subroutine fastosd240_74(llr,k,apmask,ndeep,message74,cw,nhardmin,dmin) ! ! An ordered-statistics decoder for the (240,74) code. ! Message payload is 50 bits. Any or all of a 24-bit CRC can be ! used for detecting incorrect codewords. The remaining CRC bits are ! cascaded with the LDPC code for the purpose of improving the ! distance spectrum of the code. ! ! If p1 (0.le.p1.le.24) is the number of CRC24 bits that are ! to be used for bad codeword detection, then the argument k should ! be set to 77+p1. ! ! Valid values for k are in the range [50,74]. ! character*24 c24 integer, parameter:: N=240 integer*1 apmask(N),apmaskr(N) integer*1, allocatable, save :: gen(:,:) integer*1, allocatable :: genmrb(:,:),g2(:,:) integer*1, allocatable :: temp(:),temprow(:),m0(:),me(:),mi(:) integer indices(N),indices2(N),nxor(N) integer*1 cw(N),ce(N),c0(N),hdec(N) integer*1, allocatable :: decoded(:) integer*1 message74(74) integer*1, allocatable :: sp(:) integer indx(N),ksave real llr(N),rx(N),absrx(N) logical first data first/.true./,ksave/64/ save first,ksave allocate( genmrb(k,N), g2(N,k) ) allocate( temp(k), temprow(n), m0(k), me(k), mi(k) ) allocate( decoded(k) ) if( first .or. k.ne.ksave) then ! fill the generator matrix ! ! Create generator matrix for partial CRC cascaded with LDPC code. ! ! Let p2=74-k and p1+p2=24. ! ! The last p2 bits of the CRC24 are cascaded with the LDPC code. ! ! The first p1=k-50 CRC24 bits will be used for error detection. ! if( allocated(gen) ) deallocate(gen) allocate( gen(k,N) ) gen=0 do i=1,k message74=0 message74(i)=1 if(i.le.50) then call get_crc24(message74,74,ncrc24) write(c24,'(b24.24)') ncrc24 read(c24,'(24i1)') message74(51:74) message74(51:k)=0 endif call encode240_74(message74,cw) gen(i,:)=cw enddo first=.false. ksave=k endif ! Use best k elements from the sorted list for the first basis. For the 2nd basis replace ! the nswap lowest quality symbols with the best nswap elements from the parity symbols. nswap=20 do ibasis=1,2 rx=llr apmaskr=apmask ! Hard decisions on the received word. hdec=0 where(rx .ge. 0) hdec=1 ! Use magnitude of received symbols as a measure of reliability. absrx=abs(llr) call indexx(absrx,N,indx) ! Re-order the columns of the generator matrix in order of decreasing reliability. do i=1,N genmrb(1:k,i)=gen(1:k,indx(N+1-i)) indices(i)=indx(N+1-i) enddo if(ibasis.eq.2) then do i=k-nswap+1,k temp(1:k)=genmrb(1:k,i) genmrb(1:k,i)=genmrb(1:k,i+nswap) genmrb(1:k,i+nswap)=temp(1:k) itmp=indices(i) indices(i)=indices(i+nswap) indices(i+nswap)=itmp enddo endif ! Do gaussian elimination to create a generator matrix with the most reliable ! received bits in positions 1:k in order of decreasing reliability (more or less). icol=1 indices2=0 nskipped=0 do id=1,k iflag=0 do while(iflag.eq.0) if(genmrb(id,icol).ne.1) then do j=id+1,k if(genmrb(j,icol).eq.1) then temprow=genmrb(id,:) genmrb(id,:)=genmrb(j,:) genmrb(j,:)=temprow iflag=1 endif enddo if(iflag.eq.0) then ! skip this column nskipped=nskipped+1 indices2(k+nskipped)=icol ! put icol where skipped columns go icol=icol+1 ! look at the next column endif else iflag=1 endif enddo indices2(id)=icol do j=1,k if(id.ne.j .and. genmrb(j,icol).eq.1) then genmrb(j,:)=ieor(genmrb(id,:),genmrb(j,:)) endif enddo icol=icol+1 enddo do i=k+nskipped+1,240 indices2(i)=i enddo genmrb(1:k,:)=genmrb(1:k,indices2) indices=indices(indices2) !