mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2025-11-03 21:40:52 -05:00
117 lines
2.8 KiB
Plaintext
117 lines
2.8 KiB
Plaintext
|
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(12,M)
|
||
|
|
integer Mn(4,N)
|
||
|
|
integer nrw(M),ncw(N)
|
||
|
|
integer synd(M)
|
||
|
|
real tov(4,N)
|
||
|
|
real toc(12,M)
|
||
|
|
real tanhtoc(12,M)
|
||
|
|
real zn(N)
|
||
|
|
real llr(N)
|
||
|
|
real Tmn
|
||
|
|
|
||
|
|
include "ldpc_128_90_b_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
|
||
|
|
|
||
|
|
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),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=1,4 ! 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(1:12,i)=tanh(-toc(1:12,i)/2)
|
||
|
|
enddo
|
||
|
|
|
||
|
|
do j=1,N
|
||
|
|
do i=1,ncw(j)
|
||
|
|
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
|