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1
Principles of Digital Data Transmission
Chapter 7
Lectured by Dr. Yun Q. ShiDept. of Electrical & Computer Engr.New Jersey Institute of Technology
[email protected] used for the course: <Modern Digital and Analog
Communication Systems>, 4th Edition, Lathi and Ding, Oxford
Dr. Shi Lathi & Ding-Digital Commun 2
Introduction
• A significant portion of communication in 1990s was in analog.
• It has been replaced rapidly by digital communication.
• Now most of the communications become digital, with analog communication playing a minor role.
• This chapter addresses the framework (several major aspects) of digital data transmission
• All we learnt in this course so far is utilized here.
• Hence, this chapter is naturally a conclusion of this course.
2
Dr. Shi Lathi & Ding-Digital Commun 3
Digital Communication Systems:Source
• Digital Communication Systems: Figure 7.1
Source encoder, Baseband modulation (Line coding), Digital carrier modulation, Multiplexer, Channel, Regenerative repeater, Receiver (detection).
• Source: The input– A sequence of digits
– We discuss mainly: The binary case (two symbols)
– Later in this chapter: The M-ary case (M symbols)more general case
Dr. Shi Lathi & Ding-Digital Commun 4
Figure 7.1 Fundamental building blocks of digital communication systems
Figure 7.1 Fundamental building blocks of digital communication systems
3
Dr. Shi Lathi & Ding-Digital Commun 5
Line Coder (Transmission Coder)• The output of a multiplexer is coded into
electrical pulses or waveforms for the purpose of transmission over the channel.
• This process is called a line coding or transmission coding
• Many possible ways of assigning waveforms (pulses) to the digital data.
• On-off:
↔↔
pulseno
tppulsea
0
)(1
Dr. Shi Lathi & Ding-Digital Commun 6
Line Coder …• Polar:
• Bipolar (pseudoternary or alternate mark inversion (AMI))
In short, pulses representing consecutive 1’s alternate in sign.
• All the above three could use half-width pulses. It is possible to select other width.
• Figure 7.1 (parts a, b and c).
↔↔
0
1 a pulse p (t) or - p (t) depending on whether the previous 1
is encoded by -p (t) or p (t)
no pulse
−↔↔
)(0
)(1
tppulsea
tppulsea
4
Dr. Shi Lathi & Ding-Digital Commun 7
Line Codes
Dr. Shi Lathi & Ding-Digital Commun 8
Line Coder …
• Full width pulsesare often used in some applications. – i.e., the pulse amplitude is held to a constant value
throughout the pulse interval (it does not have a chance to go to zero before the next pulse begins).
– These schemes are called non return-to-zero (NRZ) in contrast to return-to-zero (RZ)
– Figure 7.2 shows• An On-off NRZ signal• A Polar NRZ signal in d and e parts of the figure
5
Dr. Shi Lathi & Ding-Digital Commun 9
Multiplexer
• Usually, the capacity of a practical channel >> the data rate of individual sources.
• To utilize this capacity effectively, we combine several sources through a digital multiplexer using the process of interleaving.
• One way: A channel is time-shared by several messages simultaneously.
Dr. Shi Lathi & Ding-Digital Commun 10
Regenerative Repeater• Used at regularly spaced intervals along a digital
transmission line to: 1) detect the incoming digital signaland 2) regenerate new clean pulsesfor further transmission along the line.
• This process periodically eliminates, and thereby combats the accumulation of noise and signal distortion along the transmission path.
• The periodic timing information (the clock signal at R bHz) is required to sample the incoming signal at a repeater.
• R b (rate): pulses/sec
6
Dr. Shi Lathi & Ding-Digital Commun 11
Regenerative Repeater…
• The clock signal can be extractedfrom the received signal.
– e.g., the polar signalwhen rectified a clock signal at Rb Hz
• The on-off signal = A periodic signal at Rb
+ a polar signal (Figure 7.)
• When the periodic signal is applied to a resonant circuit tuned to Rb Hz, the output, a sinusoid of Rb Hz, can be used for timing.
⇒
Dr. Shi Lathi & Ding-Digital Commun 12
On-off Signal Decomposition
7
Dr. Shi Lathi & Ding-Digital Commun 13
Regenerative Repeater…
• The bipolar signal an on-off signalthe clock signal can also be extracted.
• The timing signal (the output of the resonant circuit) is sensitive to the incoming pattern, sometimes.– e.g. in on-off, or
bipolar 0 no pulse
• If there are too many zeros in a sequence, will have problem.– no signal at the input of the resonant circuit for a while– sinusoids output of the resonant circuit starts decaying
• Polar scheme has no such a problem.
⇒
↔
rectified
Dr. Shi Lathi & Ding-Digital Commun 14
Transparent Line Code
• A line code in which the bit pattern does not affect the accuracy of the timing information is said to be a transparent line code.– The polar scheme is transparent.
– The on-off and bipolar are not transparent (non-transparent).
8
Dr. Shi Lathi & Ding-Digital Commun 15
Desired Properties of Line Codes1. Transmission bandwidth: as small as possible
2. Power efficiency: for a specific bandwidth and detection error rate, transmitted power should be as small as possible.
3. Error detection and correction capability: as strong as possible
4. Favorable power spectral density: desirable to have zero PSD at (dc), called DC null, because of ac coupling requires this. If at dc, , , then dc wanders in the pulse stream, which is not desired.
