381 lines
7.7 KiB
Fortran
381 lines
7.7 KiB
Fortran
subroutine bpdecode168(llr,apmask,maxiterations,decoded,niterations)
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!
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! A log-domain belief propagation decoder for the (168,84) code.
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!
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integer, parameter:: N=168, K=84, M=N-K
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integer*1 codeword(N),cw(N),apmask(N)
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integer colorder(N)
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integer*1 decoded(K)
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integer Nm(7,M) ! 5, 6, or 7 bits per check
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integer Mn(3,N) ! 3 checks per bit
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integer synd(M)
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real tov(3,N)
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real toc(7,M)
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real tanhtoc(7,M)
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real zn(N)
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real llr(N)
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real Tmn
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integer nrw(M)
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data colorder/0,1,2,3,28,4,5,6,7,8,9,10,11,34,12,32,13,14,15,16,17, &
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18,36,29,42,31,20,21,41,40,30,38,22,19,47,37,46,35,44,33,49,24, &
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43,51,25,26,27,50,52,57,69,54,55,45,59,58,56,61,60,53,48,23,62, &
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63,64,67,66,65,68,39,70,71,72,74,73,75,76,77,80,81,78,82,79,83, &
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84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104, &
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105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125, &
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126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146, &
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147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167/
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data Mn/ &
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1,24,67, &
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2,5,71, &
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3,31,66, &
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4,50,58, &
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6,60,65, &
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7,32,76, &
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8,49,83, &
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9,36,41, &
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10,40,63, &
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11,14,62, &
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12,72,75, &
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13,23,78, &
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15,16,80, &
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17,54,64, &
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18,51,59, &
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19,30,48, &
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20,68,81, &
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21,29,70, &
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22,25,43, &
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26,34,73, &
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27,35,37, &
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28,39,44, &
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33,53,55, &
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38,52,84, &
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42,56,57, &
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45,74,82, &
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46,69,79, &
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47,61,77, &
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1,4,5, &
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2,48,52, &
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3,47,82, &
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6,26,76, &
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7,9,16, &
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8,10,78, &
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11,36,56, &
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12,38,65, &
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13,43,81, &
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14,33,68, &
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15,18,44, &
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17,59,77, &
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19,27,69, &
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20,21,58, &
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22,45,79, &
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23,34,54, &
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24,28,40, &
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25,80,84, &
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29,37,51, &
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30,42,83, &
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31,63,72, &
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32,50,66, &
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35,67,73, &
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39,55,74, &
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41,61,71, &
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46,60,62, &
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49,70,74, &
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53,64,75, &
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25,57,67, &
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1,46,64, &
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2,51,63, &
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3,14,80, &
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4,15,78, &
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5,27,74, &
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6,13,70, &
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7,19,20, &
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8,38,77, &
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9,75,83, &
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10,36,69, &
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11,22,29, &
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12,58,82, &
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16,35,60, &
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17,32,43, &
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18,42,45, &
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21,53,84, &
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23,39,48, &
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24,52,68, &
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26,33,61, &
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28,56,76, &
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30,65,66, &
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31,34,49, &
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37,47,81, &
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16,40,54, &
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41,44,65, &
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50,73,79, &
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55,59,60, &
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54,57,71, &
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23,62,72, &
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1,36,47, &
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2,32,70, &
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3,28,69, &
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4,7,33, &
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5,20,26, &
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6,14,63, &
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8,22,68, &
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9,13,67, &
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10,55,71, &
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11,15,19, &
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12,51,56, &
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17,27,52, &
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18,34,46, &
