FUNDAMENTALS
OF ELECTRONICS
COMMUNICATIONS
SYSTEMS
ELX314-EE-T | 2022_1st_Term
Technological University of the Philippines - Taguig campus
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DIGITAL MODULATION
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Introduction
Digital Modulation
is the transmittal of digitally modulated analog signals
(carriers) between two or more points in a
communication system.
digital radio because digitally modulated signals can
be propagated through Earth’s atmosphere and used
in wireless communications.
offers several outstanding advantages over traditional
analog systems, such as ease of processing, ease of
multiplexing, and noise immunity
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Introduction
Digital Modulation
include systems where relatively high-frequency
analog carriers are modulated by relatively lowfrequency digital information signals (digital radio)
and systems involving the transmission of digital
pulses (digital transmission)
digital transmission systems transport information in
digital form and require a physical facility between the
transmitter and the receiver (i.e., metallic wires, coaxial
cable, or an optical fiber)
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Modulation
Modulation is the process of varying a certain
characteristic (usually amplitude, frequency, or phase)
of a high-frequency carrier in proportion with the
characteristic of a low-frequency modulating or
information signal.
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Modulation
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Digital Modulation is ideally suited to a multitude of
communications applications, including both cable and
wireless systems.
Applications include:
1. Relatively low-speed voice-band data communications
modems, such as those found in most personal computers;
2. High-speed data transmission systems, such as broadband
digital subscriber lines (DSL);
3. Digital microwave and satellite communications systems;
4. Cellular telephone Personal Communications Systems (PCS)
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Simplified Block Diagram of a Digital Radio System
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In the transmitter, the precoder performs level conversion and
then encodes the incoming data into groups of bits that
modulate an analog carrier.
The modulated carrier is shaped (filtered), amplified, and then
transmitted through the transmission medium to the receiver.
In the receiver, the incoming signals are filtered, amplified,
and then applied to the demodulator and decoder circuits,
which extracts the original source information from the
modulated carrier.
The clock and carrier recovery circuits recover the analog
carrier and digital timing (clock) signals from the incoming
modulated wave since they are necessary to perform the
demodulation process.
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Information Capacity
Is a measure of how much information can be propagated
through a communications system and is a function of
bandwidth and transmission time.
is often expressed as a bit rate.
Bit rate – refers to the rate of change of a digital
information signal
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Baud rate
Baud – refers to the rate of change of a signal on the
transmission medium after encoding and modulation
occurred
Baud – is a unit of transmission rate, modulation rate, or
symbol rate (symbols per second) baud = 1/tS
where
baud = symbol rate (symbols per second)
tS = time for one signaling element (seconds)
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Information Theory
Is a highly theoretical study of the efficient use of bandwidth
to propagate information through ECSs and use to determine
the information capacity of a data communication system.
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Information CAPACITY
Hartley’s Law
IαBxt
Example:
A data signal has 10MHz bandwidth, what is the channel
capacity ?
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Information CAPACITY
Shannon limit law
I = B log2 (1 + S/N) or I = 3.32 B log (1 + S/N)
Example:
For a standard telephone circuit with a signal-to-noise power
ratio of 30 dB and a bandwidth of 2.7 kHz, the Shannon limit
for information capacity is
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Information CAPACITY
M-ary Encoding
M represents a digit that corresponds to the number of
conditions, levels, or combinations of a given number of bits.
The number of bits necessary to produce a given number of
conditions is expressed mathematically as
N = log2 M
where
N = number of bits necessary
M = number of conditions, levels, or combinations possible with
N bits
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Information CAPACITY
M-ary Encoding
For example, a digital signal with four possible conditions
(voltage levels, frequencies, phases, and so on) is an Msystem where M = 4.
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Information CAPACITY
H. Nyquist,
Binary signals can be propagated through an ideal noiseless
transmission medium at a rate equal to two times the
bandwidth of the medium. The minimum theoretical
bandwidth necessary to propagate a signal is called the
minimum Nyquist bandwidth or minimum Nyquist
frequency.
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Information CAPACITY
H. Nyquist,
For example, a standard telephone circuit has a bandwidth
approximately 2700 Hz, which has the capacity to
propagate 5400 bps through it.
