Z Ghassemlooy H Le-Minh Optical Communications Research Group, Faculty of Engineering and Environment, Northumbria University, UK

History of Optical Communication Alexander Graham Bell 1878 more than 25 years before Reginald Fessenden did the same thing with radio 1. 1 Alexander Graham BELL, American Journal of Sciences, Third Series, vol. XX, no.118, Oct. 1880, pp F. R. Gfeller and U. Bapst, Proceedings of the IEEE, vol. 67, pp , Diagram of photophone from Bell paper 1 Development of LASER in 60’s, optical fibre and semiconductor has made the modern communication possible. The modern era of indoor wireless optical communications was proposed in 1979 by F.R. Gfeller and U. Bapst 2. In fact it was the first LAN proposed using any medium.

History of OWC 800BC Fire beacons (ancient Greeks and Romans) 150BC Smoke signals (American Indians) 1880 Alexander Graham Bell demonstrated the photophone 1 – 1 st FSO (THE GENESIS) 1960s Invention of laser (revolutionized FSO), and optical fibre 1970s FSO mainly used in secure military applications 1979 Indoor OWM systems – F R Gfeller and G Bapst 1993 Open standard for IR data commun. The Infrared Data Association 2003 The Visible Light Communications Consortium (VLCC) – Japan 2008 “hOME Gigabit Access” (OMEGA) Project – EU - Develop global standards for home networking (infrared and VLC technologies) IEEE Call for Contributions on IEEE VLC.

54 Mbps/100 Mbps/GbE Corporate LAN UniversitiesHospitalsBusinesses Bandwidth hungry applications 100M/GbE LANS HDTV 10G & Higher 2.5G – 10G Sufficient bandwidth on most routes DWDM used to upgrade congested routes Metro Edge Metro Network TeraGig Bandwidth Abundant capacity Falling bandwidth price Long Haul Fibre Network < 10 Mbps Bottleneck AccessNetworkLASTMILE Access Network Bottleneck

RF Bandwidth Congestions

Access Network Technologies 100 Mbps DSL UWB LMDS FTTH FREE SPACE OPTICS 1 Mbps 10 Mbps 1 Gbps 10 Gbps Bandwidth Distance from metro fibre route 50 m500 m 1 km 2 km 5 km + PLC PLC DSL

OWC: Overview 1 M. Kavehrad, Scientific American Magazine, July 2007, pp Typical optical wireless system components Optical wireless connectivity 1 light beams (visible and infrared) propagated through the free space. Optical transmitter -Light Emitting Diodes (LED) -Laser Diodes (LD) Optical receiver -p-i-n Photodiodes. -Avalanche Photodiodes Links -Line-of-sight(LOS) -Non-LOS -Hybrid

OWS Source: T. Lüftner, "Edge Position Modulation for Wireless Infrared Communications," PhD thesis, Friedrich-Alexander University, 2005.

Comparison with RF PropertyRadioInfraredImplication for IR Bandwidth regulated YesNoApproval not required world-wide compatibility Passes through walls YesNoInherently secure carrier reuse in adjacent rooms. Multipath fadingYesNoSimple link design Multipath dispersion Yes Problematic at high data rates Path lossHigh Dominant noiseOther users BackgroundShort range Average power proportional to f(t)is the input signal with high peak-average radio

What OWC offers Abundance bandwidth  High data rate License free operation High Directivity  small cell size  can support multiple devices within a room Free from electromagnetic interference  suitable for hospital and library environment. cannot penetrate opaque surface like wall  Spatial confinement  Secure data transmission Compatible with optical fibre (last mile bottle neck?) Low cost of deployment Quick to deploy Small size, low cost component and low power consumptions. Simple transceiver design. No multipath fading

Applications Send signal Send and receive reflection Sensors / IR viewer Simple Source: Internet

Applications Controlling & signalling Mobile communications Functional Source: Internet

OWC- Applications Hospitals Last Mile Connectivity Multi-campus University Other applications include:  Disaster recovery  Fibre communications backup  Video conferencing  Links in difficult terrains  Intelligent transport system (car-to- car Communications, ground-to- train communications)

Optical Wireless Communications OWC Indoor Outdoor VLCIR VLCIR - Broadcasting - LOS/Diffuse (3-4m, 100Mbps) - Short range communications - Device to device - Wireless hotspot (4m, ~1Gbps) - Traffic light - Car-to-car communications (low speed) - Free space optics (2-3km, > 1Gbps)

Classification of Indoor OWC Links

LOS Links Rx Tx Advantages  Least path loss  No multipath propagation  High data rate  Suitable to point-to-point communications only. Problems  Noise is limiting factor  Possibility of blocking/shadowing  Tracking necessary  No/limited mobility  Narrow low power transmit beam  Narrow field-of-view receiver

Diffuse Links  Use multiple reflections of the optical beam on surrounding surfaces such as ceilings, walls, and furniture.  transmitter and receiver not necessarily directed one towards the other.  Robust to blocking and shadowing  Allows roaming Problems:  High path loss.  Multiple paths (reflections) -Result in inter-symbol interference (ISI).  High power penalty due to ISI.  Limited bandwidth- Due to large capacitance of the large area detectors Rx Tx

Geometry LOS propagation model d ϕ ψ Transmitter Receiver ψcψc

Propagation types and definitions Definitions Input – Transmitter parameters Average optical power transmitted (Pt) Half power angle (Φ) Lambert’s mode number (m l ) – Receiver parameters Field Of View (FOV), Ψ Receiver effective area (A eff ) Receiver sensitivity (R) Output – Average optical received power (P r ) – Geometrical attenuation – Channel gain, H(0) – Link Margin

