233 lines
5.9 KiB
Plaintext
233 lines
5.9 KiB
Plaintext
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subroutine osd174(llr,apmask,norder,decoded,cw,nhardmin,dmin)
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!
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! An ordered-statistics decoder for the (174,87) code.
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!
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include "ldpc_174_87_params.f90"
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integer*1 apmask(N),apmaskr(N)
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integer*1 gen(K,N)
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integer*1 genmrb(K,N),g2(N,K)
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integer*1 temp(K),m0(K),me(K),mi(K),misub(K),e2sub(N-K),e2(N-K)
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integer indices(N),nxor(N)
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integer*1 cw(N),ce(N),c0(N),hdec(N)
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integer*1 decoded(K)
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integer indx(N)
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real llr(N),rx(N),absrx(N)
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logical first
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data first/.true./
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save first,gen
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if( first ) then ! fill the generator matrix
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gen=0
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do i=1,M
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do j=1,22
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read(g(i)(j:j),"(Z1)") istr
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do jj=1, 4
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irow=(j-1)*4+jj
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if( btest(istr,4-jj) ) gen(irow,i)=1
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enddo
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enddo
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enddo
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do irow=1,K
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gen(irow,M+irow)=1
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enddo
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first=.false.
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endif
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! Re-order received vector to place systematic msg bits at the end.
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rx=llr(colorder+1)
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apmaskr=apmask(colorder+1)
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! Hard decisions on the received word.
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hdec=0
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where(rx .ge. 0) hdec=1
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! Use magnitude of received symbols as a measure of reliability.
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absrx=abs(rx)
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call indexx(absrx,N,indx)
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! Re-order the columns of the generator matrix in order of decreasing reliability.
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do i=1,N
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genmrb(1:K,i)=gen(1:K,indx(N+1-i))
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indices(i)=indx(N+1-i)
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enddo
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! Do gaussian elimination to create a generator matrix with the most reliable
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! received bits in positions 1:K in order of decreasing reliability (more or less).
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do id=1,K ! diagonal element indices
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do icol=id,K+20 ! The 20 is ad hoc - beware
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iflag=0
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if( genmrb(id,icol) .eq. 1 ) then
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iflag=1
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if( icol .ne. id ) then ! reorder column
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temp(1:K)=genmrb(1:K,id)
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genmrb(1:K,id)=genmrb(1:K,icol)
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genmrb(1:K,icol)=temp(1:K)
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itmp=indices(id)
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indices(id)=indices(icol)
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indices(icol)=itmp
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endif
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do ii=1,K
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if( ii .ne. id .and. genmrb(ii,id) .eq. 1 ) then
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genmrb(ii,1:N)=ieor(genmrb(ii,1:N),genmrb(id,1:N))
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endif
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enddo
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exit
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endif
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enddo
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enddo
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g2=transpose(genmrb)
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! The hard decisions for the K MRB bits define the order 0 message, m0.
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! Encode m0 using the modified generator matrix to find the "order 0" codeword.
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! Flip various combinations of bits in m0 and re-encode to generate a list of
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! codewords. A pre-processing step selects a subset of these codewords.
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! Return the member of the subset with the smallest Euclidean distance to the
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! the received word.
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hdec=hdec(indices) ! hard decisions from received symbols
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m0=hdec(1:K) ! zero'th order message
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absrx=absrx(indices)
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rx=rx(indices)
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apmaskr=apmaskr(indices)
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call mrbencode(m0,c0,g2,N,K)
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nxor=ieor(c0,hdec)
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nhardmin=sum(nxor)
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dmin=sum(nxor*absrx)
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cw=c0
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ntotal=0
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nrejected=0
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nt=40 ! Count the errors in the nt best bits in the redundancy part of the cw
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ntheta=12 ! Reject the codeword without computing distance if # errors exceeds ntheta
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! norder should be 1, 2, or 3.
