Digital Communications Fundamentals and Applications in CTE Department 2018-2019
رهاط
ةزمحلا
.م :
ةداملا
سردم
1
-
2
01
8-
20
1
9
Dr.
Hussam
Dheaa
Kamel
Al-Mustafa University
Collage
CTE Department
2018-2019
Digital
Communications
CTE Department
-3
rd
stage
Reference:
Digital
Communications
Fundamentals
and
Applications,
2
nd
Addition,
by
FernardSklar
2
Digital
Communications
CTE
Department -3
rd
stage
2- Binary
Amplitude
Shift
Keying (BASK)
or
ON- OFF keying
(OOK):
ASK
or
OOK
is the
simplest digital
modulation
technique. The ASK
waveform can
be represented
as:
𝑠
(
𝑡
)
=
√
2𝑃
𝑠
cos
(
2𝜋𝑓
0
𝑡
)
to
transmit symbol
"1",
and
pulse is transmitted.
To
transmit symbol
"0"
𝑠
(
𝑡
)
= 0
,
that is
no
signal transmitted
for
such symbol. Thus ASK waveform looks like
an
ON-OFF
of
the signal
as
shown in figure
10.
Figure
10
The ASK
wave form for
symbol "1" can
be
represented
as:
𝑠
(
𝑡
)
=
√
𝑃
𝑠
𝑇
𝑏
.
√2/𝑇
𝑏
cos
(
2𝜋𝑓
0
𝑡
)
=
√
𝑃
𝑠
𝑇
𝑏
∅
1
(𝑡)
Digital
Communications
CTE Department -3
rd
stage
3
Thus there is only one
carrier
function
∅
1
(𝑡)
.
If
we take
∅
1
(
𝑡
)
=
√
2/𝑇
𝑏
cos
(
2𝜋𝑓
0
𝑡
)
as
the orthonormal
basis
function. The signal space diagram will have two points
on
∅
1
(𝑡)
.
One will
be
at
zero and other
will
be
at
√
𝑃
𝑠
𝑇
𝑏
.
As
shown
in
Fig.11:
Figure
11
Therefore
the
distance between the
two
signal points will
be,
𝑑 =
√
𝑃
𝑠
𝑇
𝑏
=
√
𝐸
𝑏
3-1
Generation
of
BASK:
Fig.
12 shows
BASK
generator. The input
binary
sequence is applied to the product modulator. The modulator
passes
the carrier
when the input bits is "1", and block
the
carrier (zero output) when input bit is
"0".
Product
Modul
a
tor
𝐶𝑎𝑟𝑟𝑖𝑒𝑟
√
2𝑃
𝑠
cos
(
2𝜋𝑓
0
𝑡
)
Binary
ASK
signal
s(t)
Binary
Input
s(t)
Digital
Communications
CTE Department -3
rd
stage
Figure
12
The bandwidth of ASK
is
𝐵𝑊 =
𝑅(1
+
𝑟)
Where
R
is
the
bit
rate and
0 < 𝑟 < 1
is
related to how signal is
filtered.
3-2
ASK
Detector:
The ASK signal is applied to the correlator consisting
of
multiplier
and
integrator. The locally generated
coherent
carrier
and
applied to multiplier
as
shown
in
Fig.
13.
Figure
13
4
Digital
Communications
CTE
Department -3
rd
stage
The output
of
multiplier
is
integrated over the bit period. The decision device takes the decision
at
the end
of
every
bit
period.
It
compare the output
of
integrator with the
threshold.
3-
Differential Phase Shift Keying
(DPSK):
DPSK
is
differentially
coherent
modulation
method.
It
does
not
need
synchronous
(coherent)
carrier
at
the
demodulator. The input sequence
of
binary bits is modified such that the next bit depends upon
the
previous bit.
Therefore
in
the receiver the previous received bits
are
used
to
detect the present
bit.
4-1
Generator of
DPSK:
The input sequence d(t), the output sequence is b(t)
and
𝑏(𝑡
−
𝑇
𝑏
)
is the previous output delayed
by one
bit period.
