Download - Ut talk feb 2017
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Mohammad Noshad
Wireless Connectivity Through Lighting: Potential and Limitations
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Outline
Motivation
Introduction to Visible Light Communications (VLC)
Applications
Challenges
Modulation and Coding Techniques
Networking in VLC Systems
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Wi-Fi is the technology that provides wireless internet access.
It is very popular
Current frequency bands for Wi-Fi (IEEE 802.11) 2.4, 5, or 60 (mm-wave) GHz
Wi-Fi Technology Introduction Applications Challenges Modulation & Coding VLC Networks
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What is Wrong with Wi-Fi?
Throughput limitation of Wi-Fi The data-rate in Wi-Fi systems (2.4 and 5 GHz bands) is limited to 600 Mb/s This speed is divided among all users
Supports limited number of users in crowded areas Each Wi-Fi access point can support limited number of users Limited bandwidth reuse
Introduction Applications Challenges Modulation & Coding VLC Networks
What is Wrong with Wi-Fi?
Wi-Fi signals passes through the walls It has low security against eavesdropping Need password for security Currently there is no wireless internet access in some buildings because of
the security concerns
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Introduction Applications Challenges Modulation & Coding VLC Networks
What is Wrong with Wi-Fi?
Wi-Fi can cause interference on sensitive devices
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There is national RF quite zone
Introduction Applications Challenges Modulation & Coding VLC Networks
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What is Wrong with Wi-Fi?
There is an increasing health concern due to exposure to Wi-Fi: Your body is exposed to Wi-Fi 24/7 It passes through the skin – might be risky for neural system It is a concern for children and pregnant women Germany, Australia, France and some schools in Canada and US have banned
Wi-Fi use at schools
Introduction Applications Challenges Modulation & Coding VLC Networks
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Other Alternative: Optical Communications Introduction Applications Challenges Modulation & Coding VLC Networks
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Other Alternative: Optical Communications
Faster than other forms of communications Backbone of modern telecommunications Wireless optical communications has many applications
Introduction Applications Challenges Modulation & Coding VLC Networks
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White Light LEDs
LED lamps are becoming popular because of their low energy consumption and longevity
Introduction Applications Challenges Modulation & Coding VLC Networks
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Goal: Use the lighting system also for communications Transmission requires energy, yet energy is already going from infrastructure to user in the form of light.
Visible Light Communications (VLC) Introduction Applications Challenges Modulation & Coding VLC Networks
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LEDs are good candidates for VLC Low cost High-Speed Safe for eye
Two technologies in white LEDs Phosphor-based
Cheap
Trichromatic High-speed Color control
Visible Light Communications (VLC) Introduction Applications Challenges Modulation & Coding VLC Networks
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Applications and Potentials
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Advantages
Advantages of VLC Introduction Applications Challenges Modulation & Coding VLC Networks
Safe for human body
High security
Can provide high-speed connection (1 Gb/s)
No interference on RF signals
Environmentally “green”
Can providing accurate (~1 mm) indoor positioning system
(indoor GPS)
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Indoor Wireless Networks
Home Office Library Airplane Hospital Convention Center
Introduction Applications Challenges Modulation & Coding VLC Networks
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Future VLC Products
VLC-enabled LED bulbs and LED panels Introduction Applications Challenges Modulation & Coding VLC Networks
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Future VLC Products
VLC-enabled floor-lamps and desk lamps can be easily bought from the market and installed.
Introduction Applications Challenges Modulation & Coding VLC Networks
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VLC modems will be used to connect users to the VLC network
Future VLC Products Introduction Applications Challenges Modulation & Coding VLC Networks
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VLC models will be integrated into smart-phones Small modems will be presented to the marked for older
smartphones to stablish connection.
