Transcript Rectangular Codes - Haverford College

Rectangular Codes
Rule: take intersection of row-column parity error(s) –
single error correction (double error detection)
m1
n1
message
bits
x
column parity
bits
row parity bits
excess bit redundancy
1
1
1


n  1 m  1 (n  1)(m  1)
x
corner bit (use even parity checking)
If n = m (square): (n − 1)² message bits +
(2n − 1) check bits = n² total
Example: n = 11, 21% overhead
3.2
Triangular Codes
Rule: intersection of parity errors – single error correction (no
double error detection). A single parity error implies the error
was on the diagonal itself.
n1
message
bits
x
x
diagonal parity checks
n1
excess bit redundancy 
2
n 1
both row and column sum
to even parity (see book)
n(n − 1)/2 message bits + n check bits = n(n + 1)/2 total
3.3
Hyper-dimensional Codes
Arrange the bits in a cube, and include check bits not in
every linear dimension, but every plane. So we get three
edges of check bits, which intersect in a single point. Total of
n³ = (3n − 2) check bits + message.
excess bit redundancy ~ 3 / n²
Four-dimensional: check over hyperplanes (cubes)
excess bit redundancy ~ 4 / n³
k=1
b
b
b
b
k=2
k=3
3.3 cont.
Hypercubes
More generally, consider a k-dimensional n-cube.
There are n (k−1) dimension hyperplanes in each of
the k directions (each are parity checked), for a
total of n × k bits. Overhead is hence ~ nk / nk = k /
nk−1. So keeping m = nk fixed, overhead is ~ k∙k√m.
This has a minimum when its (natural) logarithm
does: log k + (ln m) / k. Taking derivatives: 1/k − (ln
m)/k² = 0 implies k = ln m is the optimal dimension,
making n = e. This is almost the highest dimensional
space possible, since n must be at least two. So try
using a k-dimensional 2-cube, with n = 2k bits. One
parity check in each direction means each of the k
different ways of splitting the cube has even parity.
3.3 cont.
Hamming Code
Design a code so the sequence of parity error bits (called
syndrome) address (point to) the erroneous bit. All zero’s is
no error. If we number the parity checks pm, ..., p1, where pi =
(1 for an error / 0 for no error), then they can point to 2m  1
erroneous bits. If we use a total of n bits, we must have n ≤ 2m
 1 to correct an error in any possible location. Clearly, each
parity check pi must sum over all positions i in the string such
that the jth bit of i is one:
In order for x1, …, xn to be a valid code word, it must include
both message bits and the proper check bits. The most
convenient choice is to locate them at positions 2j-1 in the
word x1, …, xn according to the formula:
x2j-1 = ∑ {xi : i = (bm… b1)two & bj = 1} mod 2
3.4
Example
n = 7, m = 3; parity bit positions: 1, 2, 4.
7  3 = 4 message bits at positions 3, 5, 6, 7.
positions x1 x2 x3 x4 x5 x6 x7
message
-
-
1
-
0
1
1
Codeword sent 0
1
1
0
0
1
1
0
0
1
1
error
Codeword received 0
p1 = x 1 + x 3 + x 5 + x 7 = 1
p2 = x 2 + x 3 + x 6 + x 7 = 1
x
1
0
Turn 90
(011)two
p3 = x 4 + x 5 + x 6 + x 7 = 0
error in position 3
Alternatively
1 = x2 →
0
1
0
1 = x6 →
1
1
0
1 = x7 →
1
1
1
0
1
1
3.4
Hamming distance
Let x  ( x1,, xn ) and y  ( y1,, yn ) be binary words.
n
T hend ( x , y )   xi  yi is called theHammingdistance.
i 1
This is the number of places that they differ (same as L0 norm in F2).
Any metricmust satisfy:
A.
d (x, x)  0
B.
d ( x , y )  d ( y, x )
C.
d ( x , y )  d ( y, z )  d ( x , z )
The minimum distance between
allowable code words is:
1
For uniqueness
2
single-error detection
3
single-error correction
4
double-error detection
corresponds
to
performance
of code
5 double-error correction
T hesphereof radius r about x is Sr ( x )  y  d ( x , y )  r .
n n
n r n
It's volume S0  S1    Sr  1               
1  2
 r  i 0  i 
3.6
Sphere packing
For single error correction, with minimum distance = 3, the
spheres of radius one around each code word must not overlap:
k = # of message bits
2
k

nm
n = total number of bits m = # of check bits
T otalvolume
2n
 # code words 

 n  1  2m
Sphere volume n  1
double error correction has non-overlapping spheres of radius 2:
2
nm
2n
n(n  1)
“Density” corresponds


 1  2 m to efficiency of code
n(n  1)
2
1 n 
2
When equality is achieved, the code is said to be “perfect”. E.g.
double-error correcting with m = 12, n = 90.
3.6
Double – Error Detection
Hamming code would cause 3 errors in a doubleerror situation. But if we add an extra parity check
bit over the entire word we can detect double errors:
x0 x1, …, xn
Algebraic
approach
x0 = ∑ {xi : i = 1 … n} mod 2
Hamming
syndrome
extra check
meaning
0
0
correct
0
1
error in x0
i≠0
1
error in xi
≠0
0
double error
Geometric approach – In the original code, points of distance 3 from each other
have a different # of 1’s in them (mod 2) since they differ in 3 positions, so their
extra parity checks will be set differently, and now they will differ in 4 positions!
3.7
Hamming Codes on Words
Think of doing parity checks over entire words, using a logical sum.
0 0  0
represents no parity error
a check =
anything else
represents a parity error
The same syndrome technique locates the erroneous word. Any of the parity
check failure words could now be added to the erroneous word to correct it.
parity bit
character
Consider the ASCII code with each 8-bit
word having a parity bit, and for each block
of words, a checksum word.
The parity check within a word locates the
erroneous word, and the check sum bits
locate the erroneous bit.
ASCII
code
words
x
This is equivalent to a rectangular code
checksum word
x
3.8