Digital Communication 1

DIgital Communication
ECE 422L
2013
I. Pulse Modulation

Elements of Digital
Communication System
2

• In Source and Input Transducer:
Digital Source:
• Source alphabet
• Symbol rate
• Source alphabet
probabilities
• Probabilistic dependence
of symbols in a sequence
Analog source:
• audio
• video signal
Elements of Digital
3

• Source Encoder
– use as few binary digits as possible to represent the signal.
– This sequence of binary digits is called information sequence.
– Source Encoding or Data Compression: the process of efficiently
converting the
output of wither analog or digital source into a sequence of binary digits
– Codeword – a group of bits used to represent symbols
– Blocksize – maximum number of distinct codewords
– Codeword length –number of bits used to represent each codeword
– Average data rate –
– Effeciency of the encoder
Elements of Digital
4

• Channel Encoder:
– to introduced, in controlled manner, some redundancy in the binary
information sequence that can be used at the receiver to overcome
the effects of noise and interference encountered in the
transmission on the signal through the channel.
Parameters:
– Coding rate that depends upon the number the redundant bit added
– Coding method used
– Coding efficiency
– Error control capabilities
– Feasibility of the encoder and decoder
Elements of Digital
5

• Digital Modulator:
– The binary sequence is passed to digital modulator which in turns
convert the sequence into electric signals so that we can transmit
them on channel
– Parameters:
• Transmission bandwidth
• Probability of symbol
• Synchronous or asynchronous method of detection
• Complexity of implementation
Elements of Digital
6

• Channel:
– is the physical medium that is used for transmitting signals from
transmitter to receiver.
• Digital Demodulator:
– processes the channel corrupted transmitted waveform and reduces
the waveform to the sequence of numbers that represents estimates
of the transmitted data symbols.
Elements of Digital
7

• Channel Decoder:
– attempts to reconstruct the original information sequence from the
knowledge of the code used by the channel encoder and the
redundancy contained in the received data
• Source Decoder
– At the end, if an analog signal is desired then source decoder tries
to decode the sequence from the knowledge of the encoding
algorithm. And which results in the approximate replica of the
input at the transmitter end
Elements of Digital
8

Merits of Digital
Communication
1. Digital signals are very easy to receive.
2. In digital signals, the original signal can
be reproduced accurately.
3. digital signals can be cleaned up to
restore the quality and amplified by the
regenerators.
9

4. The noise may change the shape of the pulses
but not the pattern of the pulses.
5. But digital signals can be coded so that only the
person, who is intended for, can receive them.
6. digital signals can be stored at the receiving
end.
7. The digital signals can be processed
Merits of Digital
Communication
10

• Analog data
– Takes on continuous values. Ex. Voice or video
• Digital data
– Takes on discrete values. Ex. Text and integers
• Analog Signal
– Continuously varying electromagnetic wave representing data
carried over a variety of medium
• Digital Signal
– Sequence of voltage pulses representing data transmitted over a
wire medium
Data and Signal
11

Analog or Digital Data Can Be Represented By
Either Analog or Digital Signals.
These Signals Can Then Be Propogated (Moved
Along a Medium).
Optical Fiber Only Propogates Analog Signals
Remember!
12

• Analog Data, Analog Signals
– radio
• Digital Data, Analog Signals (modem)
– broadband & wireless
• Analog Data, Digital Signals [codec]
• Frequency Division Multiplexing (FDM)
• Wave Division Multiplexing (WDM) [fiber]
• Time Division Multiplexing (TDM)
• Pulse Code Modulation (PCM)
• Delta Modulation
• Digital Data, Digital Signals (baseband)
• wired LAN, (e.g., Ethernet)
Data and Signal
13

• Digital data  Digital Signal
– Easy and simple to implement
• Analog data  Digital Signal
– Allows the use of digital transmission and switching equipment
• Digital data  Analog Signal
– Allows us of the public telephone system
– Allows use of optical fiber
• Analog Data  Analog Signal
– Easy
– Telephone system was primarily analog
Data and Signal
14

• Short distance transmissions, baseband
modulation is usually used.
• Baseband modulation is often called line coding
• For long distance and wireless transmissions,
bandpass modulation is usually used.
• Bandpass modulation is also called carrier
modulation
Remember!
15

Consist essentially of sampling analog information signals
Then converting those samples into discrete pulses
Transporting the pulses from a source to destination over a
physical transmission medium.
16

