Li-Fi modulation and networked Li-Fi attocell · PDF file23 December, 2013 Professor Harald Haas Li-Fi modulation and networked Li-Fi attocell concept Tutorial TexPoint fonts used

23 December, 2013

Professor Harald Haas

Li-Fi modulation and networked

Li-Fi attocell concept

Tutorial

TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAA

Contributions by…

Svilen Dimitrov,

Thilo Fath,

Irina Stefan,

Dobroslav Tsonev,

Stefan Videv,

Wasiu Popoola,

Enrique Poves,

Harald Burchardt,

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2013

2

Sinan Sinanovic

Nikola Serafimovski,

Abdelhamid Younis,

Mostafa Afgani,

Cheng Chen,

Yichen Li,

John Fakidis,

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

devices Source - http://www.youtube.com/t/press_statistics

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

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… leading to RF Spectrum Shortage

Global Mobile Data Traffic

How fix the problem?

1. Identify new spectrum, and/or

2. Enhance spectrum reuse

(smaller cells)

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2013

5

The electromagnetic spectrum

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2013

6

.

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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8

Optical Attocell Concept

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9

Link-Level Communication System

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Communication Scenarios

State-of-the-Art: Wi-Fi

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13

Wi-Fi Router Location

State-of-the-Art: Wi-Fi

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Wi-Fi Router Location

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Optical Attocell Network

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Data rates per device

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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

Interference Scenario

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20

user, k

desired signal

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

Optimisation Framework

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Rx FOV 85° - without lighting

constraint

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Rx FOV 85° - with lighting

constraint

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Rx FOV 45° - without lighting

constraint

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Rx FOV 45° - with lighting

constraint

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Digital Modulation

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28

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

Non-linear Characteristic

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Optical channel

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Coherence Bandwidth of Channel

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Path loss

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bi-directional reflectance

distribution function

(BRDF)

Path loss, cont’d

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Optical path gain

Electrical path gain

Path loss

Irradiance

distance Detector area

Example: Aircraft cabin / LOS

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Example: Aircraft cabin / NLOS

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Modulation Techniques

Transceiver building blocks

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Pulsed Modulation

On-OFF Keying

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Time

Intensity

1 1 1 1 0 0 0 0 0

On

Off

Thres.

Finite slope limits

achievable data

rates

Single Carrier Binary

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Single Carrier Multi-level

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PPM: BER Performance

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Spectal Efficiency:

PAM: BER performance

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Spectal Efficiency:

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OFDM-based OWC System

OFDM

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DCO-OFDM and ACO-OFDM Symbol

Structures

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DCO-OFDM

ACO-OFDM

𝑥𝑁2−1

𝑥𝑁2 𝑥∗1

𝑥∗𝑁2−1

𝑥𝑁2−1

𝑥𝑁2

𝑥∗𝑁2−1

𝑥∗1

ACO-OFDM Time Domain Signal

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QAM vs. CAP

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Not feasible if C is much

greater than the symbol

frequency

QAM

CAP

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

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OFDM Generation (Time Domain)

After the IFFT, the signal follows a zero-mean Gaussian

distribution in the time domain:

Probability density

function Time-domain signal

-6 -4 -2 0 2 4 6-6

-4

-2

0

2

4

6

Quadra

ture

In-Phase

Scatter plot

Multi-carrier Multi-level

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DC Bias and Signal Power

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𝛽DC

𝜎

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UPVLC Results (assuming single

colour µLED)

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Bussgang Theorem

X is a zero-mean Gaussian random variable with variance σ

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

linear or nonlinear.

The Bussgang theorem:

Then:

2n

2

nn

n

2222

n

2

EEVar

gEE

gEE

gE

YYY

XY

KXY

XXK

0E

g

n

n

XY

YKXX

no

old

b

2

new

o

new

b

Var YN

EK

N

E

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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 n k is the order of the polynomial in interval k, and

U(x) is the

unit step function:

I

k

n

j

kk

j

jk

k

xxxxxcx1 0

,max,min, UUg

0,1

0,0U

x

xx

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Examples

3-bit DAC:

Clipping and LED

current-to-light

conversion:

5.1UU3

U5.1U1

5.1UU1

U5.1U3g

xx

xx

xx

xxx

1UU1

3U1U571

U3U3g

2

xx

xxxx

xxx

The equations from the Bussgang analysis become:

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Closed Form Solutions

I

k

n

jt

j

kk

j

jk

k

t

xxtcK

1 00

1

max,min,

1

,2 d

,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

kk

mj

mkjk

k k

t

xxtccXg

1 00

0

max,min,

,,

2

d

,0,,,DdE

Tsonev, et al., JLT, 2013

Channel Capacity: Optimisation

Frameworks

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Results

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Spectral Efficiencies

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Spectral Efficiencies, cont’d

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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, cont‘d

22 July 2013

For 10 dB dynamic range:

(1): with optimisation, DC

bias power not included

(2): without optimisation, DC

bias power not included

(3): with optimisation, DC

bias power included

(4): without optimisation, DC

bias power included

Spatial Modulation: How does it

work?

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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:

Spatial Modulation OFDM

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Thank You!