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7,4 Hamming Codes

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word number: 9875

Time: 2024-10-02 15:37:44 +0000

Figure 0. 7,4 Hamming Venn Diagram.png

Figure 0. 7,4 Hamming Code Venn Diagram

0. Note

This is the fifth assignment from my Masters Applied Digital Information Theory Course which has never been published anywhere and I, as the author and copyright holder, license this assignment customized CC-BY-SA where anyone can share, copy, republish, and sell on condition to state my name as the author and notify that the original and open version available here.

1. Introduction

The previous 4th assignment demonstrate 1 bit error detection using 3,4 parity codes. On this assignment will be demonstrating 1 bit error correction using 7,4 hamming codes. On 3,4 parity codes we group 3 bits per block and perform exclusive or on each blocks to get a bit called the parity code bit and add it into the 4th bit of the blocks. A different approach for the 7,4 hamming codes we first group 4 bits per block, and then obtain the 3 hamming bit codes from the 4 bits for each blocks and add them which makes each blocks contained 7 bits. Suppose there are 4 bits as follows:

b1,b2,b3,b4

To get the hamming bit codes we do the following calculation:

b5=b1⊕b2⊕b3, b6=b1⊕b2⊕b4, b7=b1⊕b3⊕b4

Those bits will be added to the block:

b1,b2,b3,b4,b5,b6,b7

Following Table 1 are the complete list:

Table 1. Coding Table

Info Code Info Code
0000 0000**000** 1000 1000**110**
0001 0001**011** 1001 1001**101**
0010 0010**101** 1010 1010**011**
0011 0011**110** 1011 1011**000**
0100 0100**111** 1100 1100**001**
0101 0101**100** 1101 1101**010**
0110 0110**010** 1110 1110**100**
0111 0111**001** 1111 1111**111**

On the receiver side the hamming codes is also constructed using the received first 4 bits of each blocks, then compare them with the hamming codes produced on the receiver side. Since the hamming codes were produced using the above equation (each codes uses different elements) we can know on which bit the error occurs, and a correction is performed. For example if receiver’s b5, b6, b7 is different from transmitter’s then an error on b1, if b5, b6 is different then an error occur on b2, if b5,b7 is different then an error occur on b3, if b6, b7 then b4, if b5 only then b5, if b6 only then b6, if b7 only then b7. Following Table 2 are the complete syndromes:

Table 2. Syndrome and Correction List

Syndrome Correction Syndrome Correction Syndrome Correction Syndrome Correction
No error None b6 error b6⊕1 b6b7 error b4⊕1 b5b6 error b2⊕1
b7 error b7⊕1 b5 error b5⊕1 b5b7 error b3⊕1 all error b1⊕1

2. Memoryless Errors (Skew Tent Map)

On this simulation we will (1) generate a chaotic binary sequence from memoryless source (2) perform hamming coding (3) simulate through a noisy channel (4) perform error correction on receiver. We will calculate the practical probabilities of incorrect decoding and compare them theoretically defined by PI=1–PC where PC=7p(1-p)6 + (1-p)7 is the probability of correct decoding. The formula is derived from the probability of 1 bit error out of all possible error. Also we will compare the error probability of the binary sequence before correction and after correction. We will use critical point of c=0.499999 for generating the memoryless source, while we use c=1-p for generating the error sequence, and initial chaotic value for both is x(1)=0.333333 and various error probability p for the error sequence (noise). Generating the source will be the same as assignment 2, and generating the hamming codes is similar to 4th assignment except we input 3 extra bits in each blocks based on the initial 4 bits, which will make 7 bits. Like the previous assignment we perform exclusive or between the generated memoryless source and the error sequence to obtain the received sequence. On Table 3 is the simulation result of 1000000 (million) blocks (N) (7000000 bits) with error probability up to 0.4.

