University of Gondar

Institute of TECHNOLOGY
Electrical Engineering
Department

Digital Communication System(EEng4261)

Lecture 1
Review on Digital Communication System

By: Amanuel A.
March,2025 1
 Outline

Introduction

Challenges and Summary Elements of Digital

Communication system
of Digital Comn

Analog Vs Digital Review on ADC

Introduction to Digital Communication
• Digital communication transmits information using digital signals (0s and 1s).
• Essential for modern communication systems (internet, mobile, satellite, etc.).
• More efficient, reliable, and secure compared to analog communication.
• Key Elements of Digital Communication
1. Source – Original information (text, voice, video).
2. Encoder – Converts information into digital format.
3. Transmitter – Modulates and sends the signal.
4. Channel – Medium of transmission (fiber optics, radio waves).
5. Noise & Interference – Disturbances affecting signal quality.
6. Receiver – Captures and decodes the signal.
7. Destination – Final recipient of the message.
 Cont’d…
 Any communication system has the three basic blocks: transmitter, channel,
and receiver.
Channel Receiver
Transmitter

Fig: General block diagram of digital communication

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 Information Source:
Information bit stream, like:
Source 010001101…

 Can be analog or digital,If digital, no further processing is needed at this step

,If analog, analog to digital conversion should be done (Either way, the output
of this stage is digital. (i.e: bit streams.))
The bits represent specific symbols,constitute an alphabet for a specific source.

The information source combine symbols in different arrangements to construct a

message, in a form of bit stream.

 It passes though consecutive steps and various processing before its transmission
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through the channel.
Source Coding:

Bit stream… Bit stream…

S ource Coding R’b < R b
(Rb ) (R’b )

The aim is to reduce the size of data (data compression).

There are two main types of source coding:-
 Lossless (Eg:- Zip)
 Lossy (Eg:- J P E G , MPEG)

The other main objective of source coding is to introduce security to the message
(encryption).
 Eg: DES (Data Encryption Standard), AES (Advanced Encryption Standard),
 The field of Cryptography in general.
HW: Briefly explain:
a) One of data compression standards: Zip, J P E G , MPEG, or others.
b) One of encryption standards: DES, AES, or others. 6
 Channel Coding:
Bit stream… Channel Bit stream… R’’b >R’b
(R’b ) Coding (R’’b )

 Together with the channel decoder, the main purpose of this block is to
correct and detect bit errors.

 So, Channel encoder takes actions in advance to mitigate losses in

noisy channel.

 Channel coding involves adding redundancy to the data in order

to detect and correct errors. So, data size increases. 7
Digital Modulation:
Bit stream… Digital Digitally modulated
(R’’b ) Modulation wave form

• Modulation, in general, is a process of conveying a message signal

through another signal which is more suitable to be transmitted physically.

• In digital modulation, one of the attributes of a carrier signal is made

to change according to a pulse signal that represent bit values (1 and 0).

• the most common digital modulation techniques include: ASK, FSK,

PSK & QAM.
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Digital Modulation:

1. Amplitude Shift Keying (ASK):

Varies amplitude of the carrier
wave.
2. Frequency Shift Keying (FSK):
Varies frequency of the carrier
wave.
3. Phase Shift Keying (PSK):Varies
phase of the carrier wave.
4. Quadrature Amplitude Modulation
(QAM):Combines ASK & PSK
for higher data rates.
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Transmission Channel:
 A transmission channel is a medium through which
information is sent form transmitter to receiver. Transmission
Channel
 Transmission mediums include:
• Wired: coper, fiber… etc.
• Wireless: microwave, AM/FM radio, cellular networks…etc
Can be categorized in different ways:
 Unicast, multicast, broadcast channels…
 Simplex, half duplex, duplex communication channels… etc.
 Performance measures include: bandwidth, SNR, bit rate BER
etc.
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 Transmission Channel

Transmission
Channel

 Noise is an unwanted disturbance in an information signal.

 It is inherent in any transmission channel and in electronic devices.
 It should be distinguished from Interference and distortion.
 There are different sources of noise, such as: Thermal noise (from
electronic devices themselves),Environmental noise… etc.
 It determines the way receivers should be designed.
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 Receiver Side

Digital
Information Source Channel Demodulatio Received
Sink Decoding Decoding n/Detecting wave form

 The receiver side we need to retrieve the original message for the
destination.
 Accordingly:
 Demodulator: demodulates a received signal and produce channel
coded bit stream.
 Channel Decoder: will undo the processing done by the channel
coder.
 Source decoder, in its part, decodes the data it receives and
retrieve the original message for the destination.
 If the original message is analog and if the intended
message form for the destination is analog, then Digital to 12
Types of Digital Communication

• Serial & Parallel – Data sent one bit at a time vs. multiple bits.