5. Adequate timing content: should be possible to extract the clock signal (timing information).
6. Transparency
0=ω 0≠PSD
Dr. Shi Lathi & Ding-Digital Commun 16
PSD of Various Line Codes
• Consider a general PAM signal, as shown in Figure 7.4 (b):
– the k th pulse in the pulse train • p (t): the basic pulse• P (ω) [P(f)] : Fourier spectrum of p (t)• a k: arbitrary and random
– The on-off, polar, bipolar line codes are all special cases of the general pulse train y(t), a k = 0, +1, -1
bRb
b RT 1=bkTt =
)()( tpaty k=
9
Dr. Shi Lathi & Ding-Digital Commun 17
A Random PAM signal
New approach to determine PSD of y(t)
All 3 line codes can be represented this way
Dr. Shi Lathi & Ding-Digital Commun 18
• How to determine the PSD?– Figure 7.4– An attractive general approach
need to be studied onceThen for different p(t), different PSD– x(t): an impulse train– : a rectangular pulse train
when (Figure 7.5)
PSD of Various Line Codes …
)(),( ωτ xx SR
⇒
)(ˆ tx
,0→ε)( kk ah =⋅ε
)()(ˆ txtx →
10
Dr. Shi Lathi & Ding-Digital Commun 19
Dr. Shi Lathi & Ding-Digital Commun 20
PSD of Various Line Codes …
⇒= 20 kaR time average of 2
ka
L,. 11 += kk aaR nkkn aaR += .,L
)()()(2 ωωω Xy SPS =
)(ωyS)(ωP ⇒⇒
∑∞
−∞=
−=n
bnb
X nTRT
R )(1
)( τδτ
Different line codes different different
[ ]
+== ∑∑∞
=
∞
−∞=
−
10 )cos(2
11)(
nbn
bn
Tjnn
bX TnRR
TeR
TS b ωω ω
11
Dr. Shi Lathi & Ding-Digital Commun 21
Polar Signaling
1 p(t)
0 -p(t)↔↔ } 21, , 1k ka equally likely a⋅ = ± =
120 == kaR
0. 11 == +kk aaR
1,0 ≥= nRn
equally likelySimilar reasoning
11. 1 −=+ oraa kkQ
)(.)()(2 ωωω Xy SPS =
bTP
1.)(
2ω=
Dr. Shi Lathi & Ding-Digital Commun 22
• To be specific, assume p(t) is a rectangular pulse of width (a half-width rectangular pulse)
Polar Signaling…
2bT
2
( )
2
( )2 4
( )4 4
b
b b
b by
tp t rec t
T
T TP s in c
T TS s in c
ωω
ωω
=
=
=
1 sin :
4
4
22
b
b
bb
st zero cros g
T
T
or f RT
ω ππ ω
−⋅ = ⇒ =
⇒ = =
12
Dr. Shi Lathi & Ding-Digital Commun 23
PSD of polar signal (half-width rectangle)
Dr. Shi Lathi & Ding-Digital Commun 24
Polar Signaling …
• Comment 1: The essential bandwidth of the signal (main lobe) = 2 RbFor a full-width pulse, Rb“Polar Signaling is not bandwidth efficient.”
• Comment 2: “No error-detection or error-correction capability.”
• Comment 3: “Non-zero PSD at dc( ω = 0)”This will rule out the use of ac coupling in transmission.
• Comment 4: “Most efficient scheme from the power requirement viewpoint” � for a given power, the detection-error probability for a polar scheme is the smallest possible.
• Comment 5: “Transparent”
⇒
⇒
13
Dr. Shi Lathi & Ding-Digital Commun 25
Achieving DC Null in PSD by Pulse Shaping
dttpP
dtetpP tj
∫
∫∞
∞−
∞
∞−
−
=∴
=
)()0(
)()( ωωQ
If the area under p(t) = 0, P(0) = 0. ⇒
Dr. Shi Lathi & Ding-Digital Commun 26
Manchester(split-phase, twinned-binary) Signal
14
Dr. Shi Lathi & Ding-Digital Commun 27
On-off Signaling
• 1 p(t)0 no pulse
↔↔
∑
∑
∞
−∞=
−
∞
≠−∞=
−
+=
+=
≥=
=
n
Tjn
bb
nn
Tjn
bbX
n
b
b
eTT
eTT
S
nallforR
R
ω
ωω
4
1
4
1
4
1
2
1)(
14
12
1
0
0
Dr. Shi Lathi & Ding-Digital Commun 28
On-off Signaling
∑∞
−∞=
−+=
n bbbX T
n
TTS
πωδπω 2
4
2
4
1)(
2
−+= ∑
∞
−∞=n bbby T
n
TT
PS
πωδπωω 22
14
)()(
2
For a half-width rectangular pulse,
2 2 2( ) 1
16 4b b
ynb b
T T nS sinc
T T
ω π πω δ ω∞
= −∞
= + −
∑
Note: 1. For some derivation, refer to the next four pages.2. PSD is shown in Figure 7.8.3. PSD consists of both a discrete part and acontinuous part.
15
Dr. Shi Lathi & Ding-Digital Commun 29
Some Derivation (Lathi’s book, pp. 58-59, Example 2.12)
• A unit impulse train: )()()(00
tgtCombt TT ==δ
)(00 ωω ωCOMB≅
∑∞
−∞=
−==n T
nT
tgFTG )2
(2
)}({)(00
πωδπω
∑∞
−∞=
−==n
nffT
tgFTfG )(1
)}({)( 00
δ
∑∞
−∞=
+=n
bbb
tnTT
)cos(21 ω
∑∑∞
−∞=
∞
−∞=
==n
tjn
bn
tjnn
FSbb e
TeDtg ωω 1
)(
Dr. Shi Lathi & Ding-Digital Commun 30
Example 2.12
16
Dr. Shi Lathi & Ding-Digital Commun 31
Some Derivation…
• Fact 1 (Lathi’s book, p.83, Eq. (3.20a, 3.20b):
• Fact 2: Also,
( )( )b
tfj ffe
fb −↔
↔
δδ
π2
1
( )[ ]btj be ωωδω −↔
( )( ) bTjn
b enTt
tωδ
δ−↔−
↔ 1
Dr. Shi Lathi & Ding-Digital Commun 32
Some Derivation …
• Fact 3:
( )
( )∑∞
−∞=−=∑
∞
−∞=
−
∑∞
−∞===∑
∞
−∞=−
nb
nb
n
tb
jne
FTFT
n bTb
tb
jne
bT
FS
nb
nTt
ωωδωω
πωω
δ
44 844 76
bb
44444 344444 2144 344 21
21
17
Dr. Shi Lathi & Ding-Digital Commun 33
PSD of An On-off Signal
Dr. Shi Lathi & Ding-Digital Commun 34
On-off Signaling
• This is reasonable since, as shown in Fig. 7.2, an on-off signal can be expressed as the sum of a polar and a periodic component
• Comment 1: For a given transmitted power, it is less immune to noise interference.– Noise immunity difference of amplitudes
representing binary 0 and 1.– If a pulse of amplitude 1 or –1has energy E, then a
pulse of amplitude 2 has energy (2)2E = 4E.