Table 3. simulation result of 1000000 (million) blocks (N) (7000000 bits) with error probability up to 0.4

Error Probability 0.01 0.05 0.1 0.2 0.3 0.4
Error Before Decoding 69185 310155 651846 1489329 2095701 2817931
Error After Decoding 5661 103819 413866 1456118 2253571 2871364
Probability Error Before Decoding 0.009884 0.044308 0.093121 0.212762 0.299386 0.402562
Probability Error After Decoding 0.008087 0.014831 0.059123 0.208017 0.321939 0.410195
Incorrect Decoding 1887 33173 132352 444091 672462 810696
Probability of Incorrect Decoding 0.001887 0.033173 0.132352 0.444091 0.672462 0.810696
Theoretical Probability of Incorrect Decoding 0.002031 0.044381 0.149695 0.423283 0.670583 0.841369

3. Markov-type Errors (PWL Map)

Similar to section 2 but this time we generate the error sequence through Markov type or Piece Wise Linear (PWL) map. The theoretical incorrect error decoding is PI=1–PC and correct one:

PC = P(1)p2 (1-p)5 + 5P(0)p1 p2 (1-p1)4 + P(0)(1-p1)5 p1 + P(0)(1-p1)6

We use the same critical point and initial chaotic value. To generate the error sequence we use the PWL map with various p2 values and P1 = (1-c)/c P2, with c=1-p. On Table 4 is the theoretical result of 1000000 (million) blocks (N) (7000000 bits) with error probability up to 0.4, Table 5 is the simulation result, Table 6 is error probability before decoding, Table 7 is error probability after decoding.

Table 4. Theoretical Result

p2\p 0.01 0.05 0.1 0.2 0.3 0.4
0.1 0.013497 0.067421 0.134663 0.268465 0.400994 0.531546
0.2 0.015996 0.079888 0.159498 0.317444 0.472536 0.622384
0.3 0.017509 0.087704 0.175712 0.351731 0.525154 0.690342
0.4 0.018048 0.091159 0.184430 0.375393 0.566651 0.746271

Table 5 Simulation Result

p2\p 0.01 0.05 0.1 0.2 0.3 0.4
0.1 0.014407 0.074791 0.165392 0.288812 0.409343 0.518025
0.2 0.039309 0.084100 0.125731 0.363730 0.463654 0.633573
0.3 0.013653 0.083021 0.179077 0.365025 0.567390 0.702815
0.4 0.011958 0.100293 0.248337 0.361628 0.607297 0.741535

Table 6. Error Probability Before Decoding

p2\p 0.01 0.05 0.1 0.2 0.3 0.4
0.1 0.019189 0.050632 0.121454 0.215809 0.310955 0.387244
0.2 0.0310312 0.050779 0.073711 0.239916 0.287656 0.390070
0.3 0.0064253 0.045953 0.095724 0.199656 0.351083 0.400005
0.4 0.0073853 0.051241 0.124822 0.196768 0.334634 0.393053

Table 7. Error Probability After Decoding

p2\p 0.01 0.05 0.1 0.2 0.3 0.4
0.1 0.0182 0.050889 0.120800 0.215805 0.311266 0.387078
0.2 0.0310312 0.050020 0.071017 0.236854 0.285676 0.395473
0.3 0.0065111 0.044599 0.095300 0.196479 0.336207 0.407214
0.4 0.0055779 0.050823 0.127047 0.189369 0.317022 0.398460

4. Results

On Figure 1 shows that the error probability before and after decoding fluctuates on different error probability values (p) and type of sources (memoryless and p2s), but had one thing in common that belowp < 0.3 the error after decoding decreases, and unfortunately rises above that. The incorrect coding for memoryless is preferable when p < 0.1 but not recommended when above. For PWL with p2=0.1 shows low incorrect decoding amongst other p2. For other values there is a cutting point on p=0.2, below lower p2 shows lower incorrect decoding and viceversa.

Figure 1. Error Probability vs Error Probability Before and After Decoding.png

Figure 1. Error vs Error Before and After Decoding

Figure 2. Error Probability vs Incorrect Decoding Probability.png

Figure 2. Error vs Incorrect Decoding

5. Source Code

The source code is written in Fortran95 which is said to be a good programming language for mathematics in the old days, the first one is for memoryless, and the first one is for PWL (markov).

