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23 December, 2013

Professor Harald Haas

Li-Fi modulation and networkedLi-Fi attocell concept

Tutorial

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Contributions by

Svilen Dimitrov,

Thilo Fath,

Irina Stefan,

Dobroslav Tsonev,

Stefan Videv,

Wasiu Popoola,

Enrique Poves,

Harald Burchardt,

23 December,

2013

2

Sinan Sinanovic Nikola Serafimovski,

Abdelhamid Younis,

Mostafa Afgani,

Cheng Chen,

Yichen Li,

John Fakidis,

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Wireless data is growing

exponentially

YouTube In 2011, ~ 140 views for every person on earth (over

1 Trillion views

More video is uploaded to YouTube in one month

than the 3 major US networks created in 60 years 72 hours of video are uploaded to YouTube every

minute

25% of global YouTube views come from mobile

devicesSource - http://www.youtube.com/t/press_statistics

Internet video traffic is growing at 48% CAGRSource - Cisco Visual Networking Index: Forecast and Methodology, 2010-2015

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2013

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How fix the problem?

1. Identify new spectrum, and/or

2. Enhance spectrum reuse

(smaller cells)

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2013

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The electromagnetic spectrum

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2013

6.

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0 1 1 1 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0

1 1 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0

Li-Fi

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2013

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Optical Attocell Concept

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2013

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Link-Level Communication System

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2013

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Communication Scenarios

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State-of-the-Art: Wi-Fi

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2013

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Wi-Fi Router Location

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State-of-the-Art: Wi-Fi

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2013

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Wi-Fi Router Location

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2013

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Optical Attocell Network

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2013

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Data rates per device

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2013

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Space per desk 4 m2

100 employees per

floor

Assume a uniform

distribution of

employees

20m x 20m floor = 400

m2

Area can be covered

by a single Wi-Fi AP

Each Li-Fi AP can

cover around 4 m2

Wi-Fi data rate 600

Mbps

Li-Fi data rate 20 Mbps

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Interference Scenario

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2013

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user, k

desired signal

I. Stefan, et al., VTC 2013-Spring, 2013

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Optimisation Framework

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2013

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Rx FOV 85 - without lighting

constraint

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2013

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Rx FOV 85 - with lighting

constraint

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2013

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Rx FOV 45 - without lighting

constraint

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2013

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Rx FOV 45 - with lighting

constraint

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2013

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2013

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23 December,

2013

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Digital Modulation

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December 23,

2013

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Key differences to RF

System

RF

Incoherent

Optical

Information

carried on

electric field

carried on

optical intensity

Signal

bipolar

unipolar

non-negative

complex

valued

real

valued

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Non-linear Characteristic

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2013

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Optical channel

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2013

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Coherence Bandwidth of Channel

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2013

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Path loss

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2013

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bi-directional reflectance

distribution function(BRDF)

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Path loss, contd

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2013

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Optical path gain

Electrical path gain

Path loss

Irradiance

distanceDetector area

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Example: Aircraft cabin / LOS

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2013

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Modulation Techniques

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Transceiver building blocks

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2013

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Pulsed Modulation

On-OFF Keying

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2013

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Time

Intensity

1 1 1 10 0 00 0

On

Off

Thres.

Finite slope limits

achievable data

rates

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Single Carrier Binary

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2013

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Single Carrier Multi-level

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2013

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PPM: BER Performance

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Spectal Efficiency:

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PAM: BER performance

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Spectal Efficiency:

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December 23,2013

43

OFDM-based OWC System

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OFDM

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DCO OFDM and ACO OFDM Symbol

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DCO-OFDM and ACO-OFDM Symbol

Structures

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DCO-OFDM

ACO-OFDM

2

1

2

1

21

21

2

2

1

1

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ACO-OFDM Time Domain Signal

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QAM vs. CAP

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Not feasible if Cis much

greater than the symbol

frequency

QAM

CAP

A. H. Abdolhamid, et al. A Comparison of QAM/CAP Architectures, 1998

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December 23,2013

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OFDM Generation (Time Domain)

After the IFFT, the signal follows a zero-mean Gaussiandistribution in the time domain:

Probability density

functionTime-domain signal

- - --

-

-

-

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Multi-carrier Multi-level

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UPVLC Results (assuming single

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UPVLC Results (assuming single

colour LED)

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December 23,2013

52

Bussgang Theorem

X is a zero-mean Gaussian random variable with variance

and g(X) is an arbitrary transform on X, which could be

linear or nonlinear.

The Bussgang theorem:

Then:

2n2nnn

2222n

2

EEVar

gEE

gEE

gE

YYY

XY

KXY

XXK

0E

g

n

n

XY

YKXX

no

old

b

2

new

o

new

b

VarYN

EK

N

E

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December 23,2013

53

Redefintion of the Distortion

Any arbitrary distortion function g(x) can be represented with

a set of intervals I and a number of continuous polynomials

which describe the function in those intervals.

Then g(X) becomes:

where nkis the order of the polynomial in interval k, and

U(x) is theunit step function:

I

k

n

j

kkj

jk

k

xxxxxcx1 0

,max,min, UUg

0,1

0,0U

x

xx

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54

Examples

3-bit DAC:

Clipping and LED

current-to-light

conversion:

5.1UU3

U5.1U1

5.1UU1

U5.1U3g

xx

xx

xx

xxx

1UU13U1U571

U3U3g

2

xx

xxxx

xxx

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The equations from the Bussgang analysis become:

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55

Closed Form Solutions

I

k

n

jt

j

kk

j

jk

k

t

xxtcK

1 00

1

max,min,

1

,2d

,0,,,Dd1

I

k

n

jt

j

kk

j

jk

k

t

xxtcXg

1 00

max,min,

,d

,0,,,DdE

I

k

n

jt

n

mmj

kkmj

mkjk

k k

t

xxtccXg

1 00

0

max,min,

,,

2

d

,0,,,DdE

Tsonev, et al., JLT, 2013

Channel Capacity: Optimisation

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Channel Capacity: Optimisation

Frameworks

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Results

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Spectral Efficiencies

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Spectral Efficiencies, contd

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Implications on Dimming

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Optical Output Power

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Spectral Efficiency of OFDM

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Theoretical Capacity limits, contd

22 July 2013

For 10 dB dynamic range:

(1): with optimisation, DC

bias power notincluded(2): without optimisation, DC

bias power notincluded

(3): with optimisation, DC

bias power included

(4): without optimisation, DC

bias power included

Spatial Modulation: How does it

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Spatial Modulation: How does it

work?

12/23/201366

00(00)

01(00)

10(00)

11(00)

00(11)

01(11)

10(11)11(11)

00 (Tx0)

01 (Tx1)

10 (Tx2)

11 (Tx3)

Signal Constellation

Spatial Constellation

Re

Im

Im

Im

Re

Re

The University of Edinburgh

Spectral Efficiency:

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Spatial Modulation OFDM

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Thank You!