The attached narrated power point presentation classifies Optical Wireless Communication Systems into indoor and outdoor system types and throws light on the visible spectrum used for optical communications. Block diagram and working principle of Visible Light Communication Systems (VLC) , Architecture and Layer Model of VLC as well as the different modulation schemes used for VLC are also discussed. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
2. 2
Contents
• OWC System Classifications.
• Visible Spectrum.
• Introduction and Working Principle.
• VLC Block Diagram.
• Layer Model.
• Modulation Schemes.
3. 3
OWC Systems
• Two generic groups of OWC - indoor and
outdoor optical wireless communications.
• Unlimited bandwidth offered by OWC
attributed to different bands - IR, visible light
(VL) and UV.
• Indoor OWC uses IR/VL light for in-building
wireless solution.
• Indoor OWC systems - four configurations -
tracked, diffused, nondirected LOS, and
directed line of sight (LOS).
5. 5
OWC Systems
• Outdoor OWC employs optical carrier to
transport information from one point to
another over an unguided channel.
• OWC technology also known as a free-
space optical (FSO) communication
system.
• FSO operate at near IR frequencies,
classified into terrestrial and space optical
links.
11. 11
OWC System
• Wavelength ranges of 780–850 nm and
1520–1600 nm commonly used in current
OWC equipments.
• Wavelength ranges located in atmospheric
transmission windows where molecular
absorption is negligible.
• Wavelength windows located in the region
of four specific wavelengths - 850, 1060,
1250 and 1550 nm experience attenuation
of less than 0.2 dB/km.
12. 12
OWC System
• 850- and 1550-nm
transmission
windows coincide
with standard
transmission
windows of fiber
communication
systems.
13. 13
OWC System
• 1520–1600-nm wavelengths compatible
with EDFA technology, helps achieve high
power and high-data rate systems.
• 1520–1600-nm wavelengths enable
transmission of about 50–65 times more
average output power than can be
transmitted at 780–850 nm.
14. 14
VLC System
• Addresses challenges such as energy
efficiency, bandwidth limitation,
electromagnetic radiation, and safety in
wireless communications.
• Operates in the wavelength range of ~390–
750 nm.
• Current enhancement of LED chip design
with swift nanosecond-switching times and
extensive deployment of LEDs for energy
efficiency paves way for visible light
communication (VLC) system.
15. 15
VLC System
• Li-Fi alternative in sensitive or hazardous
environments like airplanes, hospitals, and
industrial gas production plants where the
employment of RF technology is not
permitted.
• VLC based indoor navigation services offer
very high accuracy to within a few cm.
• No harmful radiations, no public health
concern.
17. 17
VLC Transmitter
• LEDs and Lasers used as sources for
VLC.
• Use of white light based on LEDs and
wavelength converters.
• LED used when both communication and
illumination have to be performed using a
single device.
• Tetra-chromatic, dichromatic and tri-
chromatic modes for white light.
20. 20
VLC Transmitter
• RGB LED for white light generation - high
bandwidth and high data rates.
• RGB LED has high associated complexity
and modulation difficulties.
• Choice of LED based on the channel
model.
21. 21
VLC Receiver
• Amplification circuit, optical filter and optical
concentrators.
• Beam divergence due to illuminating large
areas results in attenuation.
• Optical concentrator to compensate for
attenuation.
• Light detected using a photodiode in a
stationary receiver - silicon photodiode, PIN
diode or avalanche photodiode used.
• Converted to photo current.
22. 22
VLC Receiver
• Imaging sensors employed instead of
photodiodes in the case of mobility.
• Operating imaging sensors energy
expensive and slow, hence a trade-off
between cost, speed and complexity.
• Vulnerable to interference from other
sources such as sunlight and other
illumination.
• Optical filters to mitigate DC noise
components.
25. 25
VLC Architecture
• Two integral parts of a VLC system -
transmitter and receiver.
• Layered architecture of three common
layers - Physical Layer, MAC Layer and
Application Layer.
• IEEE 802.15.7 defines only two layers
(PHY and MAC) for simplicity.
27. 27
MAC Layer Tasks
• Mobility support.
• Dimming support.
• Visibility support.
• Security support.
• Schemes for mitigation of flickering.
• Color function support.
• Network beacons generation if the device is a
coordinator.
• VPAN disassociation and association support.
• Providing a reliable link between peer MAC
entities.
29. 29
Physical Layer
• Provides:
- physical specification of device.
- relationship between the device and the
medium.
System Model
30. 30
Physical Layer
• Input bit stream passed through the
channel encoder.
• Linear block codes, convolutional codes
and turbo codes used to enhance VLC
system performance.
• Channel encoded bit stream passed
through line encoder to yield encoded bit
stream.
31. 31
Physical Layer
• Modulation (ON–OFF keying, PPM and
PWM, etc.) performed.
• Finally, data drives LED for transmission
through the optical channel.
• Wavelength Division Multiplexing (WDM)
and Subcarrier Multiplexing (SCM) for bi-
directional transmission.
• Orthogonal Frequency Division Multiplexing
(OFDM) and Quadrature Amplitude
Modulation (QAM) to increase data rate.
32. 32
Modulation Schemes
• Two factors to be considered in the design
of the modulation scheme for VLC :
(a) dimming and
(b) flickering.
• Non-linear relationship between measured
light and perceived light.
34. 34
Modulation Schemes
• Changes in brightness of modulated light
should not result in human-perceivable
fluctuations.
• IEEE 802.15.7 - switching to be done at a
rate faster than 200 Hz to avoid harmful
effects.
36. 36
On-Off Keying
• LEDs turned off and on according to bits in
the stream
• LED not turned completely off in the off
state, but reduction in intensity level.
• Easy implementation.
• Done using white LEDs (a combination of
blue emitter and yellow phosphor).
• Low bandwidth due to slow time response
of the yellow phosphor.
37. 37
On-Off Keying
• Data rate of upto 10Mbps using NRZ OOK
with a white LED.
• Combination of analogue equalization with
blue filtering done to increase data rates
up to 125 Mbps and 100 Mbps.
• Limitation of OOK low data rates
motivated researchers to develop new
modulation techniques.
38. 38
Pulse Modulation Techniques
• PWM – pulse width varied according to
dimming levels.
• Using high PWM frequency, different
dimming levels achieved between 0% and
100%.
• Limitation of PWM - low data rate upto 4.8
Kbps.
• PWM combined with Discrete Multitone
(DMT) for joint communication & dimming
control with higher data rates.
39. 39
Pulse Modulation Techniques
• PPM based on position of the pulse.
• Division of symbol duration into equal
intervals, many slots, transmission of
pulse done in any of the slots.
• PPM suffers from low data rate, other
variants of PPM developed.
• Multi-pulse PPM (MPPM) - transmission of
multiple pulses in each symbol-time, more
spectral efficiency.
40. 40
Pulse Modulation Techniques
• Expurgated PPM (EPPM) - improved
performance of peak-power limited M-ary
communication systems.
• Spectral efficiency of MPPM and EPPM
less than 1.
• Multilevel EPPM (MEPPM) for spectral
effectiveness.
42. 42
Color Shift Keying
(CSK)
• Enhanced data rates.
• Utilizes three separate LEDS - Green, Blue
and Red to produce White Light.
• Modulation using intensity of three colors in
an RGB LED source.
• CSK depends on the color space chromaticity
diagram.
• Maps all colors perceivable by eye into two
chromaticity parameters x and y.