program Memoryless_Error_Hamming

! For safe purposes
 implicit none

! Type declarations
 integer :: m, n1, n2, i, j, s1, s2, s3
 real :: c, c_error, p, practical_error_before, p_practical_error_before, practical_error_after
 real :: p_practical_error_after, incorrect_decoding, p_incorrect_decoding
 real :: p_theory_correct_decoding, p_theory_incorrect_decoding
 real, dimension(40000000) :: x
 integer, dimension(40000000) :: bit
 integer, dimension(70000000) :: bit_hamming, bit_error, bit_receiver, bit_hamming_uncorrected, bit_corrected
 character(len=20) :: firstname, lastname, text
  
 write (*,'(A)',advance='no') 'Enter number of blocks: '
 read (*,*) m
 n1 = m*4
 n2 = m*7
 c = 0.499999

! Memoryless Generated Source (here n is number of bits)
 x(1) = 0.333333
 do i = 1, n1-1
  if (x(i) >= 0 .and. x(i) < c) then
   x(i+1) = x(i)/c
   bit(i) = 0
  else
   x(i+1) = (1-x(i))/(1-c)
   bit(i) = 1
  end if
 end do

! Hamming Codes
 j = 1
 do i = 1, n2, 7
  bit_hamming(i) = bit(j)
  bit_hamming(i+1) = bit(j+1)
  bit_hamming(i+2) = bit(j+2)
  bit_hamming(i+3) = bit(j+3)
  bit_hamming(i+4) = xor(xor(bit(j),bit(j+1)),bit(j+2))
  bit_hamming(i+5) = xor(xor(bit(j),bit(j+1)),bit(j+3))
  bit_hamming(i+6) = xor(xor(bit(j+1),bit(j+2)),bit(j+3))
  j = j+4
 end do

! Error Sequence
 write (*,'(A)',advance='no') 'Enter error probability: '
 read (*,*) p
 c_error = 1 - p
 do i = 1, n2
  if (x(i) >= 0 .and. x(i) < c_error) then
   x(i+1) = x(i)/c_error
   bit_error(i) = 0
  else
   x(i+1) = (1-x(i))/(1-c_error)
   bit_error(i) = 1
  end if
 end do

! Receiver Side (hamming encoded sequence + error sequence), calculate error probability before decoding, prefill future corrected bits
 practical_error_before = 0;
 do i = 1, n2
  bit_receiver(i) = xor(bit_hamming(i), bit_error(i))
  bit_corrected(i) = bit_receiver(i)
  if (bit_receiver(i) /= bit_hamming(i)) then
   practical_error_before = practical_error_before + 1
  end if
 end do
 p_practical_error_before = practical_error_before/n2
 
! Perform Hamming coding again on the received bits
 do i = 1, n2, 7
  bit_hamming_uncorrected(i) = bit_receiver(i)
  bit_hamming_uncorrected(i+1) = bit_receiver(i+1)
  bit_hamming_uncorrected(i+2) = bit_receiver(i+2)
  bit_hamming_uncorrected(i+3) = bit_receiver(i+3)
  bit_hamming_uncorrected(i+4) = xor(xor(bit_receiver(i),bit_receiver(i+1)),bit_receiver(i+2))
  bit_hamming_uncorrected(i+5) = xor(xor(bit_receiver(i),bit_receiver(i+1)),bit_receiver(i+3))
  bit_hamming_uncorrected(i+6) = xor(xor(bit_receiver(i+1),bit_receiver(i+2)),bit_receiver(i+3))
 end do

! Error Correction based on symptoms, "&" is placed to continue next line (fortran95 cannot read long lines)
 do i = 1, n2, 7
  if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i) = xor(bit_receiver(i),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+1) = xor(bit_receiver(i+1),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+2) = xor(bit_receiver(i+2),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+3) = xor(bit_receiver(i+3),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+4) = xor(bit_receiver(i+4),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+5) = xor(bit_receiver(i+5),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+6) = xor(bit_receiver(i+6),1)  
  end if
 end do

! Error Probability After Decoding
 practical_error_after = 0;
 do i = 1, n2
  if (bit_corrected(i) /= bit_hamming(i)) then
   practical_error_after = practical_error_after + 1
  end if
 end do
 p_practical_error_after = practical_error_after/n2

