Visible Light Communications Hoa Le Minh and Zabih Ghassemlooy Optical Communications Research Group (OCRG) School of Computing, Engineering and Information Sciences Northumbria University, United Kingdom hoa.le-minh@northumbria.ac.uk (ERASMUS Framework)
Presentation Outline • Optical wireless communications backgrounds • Visible Light Communications • Light Sources • Current technologies • Challenges • Organic Light Source • Summary
Why Optical Wireless? RF spectrum: crowded, expensive OW spectrum: free, large bandwidth
Optical Wireless Applications(short range) Traffic Communications Public data broadcasting Indoor broadband broadcasting in Hospital / Supermarket / University / Office Home Access Networks Military Communications
Applications Probably the first ever applications in visible light communications Beam reflection (directional) Flame Source: Discovery Channel
Network Evolution HFC Direct Fiber Ethernet MSO/ Cable Ethernet Ethernet Wireless [FSO/RF] SONET/ SDH Ethernet Bonded Copper Ethernet Carrier 1 TDM PON Carrier 2 Ethernet Bonded T1/E1 DS3/E3 Ethernet Source: NTT Ethernet 6 Ethernet High speed data delivered to home/office/premise need ultrafast home access networks
Apps: Home Access Network Power line, radio, visible light and infrared communications
Home/Office Wireless Network • WiFi a/b/g/n – data rate R up to hundreds of Mbit/s • Bluetooth R ~ tens of Mbit/s • Optical wireless • Infra-red communications – R ~ Gbit/s • Visible light communications – R ~ hundreds of Mbit/s
OW Apps: Broadband VLC Source: Boston University Indoor broadband broadcasting in Hospital / Supermarket / University / Office
OW Apps: Indoor Broadband Source: Oxford University (OMEGA project)
Apps: Traffic Communications FSO M Kavehrad PSU, USA
Research in VLC VLCC (Casio, NEC, Panasonic Electric Works, Samsung, Sharp, Toshiba, NTT, Docomo) OMEGA (EU Framework 7) IEEE 802.15 Wireless Personal Area Network standards Boston University Siemens France Telecom Oxford University Edinburgh University Northumbria University
VISIBLE LIGHT COMMUNICATIONS Main purpose: General Lighting Added Value: Communications
General Lighting Sources • Incandescent bulb • First industrial light source • 5% light, 95% heat • Few thousand hours of life • Fluorescent lamp • White light • 25% light • 10,000s hours • Solid-state light emitting diode (LED) • Compact • 50% light • More than 50,000 hours lifespan
1.2 1 0.8 0.6 0.4 0.2 0 0.7 0.8 0.9 1.0 0.3 0.4 0.5 0.6 1.1 1.2 1.3 1.4 1.5 Light Source Spectrum Sun Incandescent Fluorescent Normalised power/unit wavelength UV IR Wavelength (m)
What is LED?
LED – Fundamental Light Emitting Diode (LED)
White-Light LED • LED types: RGBBlue chip + PhosphorOLED New technology, expensive and short life time. It is, however, very potential Well-known technology, limited use, problem with balancing each R, G, B component to create white light Popular for today general lighting, efficient and cheap
VLC System • Key Attributes • Secured communications: “you receive what you see” • Immunity to RF interference • Signals are easily confined • Unlicensed spectrum • Visible light meets eye-safe regulation • Green communications
VLC System High Signal to Noise Ratio Signal to Noise ratio: how good signal is!
VLC Transceivers DC current: for illumination (provide sufficient brightness) Signal: Data for communications
LED Frequency Response LED frequency response LED temporal impulse response 100ns/div White light • Intrinsic LED modulation bandwidth is narrow (3MHz) • Blue-part provides wider bandwidth (20 MHz) 50ns/div Blue light
How can we improve the LED frequency response?
Pre-Equalisation Equalization BER performance VLC link configuration • 45 MHz equalized bandwidth achieved • 80 Mbit/s OOK-NRZ transmission
Post-Equalisation Simple RC equalisation circuit Natural BW Equalised BW 3-time BW improvement
Complex Modulation - Code Tx Rx 50 Msym/s 4-PAM Pulse Amplitude Modulation (PAM) Orthogonal Frequency-Division Multiplexing (OFDM) Orthogonal Subcarriers are used + M-QAM Likely achieved hundreds of Mbit/s
Complex Modulation - Diversity Space Pulse Amplitude Modulation (SPAM)
Cellular VLC Transmitter θa d1 φ Board with core r1 H Indoor channel receiver r2 receiver • User is highly mobile • Cellular structure and cell handover strategy are being developed • Cell size and transmit power are optimised
High speed VLC • Summary of strategies to achieve high speed VLC (single channel) • Bandwidth expansion: equalisation • High bandwidth efficiency: complex modulation • - SNR and system dynamic range must be large to support both approaches
Gigabit VLC If the channel matrix H is full rank, it is possible to transmit data in parallel Parallel transmission: Multiple-Input-Multiple-Output
Tx1 Tx2 Tx3 Tx4 4Rx MIMO VLC Channel Matrix Issue: If there is a geometry symmetry rank(H) < 4
MIMO VLC Performance Source: Oxford University (Samsung’s project)
Organic LED (OLED)
OLED • OLEDs: • Invented by Kodak in the 1980s • Intended for use in screens (brighter, thinner, faster, lighter and less power consumption than LCDs) • Produced in large panels that illuminate a broad area. • Can be flexible with the relevant plastic substrate (create different shape)
OLED structure Source: Lumiblade
OLED Electrical modelling (equivalent circuit) Lighting Large panel better for illumination larger capacitor value Communications Larger capacitor value slow response Source: Lumiblade, Korea Institute of Industrial Technology
OLED Equalisation approach Merit: total value of serial capacitors is smaller than individual capacitor value The external Ceq minimises the effect of OLED capacitance OLED: experimentally transmit data at 2 Mbit/s over the original BW of 0.15 MHz
Other Projects in VLC Smart VLC receiver and MIMO Portable device/Smartphone VLC Dimming and VLC
Remaining Challenges Higher data rate? Uplink communications? Light dimming (asynchronous transmission)? Heat dissipation?
Conclusions Optical Wireless Communications is an emerging technology that truly delivers data at very high rate with fibre-like quality
Acknowledgements OCRG group School of CEIS Oxford University OMEGA project Samsung Electronics
Thank you