Optical Parameters Average optical power: Signal-to-noise-ratio: DC channel gain:

21 LOS/WLOS link margin analysis The channel gain (response at null frequency) is: d : distance transmitter/receiver φ: semi-angle of transmission : ψ : semi-angle of reception P t : transmitted power Geometrical attenuation in dB: Average optical received power P r : Link margin M l :

Challenges (Indoor) ChallengesCauses(Possible ) Solutions Power limitationEye and skin safety. Power efficient modulation techniques, holographic diffuser, transreceiver at 1500ns band NoiseIntense ambient light (artificial/ natural) Optical and electrical band pass filters, Error control codes Intersymbol interference (ISI) Multipath propagation (non-LOS links) Equalization, Multi-Beam Transmitter No/Limited mobilityBeam confined to small area. Wide angle optical transmitter, MIMO transceiver. Shadowing Blocking LOS linksDiffuse links/ Cellular System/ wide angle optical transmitter Limited data rateLarge area photo- detectors Bandwidth-efficient modulation techniques /Multiple small area photo-detector. Strict link set-upLOS linksDiffuse links/ wide angle transmitter

Safety Classifications - Point Source Emitter

Issue1: Eye- safety  Infrared communication currently in market works in two wavelengths: 800 nm and 1550 nm.  At 800 nm (near infrared), light passed though cornea and lens and focus on to the retina.  Invisible light  no blinking reflex.  Retina has no pain sensor  permanent eye-damage could occur.  Infrared transceivers should conform to class 1, a few W,(inherently safe) of the IEC 825 standard. The eye safety limit is a function of the viewing time, wavelength and apparent size of the optical source.  Class 3B laser can be used by passing the beam through a hologram.  1550 nm is relatively safe as the wavelength is absorbed by the cornea and lens.  However, the cheap trans-receiver optical devices available in market are in 800 nm band.

Eye- safety- Possible Solutions  Adopt to 1500 nm band (expensive solution)  Power efficient baseband modulation techniques like pulse position modulation.  Retransmission scheme and error control code.  Power efficiency is also important factor for battery powered optical wireless gadgets as the power consumption needs to be minimised.  Combining power efficient modulation scheme with the error control code can be optimum solution.

Issue 2: Artificial Light Interference (ALI) Optical power spectra of common ambient infrared sources. Spectra have been scaled to have the same maximum value.

ALI-Possible Solutions  Differential receiver 1  Differential optical filtering 2  Electrical high pass filter 3,4  Polarisers 5  Angle diversity receiver 6,7  Discrete wavelet transform based denoising 8,9 1 J. R. Barry, PhD Dissertation, University of California at Berkeley, A.J.C Moreira, R. T. Valadas, A. M. De Oliveira Duarte, Optical Free Space Communication Links, IEE Colloquium on, vol., no., pp.5/1-510, 19 Feb R. Narasimhan, M. D. Audeh, and J. M. Kahn, IEE Proceedings - Optoelectronics, vol. 143, pp , A. R. Hayes, Z. Ghassemlooy, N. L. Seed, and R. McLaughlin, IEE Proceedings - Optoelectronics vol. 147, pp , S. Lee, Microwave and Optical Technology Letters, vol. 40, pp , R. T. Valadas, A. M. R. Tavares, and A. M. Duarte, International Journal of Wireless Information Networks, vol. 4, pp , J. M. Kahn, P. Djahani, A. G. Weisbin, K. T. Beh, A. P. Tang, and R. You, IEEE Communications Magazine, vol. 36, pp , S. Rajbhandari; Z. Ghassemlooy; and M. Angelova, IJEEE, Vol. 5, no. 2,pp S. Rajbhandari; Z. Ghassemlooy; and M. Angelova, Journal of Lightwave Technology, on print.

Issue 3: Multipath induced ISI Diffuse Links offers  Robustness to blocking and shadowing  Allows roaming  Avoid complex alignment and tracking between transmitter and receiver Challenges  For most surfaces, the light wave is diffusely reflected (as from a matter surface) rather than specularly reflected (as from a mirrored surface).  Pulse spreading beyond symbol duration.  High inter-symbol interference (ISI).  Low data rate and high power penalty.

Channel Model and Performance without an Equalizer  Characterised by Channel impulse response h(t).  Developed by Carruthers and Kahn 1. where u(t) is the unit step function and D rms RMS delay spread. Normalized delay spread, T s : bit duration.  The normalized optical power requirement for the unequalized system increases exponentially with increasing delay spread.  Modulation techniques having shorter pulse duration show higher power penalties.  It is practically impossible to achieve a reasonable BER at D T > 0.5 for OOK system. 1 J. B. Carruthers and J. M. Kahn, IEEE Transaction on Communication, vol. 45, pp , 1997.

Reported Working Systems

Long Distance Systems

Common Baseband Digital Modulation Techniques OOK  Simple to implement  High average power requirement  Suitable for Bit Rate greater tha 30Mb/s  Performance detoreaites at higher bit rates PPM  Complex to implement  Lower average power requirement  Higher transmission bandwidth  Requires symbol and slot synchronisation DPIM  Higher average power requirement compared with PPM  Higher throughput  Built in symbol synchronisation  Performance midway between PPM and OOK. DH-PIM  The highest symbol throughput  Lower transmission bandwidth than PPM and DPIM  Built in symbol synchronisation  Higher average power requirement compared with PPM and DPIM.  Complex decoder