1
Digital and Analog
Communication System
2
References
1.,现代通信原理, 曹志刚
清华大学出版社
2., 通信原理, 樊昌信
国防工业出版社
Chapter One
3
Introduction
Chapter One
Why?
Information Age
Information Superhighway
4
Application
military application;
common application;
Chapter One
5
What is a communication system?
Communication systems is used to
transmit information,
1.1 Introduction
6
Key conceptions or words
(1) Information;
(2) Message;
(3) Signal;
(4) Waveforms
1.1 Introduction
7
1 Jimes C.Maxwell（ 1864）
2 Heinrich Hertz （ 1887）
3 Claude E.Shannon (1948)
4.Winner
1.1 Introduction
8
Perspective of communication
Wire
optic-fiber and networks
wireless
1G->2G->3G->B3G
WLAN; MIMO and MIMO-OFDM,
UWB,optic –wireless communication
1.1 Introduction
9
What must be considered when you
design a communication system?
1,Selection of the information-bearing
waveform
2,Bandwidth and power of the
waveform
3,Effect of system noise on the received
information
4,Cost of the system
1.1 Introduction
10
1.2 Digital and Analog source and system
? The generation of communication system
T r an s m i t t e r
I n f or m at i on
i n p u t m ( t )
S i gn al
p r oc e s s i n g
c h an n e l
S i gn al
p r oc e s s i n g
C ar r i e r
c i r c u i t s
R e c e i ve r
n oi s e
C ar r i e r
c i r c u i t s
I n f or m at i on
ou t p u t
11
1) signal processor
In analog system it may be an analog low-
pass filter; In hybrid one,it may be analog-
to-digital converter(ADC)
2)Function of signal-processing block in hybrid
one
Source coding
Channel coding(adding parity and others)
3) Function of Carrier Circuits
modulation
Function of transmitter
12
1） Classification,wire and wireless
Wire channel including twisted-pair telephone
line/Coaxial cables,waveguides and fiber-optic
cables
Wireless channel having air,vacuum and
seawater
2) Note,The general principles of digital and
analog modulation apply to all types of channels,
although channel characteristics may impose
constraints that favor a particular type of
signaling
Channel
13
The receiver takes the corrupted signal at the
channel output and converts it to a baseband
signal that can be handled by the receiver
baseband processor,and then the receiver
baseband processor estimate the source
information and output the estimation result
Receiver
14
The goal is to design communication system that
transmit information to the receiver with as little
deterioration as possible while satisfying design
constrains such as allowable transmitted
energy,allowable signal bandwidth and cost,
In digital system measure of deterioration is
taken to be bit error rate(BER),while in analog
system the performance measure is usually taken
to be the signal-to-noise ratio(SNR) at the
receiver output,
Summary
15
1.2 Digital and Analog source and system
? Model of analog communication system
T r a n s m i t t i n g
e n d
I n f o r m a t i o n
s o u r c e
m o d u l a t o r
c h a n n e l
D e m o d u -
l a t o r
S i n k
O r ( a c c e p t a n t )
R e c e i v i n g
e n d n o i s e
16
1.2 Digital and Analog source and system
(model of digital communication system)
17
1） Relatively inexpensive digital circuits
may be used
2） Privacy is preserved by using
data encryption
3） Greater dynamic range is possible
4） Data from different sources my be
transmitted over a common digital
transmission system
2 the advantage of digital system
1.2 Digital and Analog source and system
18
5） Noise does not accumulate in
long- distance systems by repeater
6） Error in detected data may be
small,even in large amount of noise
on the received signal
7） Error may often be corrected
by the use of coding
2 the advantage of digital system
1.2 Digital and Analog source and system
19
1)More bandwidth is required generally
2)Sychronization is required
3.Disadvantages of Digital communication
1.2 Digital and Analog source and system
20
