2018-06-17 08:44:10 -05:00
|
|
|
subroutine bpdecode174_91(llr,apmask,maxiterations,message77,cw,nharderror,iter)
|
2018-05-28 12:11:41 -05:00
|
|
|
!
|
|
|
|
|
! A log-domain belief propagation decoder for the (174,91) code.
|
|
|
|
|
!
|
2018-06-17 08:44:10 -05:00
|
|
|
use iso_c_binding, only: c_loc,c_size_t
|
|
|
|
|
use crc
|
2018-05-28 12:11:41 -05:00
|
|
|
integer, parameter:: N=174, K=91, M=N-K
|
2018-06-17 08:44:10 -05:00
|
|
|
integer*1 cw(N),apmask(N)
|
2018-05-28 12:11:41 -05:00
|
|
|
integer*1 decoded(K)
|
2018-06-17 08:44:10 -05:00
|
|
|
integer*1 message77(77)
|
|
|
|
|
integer nrw(M),ncw
|
2018-05-28 12:11:41 -05:00
|
|
|
integer Nm(7,M)
|
|
|
|
|
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
|
|
|
|
|
|
2018-06-09 13:05:11 -05:00
|
|
|
include "ldpc_174_91_c_reordered_parity.f90"
|
2018-05-28 12:11:41 -05:00
|
|
|
|
|
|
|
|
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
|
|
|
|
|
|
|
|
|
|
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
|
2018-06-17 08:44:10 -05:00
|
|
|
if( ncheck .eq. 0 ) then ! we have a codeword - if crc is good, return it
|
|
|
|
|
decoded=cw(1:K)
|
|
|
|
|
call chkcrc14a(decoded,nbadcrc)
|
|
|
|
|
nharderror=count( (2*cw-1)*llr .lt. 0.0 )
|
|
|
|
|
if(nbadcrc.eq.0) then
|
|
|
|
|
message77=decoded(1:77)
|
|
|
|
|
return
|
|
|
|
|
endif
|
2018-05-28 12:11:41 -05:00
|
|
|
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. 5 .and. iter .ge. 10 .and. ncheck .gt. 15) 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
|
2018-05-29 10:32:59 -05:00
|
|
|
tanhtoc(1:7,i)=tanh(-toc(1:7,i)/2)
|
2018-05-28 12:11:41 -05:00
|
|
|
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 bpdecode174_91
|
