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


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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Wi-Fi can cause interference on sensitive devices

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There is national RF quite zone


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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


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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


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White Light LEDs

LED lamps are becoming popular because of their low energy consumption and longevity


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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


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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.


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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


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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


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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


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Another Cool Application

Car to Car Communication through headlights and taillights

Intelligent Traffic System


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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


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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)


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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


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


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


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


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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


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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


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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


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Optical Peak Power

Optical wireless system model

Clipping the OFDM signal at 0 and Pmax adds a clipping noise


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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


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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


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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)


M. Noshad, M. Brandt-Pearce, IEEE Global Communications Conference, Dec 2014

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HCM Receiver

Receiver is implemented using inverse FWHT (IFWHT)


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HCM versus OFDM

BER of HCM vs ACO-OFDM and DCO-OFDM

Can we shift HCM to the left?


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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


M. Noshad, M. Brandt-Pearce, IEEE Transactions on Communications, Feb 2016.

Maite Brandt-Pearce

DC, not DR

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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


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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.


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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


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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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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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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?

ut talk feb 2017

Engineering