1.2 Digital and Analog source and system
(classification of communication system)
according to physical properties of information,
Telephone /data /image and so on
according to frequency,
baseband transmitting/ bandpass transmitting system
according to transmitting medium,
wire /wireless
according to multiplexing patterm,
Frequency division multiplexing access(FDMA)
Time division multiplexing access(TDMS)
Code division multiplexing access(CDMA)
21
1.2 Digital and Analog source and system
? Classification of communication patterm
? Peer to peer communication,
simplex;
half duplex;
duplex
? Parallel transmitting communication
? Series transmitting communication
22
1,Key conception
Digital information source produces a finite set of
possible messages,Such as typewriter and keyboard
Analog information source produces messages that are
defined on continuum,
Digital communication system transfers information
from a digital source to the intended receiver(sink)
Analog communication system transfers information
from a analog source to the intended receiver(sink)
1.2 Digital and Analog source and system
23
1) Deterministic waveform
It can be modeled as a completely
specified function of time
2) Random waveform(stochastic
waveform)
It can not be completely specified as a
function of time and must be modeled
probabilistically
1) Example,
1.3 Deterministic and Random waveforms
)c o s ()( 00 ??? ?? tAt
24
1)Property of the book
a,from easiness to difficulty
b,from concept to performance analysis
2) Methods of learning it
a,reading it again and again
b,grasping the important concept
c,understanding the basic communication
system by deriving some key formula
d,simulation using Matlab
1.4 The property of the book and
methods of learning it
25
1,Information Content
If the probabiltiy of transmitting jth
message is Pj,the information content
sent from a digital source when it was
transmitted is given by,
1.9 Information Measure
b i tP
P
I j
j
j )(l o g)
1(l o g
22 ???
26
1.9 Information Measure
From the definition,we have the following
results,
1) The message that are less likely to occur
provide more information
2) Information content depends on only the
likehood of sending the message and does
not depend on possible interpretation of
the content as to whether or not it makes
sense,
27
2 Unit of information
1.9 Information Measure
b i t sPI jb i t sj )(l o g 2,??
n a t sPI jn a t sj )l n (,??
H a r t l e yPI jh a r t l e yj )(l o g 10,??
1) Bits,Nats and Hartley
28
2 Unit of information
1.9 Information Measure
2) Relationship between Bit,Nat and Hartley
)2/ (l o g
)2/ (l n
10,
,,
h a r t l e yj
n a t sjb i tj
I
II
?
?
29
1) Definition
If there are m possible different source message
in a digital source(m is finite),and Pj is the
probabiltiy of sending the jth message,the
entropy (also called average information) is,
1.9 Information Measure
b i t s
P
PIPH
m
j j
j
m
j
jj ??
??
??
1
2
1
)1(l o g
3,Entropy for Digital Source
30
4 Example1-1,In a string of 12 symbols,
where each symbol consistis of one of four
levels,there are 412 different combinations
(words),since each levels is equally likely,
and all the different words equally likely,
Thus Pj=(1/4)12,
And
Ij=log2(1/(1/4)12)=12log2(4)=24bits
1.9 Information Measure
31
5 Source Rate
1.9 Information Measure
Source Rate is defined as
b itsTHR ?
Example,A1-2 A telephone touch-tone keypad has the digits
0 to 9 with probability of sending being 0.099 each,plus
“*” and,#” with probability of sending being 0.005 each,
If the keys are pressed at a rate of 2keys/s,compute the
data rate for this source,
Solution,b i t s
PPIPH
m
j j
j
m
j
jj ??
??
??
1
2
1
)1(l o g
k e yb i t s /38.3
0 0 5.0
1l o g)0 0 5.0(2
0 9 9.0
1l o g)0 9 9.0(10
2l o g
1
1010
10
?
?
?
?
?
?
?
?
?
??
?
???
?
??
?
??
Using define above,where T=1/2=0.5s/key,then,
sb i t sTHR /76.65.0/38.3 ???
