PRESENTED BY:-
PRATEEK ATLANI
PRATIK AGARWAL
PUNEET MANN
VISHNU VARDHAN

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DIGITAL SYTEM DESIGNMINI PROJECTCONVOLUTION CODES
DATA TRANSFER :-
SERIAL COMMUNICATIONS
PARALLEL COMMUNICATIONS
PROBLEMS:-
Noise
Electronic devices malfunctioning.
Hazards caused by noise
NOISE REMOVAL METHODS:-
1.Addition of correction bits
2.Code conversion using different techniques
Convolution codes is one of best methods for data transfer in serial communications
Basic operations:-
1.Encoding
2.Decoding
ENCODING:-
In this we convert a given code to another one with some additional bits added.
DECODING:-
The encoded output is fed to give us back the
Message signal.
Problem statement: Implement the following convolution code (encoding and decoding) on FPGA using vhdl . The code (m,n,k) is (2,2,3).
ENCODING:-The encoder is a finite state machine that consists of a M stage shift register with prescribed connections to n modulo 2-adders and a multiplexer that serializes the outputs of the adders.
The constraint length of a convolution code is the number of shifts over which a single message bit can influence the encoder output
In M stage shift register constraint length k =M+1
Consider a message signal (101100)
The polynomial representation of message signal =1+D^2+D^3
Where D represents unit delay variable
Generator polynomial for path1 =1
Generator polynomial for path2 = 1+D+D^2
Therefore the O/p polynomial of path 1 is
(1+D^2+D^3)X1=1+D^2+D^3
The o/p polynomial for path2 is
(1+D^2+D^3)X(1+D+D^2)=1+D+D^5
The encoded sequence will have a length of n(L+K-1)=2X(6+3-1)=16
Therefore the O/p sequence is
(11,01,10,10,00,01,00)
The shift register to be restored to its initial state , a terminating sequence of k-1 zeroes is appended to the last bit of the message sequence .These terminating sequence of zeroes is called tail of the message.
The total 8 clock pulses are required so the o/p carrier wave contains 8X2=16 bits
It gives the total of 16 bits
Encoding can be done in three ways
1.Code tree
2.State machine
Using state machine method encoding is realised in vhdl coding using state analysis
o/p using this method also comes as
(11,01,10,10,00,01,00)
the output of the encoder will be 16 bits with the first two bits(k-1) being zeroes.
Therefore the carrier wave sequence is
00,11,01,10,10,00,01,00
DECODING:-
The output is 6 bits because we are considering the message width of 6 bits to the encoder.
In this we use viterbi algorithm which uses trellis diagram for finding out the maximum likelihood decoder output . The reason for choosing trellis diagram is that the number of nodes does not continue to grow as the message bits increases.
We observe at level 5 ,there are two paths entering each node. Minimum distance decoder may make a decision at that point as to which of those two paths to take . this sequence goes on…
The algorithm operates by computing a discrepancy for every possible path in the trellis
The discrepancy for a path is defined as the hamming distance between the coded sequence and the received sequence.
The path with the lower discrepancy is retained.
The paths retained are called survivor paths.
If the survivor paths are equal then we made a guess to decide the correct output .In the code all possible survivor paths are displayed
Max number of survivor paths is given by 2^(k-1).
BACKGROUND
ROOTS OF THE PROJECT LIES IN SHANNON’S INFORMATION THEOREM:
IT SAYS THAT HOW MUCH NOISE THE CHANNEL MAY BE IT IS POSSIBLE TO GET THE DATA TRANSFERRED.
CODE RATE:WHAT PORTION OF TOTAL AMOUNT OF DATA TRANSFERRED IS USEFUL.
IN CASE OF OUR PROJECT IT IS NEARLY
0.90
METHODS FOR CHOOSING ERROR CORRECTING CODES.
DEPENDS UPON EXPECTED ERROR RATE.
WE CAN ADOPT COMPLEX SCHEMES FOR FASTER SYSTEMS LIKE EMBEDDED SYSTEMS.
BANDWIDTH(LENGTH OF CODE), CODING DELAY,CODING COMPLEXITY.
THESE CODES ARE USED IN SPACE COMMUNICATION AND DEEP SPACE COMMUNICATION.
ADVANTAGES:
IT IS COMPLICATED TECHNIQUE BUT IT IS USED POWERFUL CORRECTING CAPABILITY.
SINCE IT IS HAVING HIGH BANDWIDTH SO IT IS USED AS IT IS FASTER TECHNIQUE,
EXPECTED ERROR RATE IS LOW.