Depending upon values of d(t)
and
𝑏(𝑡
−
𝑇
𝑏
)
, exclusive OR gate generates the output sequence b(t)
as
shown in Fig.
14.
Figure
14: DPSK
generator
5
Digital
Communications
CTE
Department -3
rd
stage
The truth table
of
exclusive OR gate is used
to
derive the level of waveforms shown in Fig. 15, which satisfied in
any
interval b(t) is given
as,
𝑏
(
𝑡
)
=
𝑑(𝑡)
○
𝑏(𝑡
−
𝑇
𝑏
)
Figure
15: DPSK
waveform
Note that the output sequence b(t) changes level
at
the beginning
of
each interval in which
d(t)=1
and does not change level
when
d(t)=0,
so that symbol duration (T)= duration
of
two bits
2𝑇
𝑏
. The sequence b(t)
is
applied
to
the
balanced
modulator
as
shown in Fig. 14 with carrier
√
2𝑃cos(2𝜋𝑓
0
𝑡)
,
the
modulator output
is:
𝑠
(
𝑡
)
=
𝑏
(
𝑡
)
√2𝑃
cos
(
2𝜋𝑓
0
𝑡
)
= ±√2𝑃
cos
(
2𝜋𝑓
0
𝑡
)
As
shown
in in Fig.15 the phase changes only when
d(t)=1.
6
Digital
Communications
CTE Department -3
rd
stage
4-2
DPSK
Receiver:
During the transmission, the DPSK signal undergoes
some
phase
shift
𝜃
.
Therefore
the
signal received
at
the input of
receiver
is
Received signal
𝑦
(
𝑡
)
=
𝑏
(
𝑡
)
√
2𝑃
cos
(
2𝜋𝑓
0
𝑡
+
𝜃
)
The signal is multiplied with delayed version
by one
bit:
Multiplier output
=
𝑏
(
𝑡
)
𝑏
(
𝑡
− 𝑇
𝑏
)
(2𝑝)
cos
(
2𝜋𝑓
0
𝑡
+
𝜃
)
cos
(
2𝜋𝑓
0
(
𝑡
−
𝑇
𝑏
)
+
𝜃
)
Figure
16a: QPSK
receiver
This signal is fed to integrator and
to
the decision device. The
integrator and decision is controlled
by bit
synchronizer
to get
the
output binary
sequence.
The block
diagram
of
such system
is shown in
Fig.16a, and the equivalent of
this
system
is shown
in
Fig.16b.
7
Digital
Communications
CTE Department -3
rd
stage
4-3
Bandwidth
of
DPSK:
2
1
We
know
that one previous
bit
is used to decide the phase shift
of
next bit. Change in
b(t)
occurs only
if
input bitt is
at
level
'1'.
No change occurs
if
input bit is
at
level '0'. Since one previous
bit
is
always
used to
define the phase in next bit,
the
symbol
can be
said to
have
two bits. Therefore
one
symbol duration (T)
is
equivalent to
two
bits duration
(2𝑇
𝑏
)
,
so
that
Symbol duration
𝑇 =
2𝑇
𝑏
Bandwidth is
given
as,
𝐵𝑊 =
𝑇
=
𝑇
𝑏
Or
𝐵𝑊 =
𝑓
𝑏
4-4
Advantages and disadvantages
of
DPSK:
Advantages:
1-
DPSK does not need
carrier at
its receiver.
Hence
it is simple than
BPSK.
8
Digital
Communications
CTE
Department -3
rd
stage
2-
The bandwidth required of DPSK
is
reduced compared to that
of
BPSK.
Disadvantages:
1
The probability
of
bit
error
rate
of
DPSK
is
higher than
BPSK.
2
Error
in the first bit creates error
in the
second bit.
Hence
error propagation in DPSK
is
more, while in PSK
each
bit
is
independent.
3
Noise interference in DPSK
is
more.
4
Quadrature Phase Shift Keying
(QPSK):
There
are
two
important
points
in
communication
system,
transmission
power
and
channel
bandwidth.
The
channel
bandwidth depends upon the bit rate or signaling rate
𝑓
𝑏
.
The carrier
is
used
as
bandpass transmission over
a
channel.