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21,41,42, &
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24,50,80, &
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25,39,75, &
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29,54,76, &
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30,40,84, &
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31,35,58, &
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37,79,83, &
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38,43,73, &
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44,72,81, &
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7,45,62, &
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47,48,49, &
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53,57,78, &
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20,59,66, &
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28,61,64, &
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11,75,77, &
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33,54,82, &
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1,14,44, &
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2,62,73, &
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3,9,26, &
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4,37,84, &
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5,56,80, &
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6,45,71, &
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8,67,72, &
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10,76,81, &
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12,32,78, &
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13,59,82, &
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15,17,79, &
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16,42,69, &
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18,61,70, &
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19,31,64, &
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21,39,63, &
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22,30,58, &
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23,27,66, &
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24,41,49, &
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25,36,60, &
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29,65,67, &
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34,36,53, &
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35,48,76, &
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15,38,55, &
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40,43,74, &
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46,52,57, &
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50,63,77, &
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51,68,69, &
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2,44,83, &
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1,30,55, &
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3,29,78, &
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4,34,65, &
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5,31,38, &
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6,52,58, &
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7,25,51, &
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8,16,66, &
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9,46,74, &
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10,70,75, &
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11,32,84, &
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12,48,79, &
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13,50,64, &
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14,37,57, &
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17,42,72, &
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18,43,48, &
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19,24,60, &
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20,54,83, &
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21,47,62, &
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22,28,59, &
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23,61,80, &
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8,26,39, &
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27,44,53, &
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33,49,56, &
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35,68,71, &
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12,26,40/
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data Nm/ &
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1,29,58,87,116,144,0,&
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2,30,59,88,117,143,0,&
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3,31,60,89,118,145,0,&
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4,29,61,90,119,146,0,&
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2,29,62,91,120,147,0,&
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5,32,63,92,121,148,0,&
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6,33,64,90,109,149,0,&
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7,34,65,93,122,150,164,&
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8,33,66,94,118,151,0,&
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9,34,67,95,123,152,0,&
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10,35,68,96,114,153,0,&
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11,36,69,97,124,154,168,&
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12,37,63,94,125,155,0,&
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10,38,60,92,116,156,0,&
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13,39,61,96,126,138,0,&
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13,33,70,81,127,150,0,&
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14,40,71,98,126,157,0,&
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15,39,72,99,128,158,0,&
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16,41,64,96,129,159,0,&
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17,42,64,91,112,160,0,&
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18,42,73,100,130,161,0,&
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19,43,68,93,131,162,0,&
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12,44,74,86,132,163,0,&
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1,45,75,101,133,159,0,&
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19,46,57,102,134,149,0,&
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20,32,76,91,118,164,168,&
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21,41,62,98,132,165,0,&
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22,45,77,89,113,162,0,&
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18,47,68,103,135,145,0,&
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16,48,78,104,131,144,0,&
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3,49,79,105,129,147,0,&
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6,50,71,88,124,153,0,&
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23,38,76,90,115,166,0,&
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20,44,79,99,136,146,0,&
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21,51,70,105,137,167,0,&
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8,35,67,87,134,136,0,&
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21,47,80,106,119,156,0,&
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24,36,65,107,138,147,0,&
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22,52,74,102,130,164,0,&
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9,45,81,104,139,168,0,&
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8,53,82,100,133,0,0,&
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25,48,72,100,127,157,0,&
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19,37,71,107,139,158,0,&
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22,39,82,108,116,143,165,&
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26,43,72,109,121,0,0,&
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27,54,58,99,140,151,0,&
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28,31,80,87,110,161,0,&
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16,30,74,110,137,154,158,&
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7,55,79,110,133,166,0,&
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4,50,83,101,141,155,0,&
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15,47,59,97,142,149,0,&
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24,30,75,98,140,148,0,&
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23,56,73,111,136,165,0,&
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14,44,81,85,103,115,160,&
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23,52,84,95,138,144,0,&