Q ∝
18
Dr. Shi Lathi & Ding-Digital Commun 35
On-off Signaling …
• For polar: digits are transmitted per second
polar signal power
• For on-off:
• Comment 2: Not transparent
bT
1Q
∴bb T
E
TE =
= 1
bb T
E
TEpower
2
2
14 =
= Power is now as large
as twice of above.
Dr. Shi Lathi & Ding-Digital Commun 36
Bipolar Signaling(Pseudoternary or Alternate Mark Inverted (AMI))
A. 0 no pulse1 p(t) or –p(t) depending on whether the previous 1 was transmitted by –p(t) or p(t)
B. [p(t), 0 , -p(t)]: In reality, it is ternary signaling.
C. Merit: a dc null in PSDdemerit: not transparent
↔↔
19
Dr. Shi Lathi & Ding-Digital Commun 37
D.
Bipolar Signaling…
1,01
lim
0)0(8
5)1(
8)1(
8
1lim)2(
)2(
4
1)0(
4
3)1(
4
1lim)1(
2
1)0(
2)1(
2
1lim
1lim)0( 22
>==
=
+−+=
−=
+−=
=
+±==
+∞→
∞→
∞→
∞→∞→
∑
∑
naaN
R
NNN
NR
R
NN
NR
NN
Na
NR
nkk
kN
n
N
N
Nk
kN
nkkaa + 111, 101, 110, 011, 010, 001, 000
Dr. Shi Lathi & Ding-Digital Commun 38
Bipolar Signaling…
E. )()()(2 ωωω Xy SPS =
( )
2
2
01
2
2 2
2
( )
( )2 cos
( ) 1 12 ( ) cos
2 4
( ) ( )1 cos
2 2
bjn Tn
nb
n bnb
bb
bb
b b
PR e
T
PR R n T
T
PT
T
P P TT sin
T T
ωω
ωω
ωω
ω ω ωω
∞−
= −∞
∞
=
=
= +
= + ⋅ −
= − = ⋅
∑
∑
(7.10b)
(7.10c)
20
Dr. Shi Lathi & Ding-Digital Commun 39
Bipolar Sampling …
⇒
2
( ) 0
2 1( ) 0 ,
2
, ( )
y
bb
b b
b
S
Tsin at f R
T T
bandw id th R H z regard less of P
ω
ω πω
ω
=
= = ∴ = =
⇒ =
As ω=0 (dc) regardless of P(ω)A dc null (desirable for ac coupling)
For a half-width rectangular pulse, , ( )2
2
( )42
b
b
b
b
T tP t r e c t
T
TP s i n c
T
ωω
=
=
Dr. Shi Lathi & Ding-Digital Commun 40
⇒ 2 2( )4 4 2b b b
y
T T TS sinc sin
ω ωω =
Zero:
bb
bb
RfRfTT
==
==
21
21
2
24 πωπω ⇒ 1b
b
RT
essential bandwidth
=
⋅
Half that of polar or on-off signaling.Twice that of theoretical minimum bandwidth (channel b/w).
22
trect sinc sinc
ω τ ωτ ττ
τ
↔ =
Table 3.1:
ɷ1Tb=4π ɷ2Tb=2π
21
Dr. Shi Lathi & Ding-Digital Commun 41
Dr. Shi Lathi & Ding-Digital Commun 42
Merits of Bipolar Signaling
1.Spectrum has a dc dull.
2.Bandwidth is not excessive.
3.It has single-error-detection capability.
Since, a single detection error a violation of
the alternation pulse rule.
⇒
22
Dr. Shi Lathi & Ding-Digital Commun 43
Demerits of Bipolar Signaling
1. Requires twice as much power as a polar signaling.Distinction between A, -A, 0
vs.Distinction between A/2, -A/2
2. Not transparent
Dr. Shi Lathi & Ding-Digital Commun 44
Pulse Shaping
• Different signaling (line coding, ) diff.• Different pulse shaping (P(ω)) diff.
An additional factor.• Intersymbol Interference (ISI) (refer to the next slide)
– Time-limited pulses (i.e., truncation in time domain)� not band-limited in frequency domain
– Not time-limited, causes problem band-limited signal (i.e., truncation in frequency domain)
)()()(2 ωωω Xy SPS =
)cos2()(
10
2
bn
nb
TnRRT
P∑
∞
=
+= ωω
)(ωyS⇒)(ωXS
⇒ )(ωyS
[Slide 19]
23
Dr. Shi Lathi & Ding-Digital Commun 45
Intersymbol Interference (ISI) in Detection
Back to slide 44
B. Sklar’s Digital Communications 1988
Dr. Shi Lathi & Ding-Digital Commun 46
Phase Shaping…Intersymbol Interference (ISI)
• Whether we begin with time-limited pulses or band-limited pulses, it appears that ISI cannot be avoided.An inherent problem in the finite bandwidth transmission.
• Fortunately, there is a way to get away:
be able to detect pulse amplitudes correctly.
24
Dr. Shi Lathi & Ding-Digital Commun 47
Nyquist Criterion for Zero ISI• The firstmethod proposed by Nyquist.
• Example 6.1 (p.256)
• Minimum bandwidth pulse that satisfies Nyquist Criterion Fig. 7.10 (b and c)
)22.7(1,0
0,1)(
=±=
==
bbb R
TnTt
ttp
bT : separation between successive transmitted pulses
1 1( ) (2 ),
2 2 bb
p t sinc Bt B RT
π= = =Since 2πBTb=π,Tb is the 1st zero crossing.