If
two
or
more bits
are
combined
in
some
symbol instead of one bit, then the signaling rate is reduced and the
transmission channel bandwidth
is
reduced.
In
quadrature shift keying, two successive bits
in the
data sequence are
grouped
together.
In BPSK
the phase shift
occurs in
two
level only,
the carrier is
change
by
180
0
.
In
QPSK the combination
of
two
bits
form four
distinct symbols, so that the change
of
carrier phase is
45
0
(
𝜋
𝑟𝑎𝑑𝑖𝑎𝑛𝑠)
from one
symbol to
the
next
one.
9
4
Table 1:
Symbol
and
corresponding phase shift in
QPSK
Digital
Communications
CTE Department -3
rd
stage
10
1
QPSK
generator:
There are
two type
of
QPSK:
1
Offset
QPSK
(OQPSK):
Fig. 17
shows the block diagram
of
OQPSK generator. The input signal is converted
to
NRZ and
called
b(t), the
demultiplexer divides
b(t)
into separated bit streams
of
odd numbered and
even
numbered.
Digital
Communications
CTE Department -3
rd
stage
Figure
17: OQPSK
generator
The symbol duration
of
both
𝑏
𝑜
(𝑡)
and
𝑏
𝑒
(𝑡)
are
2𝑇
𝑏
as
shown
in
Fig.
18.
Note
that the first
symbol
of
𝑏
𝑜
(𝑡)
is
delayed
by
one
bit
period with respect to symbol of
𝑏
𝑒
(𝑡)
.
This delay
of
𝑇
𝑏
is called offset
so
that the change
in
level cannot
occur
in
the
same time. Each
𝑏
𝑒
(𝑡)
and
𝑏
0
(𝑡)
are
modulate
a
carrier
√
2𝑝
𝑠
sin(2𝜋𝑓
0
𝑡)
and
√
2𝑝
𝑠
cos(2𝜋𝑓
0
𝑡)
respectivlly.
11
Digital
Communications
CTE Department -3
rd
stage
12
Digital
Communications
CTE Department -3
rd
stage
𝑠
𝑒
(
𝑡
)
=
𝑏
𝑒
(𝑡)
√
2𝑝
𝑠
sin(2𝜋𝑓
0
𝑡)
𝑠
0
(
𝑡
)
=
𝑏
0
(𝑡)
√
2𝑝
𝑠
sin(2𝜋𝑓
0
𝑡)
Thus
𝑠
𝑒
(
𝑡
)
and
𝑠
0
(
𝑡
)
are
basically
BPSK
signals but
𝑇 =
2𝑇
𝑏
. The output
of
adder is OQPSK, given
as:
𝑠
(
𝑡
)
=
𝑠
𝑒
(
𝑡
)
+
𝑠
0
(
𝑡
)
13
Digital
Communications
CTE Department -3
rd
stage
=
𝑏
𝑒
(
𝑡
)
√
2𝑝
𝑠
sin
(
2𝜋𝑓
0
𝑡
)
+
𝑏
0
(𝑡)
√
2𝑝
𝑠
cos(2𝜋𝑓
0
𝑡)
Fig.19 shows the phasor diagram
of
QPSK
signal of
above equation.
Figure
19: Phasor diagram
of
QPSK
signal
14
Digital
Communications
CTE Department -3
rd
stage
Offset
QPSK is essentially
the
same
as QPSK
except that
the
I-
and Q-channel pulse trains are staggered. The modulator
and the demodulator
of
OQPSK
are
shown
in
Figure
20,
which differs from
the
QPSK
only by an
extra
delay
of T/2
seconds in the Q-channel. Its
power
spectral density
is
the
same
as
that
of
QPSK, and
its
error performance
is
also the same
as
that
of
QPSK.
Fig. 21: OQPSK
modulator
15
Digital
Communications
CTE Department -3
rd
stage
Fig. 21: OQPSK
demodulator
16
This content delves into the Digital Communications course in the CTE Department during the academic year 2018-2019. It covers topics such as Binary Amplitude Shift Keying (BASK), Generation and Detection of BASK, Differential Phase Shift Keying (DPSK), DPSK Generator, and the application of Digital Modulation Techniques in communication systems.
