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

Free space optics(Optical Wireless Communications)

Z GhassemlooyH Le-Minh

Optical Communications Research Group, Faculty of Engineering and Environment, Northumbria University,

UKhttp://soe.northumbria.ac.uk/ocr/

History of Optical Communication

• Alexander Graham Bell 1878 more than 25 years before Reginald Fessenden did the same thing with radio1.

1 Alexander Graham BELL, American Journal of Sciences, Third Series, vol. XX, no.118, Oct. 1880, pp. 305- 324.2 F. R. Gfeller and U. Bapst, Proceedings of the IEEE, vol. 67, pp. 1474- 1486, 1979.

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 photophone1 – 1st FSO (THE GENESIS)

(www.scienceclarified.com)

1960s Invention of laser (revolutionized FSO), and optical fibre 1970s FSO mainly used in secure military applications1979 Indoor OWM systems – F R Gfeller and G Bapst1993 Open standard for IR data commun. The Infrared Data Association 2003 The Visible Light Communications Consortium (VLCC) – Japan2008 “hOME Gigabit Access” (OMEGA) Project – EU - Develop global standards for home networking (infrared and VLC technologies).

2009 IEEE802.15.7 - Call for Contributions on IEEE802.15.7 VLC.

54 Mbps/100 Mbps/GbE

Corporate LANUniversitiesHospitalsBusinesses

• Bandwidth hungry applications

• 100M/GbE LANS• HDTV

10G &

High

er

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

AccessN

etwork

LAST

MILE

Access Network Bottleneck

RF Bandwidth Congestions

Access Network Technologies

100 Mbps DSL UWB

LMDS

FTTHFREE SPACE OPTICS

1 Mbps

10 Mbps

1 Gbps

10 Gbps

Bandwidth

Distance from metro fibre route50 m 500 m 1 km 2 km 5 km +

PLCDSL

OWC: Overview

1 M. Kavehrad, Scientific American Magazine, July 2007, pp. 82-87.

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

Property Radio Infrared Implication for IRBandwidth regulated

Yes No Approval not required world-wide compatibility

Passes through walls

Yes No Inherently secure carrier reuse in adjacent rooms.

Multipath fading Yes No Simple link design

Multipath dispersion

Yes Yes Problematic at high data rates

Path loss High High

Dominant noise Other 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

VLC IR VLC IR

- 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

RxTx

Geometry LOS propagation model

d

ϕ

ψ

Transmitter

Receiver

ψc

Propagation types and definitions

DefinitionsInput– Transmitter parameters

• Average optical power transmitted (Pt)• Half power angle (Φ)• Lambert’s mode number (ml)

– Receiver parameters• Field Of View (FOV), Ψ• Receiver effective area (Aeff)• Receiver sensitivity (R)

Output– Average optical received power (Pr)– 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/receiverφ: semi-angle of transmissionψ : semi-angle of receptionPt : transmitted power

Geometrical attenuation in dB:

Average optical received power Pr:

Link margin Ml:

Challenges (Indoor)

Challenges Causes (Possible ) SolutionsPower limitation Eye and skin safety. Power efficient modulation techniques,

holographic diffuser, transreceiver at 1500ns band

Noise Intense 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 mobility Beam confined to small area.

Wide angle optical transmitter , MIMO transceiver.

Shadowing Blocking

LOS links Diffuse links/ Cellular System/ wide angle optical transmitter

Limited data rate Large area photo-detectors

Bandwidth-efficient modulation techniques /Multiple small area photo-detector.

Strict link set-up LOS links Diffuse 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 receiver1

Differential optical filtering2

Electrical high pass filter3,4

Polarisers 5

Angle diversity receiver 6,7

Discrete wavelet transform based denoising8,9 1 J. R. Barry, PhD Dissertation, University of California at Berkeley, 19922 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 1996. 3 R. Narasimhan, M. D. Audeh, and J. M. Kahn, IEE Proceedings - Optoelectronics, vol. 143, pp. 347-354, 1996.4 A. R. Hayes, Z. Ghassemlooy , N. L. Seed, and R. McLaughlin, IEE Proceedings - Optoelectronics vol. 147, pp. 295-300, 2000.5S. Lee, Microwave and Optical Technology Letters, vol. 40, pp. 228-230, 2004.6R. T. Valadas, A. M. R. Tavares, and A. M. Duarte, International Journal of Wireless Information Networks, vol. 4, pp. 275-288, 1997 .7J. M. Kahn, P. Djahani, A. G. Weisbin, K. T. Beh, A. P. Tang, and R. You, IEEE Communications Magazine, vol. 36, pp. 88-94, 1998.8 S. Rajbhandari; Z. Ghassemlooy; and M. Angelova, IJEEE, Vol. 5, no. 2 ,pp102-111. 2009.9 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.

0 0.05 0.1 0.15 0.2

0

0.2

0.4

0.6

0.8

1

Time (µS)

Am

plitu

de

Transmitted singalReceived signal

Channel Model and Performance without an Equalizer

Characterised by Channel impulse response h(t). Developed by Carruthers and Kahn1.

where u(t) is the unit step function and Drms RMS delay spread.

Normalized delay spread, Ts : 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 DT > 0.5 for OOK system.

)(7)1.0(

6)1.0(6)( tu

rmsDt

rmsD

th

sTrmsD

TD

1J. B. Carruthers and J. M. Kahn, IEEE Transaction on Communication, vol. 45, pp. 1260-1268, 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