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Information CAPACITY
H. Nyquist,
Using multilevel signaling, the Nyquist formulation for
channel capacity is fb = B log2 M
where
fb = channel capacity (bps)
B = minimum Nyquist Bandwidth (hertz)
M = number of discrete or voltage levels
.
To solve for the minimum bandwidth necessary to pass M-ary
digitally modulated carriers B = (fb / log2 M) or B = fb / N
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Amplitude-Shift Keying
Simplest digital modulation technique
A binary information signal directly modulates the
amplitude of an analog carrier
Sometimes called digital amplitude modulation (DAM)
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Amplitude-Shift Keying
(a) binary input (b) ASK output waveform
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Amplitude-Shift Keying
ASK waveform is the same as the rate of change of the
binary input (bps)
thus, the bit rate equals the baud. With ASK, the bit rate
is also equal to the minimum Nyquist Bandwidth.
B = fb / 1 = fb baud = fb / 1 = fb
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Amplitude-Shift Keying
Example:
Determine the baud and minimum bandwidth necessary to
pass a 20 kbps binary signal using amplitude shift keying.
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Frequency-Shift Keying/Binary FSK
another relatively simple, low-performance type of
digital modulation
is a form of constant-amplitude angle modulation
similar to standard FM except the modulating signal is a
binary signal that varies between two discrete voltage
levels rather than a continuously changing analog
waveform
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Frequency-Shift Keying/Binary FSK
The output frequency shifts between two frequencies:
Δf (Frequency Deviation) is expressed mathematically
as Δf = |fm - fs | / 2
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Frequency-Shift Keying/Binary FSK
The baud for binary FSK, baud = fb / 1 = fb
Bandwidth = 2(Δf + fb )
almost resembles to Carson’s Rule
Example:
Determine (a) the peak frequency deviation, (b) minimum
bandwidth, and (c) baud for a binary FSK signal with a mark
frequency of 49 kHz, a space frequency of 51 kHz, and an
input bit rate of 2 kbps.
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Frequency-Shift Keying/Binary FSK
Example:
Determine (a) the peak frequency deviation, (b) minimum
bandwidth, and (c) baud for a binary FSK signal with a mark
frequency of 49 kHz, a space frequency of 51 kHz, and an
input bit rate of 2 kbps.
Solution
(a) the peak frequency deviation
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Frequency-Shift Keying/Binary FSK
Example:
Determine (a) the peak frequency deviation, (b) minimum
bandwidth, and (c) baud for a binary FSK signal with a mark
frequency of 49 kHz, a space frequency of 51 kHz, and an
input bit rate of 2 kbps.
Solution
(b) minimum bandwidth
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Frequency-Shift Keying/Binary FSK
Example:
Determine (a) the peak frequency deviation, (b) minimum
bandwidth, and (c) baud for a binary FSK signal with a mark
frequency of 49 kHz, a space frequency of 51 kHz, and an
input bit rate of 2 kbps.
Solution
(c) baud
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Phase-Shift Keying
is an M-ary digital modulation scheme similar to
conventional PM except that the input is a binary digital
signal and there are limited number of output phases
possible.
The input binary information is encoded into groups of
bits before modulating the carrier. The number of bits
in a group ranges from 1 to 12 or more.
The number of output phases is defined by M (2^N=M)
and determined by the number of bits in the group (N).
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Phase-Shift Keying
The simplest form of PSK where N=1 and M=2.  One
phase represents logic 1, and the other phase
represents a logic 0.
As the input digital signal changes state (i.e., from a 1 to
a 0 or from a 0 to a 1), the phase of the output carrier
shifts between two angles that are separated by 180°.
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Phase-Shift Keying
Truth table
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Quarternary or Quadrature PSK
QPSK is an M-ary encoding scheme where N=2 and M=4.
Four output phases representing each two-bit input: 00,
01, 10 and 11.
The four possible output phases (+45°, +135°, –45°, –135°)
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Quarternary or Quadrature PSK
Truth table
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Quarternary or Quadrature PSK
8-PSK
Three bits are encoded, forming tribits(N=3) and
producing eight different phases (M=8).
16-QAM
Is an M-ary system where M = 16 and n = 4.
The input binary data are divided into four channels: I, I’, Q,
and Q’.
