2. INTRODUCTION
Digital communication is a mode of communication where the
information or the thought is encoded digitally as discrete signals and
electronically transferred to the recipients.
Digital communication is one of the most commonly used modes of
communication now a days.
3. Example of Digital Communication System
• Manager wanted to meet all his team members at the conference
room to discuss their key responsibility areas and areas of expertise.
He didn’t have the time to go to their workstations and invite them
individually. Instead he opted an easier and cheaper mode to
communicate his idea. He sent an email marking a cc to all the
participants, inviting them for the meeting. This is an example of
Digital communication where the information was sent electronically.
4. Explanation
In digital communication
• Information flows in a digital form and the source is generally the
keyboard of the computer.
• A single individual is capable of digital communication.
• It also saves wastage of manpower and is one of the cheapest modes
of communication.
• Digital communication is also a really quick way to communicate. The
information can reach the recipient within a fraction of a second. An
individual no longer has to wait to personally meet the other individual
and share his information.
5. Communications Techniques
Digital communication covers a broad area of communications
techniques including:
• Digital transmission is the transmission of digital pulses between two
or more points in a communication system.
• Digital radio is the transmitted of digital modulated analog carriers
between two or more points in a communication system.
7. Building blocks of Digital Communication System
• Input source and input transducer
The source of information can be analog or digital, e.g. analog: audio or
video signal, digital: like teletype signal.
• Source Encoder
The signal produced by source is converted into digital signal consists of
1′s and 0′s. For this we need source encoder. We should like to use as few
binary digits as possible to represent the signal. In such a way this
efficient representation of the source output results in little or no
redundancy. This sequence of binary digits is called information
sequence.
• Source Encoding or Data Compression
The process of efficiently converting the output of analog or digital source
into a sequence of binary digits is known as source encoding.
8. • Channel Encoder:
The information sequence is passed through the channel encoder. The
purpose of the channel encoder is 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.
E.g. take k bits of the information sequence and map that k bits to unique
n bit sequence called code word. The amount of redundancy introduced is
measured by the ratio n/k and the reciprocal of this ratio (k/n) is known as
rate of code or code rate.
Building blocks of Digital Communication System
9. • 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. The
digital modulator maps the binary sequences into signal wave forms , for
example if we represent 1 by sin x and 0 by cos x then we will transmit sin x
for 1 and cos x for 0.
• Channel:
The communication channel is the physical medium that is used for
transmitting signals from transmitter to receiver.
• Digital Demodulator:
The 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.
Building blocks of Digital Communication System
10. • Channel Decoder:
This sequence of numbers then passed through the channel decoder
which 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.
• The average probability of a bit error at the output of the
decoder is a measure of the performance of the demodulator –
decoder combination
• Source Decoder:
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.
• Output Transducer:
Finally we get the desired signal in desired format analog or digital.
Building blocks of Digital Communication System
11. Channels for Digital Communications
• The modulation and coding used in a digital communication system
depend on the characteristics of the channel. The two main
characteristics of the channel are BANDWIDTH and POWER. In
addition the other characteristics are whether the channel is linear or
nonlinear, and how free the channel is free from the external
interference.
• Five channels are considered in the digital communication, namely:
Telephone channels
Coaxial cables
Optical fibers
Microwave radio, and satellite channels.
12. CAUSES OF SIGNAL LOSS
The main factors of degradation of signal in Digital Communication are:
Loss in Signal to Noise ratio.
Signal Distortion Caused By ISI.
Distance.
13. • Loss in Signal to Noise Ratio:
The SNR can be degrade into two ways.
Through Decrease of desired signal power.
Through the increase of noise power.
• Band limiting Loss:
The use of filters in the transmitter to avoid interfering with other channel
reduces the amount of energy , result in signal loss.
• Intersymbol Interference (ISI):
Filtering through the system the received pulses overlap one another; the
tail of one pulse smears in to the adjacent symbol interval causing the loss
of data in digital communication.
• DISTANCE:
The distance is large is a great chance of signal distortion. As when the
distance is large we use repeaters to amplify the signal ,it amplifies the
noise.
CAUSES OF SIGNAL LOSS
14. • Intermodulation Products (IM):
Signals having different carrier frequencies when simultaneously
present in a device the result is a interaction between the carrier
frequencies which can produce signals at all combinations of sum and
different frequencies. The energy distributed to these signals
represents a loss in the signal energy.
• Atmospheric Loss:
The magnitude of signal loss due to oxygen and water vapors is a
function of frequencies.
• AM/PM conversion:
A phase noise phenomenon in which signal amplitude fluctuations
produce phase variation that contribute phase noise to signals that will
be coherently detected causing sidebands which result in signal loss.
CAUSES OF SIGNAL LOSS
15. Comparisons of Digital and Analog Communication Systems
Digital Communication System Analog Communication System
Advantage :
inexpensive digital circuits
privacy preserved (data encryption)
can merge different data (voice, video and
data) and transmit over a common digital
transmission system
error correction by coding
Disadvantages :
expensive analog components : L&C
no privacy
can not merge data from diff. sources
no error correction capability
Disadvantages :
larger bandwidth
synchronization problem is relatively
difficult
Advantages :
smaller bandwidth
synchronization problem is relatively
easier
