Ch5 Data communication and networking by neha g. kurale

5-1 DIGITAL-TO-ANALOG CONVERSION
Digital-to-analog conversion is the process of
changing one of the characteristics of an analog
signal based on the information in digital data.
 Aspects of Digital-to-Analog Conversion
 Amplitude Shift Keying
 Frequency Shift Keying
 Phase Shift Keying
 Quadrature Amplitude Modulation
Topics discussed in this section:

Digital to Analog Conversion
 Digital data needs to be carried on an
analog signal.
 A carrier signal (frequency fc) performs
the function of transporting the digital
data in an analog waveform.
 The analog carrier signal is manipulated
to uniquely identify the digital data
being carried.

Figure 5.1 Digital-to-analog conversion

Figure 5.2 Types of digital-to-analog conversion

Bit rate, N, is the number of bits per
second (bps). Baud rate is the number of
signal
elements per second (bauds).
In the analog transmission of digital
data, the signal or baud rate is less than
or equal to the bit rate.
S=Nx1/r bauds
Where r is the number of data bits per
signal element.
Note

An analog signal carries 4 bits per signal element. If
1000 signal elements are sent per second, find the bit
rate.
Solution
In this case, r = 4, S = 1000, and N is unknown. We can
find the value of N from
Example 5.1

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?
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.

Amplitude Shift Keying (ASK)
 ASK is implemented by changing the
amplitude of a carrier signal to reflect
amplitude levels in the digital signal.
 For example: a digital “1” could not affect the
signal, whereas a digital “0” would, by
making it zero.
 The line encoding will determine the values of
the analog waveform to reflect the digital
data being carried.

Bandwidth of ASK
 The bandwidth B of ASK is proportional
to the signal rate S.
B = (1+d)S
 “d” is due to modulation and filtering,
lies between 0 and 1.

Figure 5.3 Binary amplitude shift keying

Figure 5.4 Implementation of binary ASK

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).

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 Figure 5.5. 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.

Figure 5.5 Bandwidth of full-duplex ASK used in Example 5.4

Frequency Shift Keying
 The digital data stream changes the
frequency of the carrier signal, fc.
 For example, a “1” could be
represented by f1=fc +f, and a “0”
could be represented by f2=fc-f.

Figure 5.6 Binary frequency shift keying

Bandwidth of FSK
 If the difference between the two
frequencies (f1 and f2) is 2f, then the
required BW B will be:
B = (1+d)xS +2f

Example 5.5
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 2Δf to be 50 kHz; this means

Coherent and Non Coherent
 In a non-coherent FSK scheme, when
we change from one frequency to the
other, we do not adhere to the current
phase of the signal.
 In coherent FSK, the switch from one
frequency signal to the other only
occurs at the same phase in the signal.

Multi level FSK
 Similarly to ASK, FSK can use multiple
bits per signal element.
 That means we need to provision for
multiple frequencies, each one to
represent a group of data bits.
 The bandwidth for FSK can be higher
B = (1+d)xS + (L-1)/2f = LxS

Figure 5.7 Bandwidth of MFSK used in Example 5.6

Example 5.6
We need to send data 3 bits at a time at a bit rate of 3
Mbps. The carrier frequency is 10 MHz. Calculate the
number of levels (different frequencies), the baud rate,
and the bandwidth.
Solution
We can have L = 23 = 8. The baud rate is S = 3 Mbps/3 =
1 Mbaud. This means that the carrier frequencies must be
1 MHz apart (2Δf = 1 MHz). The bandwidth is B = 8 ×
1M = 8M. Figure 5.8 shows the allocation of frequencies
and bandwidth.

Figure 5.8 Bandwidth of MFSK used in Example 5.6

Phase Shift Keyeing
 We vary the phase shift of the carrier
signal to represent digital data.
 The bandwidth requirement, B is:
B = (1+d)xS
 PSK is much more robust than ASK as it
is not that vulnerable to noise, which
changes amplitude of the signal.

Figure 5.9 Binary phase shift keying

Figure 5.10 Implementation of BASK

Quadrature PSK
 To increase the bit rate, we can code 2 or
more bits onto one signal element.
 In QPSK, we parallelize the bit stream so that
every two incoming bits are split up and PSK
a carrier frequency. One carrier frequency is
phase shifted 90o from the other - in
quadrature.
 The two PSKed signals are then added to
produce one of 4 signal elements. L = 4 here.

Figure 5.11 QPSK and its implementation

Example 5.7
Find the bandwidth for a signal transmitting at 12
Mbps for QPSK. The value of d = 0.
Solution
For QPSK, 2 bits is carried by one signal element. This
means that r = 2. So the signal rate (baud rate) is S = N ×
(1/r) = 6 Mbaud. With a value of d = 0, we have B = S = 6
MHz.

Constellation Diagrams
 A constellation diagram helps us to
define the amplitude and phase of a
signal when we are using two carriers,
one in quadrature of the other.
 The X-axis represents the in-phase
carrier and the Y-axis represents
quadrature carrier.

Figure 5.12 Concept of a constellation diagram

Example 5.8
Show the constellation diagrams for an ASK (OOK),
BPSK, and QPSK signals.
Solution
Figure 5.13 shows the three constellation diagrams.

Figure 5.13 Three constellation diagrams

Quadrature amplitude modulation is a
combination of ASK and PSK.
Note

Figure 5.14 Constellation diagrams for some QAMs

Ch5 Data communication and networking by neha g. kurale

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