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https://github.com/saitohirga/WSJT-X.git
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155 lines
3.9 KiB
Fortran
155 lines
3.9 KiB
Fortran
subroutine decode240_74(llr,Keff,maxosd,norder,apmask,message74,cw,ntype,nharderror,dmin)
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!
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! A hybrid bp/osd decoder for the (240,74) code.
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!
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! maxosd<0: do bp only
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! maxosd=0: do bp and then call osd once with channel llrs
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! maxosd>1: do bp and then call osd maxosd times with saved bp outputs
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! norder : osd decoding depth
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!
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integer, parameter:: N=240, K=74, M=N-K
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integer*1 cw(N),apmask(N)
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integer*1 nxor(N),hdec(N)
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integer*1 message74(74),m74(74)
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integer nrw(M),ncw
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integer Nm(5,M)
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integer Mn(3,N) ! 3 checks per bit
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integer synd(M)
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real tov(3,N)
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real toc(5,M)
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real tanhtoc(5,M)
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real zn(N),zsum(N),zsave(N,3)
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real llr(N)
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real Tmn
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include "ldpc_240_74_parity.f90"
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maxiterations=30
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nosd=0
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if(maxosd.gt.3) maxosd=3
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if(maxosd.eq.0) then ! osd with channel llrs
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nosd=1
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zsave(:,1)=llr
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elseif(maxosd.gt.0) then !
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nosd=maxosd
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elseif(maxosd.lt.0) then ! just bp
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nosd=0
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endif
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toc=0
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tov=0
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tanhtoc=0
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! initialize messages to checks
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do j=1,M
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do i=1,nrw(j)
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toc(i,j)=llr((Nm(i,j)))
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enddo
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enddo
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ncnt=0
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nclast=0
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zsum=0.0
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do iter=0,maxiterations
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! Update bit log likelihood ratios (tov=0 in iteration 0).
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do i=1,N
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if( apmask(i) .ne. 1 ) then
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zn(i)=llr(i)+sum(tov(1:ncw,i))
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else
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zn(i)=llr(i)
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endif
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enddo
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zsum=zsum+zn
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if(iter.gt.0 .and. iter.le.maxosd) then
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zsave(:,iter)=zsum
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endif
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! Check to see if we have a codeword (check before we do any iteration).
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cw=0
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where( zn .gt. 0. ) cw=1
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ncheck=0
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do i=1,M
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synd(i)=sum(cw(Nm(1:nrw(i),i)))
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if( mod(synd(i),2) .ne. 0 ) ncheck=ncheck+1
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enddo
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if( ncheck .eq. 0 ) then ! we have a codeword - if crc is good, return it
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m74=0
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m74(1:74)=cw(1:74)
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call get_crc24(m74,74,nbadcrc)
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if(nbadcrc.eq.0) then
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message74=cw(1:74)
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hdec=0
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where(llr .ge. 0) hdec=1
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nxor=ieor(hdec,cw)
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nharderror=sum(nxor)
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dmin=sum(nxor*abs(llr))
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ntype=1
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return
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endif
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endif
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if( iter.gt.0 ) then ! this code block implements an early stopping criterion
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! if( iter.gt.10000 ) then ! this code block implements an early stopping criterion
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nd=ncheck-nclast
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if( nd .lt. 0 ) then ! # of unsatisfied parity checks decreased
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ncnt=0 ! reset counter
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else
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ncnt=ncnt+1
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endif
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! write(*,*) iter,ncheck,nd,ncnt
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if( ncnt .ge. 5 .and. iter .ge. 10 .and. ncheck .gt. 15) then
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nharderror=-1
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exit
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endif
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endif
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nclast=ncheck
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! Send messages from bits to check nodes
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do j=1,M
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do i=1,nrw(j)
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ibj=Nm(i,j)
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toc(i,j)=zn(ibj)
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do kk=1,ncw ! subtract off what the bit had received from the check
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if( Mn(kk,ibj) .eq. j ) then
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toc(i,j)=toc(i,j)-tov(kk,ibj)
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endif
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enddo
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enddo
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enddo
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! send messages from check nodes to variable nodes
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do i=1,M
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tanhtoc(1:5,i)=tanh(-toc(1:5,i)/2)
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enddo
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do j=1,N
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do i=1,ncw
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ichk=Mn(i,j) ! Mn(:,j) are the checks that include bit j
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Tmn=product(tanhtoc(1:nrw(ichk),ichk),mask=Nm(1:nrw(ichk),ichk).ne.j)
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call platanh(-Tmn,y)
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! y=atanh(-Tmn)
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tov(i,j)=2*y
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enddo
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enddo
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enddo ! bp iterations
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do i=1,nosd
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zn=zsave(:,i)
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call osd240_74(zn,Keff,apmask,norder,message74,cw,nharderror,dminosd)
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if(nharderror.gt.0) then
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hdec=0
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where(llr .ge. 0) hdec=1
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nxor=ieor(hdec,cw)
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dmin=sum(nxor*abs(llr))
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ntype=1+nosd
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return
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endif
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enddo
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ntype=0
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nharderror=-1
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dminosd=0.0
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return
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end subroutine decode240_74
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