Chapter 5
Analog
Analog Transmission
Transmission
„
Analog
Transmission
„
Digital to Analog Encoding : is the representation of
digital information by a analog signal
„
Analog Transmission of Analog Data
„
Analog to Analog Encoding : is the representation of
analog information by a analog signal
„
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Analog Transmission of Digital Data
Encoding means transform information into signal
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Analog
Analog Transmission
Transmission of
of Digital
Digital Data
Data
„
is the representation of digital information by an
analog signal.
„
Ex : transmit data from one computer to another across a public
access phone line, the data start as digital , but because
telephone wires carry analog signals, the data must be converted.
Analog
Analog Transmission
Transmission of
of Analog
Analog Data
Data
Digital / Analog
encoding
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5.1 Digital to Analog Conversion
Digital-to-analog conversion
• Aspects of Digital-to-Analog Conversion
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
• Quadrature Amplitude Modulation
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Figure 5.2 Types of digital-to-analog conversion
Note
Bit rate is the number of bits per second.
Baud rate is the number of signal elements
per second.
In the analog transmission of digital data,
the baud rate is less than or equal to the bit rate.
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Example 5.1
Relationship between data rate and signal rate
An analog signal carries 4 bits per signal element.
If 1000 signal elements are sent per second,
find the bit rate.
„
„
„
„
S : the signal rate (baud)
N : the data rate (bps)
r : ratio : the numbers of data elements carried
in one signal element
L : the type of signal element , not the level
Solution
In this case, r = 4, S = 1000, and N is unknown.
We can find the value of N from
Example 5.2
An analog signal has a bit rate of 8000 bps and a baud
rate of 1000 baud.
How many data elements are carried by each signal
element? How many signal elements do we need?
Carrier Signal
„
In analog transmission the sending device produces a
high-frequency signal that acts as a basis for the
information signal.
Solution
In this example, S = 1000, N = 8000, and r and L are
unknown. We find first the value of r and then the value of L.
„
This base signal is called the carrier signal or carrier
frequency.
„
Digital information is then encoded onto the carrier
signal.
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Binary amplitude shift keying (BASK)
Figure 5.4 Implementation of binary ASK
S : the number of signal element
N : the data rate (bps)
d : a factor related to the condition of a line
„
„
Both frequency and phase remain constant while
the amplitude changes.
ASK transmission is highly susceptible to noise
interference.
Example 5.3
We have an available bandwidth of 100 kHz which
spans from 200 to 300 kHz. What are the carrier
frequency and the bit rate if we modulated our data by
using ASK with d = 1?
Solution
The middle of the bandwidth is located at 250 kHz.
This means that our carrier frequency can be at fc = 250 kHz.
We can use the formula for bandwidth to find the bit rate
(with d = 1 and r = 1).
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Example 5.4
In data communications, we normally use full-duplex links
with communication in both directions. We need to divide
the bandwidth into two with two carrier frequencies, as
shown in below figure .
The figure shows the positions of two carrier frequencies
and the bandwidths. The available bandwidth for each
direction is now 50 kHz, which leaves us with a data rate
of 25 kbps in each direction.
Binary frequency shift keying (BFSK)
S : the number of signal element
N : the data rate (bps)
d : a factor related to the condition of a line
∆f : the differences between two frequencies
Example 5.5
„
„
„
„
the frequency of the signal is varied to represent binary 1 or 0.
both peak amplitude and phase remain constant.
FSK avoids most of the noise problems of ASK.
The limiting factors of FSK are the physical capabilities of the
carrier.
We have an available bandwidth of 100 kHz which
spans from 200 to 300 kHz. What should be the carrier
frequency and the bit rate if we modulated our data by
using FSK with d = 1?
Solution
This problem is similar to Example 5.3, but we are modulating
by using FSK. The midpoint of the band is at 250 kHz. We
choose 2∆f to be 50 kHz; this means
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Binary phase shift keying (BPSK)
Figure 5.10 Implementation of BPSK
S : the number of signal element
N : the data rate (bps)
d : a factor related to the condition of a line
Figure 5.9
„
phase refers to the direction in which the wave begins
„
„
„
„
„
PSK constellation
2-PSK
4-PSK
8-PSK
etc
2-PSK
0 o phase wave : the waves that start by moving up and to the right.
1o phase wave : the waves that start down and to the right.
„
Every time there is a change in the binary value (0 or 1),
there is a 180 o change in the phase
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Figure 5.11
The 4-PSK characteristics
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Figure 5.12
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Figure 5.15
The 4-QAM and 8-QAM constellations
4-QAM
1 Amplitudes , 4 Phases
The 8-PSK characteristics
Time domain for an 8-QAM signal
8-QAM
2 Amplitudes , 4 Phases
Quadrature amplitude modulation is a combination of ASK and PSK
so that a maximum contrast between each signal unit
(bit, dibit, tribit, and so on) is achieved.
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Figure 5.16
16-QAM constellations
Figure 5.17
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Table 5.1 Bit and baud rate comparison
Modulation
Bits/Baud
Baud rate
Bit Rate
Bit
1
N
N
4-PSK, 44-QAM
Dibit
2
N
2N
8-PSK, 88-QAM
Tribit
3
N
3N
1616-QAM
Quadbit
4
N
4N
3232-QAM
Pentabit
5
N
5N
6464-QAM
Hexabit
6
N
6N
128128-QAM
Septabit
7
N
7N
256256-QAM
Octabit
8
N
8N
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Example
Units
ASK, FSK, 22-PSK
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Bit and baud
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A constellation diagram consists of eight equally spaced
points on a circle. If the bit rate is 4800 bps, what is the
baud rate?
Solution
The constellation indicates 8-PSK with the points 45 degrees apart.
Since 23 = 8, 3 bits are transmitted with each signal unit.
Therefore, the baud rate is
4800 / 3 = 1600 baud
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Example
Example
Compute the bit rate for a 1000-baud 16-QAM signal.
Compute the baud rate for a 72,000-bps 64-QAM signal.
Solution
Solution
A 16-QAM signal has 4 bits per signal unit since
log216 = 4.
Thus,
(1000)(4) = 4000 bps
A 64-QAM signal has 6 bits per signal unit since
log2 64 = 6.
Thus,
72000 / 6 = 12,000 baud
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Figure 5.24
Analog-to-analog modulation
5.4 Analog to Digital Conversion
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
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Figure 5.15 Types of analog-to-analog modulation
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Figure 5.18 Frequency modulation
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Figure 5.16 Amplitude modulation
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Figure 5.20 Phase modulation
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