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Transcript of Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called...
![Page 1: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/1.jpg)
Algebriac Coding Theory
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Basic DefinitionsWord : Let F be a field then the expression of
the form a1, a2, …, an where aiF i
is called a word of length n over the field F. We denote the set of all words
of length n over F by F(n).
![Page 3: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/3.jpg)
Weight :
Let a=a1, a2, …, an F(n)
Then the number ai≠01 is called the weight of the word a & is denoted by
wt(a). i.e. weight of a word is the number of non zero entries of that
word.
![Page 4: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/4.jpg)
Distance :
Let a=a1, a2, …, an & b=b1, b2, …, bn be two words of length n over the
field F. We define the distance between a & b (written d(a,b)) by
d(a,b)=i=1,…,n d(ai,bi)
Where d(ai,bi) = 0 if ai=bi
1 if ai≠bi
![Page 5: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/5.jpg)
Code :
A block (m,n) code over a field F consist of an encoding function
E : F(m) F(n) (m < n) & a decoding function
D : F(n) F(m)
such that DoE is the identity function or almost near to identity function.
![Page 6: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/6.jpg)
Elements of F(m) are called message words and the elements of image of E in F(n) are called code words.The collection of all the code words in F(n) is denoted by C. Thus C is a subset of the vector space F(n). If C becomes a subgroup of F(n) then we say that C is a Group Code.
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Message
Word
Code Word
Received
Word
Code Word
Message Word
E D
Channel
![Page 8: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/8.jpg)
Hamming code
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Procedure for constructing Hamming code:
a) Let r be a positive integer greater then 2 (r>2). Then the message word in the code C are of length m = 2r-r-1 & the code words are of length n = 2r-1.
b) Let (b1,b2,…,bn) be the code word corresponding to the message word (a1,a2,…,an) in which b2^0, b2^1,…,
b2^(r-1) are check digits
![Page 10: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/10.jpg)
And the remaining positions are filled by the digits of the message word in the order in which they are occuring in the message word.
c) Let M be a matrix of the type nr whose ith row is the binary representation of the number i. Consider the matrix equation bM=0. Let M1, M2,…,Mr be the r columns of M then the matrix equation bM=0 gives r linear equations bM1=0, bM2=0,…,bMr=0.
![Page 11: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/11.jpg)
First We prove that each of these r equations contains exactly one check digit.
Suppose b2^i occurs in the equation which is obtained by multiplying b with the kth column of the matrix M. Then (2i)th entry of kth column is 1. This entry is in the (2i)th row of M & (2i)th row of M is the binary representation of the number 2i. Now the binary representation of 2i has 1 at (i +1)th place and 0 elsewhere i.e. 1 at the (r - i)th place from the left. k=r - i
![Page 12: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/12.jpg)
Now consider the equation bMk=0Suppose this equation contains two check symbols b2^i & b2^j.Then as before k=r – i& k=r – ji.e. i = j. Thus there is atmost one check symbol present in the equation bMk=0 Again we have shown that each check symbol b2^i occurs in the linear equation bMk=0 for k=r-i. The number of check symbols & the number of
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Linear equations is same and equal to r & none of the equation have more than one check symbol so there is exactly one check symbol in each of the r linear equations.
Solving these r linear equations we get the unique value of each check symbol. Hence E(a1,a2,…,am)=(b1,b2,…,bn)
is the encoding function for the code C.
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Theorem: Prove that Hamming code is a group code.
Proof: Let C be a (m,n) hamming code over the field F. We know that m= 2r-r-1 & n=2r-1 for some positive integer r 2. The encoding function E of C is given by E: F(m) F(n)
E(a1,a2,…,am)=(b1,b2,…,bn)Where b2^0,b2^2,…,b2^(r-1) are check digits and the remaining positions are filled by the digits of the message word
![Page 15: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/15.jpg)
in the order in which they appear in the message word. The check digits are given by the matrix equation bM=0 where M is the nr matrix in which ith row is the binary representation of the number i. Supppose E(a’)=E(a1’,a2’,…,am’) =b1’,b2’,…,bn’ Now a+a’=(a1+a1’),(a2+a2’),…,(am+am’)& b+b’=(b1+b1’),(b2+b2’),…,(bn+bn’)Also bM=0 & b’M=0
![Page 16: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/16.jpg)
(b+b’)M=bM+b’M =0 + 0 = 0 Also in b+b’, b2^0+b’2^0, b2^1+b’2^1, …, b2^(r-1)+b’2^(r-1) are check digits & the remaining entries are a1+a’1, a2+a’2, …, am+a’m in their original order. Thus b+b’ corresponds to the message word a+a’. Hence E(a+a’) = b+b’ = E(a) + E(a’) Thus E is a homomorphism & so C is a group code .