Dr. Shi Lathi & Ding-Digital Commun 48
Minimum Bandwidth Pulse (Nyquist Criterion)
25
Dr. Shi Lathi & Ding-Digital Commun 49
Nyquist Criterion for Zero ISI…
1 0( ) ( )
0bb
tp t sinc R t
t nTπ
== = = ±
=
bb R
T1
=
bb Rrect
RP
πωω
2
1)(
2bR
bandwidth=
Table 3.1:
↔W
rectWtSincW
2)(
ωπ
FT
Dr. Shi Lathi & Ding-Digital Commun 50
Problems of This Method
• Impractical
1.
2. It decays too slowly at a rate 1/t. Any small timing problem (deviation) ISI
• Solution: Find a pulse satisfying Nyquist criterion (7.22), but decays faster than 1/t.
• Nyquist: Such a pulse requires a bandwidth
∞<<∞− t
⇒ ⇒
21,2
≤≤⋅ kR
k b
26
Dr. Shi Lathi & Ding-Digital Commun 51
• Let– The bandwidth of P(ω) is in (Rb /2, Rb)
– p(t) satisfies Nyquist criterion Equation (7.22)
• Sampling p(t) every Tb seconds by multiplying p(t) by an impulse train
)()( ωPtp ↔
).(tbTδ
∑
∑
∞
−∞=
∞
−∞=
−=
=−⇒
==⇒
ns
s
nb
b
T
nPT
G
Equation
nPT
FT
tttptpb
)(1
)(
)4.6(
1)(1
)()()()(
ωωω
ωω
δδb
Dr. Shi Lathi & Ding-Digital Commun 52
27
Dr. Shi Lathi & Ding-Digital Commun 53
Derivation
• |P(ω)| is odd symmetric in y1-o-y2 system.
Dr. Shi Lathi & Ding-Digital Commun 54
Derivation…
• The bandwidth of P(ω) is– : the excess bandwidth
–
• The bandwidth of P(ω) is
• P(ω), thus derived, is called a Vestigial Spectrum.
xb ωω +
2xω
bandwidthimumminltheoretica
bandwidthexcess
offfactorroll=
−γ
b
x
b
x
ωω
ωω ⋅== 2
2
10 ≤≤ r
21
22bbb
T
R)(
rRRB γ+=+=
28
Dr. Shi Lathi & Ding-Digital Commun 55
Realizability
• A physically realizable system, h(t), must be causal, i.e., h(t) = 0 , for t < 0. (The n.c. & s.c.)
• In the frequency domain, the n.c. & s.c. is known as Paley-Wiener criterion
• Note that, for a physically realizable system, H(ω) may be zero at some discrete frequencies.
But, it cannot be zero over any finite band• So, ideal filters are clearly unrealizable.
∞<+∫
∞
∞−ω
ωω
dH
21
)(ln
Dr. Shi Lathi & Ding-Digital Commun 56
• From this point of view, the vestigial spectrum P(ω) is unrealizable.
• However, since the vestigial roll-off characteristic is gradual, it can be more closely approximatedby a practical filter.
• One family of spectra that satisfies the Nyquist criterion is
−<
+>
<−
−−
=
xb
xb
xb
x
b
P
ωωω
ωωω
ωωωω
ωωπ
ω
21
20
22
)2
(sin12/1
)(
29
Dr. Shi Lathi & Ding-Digital Commun 57
Realizability …
• Comment 1: Increasing (or r) improves p(t), i.e., more gradual cutoff reduces the oscillatory nature of p(t), p(t) decays more rapidly.
===
=
=
=
)1(
)5.0(
)0(
2
4
0
r
r
r
bx
bx
x
ωω
ωω
ω Shown in the figure on next slide
xω
Dr. Shi Lathi & Ding-Digital Commun 58
Pulses satisfying Nyquist Criterion
30
Dr. Shi Lathi & Ding-Digital Commun 59
Realizability…
• Comment 2: As = 1, i.e.,
• This characteristic is known as:
The raised-cosine characteristic
The full-cosine roll-off characteristic
2b
x
ωω =
+=
bb Rrect
RP
πωωω
42cos1
2
1)(
=
bb Rrect
R πωω
44cos2
γ
Dr. Shi Lathi & Ding-Digital Commun 60
Realizability…
A. Bandwidth is (γ =1)B. p(0) =C. p(t) = 0 at all the signaling instants
&at points midway between all the signaling instants
D. p(t) decays rapidly as 1/t3 relatively insensitive to derivations of , sampling rate, timing jitter and so on.
2 2
cos( )( ) ( )
1 4b
b bb
R tp t R Sinc R t
R t
π π=−
bRbR
bR⇒
31
Dr. Shi Lathi & Ding-Digital Commun 61
Realizability…
E. Closely realizable
F. Can also be used as a duobinary pulse.
G. : channel transfer factor
: transmitted pulse
Received pulse at the detector input should be P(ω) [p(t)].
)(ωcH
)( kPi
)()()( ωωω PHP ci =⋅
Dr. Shi Lathi & Ding-Digital Commun 62
Signaling with Controlled ISI:Partial Response Signals
• The second method proposed by Nyquist to overcome ISI.
• Duobinary pulse: =
=notherallfor
nnTp b 0
1,01)(
32
Dr. Shi Lathi & Ding-Digital Commun 63
Signaling with Controlled ISI…
• Use polar signaling
• The received signal is sampled at p(t) = 0 for all n except n= 0,1.
• Clearly, such a pulse causes zero ISI with all the pulses except the succeeding pulses.
)(0
)(1
tp
tp
−↔↔
bnTt =
Dr. Shi Lathi & Ding-Digital Commun 64
Signaling with Controlled ISI…
• Consider two such successive pulses located at 0 and Tb.
– If both pulses are positive, the sample value at t = Tb
2.– If both pulses are negative, -2– If pulses are of opposite polarity, 0
• Decision Rule: If the sample value at t = Tb is• Positive 1, 1• Negative 0, 0• Zero 0, 1 or 1, 0
⇒
⇒⇒
⇒⇒⇒
33
Dr. Shi Lathi & Ding-Digital Commun 65
Signaling with Controlled ISI…
• Table 7.1
Transmitted Seq. 1 1 0 1 1 0 0 0 1 0 1 1 1Sample of x(Tb) 1 2 0 0 2 0 -2 -2 0 0 0 2 2Detected seq. 1 1 0 1 1 0 0 0 1 0 1 1 1
Dr. Shi Lathi & Ding-Digital Commun 66
Signaling with Controlled ISI:Duobinary Pulses
• If the pulse bandwidth is restricted to be it can be shown that the p(t) must be:
• Note: The raised-cosine pulse with satisfies the condition (7.3b) to be signaling with controlled ISI – refer to Figure 7.13 (slides 57).
