subroutine decode240_101(llr,Keff,maxosd,norder,apmask,message101,cw,ntype,nharderror,dmin) ! ! A hybrid bp/osd decoder for the (240,101) code. ! ! maxosd<0: do bp only ! maxosd=0: do bp and then call osd once with channel llrs ! maxosd>1: do bp and then call osd maxosd times with saved bp outputs ! norder : osd decoding depth ! integer, parameter:: N=240, K=101, M=N-K integer*1 cw(N),apmask(N) integer*1 nxor(N),hdec(N) integer*1 message101(101),m101(101) 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),zsum(N),zsave(N,3) real llr(N) real Tmn include "ldpc_240_101_parity.f90" maxiterations=30 nosd=0 if(maxosd.gt.3) maxosd=3 if(maxosd.eq.0) then ! osd with channel llrs nosd=1 zsave(:,1)=llr elseif(maxosd.gt.0) then ! nosd=maxosd elseif(maxosd.lt.0) then ! just bp nosd=0 endif 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 zsum=0.0 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 zsum=zsum+zn if(iter.gt.0 .and. iter.le.maxosd) then zsave(:,iter)=zsum endif ! 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 - if crc is good, return it m101=0 m101(1:101)=cw(1:101) call get_crc24(m101,101,nbadcrc) if(nbadcrc.eq.0) then message101=cw(1:101) hdec=0 where(llr .ge. 0) hdec=1 nxor=ieor(hdec,cw) nharderror=sum(nxor) dmin=sum(nxor*abs(llr)) ntype=1 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 exit 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 ! bp iterations do i=1,nosd zn=zsave(:,i) call osd240_101(zn,Keff,apmask,norder,message101,cw,nharderror,dminosd) if(nharderror.gt.0) then hdec=0 where(llr .ge. 0) hdec=1 nxor=ieor(hdec,cw) nharderror=sum(nxor) ! re-calculate nharderror based on input llrs dmin=sum(nxor*abs(llr)) ntype=1+i return endif enddo ntype=0 nharderror=-1 dminosd=0.0 return end subroutine decode240_101