) Low-speed feed-back link provides channel information to Tx System dynamically adapts data rate to channel state using DMT: Bit- and power-loading, handling of multipath distortion, efficient signal processing by FFT Theoretical predictions: up to ~400 Mbit/s (depending on the Rx position), with BER 2 10-3, in a 5x5x3 m 3 room with complete coverage 9

Outline Introduction to Optical Wireless Communications Motivation and Domains of Application Infrared Links Main Features and Recent Efforts Visible Light Links Main Features and Recent Efforts Summary and Conclusions J. Vučić and K.-D. Langer Fraunhofer HHI, Berlin 10

Additional Motivation for VLC Omnipresence of LEDs; signalling and illumination LEDs offer significant potential for modulation Combination of illumination or signalling with data transmission data transfer as piggyback broadcasting hot-spots Attractive where light is always on office, industrial settings, medical area, public transport, 11

VLC Key Component: White LED Phosphorous white LED: Blue LED + yellow Ph-layer white light Modulation bandwidth limited to 1-2 MHz by slow response time of the Ph layer Manipulation of LED resonant frequency Blue bandpass filter in front of the Rx suppress the slow Ph-component ~20+ MHz white LED BW (~10x) 12

Enhancing the Bandwidth - Pre-Equalization University of Oxford, 2008-9 4 x 4 LED array Pre-equalization @Tx: 3 25 MHz 40 Mbit/s, 2 m reach using NRZ-OOK Customized equalizing needed 13

VLC Transmission System in OMEGA Goal for VLC in OMEGA: ~100 Mbit/s broadcast via ceiling lighting wireless channel Corporate Technology 14

Analogue Tx Front-End (Ω demo) Commercial LED luminary (OSTAR E3B) 6 thin-film Ph white LEDs Luminous flux ~ 400 lm (@ dc = 700 ma) Integrated optic: 76 semi-angle @ half power circular light spot OSTAR E3B Custom-designed driving circuit electrical 3-dB BW: 90 khz 12 MHz normalised received electrical signal /db 0-0.5-1 -1.5-2 -2.5-3 -3.5 E2B E2B interpol. E3B E3B interpol. -4 0 10 20 30 40 modulation frequency /MHz Corporate Technology 15

Analog Rx Front-End (Ω demo) Optimized Optical filter (OF) Rejects background illumination 500 nm cut-off wavelength Suppresses Ph component; Optical concentrator (OC) Collects and concentrates incoming radiation Integrated with PD; field of view 70 Incoming radiation OF PD OC Photodetector (PD) Commercially available Si-PIN PD Effective detector area of ~110 mm² Custom-designed two-stage amplifier (TIA) Rx 3-dB bandwidth ~35 MHz Corporate Technology 16

PHY Digital Signal Processing (Ω demo) Data in (PRBS) Data out CLK Wireless VLC link O-E DMT Rx front-end (FPGA) ADC E-O front-end DAC DMT Tx (FPGA) CLK FEC decoding 16-QAM de-mapping 31 Equalization 31 128-FFT CP removal ADC DAC CP insertion (4) 31 64 64 64 64-IFFT Channel estimation 128 31 31 Power pre-equalization Conjugate symmetry 16-QAM mapping FEC coding: RS(187,207) For ~1.4 m (500 lx) FEC renders error-free performance Longer Rx-Tx distances multiple luminaries 17

Video Transmission Demo Video transmission @ 100 Mbit/s (PHY) ~1.4 m wireless link length (500 lx @ Rx) Scenario with 16 luminaries in preparation (~10 m 2 coverage) Transmission over ~2-3 m distance Corporate Technology 18

Conclusions OW technology interesting for many indoor applications Standardization efforts (PHY + MAC) IrDA, VLCC, IEEE, others Potential for high-speed transmission via both IR and VLC links Towards further performance improvements Optimization of optical frontends (Rx bandwidth, power efficiency of LED driver, handling of imperfect LED linearity ) Spectrally-efficient modulation, advanced signal processing techniques (e.g., MIMO) 19

Acknowledgement The research leading to these results received funding from the European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement 213311 also referred to as OMEGA. 20

Thank you. jelena.vucic@hhi.fraunhofer.de Fraunhofer HHI, Berlin 21