2bR
bRj
bbb
bb
b
eR
rectRR
P
tRtR
tRtp
2
22cos
2)(
)1(
)sin()(
ω
πωωω
ππ
−
=
−=
),1(,2
== rbx
ωω
34
Dr. Shi Lathi & Ding-Digital Commun 67
Signaling with Controlled ISIDuobinary Pulses…
Dr. Shi Lathi & Ding-Digital Commun 68
Use of Differential Coding• For the controlled ISI method,
0-valued sample 0 to 1 or 1 to 0 transition.• An error may be propagated.• Differential coding helps.• In differential coding, “1” is transmitted by a pulse
identical to that used for the previous bit. “0” is transmitted by a pulse negative to that used for the previous bit.
• Useful in systems that have no sense of absolute polarity. � Fig 7.17
⇒
35
Dr. Shi Lathi & Ding-Digital Commun 69
Differential Code
Dr. Shi Lathi & Ding-Digital Commun 70
Scrambling• Purpose: A scrambler tends to make the data more
random by – Removing long strings of 1’s or 0’s
– Removing periodic data strings.
– Used for preventing unauthorized access to the data.
• Example. (Structure)Scrambler:
110000
101011
53
=⊕=⊕=⊕=⊕
⊕
=⊕⊕
:
:TD
TTDTDSn T delayed by n units
Modulo 2 Sum
→ Incomplete version
36
Dr. Shi Lathi & Ding-Digital Commun 71
Dr. Shi Lathi & Ding-Digital Commun 72
Scrambling…
• Descrambler:
53
53
53
)1(
)](1[
)(
DDF
TF
TDD
TDDTR
⊕=
⊕=⊕⊕=
⊕⊕=
∆
Incomplete version
37
Dr. Shi Lathi & Ding-Digital Commun 73
Example 7.2• The data stream 101010100000111 is fed to the
scrambler in Fig. 7.19a. Find the scrambler output T, assuming the initial content of the registers to be zero.
– From Fig. 7.19a we observer that initially T = S, and the sequence S enters the register and is returned as
through the feedback path. This new sequence FS again enters the register and is returned as , and so on. Hence,
(7.41)
FSSDD =⊕ )( 53
SFFF
SFSFFSST
...)1(
...32
32
⊕⊕⊕⊕=⊕⊕⊕⊕=
SF 2
Complete version
Dr. Shi Lathi & Ding-Digital Commun 74
Example 7.2 …
• Recognizing thatWe have
• Because modulo-2 addition of any sequence with itself is zero, and
• Similarly,
53 DDF ⊕=
( )( ) 8810653532 DDDDDDDDF ⊕⊕⊕=⊕⊕=
088 =⊕ DD
( )( ) 1513119531063 DDDDDDDDF ⊕⊕⊕=⊕⊕=
1062 DDF ⊕=
38
Dr. Shi Lathi & Ding-Digital Commun 75
Example 7.2 ……… and so on ……
Hence,
Because DnS is simply the sequence S delayed by n bits, various terms in the preceding equation correspond to the following sequences:
SDDDDDDDDDT ...)1( 15131211109653 ⊕⊕⊕⊕⊕⊕⊕⊕⊕⊕=
Dr. Shi Lathi & Ding-Digital Commun 76
Example 7.2 …
010011011100011
001111010101000000000000000000
0000011100010101010000000000
000011100101010100000000000
00011101010101000000000000
0011110101010000000000000
011101010100000000000001
110100000110000001010
01000001110000010101
000001110001010101D
001111010101000
15
13
12
11
10
9
6
5
3
==
=
=
=
=
=
=
=
=
=
T
SD
SD
SD
SD
SD
SD
SD
SD
S
S
39
Dr. Shi Lathi & Ding-Digital Commun 77
• Note that the input sequence contains the periodic sequence 10101010…, as well as a long string of 0’s.
• The scrambler output effectively removes the periodic component as well as the long strings of 0’s.
• The input sequence has 15 digits. The scrambler output up to the 15th digit only is shown, because all the output digits beyond 15 depend on the input digits beyond 15, which are not given.
• We can verify that the descrambler output is indeed S when this sequence T is applied at its input.
• Homework:Learn to be able to determine where no terms needs to be considered.
Example 7.2 …
Dr. Shi Lathi & Ding-Digital Commun 78
Regenerative Repeater
• Three functions:
1. Reshaping incoming pulsesusing an equalizer.
2. Extracting timing informationRequired to sample incoming pulses at optimum instants.
3. Making decisionbased on the pulse samples
40
Dr. Shi Lathi & Ding-Digital Commun 79
Regenerative Repeater…
Dr. Shi Lathi & Ding-Digital Commun 80
Preamplifier and Equalizer
• A pulse train is attenuated and distorted by transmission medium, say, dispersioncaused by an attenuation of high-frequency components.
• Restoration of high frequency components increase of channel noise
• Fortunately, digital signals are more robust.– Considerable pulse dispersion can be tolerated
• Main concern: Pulse dispersion ISI
increase error probability in detection
⇒
⇒⇒
41
Dr. Shi Lathi & Ding-Digital Commun 81
Zero-Forcing Equalizer
• Detection decision is based solely on sample values.
No need to eliminate ISI for all t.All that is needed is to eliminate or minimize ISI at their respective sampling instants only.
• This could be done by using the transversal-filter equalizer, which forces the equalizer output pulse to have zero values at the sampling (decision-making ) instant. (refer to the diagram in next slide)
⇒
Dr. Shi Lathi & Ding-Digital Commun 82
42
Dr. Shi Lathi & Ding-Digital Commun 83
Zero-Forcing Equalizer …• Let
• Fig 7.22 (b) indicates a problem :
a1, a-1, a2, a-2, …are not negligible due to dispersion.