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Presentation Transcript
. : 1- 2018-2019 Digital Communications CTE Department -3rdstage Reference: Digital Communications Fundamentalsand Applications, 2nd Addition, byFernardSklar Dr. Hussam Dheaa Kamel Al-Mustafa University Collage CTE Department 2018-2019
CTE Department -3rdstage Digital Communications 2- BinaryAmplitude Shift Keying (BASK) or ON- OFFkeying(OOK): ASK or OOK is the simplest digital modulation technique.TheASK waveform can be represented as: ?(?) = 2??cos(2??0?) to transmit symbol "1", and pulse is transmitted. To transmit symbol "0" ?(?) = 0, that is no signal transmitted for such symbol. ThusASK waveform looks like an ON-OFF of the signal as shown in figure 10. Figure10 The ASK wave form for symbol "1" can be represented as: ?(?) = ????. 2/??cos(2??0?) = ???? 1(?) 2
CTE Department -3rdstage Digital Communications Thus there is only one carrier function 1(?). If we take 1(?) = 2/??cos(2??0?) as the orthonormal basis function. The signal space diagram will have two points on 1(?). One will be at zero and other will be at ????. As shown in Fig.11: Figure11 Therefore the distance between the two signal points will be, ? = ????= ?? 3-1 Generation of BASK: Fig. 12 shows BASK generator. The input binary sequence is applied to the product modulator. The modulator passes the carrier when the input bits is "1", and block the carrier (zero output) when input bit is "0". Product Modulator Binary Inputs(t) BinaryASK signal s(t) ??????? 3 2??cos(2??0?)
CTE Department -3rdstage Digital Communications Figure12 The bandwidth of ASK is ?? = ?(1 +?) Where R is the bit rate and 0 < ? < 1 is related to how signal is filtered. 3-2 ASK Detector: The ASK signal is applied to the correlator consisting of multiplier and integrator. The locally generated coherent carrier and applied to multiplier as shown in Fig. 13. Figure13 4
CTE Department -3rdstage Digital Communications The output of multiplier is integrated over the bit period. The decision device takes the decision at the end of every bit period. It compare the output of integrator with the threshold. 3- Differential Phase Shift Keying (DPSK): DPSK is differentially coherent modulation method. It does not need synchronous (coherent) carrier at the demodulator. The input sequence of binary bits is modified such that the next bit depends upon the previous bit. Therefore in the receiver the previous received bits are used to detect the present bit. 4-1 Generator of DPSK: The input sequence d(t), the output sequence is b(t) and ?(? ??) is the previous output delayedby one bit period. Depending upon values of d(t) and ?(? ??), exclusive OR gate generates the output sequence b(t) as shown in Fig. 14. Figure 14: DPSKgenerator 5
CTE Department -3rdstage Digital Communications The truth table of exclusive OR gate is used to derive the level of waveforms shown in Fig. 15, which satisfied in any interval b(t) is given as, ?(?) = ?(?) ?(? ??) Figure 15: DPSK waveform Note that the output sequence b(t) changes level at the beginning of each interval in which d(t)=1 and does not change level when d(t)=0, so that symbol duration (T)= duration of two bits 2??. The sequence b(t) is applied to the balanced modulator as shown in Fig. 14 with carrier 2?cos(2??0?), the modulator output is: ?(?) = ?(?) 2? cos(2??0?) = 2? cos(2??0?) As shown in in Fig.15 the phase changes only when d(t)=1. 6
CTE Department -3rdstage Digital Communications 4-2 DPSK Receiver: During the transmission, the DPSK signal undergoes some phase shift ?. Therefore the signal received at the input of receiver is Received signal ?(?) = ?(?) 2? cos(2??0?+ ?) The signal is multiplied with delayed version by one bit: Multiplier output= ?(?)?(? ??)(2?) cos(2??0?+ ?) cos(2??0(? ??) + ?) Figure 16a: QPSK receiver This signal is fed to integrator and to the decision device. The integrator and decision is controlled by bit synchronizer to get the output binary sequence. The block diagram of such system is shown in Fig.16a, and the equivalent of this system is shown in Fig.16b. 7