SAMPLING
Sampling is the process of taking samples of the
analogue signals at given interval of time. Only samples
are being transmitted.
• If sufficient samples are sent and sampling theorem
are met the original signal can be reconstructed at the
receiver
17

SAMPLING THEOREM
Sampling theorem states that, if the sampling rate in
any pulse modulation system exceeds twice the
maximum information signal frequency, the original
signal can be reconstructed in the receiver with
minimum distortion.
• This is called Nyquist Rate, fs ≥ 2fmax
• fs – sampling frequency,
• fmax – maximum freq of the modulating signal
18

This slides includes:
• Pulse Amplitude
Modulation
• Pulse Width Modulation
• Pulse Position
Modulation
• Pulse Code Modulation
The process of transmitting signals in the form of
pulses by using special techniques.
19

Analog Pulse Modulation Digital Pulse Modulation
Pulse Amplitude (PAM)
Pulse Width (PWM)
Pulse Position (PPM)
Pulse Code (PCM)
Delta (DM)
20

Analog pulse modulation
• A periodic pulse train is used as
the carrier wave
• Some characteristic feature of
each pulse is varied in a
continuous manner in
accordance with the
corresponding sample value of
the message signal
• Analog pulse-modulation
systems rely on the sampling
process to maintain continuous
amplitude representation of the
message signal
Digital pulse modulation
• The message signal is represented
in a form that is discrete in both
time and amplitude
• Its transmission in digital form as
a sequence of coded pulse
• Digital pulse-modulation system
use not only the sampling process
but also the quantization process.
• Digital modulation makes it
possible to exploit the full power
of digital signal-processing
techniques.
21

* amplitude of discrete carrier signal changes in accordance
with the instantaneous amplitude of modulating
signal(message signal) keeping width and position of carrier
constant
*The signal is sampled at regular intervals such that each
sample is proportional to the amplitude of the signal at that
sampling instant. This technique is called “sampling”.
* For minimum distortion, the sampling rate should be more
than twice the signal frequency.
22

AND
Gate
Pulse Shaping
Network
FM
Modulator
Analog
Signal
PAM - FM
Pulses at sampling frequency
HF Carrier Oscillator
PAM
23

Analog Signal
Amplitude Modulated
Pulses
24

• There are 2 types of PAM :
1. Natural sampling
2. Flat top sampling
Types of PAM
25

• Merits
– Generation and detection is easy.
• Demerits
– Added noise cannot be removed easily as
it has impact on amplitude which carries
information.
– Transmission bandwidth is too large.
Merits and Demerits of PAM
28

* the amplitude is maintained constant but the duration or
length or width of each pulse is varied in accordance with
instantaneous value of the analog signal keeping amplitude
and position of carrier constant
* The negative side of the signal is brought to the positive
side by adding a fixed d.c. voltage.
Pulse Width Modulation
29

Analog Signal
Width Modulated Pulses
30

• Merits
– Very good noise immunity.
– Its possible to separate out signal from
noise.
• Demerits
– Bandwidth requirement is large as
compared to PAM.
Merits and Demerits of PWM
32

* In this type, the sampled waveform has fixed amplitude and
width whereas the position of each pulse is varied as per
instantaneous value of the analog signal.
* PPM signal is further modification of a PWM signal. It has
positive thin pulses (zero time or width) corresponding to the
starting edge of a PWM pulse and negative thin pulses
corresponding to the ending edge of a pulse.
Pulse Position Modulation
33

* This wave can be
further amended
by eliminating the
whole positive
narrow pulses.
The remaining
pulse is called
clipped PPM.
PWM
PPM
34

• The modulation system in which
position of the discrete carrier signal
changes in accordance with the
instantaneous amplitude of modulating
signal(message signal) keeping
amplitude and Width of carrier constant
is called as PPM.
Pulse Position Modulation
35

• Merits
– High noise immunity.
• Demerit
– Generation and detection is complex.
Merits and Demerits of PPM
38

PAM, PWM and PPM at a glance:
Analog Signal
Amplitude Modulated Pulses
Width Modulated Pulses
Position Modulated Pulses
39

lets move on to
digital pulse modulation.
40

ANALOG-TO-DIGITAL
CONVERSION
•A digital signal is superior to an analog
signal because it is more robust to noise
and can easily be recovered, corrected
and amplified.
• For this reason, the tendency today is to
change an analog signal to digital data.
•Generally used two techniques are :
pulse code modulation and
delta modulation
.
41

• It is the type of pulse modulation in
which the group of pulses or codes are
transmitted which represent binary
numbers corresponding to modulating
signal voltage.
• They are a primary building block for advanced communication
systems
Pulse Code Modulation
42

PCM is the most commonly used technique in digital communications
Used in many applications:
• Telephone systems
• Digital audio recording
• CD laser disks
• voice mail
• digital video etc.
43

Trivia!
44
PCM was invented by the British engineer
Alec Reeves in 1937 in France.
It was not until about the middle of 1943
that the Bell Labs people became aware of
the use of PCM binary coding as already
proposed by Alec Reeves.

