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/ [email protected]
Nov 08, 2014
Free space optics(Optical Wireless Communications)
S. RajbhandariOptical Communications Research Group,
School of Computing, Engineering and Information Sciences, 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.
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)
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 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.
54 Mbps/100 Mbps/GbE
Corporate LAN
Universities
Hospitals
Businesses
•Bandwidth hungry applications
•100M/GbE LANS
•HDTV
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
Bo
ttlen
eck
Acce
ss
Ne
two
rk
LAST
MILE
Access Network Bottleneck
RF Bandwidth Congestions
Access Network Technologies
100 Mbps DSLUWB
LMDS
FTTHFREE SPACE OPTICS
1 Mbps
10 Mbps
1 Gbps
10 Gbps
Bandwidth
Distance from metro fibre route
50 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 IR
Bandwidth 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
EN0630 – Optical Communications System Design – Dr. Hoa Le Minh
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:
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:
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 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 trackingbetween 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
pli
tud
e
Transmitted singal
Received 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
+
=
sT
rmsD
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