Future VLC Products Introduction Applications Challenges Modulation & Coding VLC Networks
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Another Cool Application
Car to Car Communication Intelligent Traffic System
Introduction Applications Challenges Modulation & Coding VLC Networks
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Visible Light Positioning
Signals transmitted by LEDs
can be used to estimate the
position of a receiver relative
to the LEDs
Receiver receives signals from
one or more lights
Required accuracy depends on
application
Introduction Applications Challenges Modulation & Coding VLC Networks
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Visible Light Positioning Introduction Applications Challenges Modulation & Coding VLC Networks
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VLC Opens Other Potential Applications
Users are connected to lights
Lighting features can be simply controlled from the mobile device Dim the lights
Control hue of the light
Smart lighting is much easier with VLC There are photodetectors on the user device and on the luminaries
Photodetectors sense the intensity of the light
The lights can control the illumination level automatically
Internet of things can be developed on VLC networks All the electronic devices can connect to access-points using their LEDs
Access-points know the location of all devices
Introduction Applications Challenges Modulation & Coding VLC Networks
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Another Cool Application
Car to Car Communication through headlights and taillights
Intelligent Traffic System
Introduction Applications Challenges Modulation & Coding VLC Networks
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Challenges
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Shadowing
Shadowing is when the direct path from user to luminary is blocked
Channel estimation Equalizer is needed to decrease inter-symbol interference (ISI) effect Channel estimation is required to implement optimum equalizer We plan to use position of users to estimate channel response
Introduction Applications Challenges Modulation & Coding VLC Networks
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Connectivity
Users should stay connected when they move
There are overlaps between the coverage of luminaries (This is due to lighting standards)
Introduction Applications Challenges Modulation & Coding VLC Networks
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Uplink Connection
Visible light is used for downlink
It is not desirable to use white light for uplink
There are two options:
Infrared
Dimmed white light
Introduction Applications Challenges Modulation & Coding VLC Networks
Lighting Standards
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The illumination should satisfy some standards
There is a required level of light for each environment
The distribution of light should be uniform
Introduction Applications Challenges Modulation & Coding VLC Networks
Dimming Control
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Dimming is a feature required in modern lighting systems
The level of the light should be controllable by the user
Introduction Applications Challenges Modulation & Coding VLC Networks
Background Light
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Strong background light can affect the performance of the system
The system should operate when it is exposed to extreme Sun light
Introduction Applications Challenges Modulation & Coding VLC Networks
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Other Challenges Introduction Applications Challenges Modulation & Coding VLC Networks
Dimming is a feature required in modern lighting systems
The level of the light should be controllable by the user
The system should operate when it is exposed to extreme Sun light
Background light (direct Sun exposure)
Dimming The illumination should
satisfy some standards There is a required level of
light for each environment The distribution of light should
be uniform
Lighting Standards
We address all of these by modulation and coding
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Modulation and Coding
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In VLC systems, peak and average powers are given
Modulations for VLC Systems
Time
Opt
ical
Pow
er
Peak Power
Average Power
Peak to average power ratio (PAPR) is fixed
This is determined by dimming level
Adaptive modulation techniques are needed
Introduction Applications Challenges Modulation & Coding VLC Networks
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LEDs are the main sources Modulations that can be used are different from RF
systems Two families of modulations are used:
Pulsed techniques: On-off keying (OOK), Pulse amplitude modulation (PAM) Pulse position modulation (PPM)
Subcarrier techniques: Orthogonal frequency division multiplexing (OFDM) Hadamard Coded Modulation (HCM)
Modulations for VLC SystemsIntroduction Applications Challenges Modulation & Coding VLC Networks
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Orthogonal Frequency Division Multiplexing (OFDM)
Special version of subcarrier modulation where all the subcarrier frequencies are orthogonal
Efficiently utilizes the available bandwidth
Introduction Applications Challenges Modulation & Coding VLC Networks
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Original OFDM cannot be used in incoherent optical systems It generates complex-valued signals Incoherent optical sources can only transmit positive real
signals Modified forms of OFDM
OFDM for Optical Systems
Asymmetrically Clipped Optical (ACO) OFDM Apply Hermitian symmetry Modulate only half of the data
DC-biased Optical (DCO) OFDM Apply Hermitian symmetry to make signal real Add a DC bias and clip the negative part
rate of ≈1/2
rate of ≈ 1/4
Introduction Applications Challenges Modulation & Coding VLC Networks
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OFDM for Optical Systems
Optical Peak Power
Optical wireless system model
Clipping the OFDM signal at 0 and Pmax adds a clipping noise
Introduction Applications Challenges Modulation & Coding VLC Networks
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OFDM for Optical Systems
Consider two systems:1) Optical wireless communications requiring high average
power levels, such as VLC systems2) Optical systems with unconstrained energy usage
OFDM does not work well in these two cases
Low average power
High average power
Introduction Applications Challenges Modulation & Coding VLC Networks
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OFDM signal generation
Why not use another orthogonal basis function?Hadamard matrix is an option
Add a vector to make signals positive Each component of u is
M-ary PAM modulated
Hadamard Coded Modulation (HCM): An Alternative for OFDM
Introduction Applications Challenges Modulation & Coding VLC Networks
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HCM Trnasmitter
Can be implemented with fast Walsh-Hadamard transform (FWHT) FWHT has the same complexity as FFT: O(N logN)
Introduction Applications Challenges Modulation & Coding VLC Networks
M. Noshad, M. Brandt-Pearce, IEEE Global Communications Conference, Dec 2014
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HCM Receiver
Receiver is implemented using inverse FWHT (IFWHT)
Introduction Applications Challenges Modulation & Coding VLC Networks
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HCM versus OFDM
BER of HCM vs ACO-OFDM and DCO-OFDM
Can we shift HCM to the left?