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ASK, FSK, PSK, and QAM Summary
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Bandwidth Efficiency
Bandwidth Efficiency / Information Density / Spectral Density
often used to compare the performance of one digital
technique to another
is the ratio of the transmission bit rate to the minimum
bandwidth required for a particular modulation scheme
In bits / cycle
EXAMPLE
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For an 8-PSK system, operating with an information bit rate of
24 kbps, determine (a) baud, (b) minimum bandwidth, and
(c)bandwidth efficiency.
EXAMPLE
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For a 16-PSK and a transmission system with 10 kHz
bandwidth, determine the maximum bit rate.
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PULSE MODULATION
TECHNIQUES
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PULSE MODULATION
Many signals in modern communication systems are digital
Additionally, analog signals are transmitted digitally
Digitizing a signal results in reduced distortion and
improvement in signal-tonoise ratios
The process of transmitting signals in the form of pulses
(discontinuous signals) by using special techniques.
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PULSE MODULATION
Pulse Amplitude Modulation
Pulse Width Modulation
Pulse Position Modulation
Pulse Code Modulation
Delta Modulation
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Sampling
The process of transmitting signals in the form of pulses
(discontinuous signals) by using special techniques.
The signal is sampled at regular intervals such that each
sample is propotional to the amplitude of signal at that
instant.This technique is called “sampling”.
Analog signal is sampled every TS secs.
Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
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There are 3 sampling methods
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flat top - sample and hold, like natural but with single
amplitude value
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There are 3 sampling methods
Ideal - an impulse at each sampling instant
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There are 3 sampling methods
Natural - a pulse of short width with varying amplitude
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There are 3 sampling methods
Flat top - sample and hold, like natural but with single
amplitude value
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Pulse Width Modulation
In this type, the amplitude is maintained constant but
the width of each pulse is varied in accordance with
instantaneous value of the analog signal.
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Pulse Width Modulation
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Pulse Width Modulation
In pulse width
modulation (PWM),
the width of each
pulse is made directly
proportional to the
amplitude of the
information signal.
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Digital Pulse Modulation
The process of Sampling which we have already discussed
in initial slides is also adopted in Digital pulse modulation.
It is mainly of two types:
Pulse Code Modulation(PCM)
Delta Modulation(DM)
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Pulse Code Modulation(PCM)
In PCM, the available range of signal voltages is divided
into levels and each is assigned a binary number.
Each sample is represented by a binary number and
transmitted serially
The number of levels available depends upon the
number of bits used to express the sample value
The number of levels is given by: N = 2^m
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Pulse Code Modulation(PCM)
PCM consists of three steps to digitize an analog signal:
1. Sampling
2. Quantization
3. Binary encoding
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Pulse Code Modulation(PCM)
Analog to digital converter employs two techniques:
1. Sampling:
The process of generating pulses of zero width and of
amplitude equal to the instantaneous amplitude of the
analog signal. The no. of pulses per second is called
“sampling rate”.
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Pulse Code Modulation(PCM)
2. Quantization: The process of dividing the maximum value
of the analog signal into a fixed no. of levels in order to
convert the PAM into a Binary Code. The levels obtained
are called “quanization levels”.
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Pulse Code Modulation(PCM)
3. Binary Coding
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Delta Modulation
In Delta Modulation, only one bit is transmitted per
sample
That bit is a one if the current sample is more positive
than the previous sample, and a zero if it is more
negative
Since so little information is transmitted, delta
modulation requires higher sampling rates than PCM
for equal quality of reproduction
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Delta Modulation
This scheme sends only the difference between pulses,
if the pulse at time tn+1 is higher in amplitude value
than the pulse at time tn, then a single bit, say a “1”, is
used to indicate the positive value.
If the pulse is lower in value, resulting in a negative
value, a “0” is used.
This scheme works well for small changes in signal
values between samples. If changes in amplitude are
large, this will result in large errors.
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Delta Modulation
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Delta Modulation
Components of Delta Modulation
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Delta Modulation
Distortions in DM system
1. If the slope of analog signal is much higher than that of
approximated digital signal over long duration,than this
difference is called Slope overload distortion.
2. The difference between quantized signal and original
signal is called as Granular noise. It is similar to quantisation
noise.
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Delta Modulation
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