• Now, want to force a1= a-1= a2= a-2 =…= 0
• Consider ck’s assume other values (other tap setting).
settingtapkc
c
k
≠∀==
00
10
)()(
)()(
0
0
tPtP
NTtPtP
r
br
=−=⇒
If ignore delay.
Dr. Shi Lathi & Ding-Digital Commun 84
• Consider ck’s assume other values (other tap setting).
• For simplicity of notation:
• Nyquist Criterion:
Zero-Forcing Equalizer …
...210,])[()(
)()(
0
0
,,,kTnkpckTp
nTtpctp
N
Nnbrnb
N
Nn
brn
±±=−=
−=
∑
∑
−=
−=
0( ) ( )n rn
p k c p k n∞
=−∞
= −∑
01)(
00)(
0
0
==≠∀=kforkp
kkp
43
Dr. Shi Lathi & Ding-Digital Commun 85
Zero-Forcing Equalizer …
• A set of infinitely many simultaneous equations:
2N+1: cn’sImpossible to solve this set of equations.
• If, however, we specify the values of p0(k) only at 2N+1 points:
then a unique solution exists.
⇒
±±±==
=Nk
kkp
,...,2,10
01)(0
Dr. Shi Lathi & Ding-Digital Commun 86
Zero-Forcing Equalizer …
• Meaning: Zero ISI at the sampling instants of N preceding and N succeeding pulses.
• Since pulse amplitude decays rapidly, ISIbeyond the Nth pulse is not significant for N>2in general.
44
Dr. Shi Lathi & Ding-Digital Commun 87
Zero-Forcing Equalizer …
• Page 7.37
Think about when n = 2, 1, 0, -1, -2; an example in next slide
Dr. Shi Lathi & Ding-Digital Commun 88
Zero-Forcing Equalizer …
• Page 7.37
45
Dr. Shi Lathi & Ding-Digital Commun 89
Eye Diagram• A convenient way to study ISI on an oscilloscope
• Figure
Dr. Shi Lathi & Ding-Digital Commun 90
Timing Extraction
• The received signal needs to be sampled at precise instants.
• Timing is necessary.
• Three ways for synchronization:
1. Derivation from a primary or a secondary standard(a master timing source exists, both transmitter and receiver follow the master).
2. Transmitting a separate synchronizing signal(pilot clock)
3. Self synchronization(timing information is extracted from the received signal itself).
46
Dr. Shi Lathi & Ding-Digital Commun 91
Timing Extraction …• Way 1: Suitable for large volumes of data
high speed comm. Systems. � High cost.
• Way 2: Part of channel capacity is used to transmit timing information. � Suitable for a large available capacity.
• Way 3: Very efficient.• Examples:
– On-off signaling (decomposition: Fig. 7.2)– Bipolar signaling on-off signalingrectification
Dr. Shi Lathi & Ding-Digital Commun 92
Timing Jitter
• Small random deviation of the incoming pulses from their ideal locations.
• Always present, even in the most sophisticated communication systems.
• Jitter reduction is necessary about every 200 milesin a long digital link to keep the maximum jitter within reasonable limits.
• Figure 7.23
47
Dr. Shi Lathi & Ding-Digital Commun 93
Figure 7.23
Dr. Shi Lathi & Ding-Digital Commun 94
Detection –Error Probability
• Received signal = desired + AWGNExample: Polar signaling and transmission (Fig. 7.24, the next slide)
: Peak signal value
• Polar: Instead of , received signal =
• Because of symmetry, the detection threshold = 0.– If sample value > 0, “1”
– If sample value < 0, “0”
– Error in detection may take place
nAp+±Ap
Ap±
⇒⇒
48
Dr. Shi Lathi & Ding-Digital Commun 95
Figure 7.24
Dr. Shi Lathi & Ding-Digital Commun 96
Error Probability for Polar Signal
• With respect to Figure 7.24
)1/(
)0/(
∈∈
P
P = probability that n > Ap
= probability that n < -Ap2
21
2
1)(
−
= n
n
n
enp σ
σπ
( )
2
2
1
2
1
2
1( 0)
2
1
2
n
p
p
n
n
An
x pA
n
P e dn
Ae dx Q
σ
σ
σ π
σπ
− ∞
−∞
⇒ ∈ =
= =
∫
∫
n: AWGN
n
nx
σ=
49
Dr. Shi Lathi & Ding-Digital Commun 97
Error Probability for Polar Signal…where
• Similarly,
• Summary:
• Q(x): Complementary error function: erfc (x)
∫∞ −
=y
x
dxeyQ 2
2
2
1)(
π
=∈
n
pAQP
σ)1(
∞−−Symmetric
toAp
[ ]
=∈+∈=
∈+∈=∈+∈=∈
n
pAQPP
PPPP
PPP
σ)1()0(
2
1
)1()1()0()0(
)1,()0,()( Here, assume P(0)= P(1)=1/2.
2,7.0
12
1)( 2
2
2
>
−≅−
xexx
xQx
π
Dr. Shi Lathi & Ding-Digital Commun 98
Error Probability for Polar Signal…
• If )()()1()0(, kQPPPkA
n
p =∈=∈=∈=σ
kQ(k)=P(ε)
1 2 3 4 5 60.1587 0.0227 0.00135 51016.3 −× 71087.2 −× 9109.9 −×
⇒× −51016.3e.g. one of pulses will be possibly detectedwrongly
53.16 10×
50
Dr. Shi Lathi & Ding-Digital Commun 99
Error Probability for On-off Signals
• In this case, we have to distinguish between Ap
and 0.
• Threshold:
• Error Probabilities:pA
2
1
ofprobP .)0( =∈
=−<
=>
n
pp
n
pp
AQAn
AQAn
σ
σ
22
1
22
1
)()( εPP >∈
ofprobP .)1( =∈
[ ]
=∈+∈=∈
n
pAQPPP
σ2)1()0(
2
1)(
(in the case of polar signaling)
Also assume 0 and 1 equally probable
Dr. Shi Lathi & Ding-Digital Commun 100
51
Dr. Shi Lathi & Ding-Digital Commun 101
Error Probability for Bipolar Signals
v
AorA pp
00
1
↔
−↔
⇒ If the detected sample value:
1..