PCM consists of three steps to digitize an analog signal:
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”.
• Nyquist theorem
46

3 DIFFERENT SAMPLING METHODS
47

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”.
• quantizing process will produce errors called
quantizing errors or quantizing noise
48

Two types of quantization.
(a) midtread (b) midrise
49

Illustration of the quantization process
50

Nonuniform Quantizing
• Voice analog signals are more likely to have
amplitude values near zero than at the extreme
peak values allowed.
• For signals with nonuniform amplitude distribution,
the granular quantizing noise will be a serious
problem if the step size is not reduced for amplitude
values near zero and increased for extremely large
values. This is called nonuniform quantizing since a
variable step size is used.
51

Quantization Error and SNQR
• When a signal is quantized, we introduce an error -
the coded signal is an approximation of the actual
amplitude value.
• The difference between actual and coded value
(midpoint) is referred to as the quantization error.
• Signals with lower amplitude values will suffer more
from quantization error as the error range: /2, is
fixed for all signal levels.
53

• Non linear quantization is used to alleviate this
problem. Goal is to keep SNQR fixed for all sample
values.
• Two approaches:
– The quantization levels follow a logarithmic curve.
Smaller ’s at lower amplitudes and larger’s at
higher amplitudes.
– Companding: The sample values are compressed
at the sender into logarithmic zones, and then
expanded at the receiver.
54

• Qe=Resolution/2
• SNQR = minimum voltage / quantization noise voltage
• SNQR = 10 log (average signal power/average quantization
noise power)
55

PCM consists of three steps to digitize an analog signal:
3. Binary encoding:
Note:
A digital signal is described by its ‘bit rate’ whereas
analog signal is described by its ‘frequency range’.
56

Encoding
• Encoding is the process of representing the sampled
values as a binary number in the range 0 to n.
• The value of n is chosen as a power of 2, depending
on the accuracy required.
• Increasing n reduces the step size between adjacent
Quantization levels and hence reduces the
Quantization noise.
• The down side of this is that the amount of digital
data required to represent the analog signal
increases.
58

• To recover an analog signal from a digitized signal
we follow the following steps:
– We use a hold circuit that holds the amplitude value of
a pulse till the next pulse arrives.
– We pass this signal through a low pass filter with a
cutoff frequency that is equal to the highest frequency
in the pre-sampled signal.
PCM Decoder
60

• Dynamic Range
• Resolution
• Maximum allowable input amplitude
• Coding efficiency
• Ratio of the largest possible magnitude to
the smallest possible magnitude that can be
decoded
PCM Parameter
62

• Dynamic Range, DR
 Ratio of the largest possible magnitude to the
smallest possible magnitude that can be
decoded
 DR=Vmax/Vmin = Vmax/Resolution
 2n – 1 >=DR
PCM Parameter
63

64
•Is the ratio of the strongest possible signal that can be
transmitted and the weakest discernible signal
•In a linear PCM system, the maximum dynamic range is
found by:
DR = (1.76 + 6.02m) dB
Dynamic Range

Companding
• Sometimes called compassion
• used to improve dynamic range
• Compression is used on the transmitting end and
expanding is used on the receiving end
• Keep the bit rate and bandwidth low
65

• The human auditory system is believed to be a
logarithmic process in which high amplitude sounds
do not require the same resolution as low amplitude
sounds.
• The human ear is more sensitive to quantization
noise in small signals than large signals.
• A-law and µ-law coding apply a logarithmic
quantization function to adjust the data resolution in
proportion to the level of the input signal.
69
Speech Companding

• quantises the difference between the original and the
predicted signals, i.e. the difference between
successive values.
• Leads to reduction in the number of bits used per
sample over that used for PCM. Using DPCM can
reduce the bit rate of voice transmission down to 48
kbps.
Speech Companding
70

Inter Symbol Interference
• If the system impulse response h(t) extends over
more than 1 symbol period, symbols become
smeared into adjacent symbol periods
• Known as inter symbol interference (ISI)
71