Introduction Applications Challenges Modulation & Coding VLC Networks
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Improving Energy Efficiency
The first row of Hadamard matrix is all ones Adding or removing a DC value has no effect on (N-1) of the columns
We use only N-1 rows of Hadamard to modulate the data
Send (x – min x) instead of x
Then we can reduce the energy of transmitted signals
New technique is named DR-reduced (DCR) HCM
0
u2
u1
u3u4u5u6u7
Introduction Applications Challenges Modulation & Coding VLC Networks
M. Noshad, M. Brandt-Pearce, IEEE Transactions on Communications, Feb 2016.
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Improving Energy Efficiency Motivation Past Research Existing Solutions Proposed Techniques Results
HCM signal DCR-HCM signal
DC part
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DCR-HCM vs HCM
Probability mass function for HCM and DCR-HCM
M. Noshad, M. Brandt-Pearce
HCM DCR-HCM
HCM generates large signals with higher probability
Introduction Applications Challenges Modulation & Coding VLC Networks
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BER of HCM and DCR-HCM vs ACO-OFDM and DCO-OFDMSpectral efficiency = 1
HCM versus OFDMIntroduction Applications Challenges Modulation & Coding VLC Networks
M. Noshad, M. Brandt-Pearce, IEEE Transactions on Communications, Feb 2016.
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HCM versus OFDM
Maximum achievable SNR versus the spectral efficiency for HCM, DCR-HCM, ACO-OFDM and DCO-OFDM.
Introduction Applications Challenges Modulation & Coding VLC Networks
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VLC Networks
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VLC Networks
Providing network access for large number of users is essential is VLC systems
Introduction Applications Challenges Modulation & Coding VLC Networks
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Multiuser VLC Networks
LEDs are used in VLC networks Frequency-domain processing may be expensive and
complicated They are nonlinear Time-based modulations, specially pulsed techniques, are
preferred
Current Solutions for Multiuser Support Time division multiple access Code division multiple access Spatial division multiple access
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Solutions for Multiuser Support
Time division multiple access Low data-rate
Inflexible data-rates
Does not support dimming
Code division multiple access
Spatial division multiple access
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Solutions for Multiuser Support
Time division multiple access
Code division multiple access Time spreading codes are preferred Optical orthogonal codes (OOC)
Are designed for fiber-optic communications Codes have the same length and weight Cross-correlation is equal or smaller than 1
For fixed number of users it is hard to control dimming
Spatial division multiple access
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Solutions for Multiuser Support
Time division multiple access
Code division multiple access
Spatial division multiple access Uses narrow optical beams The area is divided into sub-spaces
Face problem when two users are in one sector It needs to be used with another multiple access technique
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Conclusions
Wi-Fi cannot satisfy the increasing demand for high-speed internet
access
VLC is strong contender for future wireless networks
Modulation and networking techniques need to be adapted to VLC
systems because of their constraints
There is still much to be done on the networking aspects of VLC
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Questions?