DIGITAL COMMUNICATION SYSTEM

In digital communication system, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form.

Elements of digital communications system:
Discrete information source
Source Encoder
Channel Encoder
Modulator
Electrical Communication Channel
Noise
Demodulator
Channel Decoder
Source Decoder
Destination

The overall purpose of the system is to transmit the message or sequence of symbols coming out of source to a destination point as a high rate and accuracy as possible. The source and destination point are physically separated in space and a communication channel connects the source to the destination point. The communication channel accepts electrical (i.e., electromagnetic) signals and the output of the channel is usually a smeared of destroyed version of the input due to the non-ideal nature of communication channel. In addition to this, the information bearing signal is also corrupted by unpredictable electrical signals (i.e. noise) from both man-made and natural causes. Thus, the smearing and the noise introduces errors in the information being transmitted and limits the rate at which information can be communicated from the source to the destination.

Discrete Information Source: Information source may be classified into two categories based upon the nature of their output i.e. analog Information sources and discrete information sources. In case of analog communication, the information source is analog. Analog information sources, such as microphone actuated by speech emit one or more continuous amplitude signals.
In case of digital communication system, the information source produces a message signal which is not continuously varying with time. Rather the message signal is intermittent with respect to time. The output of discrete information source such as teletype or the numerical output of the computer consists of a sequence of discrete symbols of letters. An analog information source may be transformed into a discrete information sources through the process of sampling and quantizing. Discrete information sources are characterized by the following parameters:
i. Source Alphabet : These are the letters, digits or special characters available from the information source.
ii. Symbol Rate : It is the rate at which the information source generates source alphabets. It is generally represented in symbols/sec unit.
iii. Source Alphabet Probabilities : Each source alphabet from the source has independent occurrence rate in the sequence. As an example, letters A, E, I etc. occur frequently in the sequence. Hence, probability of the occurrence of each source alphabet can become one of the important property which is useful in digital communication.
iv. Probabilistic Dependence of Symbols in a Sequence: The information carrying capacity of each source alphabet is different in a particular sequence. This parameter defined average information content of the symbols. The entropy of a source describes the average information content per symbol in long message. Entropy may be defined in terms of bits per symbol. Bit is the abbreviation for a binary digit.
This means that the source information rate is the product of symbol rate and source entropy, i.e.,
Information rate = Symbol rate * Source entropy
(Bits/sec) (symbols/sec) (Bits/Symbol)

Thus, the information rate represents minimum average data rate required to transmit information from source to the destination.

Source Encoder and Decoder:
In source coding, the encoder maps the digital signal generated at the source output into another signal in digital form. The mapping is one to one and the objective is to eliminate or reduce the redundancy so as to provide an efficient representation of the source output. The source decoder simply performs the inverse mapping and thereby delivers to the user destination, a reproduction of the digital source output. The advantage of source coding is to reduce the bandwidth of transmission.

Channel Encoder and Decoder:
The purpose of channel encoder is to map the incoming digital signal into a channel input and for the decoder to map the channel output into an output digital signal in such a way that the effect of channel noise is minimized. That is the combined roll of channel encoder and decoder is to provide reliable communication. This provision is satisfied by introducing redundancy in a prescribed fashion. In the channel encoder and exploiting it in the decoder, to reconstruct the original encoder input as accurately as possible.

Other element has described above in element of communication system.


Classification of communication
Regarding the mode of propagation, communication may be divided in the following two forms:
i. Line Communication
ii. Wireless or Radio Communication

Line Communication :
In line communication, the medium of transmission is a pair of conductors called transmission line. This is also called as line channel. This means that in line communication, the transmitter and the receiver are connected through a wire or line. However, the installation and maintenance of a transmission line is not only costly and complex but also overcrowds the open space. Apart from this, its message transmission capability is also limited.

Wireless or Radio Communication :
In wireless or radio communication, a message is transmitted through open space by electromagnetic waves called as radio waves. Radio waves are radiated from the transmitter in open space through a device called antenna. A receiving antenna intercepts the radio waves at the receiver. All the radio, TV and satellite broadcasting are wireless or radio communication. The advantages of wireless communication are cost effectiveness, possible long distance communication and simplicity.