![Page 17: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/17.jpg)
Theorem: Prove that minimum distance of a hamming code is 3.
Proof: Let C be a (m,n) hamming code over the field F. We know that m= 2r-r-1 & n=2r-1 for some positive integer r 2. The encoding function E of C is given by E: F(m) F(n)
E(a1,a2,…,am)=(b1,b2,…,bn)Where b2^0,b2^2,…,b2^(r-1) are check digits and the remaining positions are filled by the digits of the message word
![Page 18: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/18.jpg)
in the order in which they appear in the message word. The check digits are given by the matrix equation bM=0 where M is the nr matrix in which ith row is the binary representation of the number i. Again let M1, M2, …, Mr be the r columns of M then bM=0 is equivalent to r linear equations bM1=0, bM2=0, …, bMr=0. Also we know that each linear equation contains one & only one check symbol.
![Page 19: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/19.jpg)
We have also shown that C is a group code. So to prove that minimum distance of C is 3 it is sufficient to prove that each non zero code word is of weight 3. Now by definition of C all the digits of the message word apppear in the code word so we must prove that the weight of a code word corresponding to the message word of weight 1 or 2 is always greater than or equal to 3.
![Page 20: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/20.jpg)
Case (1) Let a=a1,a2,…,am be a message word of weight 1 & let b=b1,b2,…,bn be the corresponding code word. Let the non zero entry of a occurs at the ith position in b i.e. bi≠0. Since bi is a message digit so i is not a power of 2. Therefore the binary representation of i contains atleat two non zero entries i.e. ith row of M contains atleast two non zero entries. Suppose sth & tth entries of ith row are non zero.
![Page 21: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/21.jpg)
So (i,s)th and (i,t)th entry of M is 1.Consider the equations bMt=0 & bMs=0
Let b2^k be the check symbol present in the equation bMt=0 & b2^l be the check symbol present in the equation bMs=0.
Since all the entries of the message word except bi are zero. So bMt=0 is of the form b2^k+bi=0
i.e. b2^k = -bi ≠ 0.
Similarly b2^l is also non zero.
![Page 22: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/22.jpg)
Hence the code word b corresponding to the message word a of weight 1 contains atleast two non zero check digits.So wt(b)3 Case (2) Suppose the weight of the message word a=a1,a2,…,am is 2 then the corresponding code word b=b1,b2,…,bn
Contains two non zero entries corresponding to the message word a.
![Page 23: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/23.jpg)
Let these entries be bi & bj. We may also suppose that i < j. Since bi & bj are the entries of a therefore i & j are not the power of 2. Again i ≠ j. The binary representation of i & j must differ atleast at one place. Let the binary representation of i & j differs at sth place (from the left). By interchanging i & j we may suppose that (i,s)th entry of M is 1 & (j,s)th entry of
![Page 24: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/24.jpg)
M is zero. Consider the linear equation bMs=0.
This linear equation contain the unique check digit say b2^k. Since every digit of the message word except bi & bj is zero.
\bMs=0 is equivalent to b2^k+bi=0
i.e. b2^k= -bi ≠ 0.
Thus the code word b corresponding to the message word a contains atleast one non zero check symbol.
![Page 25: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/25.jpg)
Since it already contains two non zero check symbols so wt(b) 3. Thus in all the cases wt(b) 3. Now to show minimum distancs of hamming code is 3 we are to find a code word of weight 3. Let b=b1, b2, …, bn be the code word corresponding to the message word a=a1, a2, …, am where a1=1 and ai=0 for 2 i m. So b3=1.
![Page 26: Word : Let F be a field then the expression of the form a 1, a 2, …, a n where a i F i is called a word of length n over the field F. We denote the.](https://reader036.fdocuments.us/reader036/viewer/2022062409/5697bf731a28abf838c7ec7f/html5/thumbnails/26.jpg)
Consider the equation bMs=0 then the check symbol present in the equation takes the value 1 iff third entry of the sth column of M is 1. So the number of non zero check symbols in b is equal to the number of non zero entries in the third row of M. Third row of M is the binary representation of the number 3 which contains exactly 2 1’s. Thus b contains exactly two non zero check symbols and one non zero message symbol. So wt(b) = 3.
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TEST
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Do any two:
1) Construct a Hamming (4,7) code.
2) Prove that hamming code is a group code.
3) Prove that minimum distance of hamming code is 3.