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
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