CTE Department -3rdstage Digital Communications 4-3 Bandwidth of DPSK: We know that one previous bit is used to decide the phase shift of next bit. Change in b(t) occurs only if input bitt is at level '1'. No change occurs if input bit is at level '0'. Since one previous bit is always used to define the phase in next bit, the symbol can be said to have two bits.Therefore one symbol duration (T) is equivalent to two bits duration (2??), so that Symbol duration ? =2?? 2 1 Bandwidth is given as, ?? =?=?? Or ?? =?? 4-4 Advantages and disadvantages of DPSK: Advantages: 1- DPSK does not need carrier at its receiver. Hence it is simple than BPSK. 8
CTE Department -3rdstage Digital Communications 2- The bandwidth required of DPSK is reduced compared to that of BPSK. Disadvantages: 1 The probability of bit error rate of DPSK is higher than BPSK. 2 Error in the first bit creates error in the second bit. Hence error propagation in DPSK is more, while in PSK each bit is independent. 3 Noise interference in DPSK is more. 4 Quadrature Phase Shift Keying (QPSK): There are two important points in communication system, transmission power and channel bandwidth. The channel bandwidth depends upon the bit rate or signaling rate ??. The carrier is used as bandpass transmission over a channel. If two or more bits are combined in some symbol instead of one bit, then the signaling rate is reduced and the transmission channel bandwidth is reduced. In quadrature shift keying, two successive bits in the data sequence are grouped together. In BPSK the phase shift occurs in two level only, the carrier is change by 1800. In QPSK the combination of two bits form four distinct symbols, so that the change of carrier phase is 450 (????????) from one symbol to the next one. Table 1: Symbol and corresponding phase shift in QPSK Sr.No. Input successive bits Symbol 4 Phase shift in carrier ? = 1 1(1?) 0( 1?) ?1 ?/4 ? = 2 0( 1?) 0( 1?) ?2 3?/4 9
CTE Department -3rdstage Digital Communications ? =3 0( 1?) 1(1?) ?3 5?/4 ? =4 1(1?) 1(1?) ?4 7?/4 1 QPSK generator: There are two type of QPSK: 1 Offset QPSK (OQPSK): Fig. 17 shows the block diagram of OQPSK generator. The input signal is converted to NRZ and called b(t), the demultiplexer divides b(t) into separated bit streams of odd numbered and even numbered. 10
CTE Department -3rdstage Digital Communications Figure 17: OQPSK generator The symbol duration of both ??(?)and ??(?) are 2??as shown in Fig. 18. Note that the first symbol of ??(?) is delayed by one bit period with respect to symbol of ??(?). This delay of ??is called offset so that the change in level cannot occur in the same time. Each ??(?) and ?0(?) are modulate a carrier 2??sin(2??0?) and 2??cos(2??0?) respectivlly. 11
CTE Department -3rdstage Digital Communications 12
CTE Department -3rdstage Digital Communications ??(?) = ??(?) 2??sin(2??0?) ?0(?) = ?0(?) 2??sin(2??0?) Thus ??(?)and ?0(?) are basically BPSK signals but ? = 2??. The output of adder is OQPSK, givenas: ?(?) = ??(?) + ?0(?) 13
CTE Department -3rdstage Digital Communications =??(?) 2??sin(2??0?) + ?0(?) 2??cos(2??0?) Fig.19 shows the phasor diagram of QPSK signal of above equation. Figure 19: Phasor diagram of QPSKsignal 14
CTE Department -3rdstage Digital Communications Offset QPSK is essentially the same as QPSK except that the I- and Q-channel pulse trains are staggered. The modulator and the demodulator of OQPSK are shown in Figure 20, which differs from the QPSK only by an extra delay of T/2 seconds in the Q-channel. Its power spectral density is the same as that of QPSK, and its error performance is also the same as that of QPSK. Fig. 21: OQPSKmodulator 15
CTE Department -3rdstage Digital Communications Fig. 21: OQPSKdemodulator 16
