Transcript OWCx - Northumbria University

Free space optics
(Optical Wireless Communications)
Z Ghassemlooy
H Le-Minh
Optical Communications Research Group,
Faculty of Engineering and Environment, Northumbria University,
UK
http://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.
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.
R. Gfeller and U. Bapst, Proceedings of the IEEE, vol. 67, pp. 1474- 1486, 1979.
2 F.
History of OWC
800BC
150BC
1880
Fire beacons (ancient Greeks and Romans)
Smoke signals (American Indians)
Alexander Graham Bell demonstrated the photophone1 – 1st FSO
(THE GENESIS)
1960s
1970s
1979
1993
2003
2008
Invention of laser (revolutionized FSO), and optical fibre
FSO mainly used in secure military applications
Indoor OWM systems – F R Gfeller and G Bapst
Open standard for IR data commun. The Infrared Data Association
The Visible Light Communications Consortium (VLCC) – Japan
“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
Bottleneck
•Bandwidth hungry applications
•100M/GbE LANS
•HDTV
Metro
Network
•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
FTTH
FREE SPACE OPTICS
1 Gbps
DSL
100 Mbps
UWB
LMDS
10 Mbps
1 Mbps
DSL
50 m
500 m
PLC
1 km
2 km
Distance from metro fibre route
5 km +
OWC: Overview
•
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
Typical optical wireless system components
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
Bandwidth
regulated
Radio
Yes
Passes through
walls
Yes
No
Inherently secure carrier
reuse in adjacent rooms.
Multipath fading
Multipath
dispersion
Path loss
Dominant noise
Yes
Yes
No
Yes
Simple link design
Problematic at high data
rates
Average power
proportional to
High
Other
users
Infrared
Implication for IR
No
Approval not required
world-wide compatibility
High
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
Applications
Controlling & signalling
Mobile communications
Functional
Source: Internet
OWC- Applications
Other applications include:
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Disaster recovery
Fibre communications backup
Video conferencing
Links in difficult terrains
Intelligent transport system (car-tocar Communications, ground-totrain communications)
Last Mile Connectivity
Hospitals
Multi-campus University
Optical Wireless Communications
OWC
Outdoor
Indoor
VLC
- Broadcasting
- LOS/Diffuse
(3-4m, 100Mbps)
IR
- Short range
communications
- Device to device
- Wireless hotspot
(4m, ~1Gbps)
VLC
IR
- Traffic light
- Free space optics
- Car-to-car
(2-3km, > 1Gbps)
communications
(low speed)
Classification of Indoor OWC Links
LOS Links
Rx
Tx
 Narrow low power transmit beam
 Narrow field-of-view receiver
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
ψc
Receiver
Propagation types and definitions
Definitions
Input
– 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:
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
Pt : transmitted power
Geometrical attenuation in dB:
Average optical received power Pr:
Link margin Ml:
21
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)
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 photodetectors
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
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1 J.
Differential receiver1
Differential optical filtering2
Electrical high pass filter3,4
Polarisers 5
Angle diversity receiver 6,7
Discrete wavelet transform based denoising8,9
R. Barry, PhD Dissertation, University of California at Berkeley, 1992
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. 295300, 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.
2
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
Transmitted singal
Received signal
1
0.8
Amplitude
 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.
0.6
0.4
0.2
0
0
0.05
0.1
Time (µS)
0.15
0.2
Channel Model and Performance
without an Equalizer
 Characterised by Channel impulse response h(t).
 Developed by Carruthers and Kahn1.
h (t ) 
6(0.1D
rms
(t  0.1D
)6
rms
)7
u (t )
where u(t) is the unit step function and Drms RMS
delay spread.
Normalized delay spread,
D 
T
D
rms
Ts
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.
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
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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