! Probability of Correct, and incorrect decoding
 p_theory_correct_decoding = (7*p*(1-p)*(1-p)*(1-p)*(1-p)*(1-p)*(1-p))+((1-p)*(1-p)*(1-p)*(1-p)*(1-p)*(1-p)*(1-p))
 p_theory_incorrect_decoding = 1 - p_theory_correct_decoding
 incorrect_decoding = 0
 do i = 1, n2, 7
  if ((bit_corrected(i) /= bit_hamming(i)) .or. (bit_corrected(i+1) /= bit_hamming(i+1)) .or. &
(bit_corrected(i+2) /= bit_hamming(i+2)) .or. (bit_corrected(i+3) /= bit_hamming(i+3)) .or. &
(bit_corrected(i+4) /= bit_hamming(i+4)) .or. (bit_corrected(i+5) /= bit_hamming(i+5)) .or. &
(bit_corrected(i+6) /= bit_hamming(i+6))) then
   incorrect_decoding = incorrect_decoding + 1
  end if
 end do
 p_incorrect_decoding = incorrect_decoding/m

! Results
 write (*,'(A)',advance='no') 'Practical error before decoding: '
 write (*,*) practical_error_before
 write (*,'(A)',advance='no') 'Practical error after decoding: '
 write (*,*) practical_error_after
 write (*,'(A)',advance='no') 'Probability practical error before decoding: '
 write (*,*) p_practical_error_before
 write (*,'(A)',advance='no') 'Probability practical error after decoding: '
 write (*,*) p_practical_error_after
 write (*,'(A)',advance='no') 'Incorrect decoding: '
 write (*,*) incorrect_decoding
 write (*,'(A)',advance='no') 'Probability of incorrect decoding: '
 write (*,*) p_incorrect_decoding
 write (*,'(A)',advance='no') 'Theoretical probability of incorrect decoding: '
 write (*,*) p_theory_incorrect_decoding


! Debugging purposes, uncomment them to see binary sequences, max 18 blocks
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_hamming(i)
! end do
! write (*,*) ' '
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_error(i)
! end do
! write (*,*) ' '
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_receiver(i)
! end do
!  write (*,*) ' '
!  do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_corrected(i)
! end do
!  write (*,*) ' '

end program Memoryless_Error_Hamming

program Piece_Wise_Linear_Error

! For safe purposes
 implicit none

! Type declarations
 integer :: m, n1, n2, i, j, s1, s2, s3
 real :: c, c_error, p, practical_error_before, p_practical_error_before, practical_error_after
 real :: p_practical_error_after, incorrect_decoding, p_incorrect_decoding
 real :: p_theory_correct_decoding, p_theory_incorrect_decoding
 real :: p1, p2, a, a1, a2, a3, c1, c2, d1, d2
 real, dimension(40000000) :: x
 integer, dimension(40000000) :: bit
 integer, dimension(70000000) :: bit_hamming, bit_error, bit_receiver, bit_hamming_uncorrected, bit_corrected
 character(len=20) :: firstname, lastname, text
  
 write (*,'(A)',advance='no') 'Enter number of blocks: '
 read (*,*) m
 n1 = m*4
 n2 = m*7
 c = 0.499999
 x(1) = 0.333333

! Uncomment to use memoryless source
! Memoryless Generated Source (here n is number of bits)
! x(1) = 0.333333
! do i = 1, n1-1
!  if (x(i) >= 0 .and. x(i) < c) then
!   x(i+1) = x(i)/c
!   bit(i) = 0
!  else
!   x(i+1) = (1-x(i))/(1-c)
!   bit(i) = 1
!  end if
! end do

