Q.

1: Draw the block diagram of digital communication system and explain each block in
brief

The figure shows the functional elements of a digital communication system. The overall
purpose of the system is to transmit messages (or sequence of symbols) coming out of source to
destination point at as high rate and accuracy as possible. The source and destination point are
physically separated in space and a communication channel of some sort connects source to
destination point.

1. Information Source: Information Source can be classified in two categories

i. Analog Information Sources.
ii. Discrete Information Sources.

Analog Information Sources → Microphone actuated by a speech, TV Camera scanning a

scene emit one or more continuous amplitude signals.

Discrete Information Sources → These are teletype or the numerical output of computer which
consists of a sequence of discrete symbols or letters. An Analog information is transformed into
a discrete information through the process of sampling and quantizing.

Discrete information source is characterized by following four parameters

a. Source alphabet (symbols or letters): These are the letters, digits or special characters available
from the information source
b. Symbol rate: It is the rate at which the information source generates source alphabets. It is
normally represented in symbols/sec
c. Source alphabet probabilities: Each source alphabet from the source has independent
occurrence rate in the sequence. For example letters A, E, I etc. occur frequently in the sequence.
d. Probabilistic dependence of symbols in a sequence: The information carrying capacity of each
source alphabet is different in a particular sequence. This parameter defines average information
content of the symbols.
2. SOURCE ENCODER / DECODER:
The Source encoder ( or Source coder) converts the input i.e. symbol sequence into a
binary sequence of 0’s and 1’s by assigning code words to the symbols in the input sequence. For
every distinct symbol there is a unique codeword. As the number of bits are increased in each
codeword, the symbols that can be represented are increased.

For eg. :-8 bits will have 28 = 256 distinct codewords. Therefore 8 bits can be used to
represent 256 symbols. 16 bits can represent 216 = 65536 symbols and so on.
The important parameters of a source encoder are block size, code word lengths, average data
rate and the efficiency of the coder
At the receiver, the source decoder converts the binary output of the channel decoder
into a symbol sequence. The decoder for a system using fixed – length code words is quite
simple, but the decoder for a system using variable – length code words will be very complex.
Aim of the source coding is to remove the redundancy in the transmitting information, so that
bandwidth required for transmission is minimized. Based on the probability of the symbol code
word is assigned. Higher the probability, shorter is the codeword.
Ex: Huffman coding.

3. CHANNEL ENCODER / DECODER:

Error control is accomplished by the channel coding operation that consists of
systematically adding extra bits to the output of the source coder. These extra bits do not convey
any information but helps the receiver to detect and / or correct some of the errors in the
information bearing bits.
There are two methods of channel coding:
1. Block Coding: The encoder takes a block of ‘k’ information bits from the source encoder and
adds ‘r’ error control bits, where ‘r’ is dependent on ‘k’ and error control capabilities desired.
2. Convolution Coding: The information bearing message stream is encoded in a continuous
fashion by continuously interleaving information bits and error control bits.

The Channel decoder recovers the information bearing bits from the coded binary stream. Error
detection and possible correction is also performed by the channel decoder. The important
parameters of coder / decoder are: Method of coding, efficiency, error control capabilities and
complexity of the circuit.

4. MODULATOR:The Modulator converts the input bit stream into an electrical waveform
suitable for transmission over the communication channel. Modulator can be effectively used to
minimize the effects of channel noise, to match the frequency spectrum of transmitted signal
with channel characteristics, to provide the capability to multiplex many signals.
The important parameters of modulator are types of waveforms used, the duration of
the waveforms, the power level, and the bandwidth used. The modulator accomplishes the task
of minimizing the effects of channel noise by the use of large signal power and bandwidth, and
by the use of waveforms that last for longer duration
5. DEMODULATOR: The extraction of the message from the information bearing waveform
produced bythe modulation is accomplished by the demodulator. The output of the demodulator
is bitstream. The important parameter is the method of demodulation.

