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,

23 December,

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

23 December,

2013

3

23 December,

2013

4

… 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)

23 December,

2013

5

The electromagnetic spectrum

23 December,

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

23 December,

2013

8

Optical Attocell Concept

23 December,

2013

9

Link-Level Communication System

23 December,

2013

12

Communication Scenarios

State-of-the-Art: Wi-Fi

23 December,

2013

13

Wi-Fi Router Location

State-of-the-Art: Wi-Fi

23 December,

2013

14

Wi-Fi Router Location

23 December,

2013

15

Optical Attocell Network

23 December,

2013

16

Data rates per device

23 December,

2013

17

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

23 December,

2013

20

user, k

desired signal

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

Optimisation Framework

23 December,

2013

21

Rx FOV 85° - without lighting

constraint

23 December,

2013

22

Rx FOV 85° - with lighting

constraint

23 December,

2013

23

Rx FOV 45° - without lighting

constraint

23 December,

2013

24

Rx FOV 45° - with lighting

constraint

23 December,

2013

25

23 December,

2013

26

23 December,

2013

27

Digital Modulation

December 23,

2013

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

23 December,

2013

29

Optical channel

23 December,

2013

30

Coherence Bandwidth of Channel

23 December,

2013

31

Path loss

23 December,

2013

32

bi-directional reflectance

distribution function

(BRDF)

Path loss, cont’d

23 December,

2013

33

Optical path gain

Electrical path gain

Path loss

Irradiance

distance Detector area

Example: Aircraft cabin / LOS

23 December,

2013

34

Example: Aircraft cabin / NLOS

23 December,

2013

35

23 December,

2013

36

Modulation Techniques

Transceiver building blocks

23 December,

2013

37

Pulsed Modulation

On-OFF Keying

December 23,

2013

38

Time

Intensity

1 1 1 1 0 0 0 0 0

On

Off

Thres.

Finite slope limits

achievable data

rates

Single Carrier Binary

23 December,

2013

39

Single Carrier Multi-level

23 December,

2013

40

PPM: BER Performance

23 December,

2013

41

Spectal Efficiency:

PAM: BER performance

23 December,

2013

42

Spectal Efficiency:

December 23,

2013

43

OFDM-based OWC System

OFDM

23 December,

2013

44

DCO-OFDM and ACO-OFDM Symbol

Structures

23 December,

2013

45

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

23 December,

2013

46

QAM vs. CAP

23 December,

2013

47

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

December 23,

2013

48

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

23 December,

2013

49

DC Bias and Signal Power

23 December,

2013

50

𝛽DC

𝜎

23 December,

2013

51

UPVLC Results (assuming single

colour µLED)

December 23,

2013

52

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

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 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

December 23,

2013

54

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:

December 23,

2013

55

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

23 December,

2013

56

Results

23 December,

2013

57

Spectral Efficiencies

23 December,

2013

58

Spectral Efficiencies, cont’d

23 December,

2013

59

Implications on Dimming

23 December,

2013

60

Optical Output Power

23 December,

2013

62

Spectral Efficiency of OFDM

23 December,

2013

63

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?

12/23/2013 66

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

23 December,

2013

67

23 December,

2013

68

Thank You!