DIGITAL COMMUNICATION SYSTEM

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.

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.

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.

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.

Building blocks of Digital Communication System

Building blocks of Digital Communication System

• Input source and input transducerThe source of information can be analog or digital, e.g. analog: audio or video signal, digital: like teletype signal. • Source EncoderThe 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.

• 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

• 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

• 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

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.

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.

 

• 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

• 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

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 digitaltransmission 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