6. ELECTRICAL COMMUNICATION CHANNEL: The Channel provides the electrical

connection between the source and destination.The different channels are: Pair of wires, Coaxial
cable, Optical fibre, Radio channel,Satellite channel or combination of any of these.The
communication channels have only finite Bandwidth, non-ideal frequencyresponse, the signal
often suffers amplitude and phase distortion as it travels over thechannel. Also, the signal power
decreases due to the attenuation of the channel. The signalis corrupted by unwanted,
unpredictable electrical signals referred to as noise.The important parameters of the channel are
Signal to Noise power Ratio (SNR),usable bandwidth, amplitude and phase response and the
statistical properties of noise.

Q. 2: State advantages of digital communication system over analog communication

system.

Advantages of Digital Communication

1. The effect of distortion, noise and interference is less in a digital communication system. This
is because the disturbance must be large enough to change the pulse from one state to the other.
2. Regenerative repeaters can be used at fixed distance along the link, to identify and regenerate
a pulse before it is degraded to an ambiguous state.
3. Digital circuits are more reliable and cheaper compared to analog circuits.

4. The Hardware implementation is more flexible than analog hardware because of the use of
microprocessors, VLSI chips etc.

5. Signal processing functions like encryption, compression can be employed to maintain the
secrecy of the information.
6. Error detecting and Error correcting codes improve the system performance by reducing the
probability of error.
7. Combining digital signals using TDM is simpler than combining analog signals using FDM.
The different types of signals such as data, telephone, TV can be treated as identical signals in
transmission and switching in a digital communication system.
8. We can avoid signal jamming using spread spectrum technique.

Disadvantages of Digital Communication:

1. Large System Bandwidth:- Digital transmission requires a large system bandwidth to
communicate the same information in a digital format as compared to analog format.
2. System Synchronization:- Digital detection requires system synchronization whereas the
analog signals generally have no such requirement.
 Line code

A line code is the code used for data transmission of a digital signal over a transmission line.
This process of coding is chosen so as to avoid overlap and distortion of signal such as
inter-symbol interference.

Following are the properties of line coding −

●​ As the coding is done to make more bits transmit on a single signal, the bandwidth used
is much reduced.
●​ For a given bandwidth, the power is efficiently used.
●​ The probability of error is much reduced.
●​ Error detection is done and the bipolar too has a correction capability.
●​ Power density is much favorable.
●​ The timing content is adequate.
●​ Long strings of 1s and 0s is avoided to maintain transparency.

Types of Line Coding

There are 3 types of Line Coding

●​ Unipolar
●​ Polar
●​ Bi-polar

Unipolar Signaling

Unipolar signaling is also called as On-Off Keying or simply OOK.

The presence of pulse represents a 1 and the absence of pulse represents a 0.

There are two variations in unipolar signaling −

●​ Non Return to Zero NRZ

●​ Return to Zero RZ

Unipolar Non-Return to Zero NRZ

In this type of unipolar signaling, a High in data is represented by a positive pulse called as
Mark, which has a duration T0 equal to the symbol bit duration. A Low in data input has no
pulse.

The following figure clearly depicts this.

Advantages

●​ It is simple.
●​ A lesser bandwidth is required.

Disadvantages

●​ No error correction done.

●​ Presence of low frequency components may cause the signal droop.
●​ No clock is present.
●​ Loss of synchronization is likely to occur (especially for long strings of 1s and 0s).

Unipolar Return to Zero RZ

In this type of unipolar signaling, a High in data, though represented by a Mark pulse, its
duration T0 is less than the symbol bit duration. Half of the bit duration remains high but it
immediately returns to zero and shows the absence of pulse during the remaining half of the bit
duration.

It is clearly understood with the help of the following figure.

Advantages

●​ It is simple.
●​ The spectral line present at the symbol rate can be used as a clock.