! PWL Generated Source
 p1 = (1-c)*p2/c
 a = 1/(1-(p1+p2))
 if (p1+p2 < 1) then
  c1 = c-(c/a)
  c2 = c+((1-c)/a)
  d1 = c1*(1-c)
  d2 = 1-((1-c2)*c)
  a1 = -c/(c1-d1)
  a2 = a
  a3 = (c-1)/(d2-c2)
  do i = 1, n2
   if (x(i) >= 0 .and. x(i) < c1) then
    x(i+1) = (a1*(x(i)-d1))+c
   else if (x(i) >= c1 .and. x(i) < c2) then
    x(i+1) = a2*(x(i)-c1)
   else
    x(i+1) = (a3*(x(i)-c2))+1
   end if
   if (x(i) >= 0 .and. x(i) < c) then
    bit(i) = 0
   else
    bit(i) = 1
   end if
  end do
 else
  c1 = c-((c-1)/a)
  c2 = c-(c/a)
  d1 = c1*(1-c)
  d2 = 1-((1-c2)*(1-c))
  a1 = -c/(c1-d1)
  a2 = a
  a3 = c/(d2-c2)
  do i = 1, n2
   if (x(i) >= 0 .and. x(i) < c1) then
    x(i+1) = (a1*(x(i)-d1))+c
   else if (x(i) >= c1 .and. x(i) < c2) then
    x(i+1) = (a2*(x(i)-c1))+1
   else
    x(i+1) = a3*(x(i)-c2)
   end if
   if (x(i) >= 0 .and. x(i) < c) then
    bit(i) = 0
   else
    bit(i) = 1
   end if
  end do
 end if

! Hamming Codes
 j = 1
 do i = 1, n2, 7
  bit_hamming(i) = bit(j)
  bit_hamming(i+1) = bit(j+1)
  bit_hamming(i+2) = bit(j+2)
  bit_hamming(i+3) = bit(j+3)
  bit_hamming(i+4) = xor(xor(bit(j),bit(j+1)),bit(j+2))
  bit_hamming(i+5) = xor(xor(bit(j),bit(j+1)),bit(j+3))
  bit_hamming(i+6) = xor(xor(bit(j+1),bit(j+2)),bit(j+3))
  j = j+4
 end do

! Error Sequence
 write (*,'(A)',advance='no') 'Enter error probability: '
 read (*,*) p
 c_error = 1 - p
 write (*,'(A)',advance='no') 'Enter p2: '
 read (*,*) p2
 p1 = (1-c_error)*p2/c_error
 a = 1/(1-(p1+p2))
 if (p1+p2 < 1) then
  c1 = c_error-(c_error/a)
  c2 = c_error+((1-c_error)/a)
  d1 = c1*(1-c_error)
  d2 = 1-((1-c2)*c_error)
  a1 = -c_error/(c1-d1)
  a2 = a
  a3 = (c_error-1)/(d2-c2)
  do i = 1, n2
   if (x(i) >= 0 .and. x(i) < c1) then
    x(i+1) = (a1*(x(i)-d1))+c_error
   else if (x(i) >= c1 .and. x(i) < c2) then
    x(i+1) = a2*(x(i)-c1)
   else
    x(i+1) = (a3*(x(i)-c2))+1
   end if
   if (x(i) >= 0 .and. x(i) < c_error) then
    bit_error(i) = 0
   else
    bit_error(i) = 1
   end if
  end do
 else
  c1 = c_error-((c_error-1)/a)
  c2 = c_error-(c_error/a)
  d1 = c1*(1-c_error)
  d2 = 1-((1-c2)*(1-c_error))
  a1 = -c_error/(c1-d1)
  a2 = a
  a3 = c_error/(d2-c2)
  do i = 1, n2
   if (x(i) >= 0 .and. x(i) < c1) then
    x(i+1) = (a1*(x(i)-d1))+c_error
   else if (x(i) >= c1 .and. x(i) < c2) then
    x(i+1) = (a2*(x(i)-c1))+1
   else
    x(i+1) = a3*(x(i)-c2)
   end if
   if (x(i) >= 0 .and. x(i) < c_error) then
    bit_error(i) = 0
   else
    bit_error(i) = 1
   end if
  end do
 end if

! Receiver Side (hamming encoded sequence + error sequence), calculate error probability before decoding, prefill future corrected bits
 practical_error_before = 0;
 do i = 1, n2
  bit_receiver(i) = xor(bit_hamming(i), bit_error(i))
  bit_corrected(i) = bit_receiver(i)
  if (bit_receiver(i) /= bit_hamming(i)) then
   practical_error_before = practical_error_before + 1
  end if
 end do
 p_practical_error_before = practical_error_before/n2
 