• Synchronous & Asynchronous – Data transmission with or without

a timing signal.

• Wired & Wireless – Transmission via cables vs. radio waves.

Primary Communication resources
 In a communication systems, two primary resources are

1. Transmitted power

2. Channel bandwidth

 Why these two become more important ?

 In communication systems Frequency spectrum is very expensive and controlled by universal organizations.

 Bandwidth is directly proportional to the speed of information transmission.

 Transmission power directly related to the quality of transmission.

 Increasing transmission power reduce the effects of channel noise and improve transmission distance.
Modulation process
The purpose of communication system is to deliver a message signal from the
source of information to the destination.

The process performed by the transmitter to modify the message signal is called
modulation, which involves varying some parameters of a carrier wave in
accordance with the message signal.

 The receiver re-create the message signal from the degraded version of the
transmitted signal after propagation over the channel is accomplished by the process
of demodulation.
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Cont’d…
Why we need modulation process in communication systems?
 To radiation the signal with reasonable antenna size
 To allow simultaneous signal transmission
 For effective radiation
 For long distance communication
 For multiplexing
 For Improving the quality of reception
 Avoid mixing up of other signals
Analog to Digital Conversion
 It is performed by ADC (Analog to Digital Converter).
 Can be accomplished in three steps:
1. Sampling
2. Quantization
3. Encoding
 This process is also called P C M (Pulse Code Modulation).

Continuous Digital signal

wave signal Sampling Quantization E n coding (bits)

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Sampling
The analog (continuous time – continuous amplitude) signal is sampled
at a rate of fs(sampling frequency).
The result is a discrete time – continuous amplitude signal.
To avoid aliasing, the sampling frequency should be at least twice of
the highest frequency component of the signal.
fs > 2fc
This is known as Nyquist criterion.

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Quantization
In this step, the sampled amplitudes which have a continuous value will be
replaced by quantized values.

The quantization levels are finite in number, and they constitute the symbols
for the source.

The resulting signal will be a discrete time – discrete amplitude.

Methods like rounding, truncation (limiting the number of digits) can be used.

This process introduces a quantization error.

The error can be minimized by increasing the size (number) of quantization

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levels.
E ncoding
Encoding is a process of representing the quantization levels
(symbols) by combination of bits.
The number of bits per value(symbol) can be arranged in
different forms.
But the usual and common way is to use a fixed length code
in which: n = log2L
Where n – number of bits
L – number of quantization levels (symbols)
Eg:- if we have 16 finite levels, we need 4 bits to represent the
symbols.
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Merits of Analog Communication
 Transmitters and Receivers are simple
 Low bandwidth requirement
 FDM can be used
D em erits Analog of Communication
 Noise affects the signal quality
 It is not possible to separate noise and signal
 Repeaters can‘t be used between transmitters and receivers
 Coding is not possible
 It is not suitable for the transmission of secret information
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Cont’d.….
 Power efficiency :
 Ability of a modulation technique to preserve the fidelity of digital
message at low power
 Designer can increase noise immunity, by increasing signal power
 Bandwidth Efficiency B
 Trade off between data rate and pulse width
 Easy to implement and cost effective to operate

•A
 NDWI EFFICIENCY

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Advantages of Digital Communication

• Higher noise immunity.

• Efficient bandwidth utilization.

• Better security (encryption).

• Easy signal regeneration.

• Supports error detection & correction.

D emerits Digital of Communication :
 More transmission BW is required.
 Synchronization may be required. ( in synchronous demodulation)
 Quantization error is introduced in analog to digital conversion.
 The system, and involved devices and circuits are more complex
and sophisticated.
 High power consumption due to increased processing stages.

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Summary & Key Takeaways
• Digital communication is the backbone of modern technology.
• It offers efficient, secure, and reliable data transmission.
• Digital modulation techniques improve performance.
• Applications range from mobile networks to satellite
communication.
Challenges in Digital Communication
• Bandwidth constraints.
• High initial cost for infrastructure.
• Data security threats.
• Synchronization issues in transmission.
Applications of Digital Communication

📶 Mobile Networks (3G, 4G, 5G).

🌐 Internet & Data Transmission.
📡 Satellite & Optical Fiber Communication.
🎥 Digital TV & Video Streaming.
🔒 Secure Communication (Military, Banking).
References
• Digital Communication by Simon Haykin
• Communication Systems by B.P. Lathi
• IEEE Research Papers on Digital Modulation