Disadvantages

●​ No error correction.
●​ Occupies twice the bandwidth as unipolar NRZ.
●​ The signal droop is caused at the places where signal is non-zero at 0 Hz.

Polar Signaling

There are two methods of Polar Signaling. They are −

●​ Polar NRZ
●​ Polar RZ

Polar NRZ

In this type of Polar signaling, a High in data is represented by a positive pulse, while a Low in
data is represented by a negative pulse. The following figure depicts this well.
Advantages

●​ It is simple.
●​ No low-frequency components are present.

Disadvantages

●​ No error correction.
●​ No clock is present.
●​ The signal droop is caused at the places where the signal is non-zero at 0 Hz.

Polar RZ

In this type of Polar signaling, a High in data, though represented by a Mark pulse, its duration
T0 is less than the symbol bit duration. Half of the bit duration remains high but it immediately
returns to zero and shows the absence of pulse during the remaining half of the bit duration.

However, for a Low input, a negative pulse represents the data, and the zero level remains same
for the other half of the bit duration. The following figure depicts this clearly.
Advantages

●​ It is simple.
●​ No low-frequency components are present.

Disadvantages

●​ No error correction.
●​ No clock is present.
●​ Occupies twice the bandwidth of Polar NRZ.
●​ The signal droop is caused at places where the signal is non-zero at 0 Hz.

Bipolar Signaling

This is an encoding technique which has three voltage levels namely +, - and 0. Such a signal is
called as duo-binary signal.

An example of this type is Alternate Mark Inversion AMI. For a 1, the voltage level gets a
transition from + to – or from – to +, having alternate 1s to be of equal polarity. A 0 will have a
zero voltage level.

Even in this method, we have two types.

●​ Bipolar NRZ
●​ Bipolar RZ
From the models so far discussed, we have learnt the difference between NRZ and RZ. It just
goes in the same way here too. The following figure clearly depicts this.

The above figure has both the Bipolar NRZ and RZ waveforms. The pulse duration and symbol
bit duration are equal in NRZ type, while the pulse duration is half of the symbol bit duration in
RZ type.

Advantages

●​ It is simple.
●​ No low-frequency components are present.
●​ Occupies low bandwidth than unipolar and polar NRZ schemes.
●​ This technique is suitable for transmission over AC coupled lines, as signal drooping
doesn’t occur here.
●​ A single error detection capability is present in this.

Disadvantages

●​ No clock is present.
●​ Long strings of data causes loss of synchronization.

​ Difference between Unipolar, Polar and Bipolar Line Coding Schemes.

Data as well as signals that represents data can either be digital or analog. Line coding is the
process of converting digital data to digital signals. By this technique we converts a sequence of
bits to a digital signal. At the sender side digital data are encoded into a digital signal and at the
receiver side the digital data are recreated by decoding the digital signal.

We can roughly divide line coding schemes into five categories:

1.​ Unipolar (eg. NRZ scheme).

2.​ Polar (eg. NRZ-L, NRZ-I, RZ, and Biphase – Manchester and differential Manchester).
3.​ Bipolar (eg. AMI and Pseudoternary).
4.​ Multilevel
5.​ Multitransition
But, before learning difference between first three schemes we should first know the
characteristic of these line coding techniques:

●​ There should be self-synchronizing i.e., both receiver and sender clock should be
synchronized.
●​ There should have some error-detecting capability.
●​ There should be immunity to noise and interference.
●​ There should be less complexity.
●​ There should be no low frequency component (DC-component) as long distance transfer
is not feasible for low frequency component signal.
●​ There should be less base line wandering.

Unipolar scheme –

In this scheme, all the signal levels are either above or below the axis.

Non return to zero (NRZ) – It is unipolar line coding scheme in which positive voltage defines bit
1 and the zero voltage defines bit 0. Signal does not return to zero at the middle of the bit thus
it is called NRZ. For example: Data = 10110.
But this scheme uses more power as compared to polar scheme to send one bit per unit line
resistance. Moreover for continuous set of zeros or ones there will be self-synchronization and
base line wandering problem.