! Perform Hamming coding again on the received bits
  
 do i = 1, n2, 7
  bit_hamming_uncorrected(i) = bit_receiver(i)
  bit_hamming_uncorrected(i+1) = bit_receiver(i+1)
  bit_hamming_uncorrected(i+2) = bit_receiver(i+2)
  bit_hamming_uncorrected(i+3) = bit_receiver(i+3)
  bit_hamming_uncorrected(i+4) = xor(xor(bit_receiver(i),bit_receiver(i+1)),bit_receiver(i+2))
  bit_hamming_uncorrected(i+5) = xor(xor(bit_receiver(i),bit_receiver(i+1)),bit_receiver(i+3))
  bit_hamming_uncorrected(i+6) = xor(xor(bit_receiver(i+1),bit_receiver(i+2)),bit_receiver(i+3))
 end do

! Error Correction based on symptoms, "&" is placed to continue next line (fortran95 cannot read long lines)
 do i = 1, n2, 7
  if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i) = xor(bit_receiver(i),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+1) = xor(bit_receiver(i+1),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+2) = xor(bit_receiver(i+2),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+3) = xor(bit_receiver(i+3),1)
  else if ((bit_hamming_uncorrected(i+4) /= bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+4) = xor(bit_receiver(i+4),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) /= bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) == bit_receiver(i+6))) then
   bit_corrected(i+5) = xor(bit_receiver(i+5),1)
  else if ((bit_hamming_uncorrected(i+4) == bit_receiver(i+4)) .and. (bit_hamming_uncorrected(i+5) == bit_receiver(i+5)) &
.and. (bit_hamming_uncorrected(i+6) /= bit_receiver(i+6))) then
   bit_corrected(i+6) = xor(bit_receiver(i+6),1)  
  end if
 end do

! Error Probability After Decoding
 practical_error_after = 0;
 do i = 1, n2
  if (bit_corrected(i) /= bit_hamming(i)) then
   practical_error_after = practical_error_after + 1
  end if
 end do
 p_practical_error_after = practical_error_after/n2

! Probability of Correct, and incorrect decoding
 p_theory_correct_decoding = (((1-c_error)*p2*((1-p1)**5)) + (5*c_error*p1*p2*((1-p1)**4)) + (c_error*((1-p1)**5)*p1) + &
(c_error*((1-p1)**6)))
 p_theory_incorrect_decoding = 1 - p_theory_correct_decoding
 incorrect_decoding = 0
 do i = 1, n2, 7
  if ((bit_corrected(i) /= bit_hamming(i)) .or. (bit_corrected(i+1) /= bit_hamming(i+1)) .or. &
(bit_corrected(i+2) /= bit_hamming(i+2)) .or. (bit_corrected(i+3) /= bit_hamming(i+3)) .or. &
(bit_corrected(i+4) /= bit_hamming(i+4)) .or. (bit_corrected(i+5) /= bit_hamming(i+5)) .or. &
(bit_corrected(i+6) /= bit_hamming(i+6))) then
   incorrect_decoding = incorrect_decoding + 1
  end if
 end do
 p_incorrect_decoding = incorrect_decoding/m

! Results
 write (*,'(A)',advance='no') 'Practical error before decoding: '
 write (*,*) practical_error_before
 write (*,'(A)',advance='no') 'Practical error after decoding: '
 write (*,*) practical_error_after
 write (*,'(A)',advance='no') 'Probability practical error before decoding: '
 write (*,*) p_practical_error_before
 write (*,'(A)',advance='no') 'Probability practical error after decoding: '
 write (*,*) p_practical_error_after
 write (*,'(A)',advance='no') 'Incorrect decoding: '
 write (*,*) incorrect_decoding
 write (*,'(A)',advance='no') 'Probability of incorrect decoding: '
 write (*,*) p_incorrect_decoding
 write (*,'(A)',advance='no') 'Theoretical probability of incorrect decoding: '
 write (*,*) p_theory_incorrect_decoding


! Debugging purposes, uncomment them to see binary sequences, max 18 blocks
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_hamming(i)
! end do
! write (*,*) ' '
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_error(i)
! end do
! write (*,*) ' '
! do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_receiver(i)
! end do
!  write (*,*) ' '
!  do i = 1, n2
!  write(*,'(1i1.1)',advance='no') bit_corrected(i)
! end do
!  write (*,*) ' '

end program Piece_Wise_Linear_Error

Mirrors