32
For digital systems,the optimum system might be
defined as the system that minimizes the BER at
the system output subject to constraints on
transmitted energy and channel bandwidth,Thus
bit error rate and signal bandwidth are of prime
importance in this course,
1.10 Channel capacity
Shannon gives channel capacity as
?
?
?
?
?
? ??
N
S
BC 1l o g 2
33
1.10 Channel capacity
Where B is the channel bandwidth in hertz,S/N is
the signal-to-noise power ratio at the input to the
digital receiver,And showed that if the rate of
information R(bits/s) was less than C,the BER
would approach zero,
34
In analog system,the optimum system might be
defined as the one that achieves the largest signal-
to-noise ratio at the receiver output,subject to
design constraints such as channel bandwidth
and transmitted power,
Nyquist showed that if a pulse represents one bit
of data,noninterfering pulse could be sent over a
channel no faster than 2B pulses/s,where B is the
channel bandwidth in hertz,That is now known
as the dimensionality theorem,
1.10 Channel Capacity
35
Example SA1-3,A computer user plans to buy a higher-
speed modem for sending data over his or her analog
telephone line,The telephone line has a signal-to-noise
ratio(SNR) of 25dB and passes audio frequencies over the
range,from 300 to 3200Hz,Calculate the maximum data
rate that could be sent over the telephone line when there
are on errors at the receiving end,
Solution,In the term of a power ratio,the
SNR=10(25/10)=316.2,the bandwidth is B=3200-300= 2900
Hz,we get,
1.10 Channel Capacity
? ? sb i t s
N
SBC /2 4 0 9 7)2.3 1 61(log2 9 0 01log
22 ?????
??
?
? ??
Problem,Can a 28.8kbit/s modem work on this telephone line?
And a 14.4kbits/s modem?
36
Problems,1.5,1.7,1.9,1.10,
1.12,1.13 1.15
Reading,page 1-2,§ 1-2,§ 1-3,§ 1-6
§ 1-9,§ 1-10
Homework
Digital and Analog
Communication System
2
References
1.,现代通信原理, 曹志刚
清华大学出版社
2., 通信原理, 樊昌信
国防工业出版社
Chapter One
3
Introduction
Chapter One
Why?
Information Age
Information Superhighway
4
Application
military application;
common application;
Chapter One
5
What is a communication system?
Communication systems is used to
transmit information,
1.1 Introduction
6
Key conceptions or words
(1) Information;
(2) Message;
(3) Signal;
(4) Waveforms
1.1 Introduction
7
1 Jimes C.Maxwell（ 1864）
2 Heinrich Hertz （ 1887）
3 Claude E.Shannon (1948)
4.Winner
1.1 Introduction
8
Perspective of communication
Wire
optic-fiber and networks
wireless
1G->2G->3G->B3G
WLAN; MIMO and MIMO-OFDM,
UWB,optic –wireless communication
1.1 Introduction
9
What must be considered when you
design a communication system?
