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subroutine bpdecode40(llr,maxiterations,decoded,niterations)
!
! A log-domain belief propagation decoder for the msk40 code.
! The code is a regular (32,16) code with column weight 3, row weights 5,6,7.
! k9an August, 2016
!
integer, parameter:: N=32, K=16, M=N-K
integer*1 codeword(N),cw(N)
integer*1 colorder(N)
integer*1 decoded(K)
integer Nm(7,M) ! 5,6 or 7 bits per check
integer Mn(3,N) ! 3 checks per bit
integer synd(M)
real tov(3,N)
real toc(7,M)
real tanhtoc(7,M)
real zn(N)
real llr(N)
real Tmn
integer nrw(M)
data colorder/ &
4, 1, 2, 3, 0, 8, 6, 10, &
13, 28, 20, 23, 17, 15, 27, 25, &
16, 12, 18, 19, 7, 21, 22, 11, &
24, 5, 26, 14, 9, 29, 30, 31/
data Mn/ &
1, 6, 13, &
2, 3, 14, &
4, 8, 15, &
5, 11, 12, &
7, 10, 16, &
6, 9, 15, &
1, 11, 16, &
2, 4, 5, &
3, 7, 9, &
8, 10, 12, &
8, 13, 14, &
1, 4, 12, &
2, 6, 10, &
3, 11, 15, &
5, 9, 14, &
7, 13, 15, &
12, 14, 16, &
1, 2, 8, &
3, 5, 6, &
4, 9, 11, &
1, 7, 14, &
5, 10, 13, &
3, 4, 16, &
2, 15, 16, &
6, 7, 12, &
7, 8, 11, &
1, 9, 10, &
2, 11, 13, &
3, 12, 13, &
4, 6, 14, &
1, 5, 15, &
8, 9, 16/
data Nm/ &
1, 7, 12, 18, 21, 27, 31, &
2, 8, 13, 18, 24, 28, 0, &
2, 9, 14, 19, 23, 29, 0, &
3, 8, 12, 20, 23, 30, 0, &
4, 8, 15, 19, 22, 31, 0, &
1, 6, 13, 19, 25, 30, 0, &
5, 9, 16, 21, 25, 26, 0, &
3, 10, 11, 18, 26, 32, 0, &
6, 9, 15, 20, 27, 32, 0,&
5, 10, 13, 22, 27, 0, 0, &
4, 7, 14, 20, 26, 28, 0, &
4, 10, 12, 17, 25, 29, 0, &
1, 11, 16, 22, 28, 29, 0, &
2, 11, 15, 17, 21, 30, 0, &
3, 6, 14, 16, 24, 31, 0, &
5, 7, 17, 23, 24, 32, 0/
data nrw/7,6,6,6,6,6,6,6,6,5,6,6,6,6,6,6/
ncw=3
toc=0
tov=0
tanhtoc=0
! initialize messages to checks
do j=1,M
do i=1,nrw(j)
toc(i,j)=llr((Nm(i,j)))
enddo
enddo
do iter=0,maxiterations
! Update bit log likelihood ratios (tov=0 in iteration 0).
do i=1,N
zn(i)=llr(i)+sum(tov(1:ncw,i))
enddo
! Check to see if we have a codeword (check before we do any iteration).
cw=0
where( zn .gt. 0. ) cw=1
ncheck=0
do i=1,M
synd(i)=sum(cw(Nm(1:nrw(i),i)))
if( mod(synd(i),2) .ne. 0 ) ncheck=ncheck+1
enddo
if( ncheck .eq. 0 ) then ! we have a codeword - reorder the columns and return it
niterations=iter
codeword=cw(colorder+1)
decoded=codeword(M+1:N)
return
endif
! Send messages from bits to check nodes
do j=1,M
do i=1,nrw(j)
ibj=Nm(i,j)
toc(i,j)=zn(ibj)
do kk=1,ncw ! subtract off what the bit had received from the check
if( Mn(kk,ibj) .eq. j ) then
toc(i,j)=toc(i,j)-tov(kk,ibj)
endif
enddo
enddo
enddo
! send messages from check nodes to variable nodes
do i=1,M
tanhtoc(1:7,i)=tanh(-toc(1:7,i)/2)
enddo
do j=1,N
do i=1,ncw
ichk=Mn(i,j) ! Mn(:,j) are the checks that include bit j
Tmn=product(tanhtoc(1:nrw(ichk),ichk),mask=Nm(1:nrw(ichk),ichk).ne.j)
call platanh(-Tmn,y)
tov(i,j)=2*y
enddo
enddo
enddo
niterations=-1
return
end subroutine bpdecode40
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