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25,35,77,97,120,166,0,&
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25,57,85,111,140,156,0,&
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4,42,69,105,131,148,0,&
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15,40,84,112,125,162,0,&
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5,54,70,84,134,159,0,&
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28,53,76,113,128,163,0,&
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10,54,86,109,117,161,0,&
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9,49,59,92,130,141,0,&
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14,56,58,113,129,155,0,&
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5,36,78,82,135,146,0,&
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3,50,78,112,132,150,0,&
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1,51,57,94,122,135,0,&
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17,38,75,93,142,167,0,&
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27,41,67,89,127,142,0,&
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18,55,63,88,128,152,0,&
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2,53,85,95,121,167,0,&
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11,49,86,108,122,157,0,&
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20,51,83,107,117,0,0,&
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26,52,55,62,139,151,0,&
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11,56,66,102,114,152,0,&
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6,32,77,103,123,137,0,&
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28,40,65,114,141,0,0,&
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12,34,61,111,124,145,0,&
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27,43,83,106,126,154,0,&
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13,46,60,101,120,163,0,&
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17,37,80,108,123,0,0,&
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26,31,69,115,125,0,0,&
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7,48,66,106,143,160,0,&
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24,46,73,104,119,153,0/
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data nrw/ &
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6,6,6,6,6,6,6,7,6,6,6,7,6,6,6,6,6,6,6,6,6, &
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6,6,6,6,7,6,6,6,6,6,6,6,6,6,6,6,6,6,6,5,6, &
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6,7,5,6,6,7,6,6,6,6,6,7,6,6,6,6,6,6,6,6,6, &
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6,6,6,6,6,6,6,6,6,5,6,6,6,5,6,6,6,5,5,6,6/
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ncw=3
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toc=0
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tov=0
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tanhtoc=0
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!write(*,*) llr
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! initialize messages to checks
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do j=1,M
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do i=1,nrw(j)
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toc(i,j)=llr((Nm(i,j)))
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enddo
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enddo
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ncnt=0
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do iter=0,maxiterations
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! Update bit log likelihood ratios (tov=0 in iteration 0).
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do i=1,N
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if( apmask(i) .ne. 1 ) then
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zn(i)=llr(i)+sum(tov(1:ncw,i))
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else
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zn(i)=llr(i)
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endif
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enddo
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! Check to see if we have a codeword (check before we do any iteration).
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cw=0
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where( zn .gt. 0. ) cw=1
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ncheck=0
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do i=1,M
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synd(i)=sum(cw(Nm(1:nrw(i),i)))
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if( mod(synd(i),2) .ne. 0 ) ncheck=ncheck+1
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! if( mod(synd(i),2) .ne. 0 ) write(*,*) 'check ',i,' unsatisfied'
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enddo
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!write(*,*) 'number of unsatisfied parity checks ',ncheck
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if( ncheck .eq. 0 ) then ! we have a codeword - reorder the columns and return it
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niterations=iter
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codeword=cw(colorder+1)
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decoded=codeword(M+1:N)
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return
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endif
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if( iter.gt.0 ) then ! this code block implements an early stopping criterion
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nd=ncheck-nclast
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if( nd .lt. 0 ) then ! # of unsatisfied parity checks decreased
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ncnt=0 ! reset counter
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else
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ncnt=ncnt+1
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endif
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! write(*,*) iter,ncheck,nd,ncnt
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if( ncnt .ge. 3 .and. iter .ge. 5 .and. ncheck .gt. 10) then
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niterations=-1
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return
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endif
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endif
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nclast=ncheck
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! Send messages from bits to check nodes
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do j=1,M
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do i=1,nrw(j)
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ibj=Nm(i,j)
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toc(i,j)=zn(ibj)
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do kk=1,ncw ! subtract off what the bit had received from the check
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if( Mn(kk,ibj) .eq. j ) then
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toc(i,j)=toc(i,j)-tov(kk,ibj)
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endif
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enddo
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enddo
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enddo
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! send messages from check nodes to variable nodes
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do i=1,M
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tanhtoc(1:7,i)=tanh(-toc(1:7,i)/2)
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enddo
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do j=1,N
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do i=1,ncw
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ichk=Mn(i,j) ! Mn(:,j) are the checks that include bit j
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Tmn=product(tanhtoc(1:nrw(ichk),ichk),mask=Nm(1:nrw(ichk),ichk).ne.j)
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call platanh(-Tmn,y)
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! y=atanh(-Tmn)
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tov(i,j)=2*y
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enddo
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enddo
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enddo
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niterations=-1
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return
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end subroutine bpdecode168
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