Optical Transmitters

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2 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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1

Prof. Brandt
-
Pearce


Lecture 3

Transmitters, Receivers, and
Modulation Techniques

Optical Wireless
Communications



Optical Transmitter


LED


Laser


Lamp



Optical Receiver


Detection Techniques:



Direct Detection



Coherent Detection


Photodetectors


p
-
i
-
n


Avalanche Photo Diode (APD)


Photo Multiplier Tube (PMT)



Modulation Techniques


Transmitters/Receivers and
Modulation in FSO Systems

2

3



LED


S
emiconductor device


Medium modulation speed


Incoherent output light


M
ainly used for short range FSO systems (shorter than 1 km)


Laser


H
ighly directional beam profile


Used for long range FSO systems


High modulation speed


Coherent output light


Lamp


Lower efficiency compared to LED and laser


Lower cost


Low modulation speed


Incoherent output light


Provides higher power

Optical Transmitters

4


A

semiconductor

p

n

junction

device

that

gives

off

spontaneous

optical

radiation

when

subjected

to

electronic

excitation


The

electro
-
optic

conversion

process

is

fairly

efficient,

thus

resulting

in

very

little

heat

compared

to

incandescent

lights


M
ainly

used

for

short
-
range

FSO

systems

(shorter

than

1

km)


Ultraviolet

communications


Indoor

FSO

systems

Optical Transmitters: LED




Illustration

of

the

radiated

optical

power

against

driving

current

of

an

LED

5



LED

Types

Optical Transmitters: LED


Dome

LED



Edge
-
Emitting

LED



Planar

LED


6


Laser
:

light

amplification

by

stimulated

emitted

radiation


Has highly directional beam profile


Is used for long range FSO systems


Has narrow spectral width compared to LED


Optical Transmitters: Laser



Laser

output

power

against

drive

current

plot

7



Laser

Types

Optical Transmitters: Laser


Fabry
-
Perot

Laser



Distributed

Feedback

Laser



Vertical
-
cavity

surface
-
emitting

Laser

(VCSEL)


Optical Transmitters

8

9


Can

be

used

in

FSO

communications,

not

in

fiber

optics


Wideband

and

continuous

spectrum


Have

very

high

power,

but

undirected


The

electro
-
optic

process

is

inefficient,

and

huge

amount

of

energy

is

dissipated

as

heat

(causes

high

temperature

in

lamps)


Has

very

low

modulation

bandwidth


D
ivided

as

follows


Carbon

button

lamp



Halogen

lamps



Globar



Nernst

lamp

Optical Transmitters: Lamp

Optical Receivers

The
purpose of the receiver is:


To
convert the optical signal to
electrical domain


Recover
data




Direct
-
Detection Receiver:

10

Coherent
-
Detection Receiver


For

detecting

weak

signal,

coherent

detection

scheme

is

applied

where

the

signal

is

mixed

with

a

single
-
frequency

strong

local

oscillator

signal
.



The

mixing

process

converts

the

weak

signal

to

an

intermediate

frequency

(IF)

in

the

RF

for

improved

detection

and

processing
.



11

Optical Receivers

Photodetectors

12


A

square
-
law

optoelectronic

transducer

that

generates

an

electrical

signal

proportional

to

the

square

of

the

instantaneous

optical

field

incident

on

its

surface



The

ratio

of

the

number

of

electron

hole

(e

h)

pairs

generated

by

a

photodetector

to

the

incident

photons

in

a

given

time

is

termed

the

quantum

efficiency
,

η





Dark current
: the current through the photodiode in the absence of light



Noise
-
equivalent

power

(NEP)
:

the

minimum

input

optical

power

to

generate

photocurrent

equal

to

the

root

mean

square

(RMS)

noise

current

in

a

1

Hz

bandwidth



Responsivity
:

photocurrent

generated

per

unit

incident

optical

power









(W/A)


13

Photodetectors



p
-
i
-
n

photodetector


C
onsists

of

p
-

and

n
-
type

semiconductor

materials

separated

by

a

very

lightly

n
-
doped

intrinsic

region


In

normal

operating

conditions,

a

sufficiently

large

reverse

bias

voltage

is

applied

across

the

device




The

reverse

bias

ensures

that

the

intrinsic

region

is

depleted

of

any

charge

carriers

14

Photodetectors



Avalanche Photo
-
Diode (APD)


provides

an

inherent

current

gain

through

the

process

called

repeated

electron


This

culminates

in

increased

sensitivity

since

the

photocurrent

is

now

multiplied

before

encountering

the

thermal

noise

associated

with

the

receiver

circuit




Multiplication

(or

gain)

factor
:



𝐼
𝑇
:

the

average

value

of

the

total

output

current




𝐼
𝑃
=


𝑅
:

the

primary

unmultiplied

photocurrent


Typical

gain

values

lie

in

the

range

50

300



Excess

noise

factor
:

𝐹
=
𝜅
+
2

1

1

𝜅



𝜅
:

the

ratio

of

the

hole

impact

ionization


rate

to

that

of

electrons


15

Photodetectors



APD
vs

p
-
i
-
n

diode


16

Photodetectors



Photo Multiplier Tube (PMT)


Multiplies

the

current

produced

by

incident

light

by

as

much

as

100

million

times

(i
.
e
.
,

160

dB),

in

multiple

dynode

stages



Enables

individual

photons

to

be

detected

when

the

incident

flux

of

light

is

very

low



S
uperior

in

response

speed

and

sensitivity

(low

light
-
level

detection)



Has

low

quantum

efficiency

and

high

dark

current



Shot Noise


Present

in

all

photon

detectors


Is

associated

with

the

quantum

nature

of

light


The

number

of

photons

emitted

by

all

optical

sources,

including

coherent

source

in

a

given

time

is

never

constant



For

a

constant

power

optical

source,

the

mean

number

of

photons

generated

per

second

is

constant
;

yet

the

actual

number

of

photons

per

second

follows

the

Poisson

distribution


Shot

noise

in

p
-
i
-
n
:

(
A
2

)



Shot

noise

in

APD
:

(A
2

)



q
:

Electron

charge

(coulombs)



B
:

Receiver

equivalent

bandwidth

(Hz)


𝑖
:

mean

of

generated

photo
-
current

(A)

17

Noise in Optical
Receivers

𝜎

2
=
2

𝑖
𝐵

𝜎

2
=
2
𝑖
𝐵𝐹

2


Thermal Noise


Also

known

as

Johnson

noise


Occurs

in

all

conducting

materials


C
aused

by

the

thermal

fluctuation

of

electrons

in

any

receiver

circuit

of

equivalent

resistance



(
Ω
)

and

temperature

𝑇

(K)



W
hite

noise

since

the

power

spectral

density

(PSD)

is

independent

of

frequency



Distributed

as

a

zero

mean

Gaussian

process



Thermal

noise

variance
:

𝜎
𝑇
2
=
4


𝑇
𝑒
𝐵
𝑅
𝐿

(A
2
)



K
:

Boltzmann

Coefficient

(m
2

kg

s
-
2
)

18

Noise in Optical
Receivers


Amplified Spontaneous Emission (ASE) Noise


Produced

by

spontaneous

emission

that

has

been

optically

amplified

by

the

process

of

stimulated

emission

in

a

gain

medium


Inherent

in

lasers

and

optical

amplifiers


ASE

usually

limiting

noise

source

for

high

power

levels


ASE

is

added

to

the

optical

signal

when

it

is

amplified




In

a

nonlinear

medium

interacts

with

signal

and

generates

a

random

output



σ
2
sig
-
sp
:

generated

due

to

the

interaction

of

ASE

and

main

signal


σ
2
sp
-
sp
:


generated

due

to

the

interaction

of

ASE

with

itself

19

Noise in Optical
Receivers


Receiver performance


Definition

of

SNR

given

received

signal

r
(
t
)
:

SNR
=


(

)
2

(

)
2
,
or
power

of

signal
power

of

noise



For

an

optical

receiver

without

any

optical

amplifier,

SNR

can

be

calculated

as
:







SNR =I
p
2

/ (
σ
2
T

+
σ
2
s
)



For

an

optical

receiver

containing

a

p
-
i
-
n

diode

preceded

by

an

EDFA,

SNR

can

be

calculated

as
:







SNR
=I
p
2

/ (
σ
2
T

+
σ
2
s
+
σ
2
sig
-
sp
+
σ
2
sp
-
sp
)



20

Signal to Noise Ratio in Optical
Receivers



Bit

Error

Rate

(BER
)

is

defined

as

the

ratio

of

the

number

of

wrong

bits

over

the

number

of

total

bits
.


Probability

of

error

is

the

theoretically

predicted

expected

BER
.


The

more

the

signal

is

affected,

the

more

bits

are

incorrect
.


The

BER

is

the

fundamental

specification

of

the

performance

requirement

of

a

digital

communication

system


It

is

an

important

concept

to

understand

in

any

digital

transmission

system

since

it

is

a

major

indicator

of

the

health

of

the

system
.



It’s

important

to

know

what

portion

of

the

bits

are

in

error

so

you

can

determine

how

much

margin

the

system

has

before

failure
.

