149 lines
3.1 KiB
Fortran
149 lines
3.1 KiB
Fortran
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subroutine bpdecode40(llr,maxiterations,decoded,niterations)
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!
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! A log-domain belief propagation decoder for the msk40 code.
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! The code is a regular (32,16) code with column weight 3, row weights 5,6,7.
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! k9an August, 2016
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!
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integer, parameter:: N=32, K=16, M=N-K
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integer*1 codeword(N),cw(N)
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integer*1 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/ &
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4, 1, 2, 3, 0, 8, 6, 10, &
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13, 28, 20, 23, 17, 15, 27, 25, &
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16, 12, 18, 19, 7, 21, 22, 11, &
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24, 5, 26, 14, 9, 29, 30, 31/
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data Mn/ &
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1, 6, 13, &
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2, 3, 14, &
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4, 8, 15, &
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5, 11, 12, &
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7, 10, 16, &
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6, 9, 15, &
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1, 11, 16, &
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2, 4, 5, &
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3, 7, 9, &
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8, 10, 12, &
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8, 13, 14, &
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1, 4, 12, &
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2, 6, 10, &
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3, 11, 15, &
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5, 9, 14, &
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7, 13, 15, &
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12, 14, 16, &
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1, 2, 8, &
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3, 5, 6, &
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4, 9, 11, &
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1, 7, 14, &
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5, 10, 13, &
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3, 4, 16, &
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2, 15, 16, &
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6, 7, 12, &
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7, 8, 11, &
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1, 9, 10, &
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2, 11, 13, &
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3, 12, 13, &
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4, 6, 14, &
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1, 5, 15, &
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8, 9, 16/
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data Nm/ &
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1, 7, 12, 18, 21, 27, 31, &
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2, 8, 13, 18, 24, 28, 0, &
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2, 9, 14, 19, 23, 29, 0, &
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3, 8, 12, 20, 23, 30, 0, &
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4, 8, 15, 19, 22, 31, 0, &
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1, 6, 13, 19, 25, 30, 0, &
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5, 9, 16, 21, 25, 26, 0, &
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3, 10, 11, 18, 26, 32, 0, &
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6, 9, 15, 20, 27, 32, 0,&
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5, 10, 13, 22, 27, 0, 0, &
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4, 7, 14, 20, 26, 28, 0, &
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4, 10, 12, 17, 25, 29, 0, &
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1, 11, 16, 22, 28, 29, 0, &
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2, 11, 15, 17, 21, 30, 0, &
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3, 6, 14, 16, 24, 31, 0, &
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5, 7, 17, 23, 24, 32, 0/
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data nrw/7,6,6,6,6,6,6,6,6,5,6,6,6,6,6,6/
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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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! 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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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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zn(i)=llr(i)+sum(tov(1:ncw,i))
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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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enddo
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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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! 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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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 bpdecode40
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