1,Selection of the information-bearing
waveform
2,Bandwidth and power of the
waveform
3,Effect of system noise on the received
information
4,Cost of the system
1.1 Introduction
10
1.2 Digital and Analog source and system
? The generation of communication system
T r an s m i t t e r
I n f or m at i on
i n p u t m ( t )
S i gn al
p r oc e s s i n g
c h an n e l
S i gn al
p r oc e s s i n g
C ar r i e r
c i r c u i t s
R e c e i ve r
n oi s e
C ar r i e r
c i r c u i t s
I n f or m at i on
ou t p u t
11
1) signal processor
In analog system it may be an analog low-
pass filter; In hybrid one,it may be analog-
to-digital converter(ADC)
2)Function of signal-processing block in hybrid
one
Source coding
Channel coding(adding parity and others)
3) Function of Carrier Circuits
modulation
Function of transmitter
12
1） Classification,wire and wireless
Wire channel including twisted-pair telephone
line/Coaxial cables,waveguides and fiber-optic
cables
Wireless channel having air,vacuum and
seawater
2) Note,The general principles of digital and
analog modulation apply to all types of channels,
although channel characteristics may impose
constraints that favor a particular type of
signaling
Channel
13
The receiver takes the corrupted signal at the
channel output and converts it to a baseband
signal that can be handled by the receiver
baseband processor,and then the receiver
baseband processor estimate the source
information and output the estimation result
Receiver
14
The goal is to design communication system that
transmit information to the receiver with as little
deterioration as possible while satisfying design
constrains such as allowable transmitted
energy,allowable signal bandwidth and cost,
In digital system measure of deterioration is
taken to be bit error rate(BER),while in analog
system the performance measure is usually taken
to be the signal-to-noise ratio(SNR) at the
receiver output,
Summary
15
1.2 Digital and Analog source and system
? Model of analog communication system
T r a n s m i t t i n g
e n d
I n f o r m a t i o n
s o u r c e
m o d u l a t o r
c h a n n e l
D e m o d u -
l a t o r
S i n k
O r ( a c c e p t a n t )
R e c e i v i n g
e n d n o i s e
16
1.2 Digital and Analog source and system
(model of digital communication system)
17
1） Relatively inexpensive digital circuits
may be used
2） Privacy is preserved by using
data encryption
3） Greater dynamic range is possible
4） Data from different sources my be
transmitted over a common digital
transmission system
2 the advantage of digital system
1.2 Digital and Analog source and system
18
5） Noise does not accumulate in
long- distance systems by repeater
6） Error in detected data may be
small,even in large amount of noise
on the received signal
7） Error may often be corrected
by the use of coding
2 the advantage of digital system
1.2 Digital and Analog source and system
19
1)More bandwidth is required generally
2)Sychronization is required
3.Disadvantages of Digital communication
1.2 Digital and Analog source and system
20
1.2 Digital and Analog source and system
(classification of communication system)
according to physical properties of information,
Telephone /data /image and so on
according to frequency,
baseband transmitting/ bandpass transmitting system
according to transmitting medium,
wire /wireless
according to multiplexing patterm,
Frequency division multiplexing access(FDMA)
Time division multiplexing access(TDMS)
Code division multiplexing access(CDMA)
21
1.2 Digital and Analog source and system
? Classification of communication patterm
? Peer to peer communication,
simplex;
half duplex;
duplex
? Parallel transmitting communication
? Series transmitting communication
22
1,Key conception
Digital information source produces a finite set of
possible messages,Such as typewriter and keyboard
Analog information source produces messages that are
defined on continuum,
Digital communication system transfers information
from a digital source to the intended receiver(sink)
Analog communication system transfers information
from a analog source to the intended receiver(sink)
1.2 Digital and Analog source and system
23
1) Deterministic waveform
It can be modeled as a completely
specified function of time
2) Random waveform(stochastic
waveform)
It can not be completely specified as a
function of time and must be modeled
probabilistically
1) Example,
1.3 Deterministic and Random waveforms
)c o s ()( 00 ??? ?? tAt
24
1)Property of the book
a,from easiness to difficulty
b,from concept to performance analysis
2) Methods of learning it
a,reading it again and again
b,grasping the important concept
c,understanding the basic communication
system by deriving some key formula
d,simulation using Matlab
1.4 The property of the book and
methods of learning it
25
1,Information Content
If the probabiltiy of transmitting jth
message is Pj,the information content
sent from a digital source when it was
transmitted is given by,
1.9 Information Measure
b i tP
P
I j
j
j )(l o g)
1(l o g
22 ???
26
1.9 Information Measure
From the definition,we have the following
results,
1) The message that are less likely to occur
provide more information
2) Information content depends on only the
likehood of sending the message and does
not depend on possible interpretation of
the content as to whether or not it makes
sense,
27
2 Unit of information
1.9 Information Measure
b i t sPI jb i t sj )(l o g 2,??
n a t sPI jn a t sj )l n (,??