Polar schemes –

In polar schemes, the voltages are on the both sides of the axis.

▪​ NRZ-L and NRZ-I – These are somewhat similar to unipolar NRZ scheme but here we use two
levels of amplitude (voltages). For NRZ-L(NRZ-Level), the level of the voltage determines the
value of the bit, typically binary 1 maps to logic-level high, and binary 0 maps to logic-level
low, and for NRZ-I(NRZ-Invert), two-level signal has a transition at a boundary if the next bit
that we are going to transmit is a logical 1, and does not have a transition if the next bit that
we are going to transmit is a logical 0.
Note – For NRZ-I we are assuming in the example that previous signal before starting of data
set “01001110” was positive. Therefore, there is no transition at the beginning and first bit
“0” in current data set “01001110” is starting from +V. Example: Data = 01001110.

Comparison between NRZ-L and NRZ-I: Baseline wandering is a problem for both of them,
but for NRZ-L it is twice as bad as compared to NRZ-I. This is because of transition at the
boundary for NRZ-I (if the next bit that we are going to transmit is a logical 1). Similarly
self-synchronization problem is similar in both for long sequence of 0’s, but for long
sequence of 1’s it is more severe in NRZ-L.

▪​ Return to zero (RZ) – One solution to NRZ problem is the RZ scheme, which uses three
values positive, negative and zero. In this scheme signal goes to 0 in the middle of each bit.
 Note – The logic we are using here to represent data is that for bit 1 half of the signal is
represented by +V and half by zero voltage and for bit 0 half of the signal is represented by
-V and half by zero voltage. Example: Data = 01001.

Main disadvantage of RZ encoding is that it requires greater bandwidth. Another problem is

the complexity as it uses three levels of voltage. As a result of all these deficiencies, this
scheme is not used today. Instead, it has been replaced by the better-performing
Manchester and differential Manchester schemes.

▪​ Biphase (Manchester and Differential Manchester) – Manchester encoding is somewhat

combination of the RZ (transition at the middle of the bit) and NRZ-L schemes. The duration
of the bit is divided into two halves. The voltage remains at one level during the first half
and moves to the other level in the second half. The transition at the middle of the bit
provides synchronization.
Differential Manchester is somewhat combination of the RZ and NRZ-I schemes. There is
always a transition at the middle of the bit but the bit values are determined at the
beginning of the bit. If the next bit is 0, there is a transition, if the next bit is 1, there is no
transition.

Note –

1. The logic we are using here to represent data using Manchester is that for bit 1 there is
transition form -V to +V volts in the middle of the bit and for bit 0 there is transition from +V
to -V volts in the middle of the bit.

2. For differential Manchester we are assuming in the example that previous signal before
starting of data set “010011” was positive. Therefore there is transition at the beginning and
first bit “0” in current data set “010011” is starting from -V. Example: Data = 010011.

The Manchester scheme overcomes several problems associated with NRZ-L, and
differential Manchester overcomes several problems associated with NRZ-I as there is no
 baseline wandering and no DC component because each bit has a positive and negative
voltage contribution.

Only limitation is that the minimum bandwidth of Manchester and differential Manchester
is twice that of NRZ.

Bipolar schemes –

In this scheme there are three voltage levels positive, negative, and zero. The voltage level for
one data element is at zero, while the voltage level for the other element alternates between
positive and negative.

Alternate Mark Inversion (AMI) – A neutral zero voltage represents binary 0. Binary 1’s are
represented by alternating positive and negative voltages.

Pseudoternary – Bit 1 is encoded as a zero voltage and the bit 0 is encoded as alternating
positive and negative voltages i.e., opposite of AMI scheme. Example: Data = 010010.
The bipolar scheme is an alternative to NRZ.This scheme has the same signal rate as NRZ,but
there is no DC component as one bit is represented by voltage zero and other alternates every
time.