Bit Error Rate and Bit Error Probability

22



Received

signal

is

a

function

of

time

corrupted

by

additive

noise



=


+
𝑛



Optimal

detector

assuming

ideal

channel

and

Gaussian

noise

is

the

matched

filter

(MF)


Often

use

a

low

pass

filter

(LPF)

or

integrator

and

sample
:


Detector for OOK

r
(t)

MF or LPF

X

T
s

Threshold

Decision statistic



Assuming

a

Gaussian

additive

noise

the

probability

of

the

received

signal,

x
,

conditioned

on


0


and


1


are

as

follows

Probability of Error for OOK

μ
1

x

p
1
(
x
)

σ
1
2

μ
0

x

p
0
(
x
)

σ
0
2


μ
1

:

mean

of

x

when

bit


1


is

transmitted




μ
0

:

mean

of

x

when

bit


0


is

transmitted




σ
1
2

:

variance

of

x

when

bit


1


is

transmitted




σ
0
2

:

variance

of

x

when

bit


0


is

transmitted



σ
1
2

can

be

different

from

σ
0
2

(in

most

optical

systems

it

is)



We

need

a

threshold

to

decide

between

bit


0


and

bit


1




The

rule

is
:



If

x

>

“Threshold”,

then

decide

bit


1


was

sent



If

x

<

“Threshold”,

then

decide

bit


0


was

sent

Probability of Error
for OOK

μ
1

p

(
x
)

σ
1
2

μ
0

x

σ
0
2

Optimum Threshold



So

the

error

probability

is




We

need

to

choose

Threshold

such

that

BER

is

minimized




When

μ
0
=
0
,

μ
1
=
A

and

σ
1
2

=
σ
0
2

=
σ
2

,

the

optimal

threshold

is

A
/
2
,

and

BER

becomes

P
e
= Q(
A
/
2
σ
)

where

Q(
.
)

is

Gaussian

error

function






A
2

is

the

energy

received

for

bit


1




σ
2

is

the

energy

of

the

noise


A
2

/
σ
2


is

called

signal

to

noise

ratio

(SNR)

and

A
/
2
σ

is

called

Q
-
factor

(Quality

factor)

Probability of Error
for OOK

A

A/
2

0

Threshold

Decide b=
1

Decide b=
0

26



When

μ
0



0
,

and/or

σ
1
2



σ
0
2
,

the

optimal

threshold

becomes




Then

the

probability

of

error

approximates

as





where

Q(
.
)

is

Gaussian

error

function



Same

as

for

fiber

systems!








Probability of Error
for OOK

Probability of Error
for OOK

28

Modulation Techniques

29


Power

Efficiency


In

portable

battery
-
powered

equipment,

it

is

desirable

to

keep

the

electrical

power

consumption

to

a

minimum,

which

also

imposes

limitations

on

the

optical

transmit

power


Power

efficiency,

𝜂
𝑝
:

the

average

power

required

to

achieve

a

given

BER

at

a

given

data

rate




Peak

to

Average

Power

Ratio

(PAPR)


The

average

optical

power

emitted

by

an

optical

wireless

transceiver

is

limited

due

to

the

eye

and

skin

safety

regulations,

a
nd

power

utilization


Optical

Sources

such

as

laser

and

LED

have

limited

peak

power


PAPR

=
Peak

Power
Average

Power

Important Criteria in FSO

30


Spectral

Efficiency

(Bandwidth

Efficiency)


Although

the

optical

carrier

can

be

theoretically

considered

as

having

an

‘unlimited

bandwidth’,

the

other

constituents

(optical

source

rise
-
time,

photodetector

area)

in

the

system

limit

the

amount

of

bandwidth

that

is

practically

available

for

a

distortion
-
free

communication

system


Also,

the

ensuing

multipath

propagation

in

diffuse

link/
nondirected

LOS

limits

the

available

channel

bandwidth


Spectral

efficiency,

𝜂
𝐵
:

Acheivable

Bit−Rate
Bandwidth

of

the

Transceiver

or

Channel



Reliability


A

modulation

technique

should

be

able

to

offer

a

minimum

acceptable

error

rate

in

adverse

conditions

as

well

as

show

resistance

to

the

multipath
-
induced

inter
-
symbol

interference

(ISI)

(e
.
g
.
,

five

9
s

reliability)


SNR

is

desired

to

be

large

and

BER

be

smaller

than

some

specification

(after

coding)

Important Criteria in FSO

31


Preferred

Modulation

Techniques

in

FSO

Systems


On
-
Off

Keying

(OOK)


Most

common

technique

for

intensity
-
modulation/direct
-
detection

(IM/DD
)