Time (bit periods)
0 2 4 6
amplitude
0.5
1.0
Time (bit periods)
0 2 4 6
amplitude
0.5
1.0
Modulator input Slicer input
Binary ‘1’ Binary ‘1’
72
Inter Symbol Interference

Noise in PCM Systems
• The performance of a PCM system is influenced by
two noise sources:
• (1) channel noise
• introduce bit errors into the received signal. The
presence of this noise can be measured in terms of
probability of symbol error or bit error rate
(BER).
• can be made practically negligible by using high
signal energy-to-noise density ratio through short
spacing between regenerative repeaters.
73

Noise in PCM Systems
(2) quantization noise.
• can be made negligible by increasing the number
of levels L
• selecting a compressor-expander pair that is
matched to the message signal characteristics.
74

Limitations of PCM systems
• Choosing a discrete value near the analog
signal for each sample leads to quantization
error
• Between samples no measurement of the
signal is made;
• Accurate clock is required for accurate
reproduction
75

• Merits
– Secured.
– Encoding is possible.
– Very high noise immunity.
– Convenient for long distance communication.
– Good signal to noise ratio.
Merits and Demerits of PCM
76

• Demerits
– Complex circuitry.
– Requires large bandwidth.
– Synchronization is required between
transmitter & receiver.
77
Merits and Demerits of PCM

• 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
Delta Modulation (DM)
78

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Delta Modulation (DM)
80

Slope overload distortion
And Granular noise
82

• Merits
– One bit code word for output.
– Low signaling rate.
– Low channel bandwidth.
– No ADC is required
• Demerits
1. Slope overload present.
2. Granular noise present.
Merits and Demerits of DM
83

Delta-Sigma Modulation
• Alternatively known as Pulse Density
modulation or Pulse Frequency modulation
• Modification of the delta modulation
84

• Conventional delta modulation - Quantizer
input is an approximation of the derivative
of the input message signal m(t).
• Results in the accumulation of error (noise)
– accumulated noise (transmission disturbances)
at the receiver (cumulative error).
• Possible solution: integrating the message
before delta modulation – called delta sigma
modulation
85

86
• The message signal is defined in its
continuous form – so pulse modulator
contains a hard limiter and a pulse generator
to produce a 1-bit encoded signal
• integration at the tx requires differentiation
at the rx side.
• But: As in conventional DM the message
has to be integrated at the final stage this
eliminates the need of differentiation here.

Delta-Sigma Modulation System
Figure 3.25
87

• Merits
– Low frequency component of input signal
is boosted
– Correlation between adjacent samples of
delta modulator is increased
– Simplifies the receiver design
• Demerits
– Requires sampling rate far in excess of
the Nyquist rate
Merits and Demerits of DSM
88

• This is the advanced version of DM.
• Avoid the problem on slope over load error
and granular noise problem.
• step size is adapted to the slope
(variation) of the message signal.
89
Adaptive Delta Modulation

• If successive errors are of opposite polarity, then
the delta modulator is operating in the granular
mode; in such a case it is advantageous to use
reduced step size.
• If successive errors are of the same polarity,
then the delta modulator is operating in its slope-
overload mode; in this case, the step size should
be increased.
• .
90
Adaptive Delta Modulation

• Merits
– Improved SNR.
– Low signaling rate.
– Wider dynamic range
– Better bandwidth utilization
– Reduction in slope overload and granular
noise.
Merits and Demerits of ADM
93

• Voice and video signals represented in PCM
exhibit high correlation, which means that
PCM signals contain redundant
information. The result is an inefficient
coding.
• By removing the PCM information
redundancy a more efficient coded signal
may be obtained.
Differential Pulse Code
Modulation (DPCM)
94

• If the prediction is well performed, then
the variance of e(k) will be much smaller
than the variance of m(k), which results
into a smaller number of levels to
quantize e(k).
• DPCM can be described as a predictive
coding scheme.
96
DPCM System

Type of Predictor
• One-tap predictor
• N-tap predictor
97

• Merits
1. Less signaling rate.
2. Less bandwidth.
3. Requires less quantization levels
• Demerits
1. High bit rate.
2. Needs the predictor circuit to be used
which is complex.
Merits and Demerits of DPCM
98

Reference
• Digital Communication
– by Sanjay Sharma
• Advance Electronic Communication
– by Robert Tomasi
• World Wide Web
99

Digital Communication 1

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