mirror of https://github.com/saitohirga/WSJT-X.git
293 lines
8.0 KiB
Fortran
293 lines
8.0 KiB
Fortran
subroutine fastosd240_74(llr,k,apmask,ndeep,message74,cw,nhardmin,dmin)
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!
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! An ordered-statistics decoder for the (240,74) code.
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! Message payload is 50 bits. Any or all of a 24-bit CRC can be
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! used for detecting incorrect codewords. The remaining CRC bits are
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! cascaded with the LDPC code for the purpose of improving the
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! distance spectrum of the code.
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!
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! If p1 (0.le.p1.le.24) is the number of CRC24 bits that are
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! to be used for bad codeword detection, then the argument k should
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! be set to 77+p1.
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!
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! Valid values for k are in the range [50,74].
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!
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character*24 c24
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integer, parameter:: N=240
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integer*1 apmask(N),apmaskr(N)
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integer*1, allocatable, save :: gen(:,:)
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integer*1, allocatable :: genmrb(:,:),g2(:,:)
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integer*1, allocatable :: temp(:),temprow(:),m0(:),me(:),mi(:)
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integer indices(N),indices2(N),nxor(N)
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integer*1 cw(N),ce(N),c0(N),hdec(N)
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integer*1, allocatable :: decoded(:)
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integer*1 message74(74)
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integer*1, allocatable :: sp(:)
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integer indx(N),ksave
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real llr(N),rx(N),absrx(N)
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logical first
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data first/.true./,ksave/64/
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save first,ksave
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allocate( genmrb(k,N), g2(N,k) )
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allocate( temp(k), temprow(n), m0(k), me(k), mi(k) )
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allocate( decoded(k) )
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if( first .or. k.ne.ksave) then ! fill the generator matrix
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!
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! Create generator matrix for partial CRC cascaded with LDPC code.
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!
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! Let p2=74-k and p1+p2=24.
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!
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! The last p2 bits of the CRC24 are cascaded with the LDPC code.
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!
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! The first p1=k-50 CRC24 bits will be used for error detection.
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!
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if( allocated(gen) ) deallocate(gen)
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allocate( gen(k,N) )
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gen=0
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do i=1,k
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message74=0
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message74(i)=1
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if(i.le.50) then
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call get_crc24(message74,74,ncrc24)
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write(c24,'(b24.24)') ncrc24
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read(c24,'(24i1)') message74(51:74)
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message74(51:k)=0
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endif
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call encode240_74(message74,cw)
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gen(i,:)=cw
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enddo
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first=.false.
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ksave=k
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endif
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! Use best k elements from the sorted list for the first basis. For the 2nd basis replace
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! the nswap lowest quality symbols with the best nswap elements from the parity symbols.
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nswap=20
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do ibasis=1,2
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rx=llr
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apmaskr=apmask
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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(llr)
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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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if(ibasis.eq.2) then
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do i=k-nswap+1,k
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temp(1:k)=genmrb(1:k,i)
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genmrb(1:k,i)=genmrb(1:k,i+nswap)
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genmrb(1:k,i+nswap)=temp(1:k)
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itmp=indices(i)
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indices(i)=indices(i+nswap)
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indices(i+nswap)=itmp
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enddo
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endif
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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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icol=1
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indices2=0
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nskipped=0
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do id=1,k
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iflag=0
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do while(iflag.eq.0)
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if(genmrb(id,icol).ne.1) then
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do j=id+1,k
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if(genmrb(j,icol).eq.1) then
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temprow=genmrb(id,:)
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genmrb(id,:)=genmrb(j,:)
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genmrb(j,:)=temprow
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iflag=1
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endif
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enddo
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if(iflag.eq.0) then ! skip this column
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nskipped=nskipped+1
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indices2(k+nskipped)=icol ! put icol where skipped columns go
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icol=icol+1 ! look at the next column
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endif
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else
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iflag=1
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endif
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enddo
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indices2(id)=icol
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do j=1,k
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if(id.ne.j .and. genmrb(j,icol).eq.1) then
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genmrb(j,:)=ieor(genmrb(id,:),genmrb(j,:))
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endif
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enddo
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icol=icol+1
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enddo
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do i=k+nskipped+1,240
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indices2(i)=i
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enddo
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genmrb(1:k,:)=genmrb(1:k,indices2)
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indices=indices(indices2)
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!************************************
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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. Return the member of the list that has the smallest Euclidean
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! distance to 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=abs(llr)
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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 mrbencode74(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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np=32
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if(ibasis.eq.1) allocate(sp(np))
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cw=c0
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ntotal=0
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nrejected=0
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xlambda=0.0
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if(ndeep.eq.0) goto 998 ! norder=0
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if(ndeep.gt.4) ndeep=4
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if( ndeep.eq. 1) then
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nord=1
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xlambda=0.0
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nsyncmax=np
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elseif(ndeep.eq.2) then
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nord=2
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xlambda=0.0
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nsyncmax=np
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elseif(ndeep.eq.3) then
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nord=3
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xlambda=4.0
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nsyncmax=11
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elseif(ndeep.eq.4) then
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nord=4
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xlambda=3.4
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nsyndmax=12
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endif
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s1=sum(absrx(1:k))
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s2=sum(absrx(k+1:N))
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rho=s1/(s1+xlambda*s2)
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rhodmin=rho*dmin
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nerr64=-1
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do iorder=1,nord
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!beta=0.0
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!if(iorder.ge.3) beta=0.4
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!spnc_order=sum(absrx(k-iorder+1:k))+beta*(N-k)
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!if(dmin.lt.spnc_order) cycle
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mi(1:k-iorder)=0
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mi(k-iorder+1:k)=1
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iflag=k-iorder+1
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do while(iflag .ge.0)
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ntotal=ntotal+1
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me=ieor(m0,mi)
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d1=sum(mi(1:k)*absrx(1:k))
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if(d1.gt.rhodmin) exit
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call partial_syndrome(me,sp,np,g2,N,K)
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nwhsp=sum(ieor(sp(1:np),hdec(k:k+np-1)))
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if(nwhsp.le.nsyndmax) then
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call mrbencode74(me,ce,g2,N,k)
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nxor=ieor(ce,hdec)
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dd=sum(nxor*absrx(1:N))
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if( dd .lt. dmin ) then
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dmin=dd
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rhodmin=rho*dmin
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cw=ce
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nhardmin=sum(nxor)
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nwhspmin=nwhsp
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nerr64=sum(nxor(1:K))
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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 nextpat74(mi,k,iorder,iflag)
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enddo
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enddo
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998 continue
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! Re-order the codeword to [message bits][parity bits] format.
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cw(indices)=cw
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hdec(indices)=hdec
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message74=cw(1:74)
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call get_crc24(message74,74,nbadcrc)
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if(nbadcrc.eq.0) exit
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nhardmin=-nhardmin
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enddo ! basis loop
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return
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end subroutine fastosd240_74
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subroutine mrbencode74(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 mrbencode74
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subroutine partial_syndrome(me,sp,np,g2,N,K)
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integer*1 me(K),sp(np),g2(N,K)
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! compute partial syndrome
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sp=0
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do i=1,K
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if( me(i) .eq. 1 ) then
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sp=ieor(sp,g2(K:K+np-1,i))
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endif
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enddo
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return
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end subroutine partial_syndrome
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subroutine nextpat74(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 nextpat74
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