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! if norder = 1, do one loop, no pre-processing
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! if norder = 2, do norder=1, then norder=2 using first W&H pre-processing rule
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! if norder = 3, do norder=2, then norder=3 using first W&H pre-processing rule
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if(norder.lt.1) goto 998 ! norder=0
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if(norder.gt.3) norder=3
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if( norder.eq. 1) then
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nord=1
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npre=0
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elseif(norder.eq.2) then
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nord=1
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npre=1
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elseif(norder.eq.3) then
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nord=2
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npre=1
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endif
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do iorder=1,nord
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if( iorder.eq. 1 ) then
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misub(1:K-1)=0
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misub(K)=1
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iflag=K
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elseif( iorder.eq. 2 ) then
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misub(1:K-2)=0
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misub(K-1:K)=1
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iflag=K-1
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endif
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do while(iflag .ge.0)
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if(iorder.eq.nord .and. npre.eq.0) then
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iend=iflag
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else
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iend=1
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endif
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do n1=iflag,iend,-1
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mi=misub
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mi(n1)=1
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if(any(iand(apmaskr(1:K),mi).eq.1)) cycle
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ntotal=ntotal+1
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me=ieor(m0,mi)
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if(n1.eq.iflag) then
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call mrbencode(me,ce,g2,N,K)
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e2sub=ieor(ce(K+1:N),hdec(K+1:N))
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e2=e2sub
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nd1Kpt=sum(e2sub(1:nt))+1
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d1=sum(ieor(me(1:K),hdec(1:K))*absrx(1:K))
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else
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e2=ieor(e2sub,g2(K+1:N,n1))
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nd1Kpt=sum(e2(1:nt))+2
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endif
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if(nd1Kpt .le. ntheta) then
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call mrbencode(me,ce,g2,N,K)
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nxor=ieor(ce,hdec)
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if(n1.eq.iflag) then
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dd=d1+sum(e2sub*absrx(K+1:N))
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else
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dd=d1+ieor(ce(n1),hdec(n1))*absrx(n1)+sum(e2*absrx(K+1:N))
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endif
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if( dd .lt. dmin ) then
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dmin=dd
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cw=ce
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nhardmin=sum(nxor)
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nd1Kptbest=nd1Kpt
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endif
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else
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nrejected=nrejected+1
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endif
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enddo
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! Get the next test error pattern, iflag will go negative
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! when the last pattern with weight iorder has been generated.
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call nextpat(misub,k,iorder,iflag)
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enddo
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enddo
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!write(*,*) 'rejected, total, nd1Kptbest: ',nrejected, ntotal, nd1Kptbest
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998 continue
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! Re-order the codeword to place message bits at the end.
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cw(indices)=cw
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hdec(indices)=hdec
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decoded=cw(K+1:N)
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cw(colorder+1)=cw ! put the codeword back into received-word order
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return
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end subroutine osd174
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subroutine mrbencode(me,codeword,g2,N,K)
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integer*1 me(K),codeword(N),g2(N,K)
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! fast encoding for low-weight test patterns
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codeword=0
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do i=1,K
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if( me(i) .eq. 1 ) then
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codeword=ieor(codeword,g2(1:N,i))
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endif
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enddo
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return
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end subroutine mrbencode
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subroutine nextpat(mi,k,iorder,iflag)
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integer*1 mi(k),ms(k)
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! generate the next test error pattern
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ind=-1
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do i=1,k-1
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if( mi(i).eq.0 .and. mi(i+1).eq.1) ind=i
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enddo
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if( ind .lt. 0 ) then ! no more patterns of this order
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iflag=ind
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return
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endif
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ms=0
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ms(1:ind-1)=mi(1:ind-1)
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ms(ind)=1
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ms(ind+1)=0
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if( ind+1 .lt. k ) then
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nz=iorder-sum(ms)
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ms(k-nz+1:k)=1
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endif
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mi=ms
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do i=1,k ! iflag will point to the lowest-index 1 in mi
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if(mi(i).eq.1) then
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iflag=i
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exit
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endif
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enddo
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return
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end subroutine nextpat
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