1. 4S Communica(ons Believes
In
Engineering
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2. Modula2on
Schemes
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3. What
is
Modula2on?
Modula'on
is
a
process
of
mixing
a
signal
with
a
sinusoid
to
produce
a
new
signal.
This
new
signal,
possibly,
will
have
certain
benefits
of
an
un-­‐modulated
signal,
especially
during
transmission.
If
we
look
at
a
general
func?on
for
a
sinusoid:
Information Signal Modulated Signal
we
can
see
that
this
sinusoid
has
3
parameters
that
can
be
altered,
to
Modulator
affect
the
shape
of
the
graph.
The
first
term,
A,
is
called
the
magnitude,
or
amplitude
of
the
sinusoid.
The
next
term,
is
known
as
the
frequency,
and
the
last
term,
is
known
as
the
phase
angle.
All
3
parameters
can
be
altered
to
transmit
data. Carrier Signal
The
sinusoidal
signal
that
is
used
in
the
modula?on
is
known
as
the
carrier
signal,
or
simply
"the
carrier".
The
signal
that
is
used
in
modula?ng
the
carrier
signal(or
sinusoidal
signal)
is
known
as
the
"data
signal"
or
the
"message
signal".
It
is
important
to
no?ce
that
a
simple
sinusoidal
carrier
contains
no
informa?on
of
its
own.
In
other
words
we
can
say
that
modula?on
is
used
because
the
some
data
signals
are
not
always
suitable
for
direct
transmission,
but
the
modulated
signal
may
be
more
suitable.
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4. Types
of
Modula2on
There
are
3
basic
types
of
modula6on:
1.
AM
(Amplitude
Modula6on:
a
type
of
modula6on
where
the
amplitude
of
the
carrier
signal
is
modulated
(changed)
in
propor6on
to
the
message
signal
while
the
frequency
and
phase
are
kept
constant)
2.
FM
(Frequency
Modula6on:
a
type
of
modula6on
where
the
frequency
of
the
carrier
signal
is
modulated
(changed)
in
propor6on
to
the
message
signal
while
the
amplitude
and
phase
are
kept
constant)
3.
PM
(Phase
Modula6on:
a
type
of
modula6on
where
the
phase
of
the
carrier
signal
is
varied
accordance
to
the
low
frequency
of
the
message
signal
is
known
as
phase
modula6on)
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5. Amplitude
Modula2on
 Amplitude
modula6on
(AM)
occurs
Carrier#Signal# Actual#Signal#
when
the
amplitude
of
a
carrier
wave
2#
is
modulated,
to
correspond
to
a
1.5#
1#
informa6on
signal. 0.5#
0#
!0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200#
 Amplitude
modula6on
requires
a
high
!1#
!1.5#
frequency
constant
carrier
and
a
low
!2#
frequency
modula6on
(informa6on)
3.5%
Amplitude%Modula8on%
signal. 2.5%
1.5%
0.5%
 Normalized
equa6on
for
amplitude
!0.5% 0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200%
modula6on: !1.5%
!2.5%
!3.5%
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6. Frequency
Modula2on
Carrier#Signal# Actual#Signal#
2#
 Frequency
modula6on
(AM)
occurs
1.5#
1#
when
the
frequency
of
a
carrier
0.5#
wave
is
modulated,
to
correspond
to
0#
!0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200#
a
informa6on
signal. !1#
!1.5#
 In
general
the
frequency
of
the
!2#
Frequency#Modula5on#
carrier
wave
is
varied
in
accordance
2#
with
the
amplitude
and
phase
of
the
1.5#
1#
input
signal,
the
amplitude
of
the
0.5#
0#
carrier
remaining
unchanged. !0.5# 0# 50# 100# 150# 200#
!1#
!1.5#
!2#
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7. Phase
Modula2on
Carrier#Signal# Actual#Signal#
 Phase
modula6on
(AM)
occurs
2#
1.5#
when
the
phase
of
a
carrier
wave
1#
0.5#
is
modulated,
to
correspond
to
a
0#
!0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200#
informa6on
signal. !1#
!1.5#
 In
general
the
phase
of
a
carrier
!2#
Phase#Modula5on#
wave
is
varied
by
an
amount
2#
propor6onal
to
the
instantaneous
1.5#
1#
amplitude
of
the
modula6ng
0.5#
(informa6on)
signal 0#
0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200#
!0.5#
!1#
!1.5#
!2#
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8. Phase
shiB
keying
(PSK)
In
PSK,
we
change
the
phase
of
the
sinusoidal
carrier
to
indicate
informa6on.
Phase
in
this
context
is
the
star6ng
angle
at
which
the
sinusoid
starts.
To
transmit
0,
we
shiO
the
phase
of
the
sinusoid
by
180°.
Phase
shiO
represents
the
change
in
the
state
of
the
informa6on
in
this
case.
1
-1
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9. Binary
phase
shiB
keying
(BPSK)
 BPSK
(also
called
2PSK)
is
the
simplest
form
of
phase
shiO
keying
(PSK).
It
uses
two
phases
which
are
separated
by
180°
and
so
can
also
be
termed
2-­‐PSK.