02
,2
↔
↔
−∈
wo
AA pp
=
>=
−<+
>=
>=∈
n
pp
pp
p
AQ
Anprob
Anprob
Anprob
AnprobP
σ22
2.2
2.
2.
2.)0(
Dr. Shi Lathi & Ding-Digital Commun 102
Error Prob. for Bipolar Signals…
[ ]
signalingoffonforPA
Q
PPPPPPP
AQ
usedpulsenegativewhenA
nprobor
usedpulsepositivewhenA
nprobP
n
p
n
p
p
p
−>
=
∈+∈=∈+∈=∈
=
>
−<=∈
)(2
5.1
)1()0(2
1)1()1()0()0()(
2
2.
2.)1(
εσ
σ
52
Dr. Shi Lathi & Ding-Digital Commun 103
Detection Error Probability…
• Summary, polar is the best,
on-off is in middle,
bipolar is the worst,
• Another factor:
decreases exponentially with the signal power.
Assumption:“0” and “1”Equally likely
n
p
n
p
n
p
AQ
AQ
AQ
σ
σ
σ
25.1
2
)(∈P
Dr. Shi Lathi & Ding-Digital Commun 104
Comparison among three line codes
• To obtain:
• We need:
for bipolar case
for polar case
for on-off case
610286.0)( −×=∈P
16.10
10
5
=
=
=
n
p
n
p
n
p
A
A
A
σ
σ
σ6(5) 0.286 10Q −= ×Q
=∈
n
pAQP
σ2)(Q
08.52
10286.0
25.1)(
6
=
×=
=∈
−
n
p
n
p
A
AQP
σ
σ
c
Q
53
Dr. Shi Lathi & Ding-Digital Commun 105
Comparison among three line codes…
• For the same error rate, required SNR ,
Polar < On-off < Bipolar (from previous example)
• For the same SNR , the caused error ratePolar < On-off < Bipolar (from next example)
• Polar is most efficient in terms of SNR vs. error rate
• Between, on-off and bipolar:
Only 16% improvement of on-off over bipolar
• Performance of bipolar case performance of on-off case in terms of error rate.
n
pA
σ
n
pA
σ
≈
Dr. Shi Lathi & Ding-Digital Commun 106
Example
a) Polar binary pulses are received with peak amplitude Ap = 1 mV. The channel noise rms amplitude is 192.3 µV. Threshold detection is used, and 1 and 0 are equally likely.
b) Find the error probability for
(i) the polar case, and the on-off case
(ii) the bipolar case if pulses of the same shape as in part (a) are used, but their amplitudes are adjusted so that the transmitted power is the same as in part (a)
54
Dr. Shi Lathi & Ding-Digital Commun107
Example …
a) For the polar case
From Table 8.2 (4th ed), we find
b) Because half the bits are transmitted by no-pulse, there are, on the average, only half as many pulses in the on-off case (compared to the polar). Now, doubling the pulse energy is accomplished by multiplying the pulse by
2.5)10(3.192
106
3
== −
−
n
pA
σ
7109964.0)2.5()( −×==∈ QP
2
(in order to keep the power dissipation same)
Dr. Shi Lathi & Ding-Digital Commun 108
Example…
Thus, for on-off Ap is times the Ap in the polar case.
•Therefore, from Equation (7.53)
•As seen earlier, for a given power, the for both the on-off and the bipolar cases are identical. Hence, from equation (7.54)
2
41066.1)68.3(2
)( −×==
=∈ Q
AQP
n
p
σ
410749.12
5.1)( −×=
=∈
n
pAQP
σ
pA
55
Dr. Shi Lathi & Ding-Digital Commun 109
• Digital communications use only a finite number of symbols
• Information transmitted by each symbol increases with M.
• Transmitted power increases as M2 , i.e., to increase the rate of communication by a factor of , the power required increases as M2.
(see an example below)
M2log
MIM 2log= bitsMI : information transmitted
by an M-ary symbol
M-ary Communication
Dr. Shi Lathi & Ding-Digital Commun 110
M-ary Communication…
• Most of the terrestrial digital telephone network: Binary
• The subscriber loop portion of the integrated services digital network (ISDN) uses the quarternary code 2BIQ shown in Figure. 7.28
56
Dr. Shi Lathi & Ding-Digital Commun 111
Dr. Shi Lathi & Ding-Digital Commun 112
Pulse Shaping in Multi-amplitude Case
• Nyquist CriterionControlled ISI
• Figure 7.28: One possible M-ary Scheme
• Another Scheme: Use M orthogonal pulses:
• Definition:
can be used for M-ary case
)(),...,(),( 21 ttt Mϕϕϕ
≠=
=∫ ji
jictt
bT
ji 0)()(
0ϕϕ
57
Dr. Shi Lathi & Ding-Digital Commun 113
Pulse Shaping in Multi-amplitude Case
• The figure in next slide: One example in M orthogonal pulses:
• In the set, pulse frequency:
2sin , 0 1,2,...,
( )
0,
bbk
k tt T k M
Tt
otherwise
πϕ
⋅ ⋅ < < ==
bbbb T
M
TTTk ,...,
2,
1:
1M times that of the binary
scheme (see an example below)
Dr. Shi Lathi & Ding-Digital Commun 114
58
Dr. Shi Lathi & Ding-Digital Commun 115
Pulse Shaping in Multi-amplitude Case
• In general, it can be shown that the bandwidth of an orthogonal M-ary scheme is M times that of the binary scheme– In an M-ary orthogonal scheme, the rate of
communication is increased by a factor of at the cost of an increase in transmission bandwidth by a factor of M.
M2log
Dr. Shi Lathi & Ding-Digital Commun 116
Digital Carrier Systems
• So far: Baseband digital systems– Signals are transmitted directly without any shift in
frequency.– Suitable for transmission over wires, cables, optical
fibers.
