Model Digital Communication System :

     The purpose of this system is to transmit the message or square of symbols coming out of the source to a destination point.
      The source and destination points are separated through a communication channel
Model of Digital communication system

* Discrete information source :

The information source generates the message signal to be transmitted an analof information source may be transformed into a discrete information source through the process of sampling and quantization. The discrete information sources are charectarized by the following Parameters:
 
  • Source : These are the letter, digits or special charectar available in the information source.
  • Source Alphabet Probability : Each source alphabet  from information source has independent occurrence rate in the sequence.
    Example: Letter or words (a, e, i, o, u) occurs frequently in the source.
  • Symbol Rate: It is the rate at which the information source generates source alphabet and represented in symbol/ sec unit.
  • Information Rate: This parameter defines the average information content of the symbols. Entropy describes the average information content per symbol, it is defined in terms of bits / symbols.
            The source information rate is the product of symbol rate and source entropy
    information rate = Symbol rate * source entropy
    (bits/sec) = (Symbol /sec) * (bits/symbol).

Source encoder and decoder:

The symbols produced by information source converted into digital form (binary 0, 1). The group of bits is called a code word.The source encoder asigns code word to the symbols.

  • Block Size: It describes the maximum number of different code words which can be represented by the source encoder.
    Example: The block size of 8- bits will be

                      28 = 256 code words.

     

  • Code word Length : This is the number of  bits used to represent each word .

  • Average Data Rate: The source encoder assigns multiple number of bits to each input symbols.
    Example: If we consider that the symbols are given to the source encoder at the rate of 10 symbols/sec and the length of code word is 8- bits then the output data from the source encoder is 
                    Data rate = 10*8
                                       = 80 bits/sec.

Source Decoder:

At the receiver side the decoder is used to perform the reverse operation of source encoder. Here it converts the binary output of the channel decoder into a symbol sequence. 

 
 
 

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