Introduction to Optical Wireless communicaitons - Northumbria ...

Free space optics
(Optical Wireless Communications)
S. Rajbhandari
Optical Communications Research Group,
School of Computing, Engineering and Information Sciences,
Northumbria University, UK
http://soe.northumbria.ac.uk/ocr/
Sujan.rajbhandari@northumbria.ac.uk

History of Optical Communication
• Alexander Graham Bell 1878 more
than 25 years before Reginald
Fessenden did the same thing
with radio 1 .
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.
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.

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

Access Network Bottleneck
LAST
MILE
54 Mbps/100 Mbps/GbE
TeraGig Bandwidth
Corporate LAN
Universities
Hospitals
Businesses
Long Haul Fibre Network
Access
Network
2.5G – 10G
Metro Edge
Metro
Network
•Bandwidth hungry applications
•100M/GbE LANS
•HDTV
Bottleneck
•Sufficient bandwidth on most
routes
•DWDM used to upgrade
congested routes
•Abundant capacity
•Falling bandwidth
price

RF Bandwidth Congestions

Access Network Technologies
Bandwidth
10 Gbps
FREE SPACE OPTICS
FTTH
1 Gbps
100 Mbps DSL
UWB
10 Mbps
LMDS
1 Mbps
DSL
PLC
50 m 500 m 1 km 2 km 5 km +
Distance from metro fibre route

OWC: Overview
• light beams (visible and infrared)
• propagated through the free space.
• Optical transmitter
- Light Emitting Diodes (LED)
- Laser Diodes (LD)
Typical optical wireless system components
• Optical receiver
- p-i-n Photodiodes.
- Avalanche Photodiodes
• Links
- Line-of-sight(LOS)
- Non-LOS
- Hybrid
Optical wireless connectivity 1
1
M. Kavehrad, Scientific American Magazine, July 2007, pp. 82-87.

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 IR
Bandwidth
regulated
Passes through
walls
Yes No Approval not required
world-wide compatibility
Yes No Inherently secure carrier
reuse in adjacent rooms.
Multipath fading Yes No Simple link design
Multipath Yes Yes Problematic at high data
dispersion
rates
Path loss High High
Dominant noise
Average power
proportional to
Other
users
BackgroundShort range
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
Simple
Sensors / IR viewer
Source: Internet
EN0630 – Optical Communications System Design – Dr. Hoa Le Minh

Applications
Controlling & signalling
Functional
Mobile communications
Source: Internet

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

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
Narrow low power transmit beam
Narrow field-of-view receiver
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

Diffuse Links
Tx
Rx
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

Geometry LOS propagation model
Transmitter
ϕ
d
ψ
ψ c
Receiver

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:

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

Challenges (Indoor)
Challenges Causes (Possible ) Solutions
Power 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)
No/Limited mobility
Shadowing
Blocking
Limited data rate
Multipath propagation
(non-LOS links)
Beam confined to small
area.
LOS links
Large area photodetectors
Equalization, Multi-Beam Transmitter
Wide angle optical transmitter , MIMO
transceiver.
Diffuse links/ Cellular System/ wide
angle optical transmitter
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 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, 1992
2
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.
5
S. Lee, Microwave and Optical Technology Letters, vol. 40, pp. 228-230, 2004.
6
R. T. Valadas, A. M. R. Tavares, and A. M. Duarte, International Journal of Wireless Information Networks, vol. 4, pp.
275-288, 1997 .
7
J. 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.
Amplitude
1
0.8
0.6
0.4
0.2
0
Transmitted singal
Received signal
0 0.05 0.1 0.15 0.2
Time (µS)

Channel Model and Performance
without an Equalizer
Characterised by Channel impulse response h(t).
Developed by Carruthers and Kahn 1 .
6(0.1D
)
6
h( t)
=
rms
u(
t)
( t + 0.1D
)
7
rms
where u(t) is the unit step function and D rms RMS
delay spread.
Normalized delay spread,
D =
T
D
rms
T s
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. 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