WSJT-X/lib/bpdecode128_90.f90
2020-03-19 11:00:21 -04:00

118 lines
2.8 KiB
Fortran

subroutine bpdecode128_90(llr,apmask,maxiterations,message77,cw,nharderror,iter)
!
! A log-domain belief propagation decoder for the (128,90) code.
!
use iso_c_binding, only: c_loc,c_size_t
use crc
integer, parameter:: N=128, K=90, M=N-K
integer*1 cw(N),apmask(N)
integer*1 decoded(K)
integer*1 message77(77)
integer Nm(11,M)
integer Mn(3,N)
integer nrw(M)
integer synd(M)
real tov(4,N)
real toc(11,M)
real tanhtoc(11,M)
real zn(N)
real llr(N)
real Tmn
include "ldpc_128_90_reordered_parity.f90"
decoded=0
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
ncnt=0
nclast=0
do iter=0,maxiterations
! Update bit log likelihood ratios (tov=0 in iteration 0).
do i=1,N
if( apmask(i) .ne. 1 ) then
zn(i)=llr(i)+sum(tov(1:ncw,i))
else
zn(i)=llr(i)
endif
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
! if( mod(synd(i),2) .ne. 0 ) write(*,*) 'check ',i,' unsatisfied'
enddo
! write(*,*) 'number of unsatisfied parity checks ',ncheck
if( ncheck .eq. 0 ) then ! we have a codeword - reorder the columns and return it
decoded=cw(1:K)
call chkcrc13a(decoded,nbadcrc)
if(nbadcrc.eq.0) then
message77=decoded(1:77)
nharderror=count( (2*cw-1)*llr .lt. 0.0 )
return
endif
endif
if( iter.gt.0 ) then ! this code block implements an early stopping criterion
! if( iter.gt.10000 ) then ! this code block implements an early stopping criterion
nd=ncheck-nclast
if( nd .lt. 0 ) then ! # of unsatisfied parity checks decreased
ncnt=0 ! reset counter
else
ncnt=ncnt+1
endif
! write(*,*) iter,ncheck,nd,ncnt
if( ncnt .ge. 3 .and. iter .ge. 5 .and. ncheck .gt. 10) then
nharderror=-1
return
endif
endif
nclast=ncheck
! 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(1:11,i)=tanh(-toc(1:11,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)
! y=atanh(-Tmn)
tov(i,j)=2*y
enddo
enddo
enddo
nharderror=-1
return
end subroutine bpdecode128_90