subroutine bpdecode174_74(llr,apmask,maxiterations,message50,cw,nharderror,iter,ncheck) ! ! A log-domain belief propagation decoder for the (174,74) code. ! integer, parameter:: N=174, K=74, M=N-K integer*1 cw(N),apmask(N) integer*1 decoded(K) integer*1 message50(50) integer nrw(M),ncw integer Nm(6,M) integer Mn(3,N) ! 3 checks per bit integer synd(M) real tov(3,N) real toc(6,M) real tanhtoc(6,M) real zn(N) real llr(N) real Tmn include "ldpc_174_74_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 if( ncheck .eq. 0 ) then ! we have a codeword - if crc is good, return it decoded=cw(1:74) call get_crc24(decoded,74,nbadcrc) nharderror=count( (2*cw-1)*llr .lt. 0.0 ) if(nbadcrc.eq.0) then message50=decoded(1:50) 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. 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 tanhtoc(1:6,i)=tanh(-toc(1:6,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 bpdecode174_74