187 lines
4.7 KiB
Plaintext
187 lines
4.7 KiB
Plaintext
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)
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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 decode 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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! reliability will not be strictly decreasing because column re-ordering is needed
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! to put the generator matrix in systematic form. the "indices" array tracks
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! column permutations caused by reliability sorting and gaussian elimination.
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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. Test all such codewords against the received word to decide which
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! codeword is most likely to be correct.
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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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s1=sum(absrx(1:K))
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s2=sum(absrx(K+1:N))
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xlam=7.0 ! larger values reject more error patterns
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rho=s1/(s1+xlam*s2)
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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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thresh=rho*dmin
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cw=c0
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nt=0
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nrejected=0
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do iorder=1,norder
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mi(1:K-iorder)=0
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mi(K-iorder+1:K)=1
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iflag=0
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do while(iflag .ge. 0 )
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if(all(iand(apmaskr(1:K),mi).eq.0)) then ! reject patterns with ap bits
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dpat=sum(mi*absrx(1:K))
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nt=nt+1
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if( dpat .lt. thresh ) then ! reject unlikely error patterns
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me=ieor(m0,mi)
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call mrbencode(me,ce,g2,N,K)
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nxor=ieor(ce,hdec)
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dd=sum(nxor*absrx)
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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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thresh=rho*dmin
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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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endif
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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(mi,k,iorder,iflag)
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enddo
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enddo
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!write(*,*) 'nhardmin ',nhardmin
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!write(*,*) 'total patterns ',nt,' number rejected ',nrejected
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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(M+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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iflag=ind
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return
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end subroutine nextpat
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