• Baseband signals cannot be transmitted over a radio link or satellites.– Since it needs impractically large size for antennas– Modulation (Shifting signal spectrum to higher
frequencies is needed)
59
Dr. Shi Lathi & Ding-Digital Commun 117
Digital Carrier Systems…
• A spectrum shift to higher frequencies is also required when transmitting several messages simultaneously by sharing the large bandwidth of the transmission medium,
• FDM: Frequency-division Multiplexing.
(FDMA: Frequency-division Multiplexing Access)
Dr. Shi Lathi & Ding-Digital Commun 118
Several Types of Modulation
• Amplitude-Shift Keying (ASK),also known as on-off keying (OOK)
:)cos()( ttm cω )(tm
)cos( tcω: on-off baseband signal (modulating signal)
: carrier
60
Dr. Shi Lathi & Ding-Digital Commun 119
Modulation Types…• Phase-Shift Keying (PSK)
: polar Signal)(tm
)cos()()cos()()cos()(0),cos()(1 πωωπωω −=−=−↔↔ ttpttpttpttp cccc
Dr. Shi Lathi & Ding-Digital Commun 120
Modulation Types…
• Frequency-Shift Keying (FSK)
– Frequency is modulation by the base band signal.
– Information bit resides in the carrier frequency.
010,1 cc ωω ↔↔
61
Dr. Shi Lathi & Ding-Digital Commun 121
PSD of ASK, PSK & FSK
Dr. Shi Lathi & Ding-Digital Commun 122
FSK
• FSK signal may be viewed as a sum of two interleaved ASK signals, one with a modulating frequency , the other, – Spectrum of FSK = Sum of the two ASK
– Bandwidth of FSK is higher than that of ASK or PSK
1cω
0cω
62
Dr. Shi Lathi & Ding-Digital Commun 123
DemodulationA. Review of Analog Demodulation
• Baseband signal and bandpass signal The end of the 2nd part of the slides of Ch. 2 and Ch. 3
• Double-sideband Suppressed Carrier (DSB-SC) Modulation
[ ]
[ ]
( ) ( )
1cos( ) ( ) ( )
21
( )cos( ) ( ) ( )2
c c c
c c c
m t M message signal
t carrier
m t t M M modulated signal
ω
ω δ ω ω δ ω ω
ω ω ω ω ω
↔
↔ + + −
↔ + + −
124
Figure 4.1 (a), (b), (c) DSB-SC Modulation
63
Dr. Shi 125
DSB-SC Demodulation
• Demodulation: Synchronization Detection(Coherent Detection)
• Figure 4.1 (e) and (d)
Using a carrier of
exactly the same frequency & pulse
Dr. Shi 126
DSB-SC Demodulation …
[ ] [ ]
[ ]
)(2
1
)2()2(4
1)(
2
1)(
)2cos()()(2
1)()cos()cos()(
ω
ωωωωωω
ωωω
MLPFAftera
MMME
ttmtmtetttm
cc
ccc
⇒
−+++=
+==
)(2
1tm
64
127
Figure 4.1 (d), (e) DSB-SC demodulation
Dr. Shi 128
AM Signal
• Motivation:– Requirements of a carrier in frequency and phase
synchronism with carrier at the transmitter is too sophisticated and quite costly
– Sending the carrier a. eases the situationb. requires more power in
transmission
c. suitable for broadcasting
⇒
65
Dr. Shi 129
Amplitude Modulation (AM)
)cos()()cos()( ttmtAt ccAM ωωϕ +=[ ] )cos()( ttmA cω+=
[ ])()(2
1)( ccAM MMt ωωωωϕ −++⇔
[ ])()( ccA ωωδωωδπ −++⋅+
130
• AM Signal and its Envelope
66
Dr. Shi 131
• Demodulation of AM signals
– Rectifier Detection: Figure 4.11
Output:
– Envelope Detector: Figure 4.12
Output:
)(1
tmπ
)(tmA+
132
67
133
Dr. Shi Lathi & Ding-Digital Commun 134
B. Back to Digital Communication System:
Demodulation
• Demodulation of Digital-Modulated Signals is similar to Demodulation of Analog-Modulated Signals
• Demodulation of ASK� Synchronous (Coherent) Detection
� Non-coherent Detection: Say, envelope detection
68
Dr. Shi Lathi & Ding-Digital Commun 135
Demodulation of PSK
• Cannot be demodulated non-coherently (envelope detection).– Since, envelope is the same for 0 and 1
• Can be demodulated coherently.
• PSK may be demodulated non-coherently if use: Differential coherent PSK (DPSK)
Dr. Shi Lathi & Ding-Digital Commun 136
Differential Encoding
• Differential coding:– “1” � encoded by the same pulse used to encode
the previous data bit � no transition
– “0” � encoded by the negative of pulse used to encode the previous data bit � transition
• Figure 7.30 (a)
• Modulated signal consists of pulses with a possible sign ambiguity
)cos( tA cω±
69
Dr. Shi Lathi & Ding-Digital Commun 137
Differential Encoding
• Figure
Dr. Shi Lathi & Ding-Digital Commun 138
Differential Encoding…
• : one-bit interval
• If the received pulse is identical to the previous pulse �1,
• If 0 is transmitted
bT
[ ]
2)(
)2cos(12
)(cos)(
2
222
Atz
tA
tAtY cc
=
+== ωω
[ ]
2)(
)2cos(12
)(cos)(
2
222
Atz
tA
tAtY cc
−=
+−=−= ωω
70
Dr. Shi Lathi & Ding-Digital Commun 139
Demodulation of FSK
• FSK can be viewed as two interleaved ASK signals with carrier frequency and , respectively.
• Hence, FSK can be detected by non-coherent detection (envelope) or coherent detection technique
1cω
0cω
Dr. Shi Lathi & Ding-Digital Commun 140
Demodulation of FSK
71
Dr. Shi Lathi & Ding-Digital Commun 141
Demodulation of FSK
• In (a), non-coherent detection (envelope),– H0(ω)
Turned to respectively
– H1(ω)
• Comparison: above or bottomWhose output is large � 0 or 1
1cω
0cω