H a r t l e yPI jh a r t l e yj )(l o g 10,??
1) Bits,Nats and Hartley
28
2 Unit of information
1.9 Information Measure
2) Relationship between Bit,Nat and Hartley
)2/ (l o g
)2/ (l n
10,
,,
h a r t l e yj
n a t sjb i tj
I
II
?
?
29
1) Definition
If there are m possible different source message
in a digital source(m is finite),and Pj is the
probabiltiy of sending the jth message,the
entropy (also called average information) is,
1.9 Information Measure
b i t s
P
PIPH
m
j j
j
m
j
jj ??
??
??
1
2
1
)1(l o g
3,Entropy for Digital Source
30
4 Example1-1,In a string of 12 symbols,
where each symbol consistis of one of four
levels,there are 412 different combinations
(words),since each levels is equally likely,
and all the different words equally likely,
Thus Pj=(1/4)12,
And
Ij=log2(1/(1/4)12)=12log2(4)=24bits
1.9 Information Measure
31
5 Source Rate
1.9 Information Measure
Source Rate is defined as
b itsTHR ?
Example,A1-2 A telephone touch-tone keypad has the digits
0 to 9 with probability of sending being 0.099 each,plus
“*” and,#” with probability of sending being 0.005 each,
If the keys are pressed at a rate of 2keys/s,compute the
data rate for this source,
Solution,b i t s
PPIPH
m
j j
j
m
j
jj ??
??
??
1
2
1
)1(l o g
k e yb i t s /38.3
0 0 5.0
1l o g)0 0 5.0(2
0 9 9.0
1l o g)0 9 9.0(10
2l o g
1
1010
10
?
?
?
?
?
?
?
?
?
??
?
???
?
??
?
??
Using define above,where T=1/2=0.5s/key,then,
sb i t sTHR /76.65.0/38.3 ???
32
For digital systems,the optimum system might be
defined as the system that minimizes the BER at
the system output subject to constraints on
transmitted energy and channel bandwidth,Thus
bit error rate and signal bandwidth are of prime
importance in this course,
1.10 Channel capacity
Shannon gives channel capacity as
?
?
?
?
?
? ??
N
S
BC 1l o g 2
33
1.10 Channel capacity
Where B is the channel bandwidth in hertz,S/N is
the signal-to-noise power ratio at the input to the
digital receiver,And showed that if the rate of
information R(bits/s) was less than C,the BER
would approach zero,
34
In analog system,the optimum system might be
defined as the one that achieves the largest signal-
to-noise ratio at the receiver output,subject to
design constraints such as channel bandwidth
and transmitted power,
Nyquist showed that if a pulse represents one bit
of data,noninterfering pulse could be sent over a
channel no faster than 2B pulses/s,where B is the
channel bandwidth in hertz,That is now known
as the dimensionality theorem,
1.10 Channel Capacity
35
Example SA1-3,A computer user plans to buy a higher-
speed modem for sending data over his or her analog
telephone line,The telephone line has a signal-to-noise
ratio(SNR) of 25dB and passes audio frequencies over the
range,from 300 to 3200Hz,Calculate the maximum data
rate that could be sent over the telephone line when there
are on errors at the receiving end,
Solution,In the term of a power ratio,the
SNR=10(25/10)=316.2,the bandwidth is B=3200-300= 2900
Hz,we get,
1.10 Channel Capacity
? ? sb i t s
N
SBC /2 4 0 9 7)2.3 1 61(log2 9 0 01log
22 ?????
??
?
? ??
Problem,Can a 28.8kbit/s modem work on this telephone line?
And a 14.4kbits/s modem?
36
Problems,1.5,1.7,1.9,1.10,
1.12,1.13 1.15
Reading,page 1-2,§ 1-2,§ 1-3,§ 1-6
§ 1-9,§ 1-10
Homework