************************************ g2=transpose(genmrb) ! The hard decisions for the k MRB bits define the order 0 message, m0. ! Encode m0 using the modified generator matrix to find the "order 0" codeword. ! Flip various combinations of bits in m0 and re-encode to generate a list of ! codewords. Return the member of the list that has the smallest Euclidean ! distance to the received word. hdec=hdec(indices) ! hard decisions from received symbols m0=hdec(1:k) ! zero'th order message absrx=abs(llr) absrx=absrx(indices) rx=rx(indices) apmaskr=apmaskr(indices) call mrbencode74(m0,c0,g2,N,k) nxor=ieor(c0,hdec) nhardmin=sum(nxor) dmin=sum(nxor*absrx) np=32 if(ibasis.eq.1) allocate(sp(np)) cw=c0 ntotal=0 nrejected=0 xlambda=0.0 if(ndeep.eq.0) goto 998 ! norder=0 if(ndeep.gt.4) ndeep=4 if( ndeep.eq. 1) then nord=1 xlambda=0.0 nsyncmax=np elseif(ndeep.eq.2) then nord=2 xlambda=0.0 nsyncmax=np elseif(ndeep.eq.3) then nord=3 xlambda=4.0 nsyncmax=11 elseif(ndeep.eq.4) then nord=4 xlambda=3.4 nsyndmax=12 endif s1=sum(absrx(1:k)) s2=sum(absrx(k+1:N)) rho=s1/(s1+xlambda*s2) rhodmin=rho*dmin nerr64=-1 do iorder=1,nord !beta=0.0 !if(iorder.ge.3) beta=0.4 !spnc_order=sum(absrx(k-iorder+1:k))+beta*(N-k) !if(dmin.lt.spnc_order) cycle mi(1:k-iorder)=0 mi(k-iorder+1:k)=1 iflag=k-iorder+1 do while(iflag .ge.0) ntotal=ntotal+1 me=ieor(m0,mi) d1=sum(mi(1:k)*absrx(1:k)) if(d1.gt.rhodmin) exit call partial_syndrome(me,sp,np,g2,N,K) nwhsp=sum(ieor(sp(1:np),hdec(k:k+np-1))) if(nwhsp.le.nsyndmax) then call mrbencode74(me,ce,g2,N,k) nxor=ieor(ce,hdec) dd=sum(nxor*absrx(1:N)) if( dd .lt. dmin ) then dmin=dd rhodmin=rho*dmin cw=ce nhardmin=sum(nxor) nwhspmin=nwhsp nerr64=sum(nxor(1:K)) endif endif ! Get the next test error pattern, iflag will go negative ! when the last pattern with weight iorder has been generated. call nextpat74(mi,k,iorder,iflag) enddo enddo 998 continue ! Re-order the codeword to [message bits][parity bits] format. cw(indices)=cw hdec(indices)=hdec message74=cw(1:74) call get_crc24(message74,74,nbadcrc) if(nbadcrc.eq.0) exit nhardmin=-nhardmin enddo ! basis loop return end subroutine fastosd240_74 subroutine mrbencode74(me,codeword,g2,N,K) integer*1 me(K),codeword(N),g2(N,K) ! fast encoding for low-weight test patterns codeword=0 do i=1,K if( me(i) .eq. 1 ) then codeword=ieor(codeword,g2(1:N,i)) endif enddo return end subroutine mrbencode74 subroutine partial_syndrome(me,sp,np,g2,N,K) integer*1 me(K),sp(np),g2(N,K) ! compute partial syndrome sp=0 do i=1,K if( me(i) .eq. 1 ) then sp=ieor(sp,g2(K:K+np-1,i)) endif enddo return end subroutine partial_syndrome subroutine nextpat74(mi,k,iorder,iflag) integer*1 mi(k),ms(k) ! generate the next test error pattern ind=-1 do i=1,k-1 if( mi(i).eq.0 .and. mi(i+1).eq.1) ind=i enddo if( ind .lt. 0 ) then ! no more patterns of this order iflag=ind return endif ms=0 ms(1:ind-1)=mi(1:ind-1) ms(ind)=1 ms(ind+1)=0 if( ind+1 .lt. k ) then nz=iorder-sum(ms) ms(k-nz+1:k)=1 endif mi=ms do i=1,k ! iflag will point to the lowest-index 1 in mi if(mi(i).eq.1) then iflag=i exit endif enddo return end subroutine nextpat74