It
does
not
par6cularly
maTer
exactly
where
the
constella6on
points
are
posi6oned,
and
in
this
figure
they
are
shown
on
the
real
axis,
at
0°
and
180°.
This
modula6on
is
the
most
robust
of
all
the
PSKs
since
it
takes
the
highest
level
of
noise
or
distor6on
to
make
the
demodulator
reach
an
incorrect
decision.
It
is,
however,
only
able
to
modulate
at
1
bit/symbol
and
so
is
unsuitable
for
high
data-­‐rate
applica6ons
when
bandwidth
is
limited. Informa'on
Bit:
0
1
0
1
0
0
1
0
1
0
1
0
 Modula6ng
angle
is
defined
as:
(for
BPSK:
M=
); 0
1
Where
M
=
Modula6on
Order
Constella'on
Diagram
For
BPSK:
n
=
1(implies
you
can
only
transmit
1
bit)
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10. Quadrature
phase
shiB
keying
(QPSK)
 Quadrature
Phase
ShiO
Keying
(QPSK)
is
a
form
of
Phase
ShiO
Keying
in
which
two
bits
are
modulated
at
once,
selec6ng
one
of
four
possible
carrier
phase
shiOs
(0,
90,
180,
or
270
degrees).
QPSK
allows
the
signal
to
carry
twice
as
much
informa6on
as
ordinary
PSK
using
the
same
bandwidth.
Using
QPSK
we
can
transmit
2
bits/symbol.
10
 Modula6ng
angle
is
defined
as:
(for
QPSK:
M=
) 11 00
01
Where
M
=
Modula6on
Order
Constella'on
Diagram
For
BPSK:
n
=
2
(implies
you
can
only
transmit
2
bit)
Informa6on
Bit
Example:
01
01
00
10
00
10
11
10
10
10
10
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11. Other
Modula2ons
Used
 8-­‐PSK
 16QAM
 64QAM
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12. Mul2ple
Access
Schemes
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13. Introduc2on
to
Mul2ple
Access
 Different
Mul6plex
Schemes
 FDMA
 TDMA
 OFDMA
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14. Frequency
Division
Mul2ple
Access
(FDMA)
• In
FDMA
users
are
assigned
specific
frequency
bands.
Once
assigned,
the
user
has
the
sole
right
of
using
the
frequency
band
for
the
en6re
dura6on
of
a
call.
• Typically
many
users
are
supported,
due
to
the
rela6vely
narrow
spectrum
alloca6on
per
user.
The
Uplink
or
Downlink
receiver
must
use
filtering
to
mi6gate
interference
from
other
users.
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15. Time
Division
Mul2ple
Access
(TDMA)
• Each
user
is
allowed
to
transmit
only
within
specified
6me
intervals
(Time
Slots).
Different
users
transmit
in
different
Time
Slots.
• When
users
transmit,
they
occupy
the
whole
frequency
bandwidth
(separa6on
among
users
is
performed
in
the
6me
domain).
• Like
FDMA,
filtering
is
required
for
both
the
Uplink
and
Downlink
receiver
to
separate
adjacent
carriers.
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16. Orthogonal
Frequency
Division
Mul2plexing
(OFDM)
• OFDM
uses
a
large
number
of
closely
spaced
narrowband
carriers.
In
a
conven6onal
FDM
system,
the
frequency
spacing
between
carriers
is
chosen
with
a
sufficient
guard
band
to
ensure
that
interference
is
minimized
and
can
be
cost
effec6vely
filtered.
In
OFDM,
however,
the
carriers
are
packed
much
closer
together.
This
increases
spectral
efficiency
by
u6lizing
a
carrier
spacing
that
is
the
inverse
of
the
symbol
or
modula6on
rate.
• High
data
rates
are
achieved
in
OFDM
by
alloca6ng
a
single
data
stream
in
a
parallel
manner
across
mul6ple
subcarriers.
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17. FDMA
vs
OFDM
FDMA
Carrier1 Carrier2 Carrier3 Carrier4 Carrier5
OFDM
Spectral Efficiency
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18. OFDM
to
OFDMA
• Orthogonal Frequency Division Multiple Access (OFDMA) is a form of OFDM. The
description of OFDM up to this point has defined that all subcarriers are assigned to a single
user during each subframe, where a subframe is some number of OFDM symbols. In
OFDMA, however, multiple users can be assigned subcarriers during the same subframe.
Sub Carriers
OFDM OFDMA
Frequency
Frequency
Sub Carriers
Subframe3
Subframe1 Subframe2 Subframe1 Subframe2 Subframe3
Time Time
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19. Example
on
how
sub
carriers
are
assigned:
• For
example,
consider
a
system
consis6ng
of
18
subcarriers.
• In
OFDM
all
18
subcarriers
would
be
assigned
to
a
single
user
during
each
subframe.
• In
OFDMA,
based
on
both
user
demand
and
channel
condi6ons,
different
users
can
be
given
different
groups
of
subcarriers
during
each
subframe.
• For
example,
3
users
may
be
assigned
6
subcarriers
each
during
one
subframe.
During
the
next
subframe,
user
1
may
be
assigned
12
subcarriers,
user
2
may
be
assigned
6,
and
user
3
may
be
assigned
0.
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20. Ques2ons???
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