Simple

to

implement,

easy

detection


Requires

a

threshold

to

make

an

optimal

decision
:

a

problem

due

to

time
-
varying

fading


Return
-
to
-
Zero

(
RZ)
:

the

pulse

occupies

only

the

partial

duration

of

bit


Non
-
Return
-
to
-
Zero

(NRZ)
:

a

pulse

with

duration

equal

to

the

bit

duration

is

transmitted

to

represent

1



Transmitted

waveforms

for

OOK
:

(a)

NRZ

and

(b)

RZ


Modulation Techniques: OOK

32


BER

against

the

average

photoelectron

count

per

bit

for

OOK
-
FSO

in

a

Poisson

atmospheric

turbulence

channel

Modulation Techniques: OOK

33


Preferred

Modulation

Techniques

in

FSO

Systems


Pulse
-
Position

Modulation

(PPM)


Orthogonal

modulation

technique


The

symbol

time

divided

into



equal

timeslots



Only

one

of

these

time

slots

contains

a

pulse


Low

spectral

efficiency
:

is

used

in

FSO

links

where

the

requirement

for

the

bandwidth

is

not

of

a

major

concern


Does

not

require

a

threshold

to

make

an

optimal

decision




Transmitted

energy

per

symbol

decreases

in

peak

power

limited

systems

Modulation Techniques: PPM

Symbol
𝑘



For

PPM

we

integrate

over

all

chip

times

and

then

choose

the

maximum


Probability of Error
for PPM



The

error

probability

can

be

written

as





Lets

denote

sampled

value

in

time

chip

i

by

x
i

,

then





This

is

called

union

bound

35

Binary PPM, No Turbulence

For

short
-
range

FSO

systems,

the

BER

is

36

Binary PPM, Turbulence

In

the

presence

of

turbulence,

the

BER

is

bounded

by

37

Modulation Techniques: PPM

BER

versus

the

scintillation

index

38

Preferred Modulation Techniques in FSO Systems


Orthogonal
Frequency Division Multiplexing (
OFDM
)


Harmonically related narrowband sub
-
carriers


S
ub
-
carriers
spaced by
1
/Ts


T
he peak of each sub
-
carrier coincides with
trough

of other sub
-
carriers









Splitting a high
-
speed data stream into a number
of
low
-
speed streams


Different sub
-
carrier transmitted
simultaneously


Guard intervals (CP) are added to reduce ISI effect



Modulation Techniques: OFDM

39


OFDM


Efficiently

utilizes

the

available

bandwidth


Special

version

of

subcarrier

modulation

where

all

the

subcarrier

frequencies

are

orthogonal


Serial

data

streams

are

grouped

and

mapped

into




constellation

symbols,

𝑋
0
,
𝑋
1
,

,
𝑋
[



1
]
,

using

BPSK,

QPSK

or

M
-
QAM
.






:

Number

of

constellation

symbols


N

:

Number

of

orthogonal

subcarriers


Block

diagram

of

an

optical

OFDM




Modulation Techniques: OFDM

40


Challenges

and

problems

with

FSO

systems


Nonlinearity

of

optical

devices

cause

distortion


The

main

drawback

of

OFDM

with

IM/DD

is

its


poor

optical

average

power

efficiency


This

is

because

the

OFDM

electrical

signal


has

both

positive

and

negative

values

and

must

take

on

both

values


A

DC

offset

must

be

added


As

the

number

of

subcarrier

signals

increase,

the

minimum

value

of

the

OFDM

signal

decreases,

becoming

more

negative


Consequently

the

required

DC

bias

increases,

thus

resulting

in

further

deterioration

of

the

optical

power

efficiency


Regarding

the

restrictions

on

the

average

transmitted

optical

power

in

FSO

system,

the

number

of

subcarriers

is

limited


Modulation Techniques: OFDM

41

Modulation Techniques: OFDM

42

M
-
ary

PAM

M
-
ary

PPM

OOK

2

M

2

PAPR

log
2

M

log
2
M/M

1

Spectral Efficiency

Modulation Techniques

Optical

power

gain

over

OOK

versus

bandwidth

efficiency

(first

spectral

null)

for

conventional

modulation

schemes

43



Error

control

coding

(ECC)

is

required

in

communication

systems

to

improve

error

rate
.



Extra

parity

bits

are

added

at

the

transmitter,

so

improved

performance

at

the

expense

of

reduced

spectral

efficiency



At

the

decoder,

errors

can

be

corrected

using

the

redundant

bits



Reed
-
Solomon

and

convolutional

codes

are

conventional

forward

error

correction

(FEC)

schemes

in

optical

links
.



New
:

LDPC

codes

Modulation Techniques

Error Control Coding