WSJT-X/lib/fst240/osd240_74.f90

404 lines
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Fortran
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subroutine osd240_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(:),m0(:),me(:),mi(:),misub(:),e2sub(:),e2(:),ui(:)
integer*1, allocatable :: r2pat(:)
integer indices(N),nxor(N)
integer*1 cw(N),ce(N),c0(N),hdec(N)
integer*1, allocatable :: decoded(:)
integer*1 message74(74)
integer indx(N)
real llr(N),rx(N),absrx(N)
logical first,reset
data first/.true./
save first
allocate( genmrb(k,N), g2(N,k) )
allocate( temp(k), m0(k), me(k), mi(k), misub(k), e2sub(N-k), e2(N-k), ui(N-k) )
allocate( r2pat(N-k), decoded(k) )
if( first ) 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.
!
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.
endif
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(rx)
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
! 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).
do id=1,k ! diagonal element indices
do icol=id,k+20 ! The 20 is ad hoc - beware
iflag=0
if( genmrb(id,icol) .eq. 1 ) then
iflag=1
if( icol .ne. id ) then ! reorder column
temp(1:k)=genmrb(1:k,id)
genmrb(1:k,id)=genmrb(1:k,icol)
genmrb(1:k,icol)=temp(1:k)
itmp=indices(id)
indices(id)=indices(icol)
indices(icol)=itmp
endif
do ii=1,k
if( ii .ne. id .and. genmrb(ii,id) .eq. 1 ) then
genmrb(ii,1:N)=ieor(genmrb(ii,1:N),genmrb(id,1:N))
endif
enddo
exit
endif
enddo
enddo
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=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)
cw=c0
ntotal=0
nrejected=0
npre1=0
npre2=0
if(ndeep.eq.0) goto 998 ! norder=0
if(ndeep.gt.6) ndeep=6
if( ndeep.eq. 1) then
nord=1
npre1=0
npre2=0
nt=40
ntheta=12
elseif(ndeep.eq.2) then
nord=1
npre1=1
npre2=0
nt=40
ntheta=12
elseif(ndeep.eq.3) then
nord=1
npre1=1
npre2=1
nt=40
ntheta=12
ntau=14
elseif(ndeep.eq.4) then
nord=2
npre1=1
npre2=1
nt=40
ntheta=12
ntau=17
elseif(ndeep.eq.5) then
nord=3
npre1=1
npre2=1
nt=40
ntheta=12
ntau=15
elseif(ndeep.eq.6) then
nord=4
npre1=1
npre2=1
nt=95
ntheta=12
ntau=15
endif
do iorder=1,nord
misub(1:k-iorder)=0
misub(k-iorder+1:k)=1
iflag=k-iorder+1
do while(iflag .ge.0)
if(iorder.eq.nord .and. npre1.eq.0) then
iend=iflag
else
iend=1
endif
d1=0.
do n1=iflag,iend,-1
mi=misub
mi(n1)=1
if(any(iand(apmaskr(1:k),mi).eq.1)) cycle
ntotal=ntotal+1
me=ieor(m0,mi)
if(n1.eq.iflag) then
call mrbencode74(me,ce,g2,N,k)
e2sub=ieor(ce(k+1:N),hdec(k+1:N))
e2=e2sub
nd1kpt=sum(e2sub(1:nt))+1
d1=sum(ieor(me(1:k),hdec(1:k))*absrx(1:k))
else
e2=ieor(e2sub,g2(k+1:N,n1))
nd1kpt=sum(e2(1:nt))+2
endif
if(nd1kpt .le. ntheta) then
call mrbencode74(me,ce,g2,N,k)
nxor=ieor(ce,hdec)
if(n1.eq.iflag) then
dd=d1+sum(e2sub*absrx(k+1:N))
else
dd=d1+ieor(ce(n1),hdec(n1))*absrx(n1)+sum(e2*absrx(k+1:N))
endif
if( dd .lt. dmin ) then
dmin=dd
cw=ce
nhardmin=sum(nxor)
nd1kptbest=nd1kpt
endif
else
nrejected=nrejected+1
endif
enddo
! Get the next test error pattern, iflag will go negative
! when the last pattern with weight iorder has been generated.
call nextpat74(misub,k,iorder,iflag)
enddo
enddo
if(npre2.eq.1) then
reset=.true.
ntotal=0
do i1=k,1,-1
do i2=i1-1,1,-1
ntotal=ntotal+1
mi(1:ntau)=ieor(g2(k+1:k+ntau,i1),g2(k+1:k+ntau,i2))
call boxit74(reset,mi(1:ntau),ntau,ntotal,i1,i2)
enddo
enddo
ncount2=0
ntotal2=0
reset=.true.
! Now run through again and do the second pre-processing rule
misub(1:k-nord)=0
misub(k-nord+1:k)=1
iflag=k-nord+1
do while(iflag .ge.0)
me=ieor(m0,misub)
call mrbencode74(me,ce,g2,N,k)
e2sub=ieor(ce(k+1:N),hdec(k+1:N))
do i2=0,ntau
ntotal2=ntotal2+1
ui=0
if(i2.gt.0) ui(i2)=1
r2pat=ieor(e2sub,ui)
778 continue
call fetchit74(reset,r2pat(1:ntau),ntau,in1,in2)
if(in1.gt.0.and.in2.gt.0) then
ncount2=ncount2+1
mi=misub
mi(in1)=1
mi(in2)=1
if(sum(mi).lt.nord+npre1+npre2.or.any(iand(apmaskr(1:k),mi).eq.1)) cycle
me=ieor(m0,mi)
call mrbencode74(me,ce,g2,N,k)
nxor=ieor(ce,hdec)
dd=sum(nxor*absrx)
if( dd .lt. dmin ) then
dmin=dd
cw=ce
nhardmin=sum(nxor)
endif
goto 778
endif
enddo
call nextpat74(misub,k,nord,iflag)
enddo
endif
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.ne.0) nhardmin=-nhardmin
return
end subroutine osd240_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 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
subroutine boxit74(reset,e2,ntau,npindex,i1,i2)
integer*1 e2(1:ntau)
integer indexes(5000,2),fp(0:525000),np(5000)
logical reset
common/boxes/indexes,fp,np
if(reset) then
patterns=-1
fp=-1
np=-1
sc=-1
indexes=-1
reset=.false.
endif
indexes(npindex,1)=i1
indexes(npindex,2)=i2
ipat=0
do i=1,ntau
if(e2(i).eq.1) then
ipat=ipat+ishft(1,ntau-i)
endif
enddo
ip=fp(ipat) ! see what's currently stored in fp(ipat)
if(ip.eq.-1) then
fp(ipat)=npindex
else
do while (np(ip).ne.-1)
ip=np(ip)
enddo
np(ip)=npindex
endif
return
end subroutine boxit74
subroutine fetchit74(reset,e2,ntau,i1,i2)
integer indexes(5000,2),fp(0:525000),np(5000)
integer lastpat
integer*1 e2(ntau)
logical reset
common/boxes/indexes,fp,np
save lastpat,inext
if(reset) then
lastpat=-1
reset=.false.
endif
ipat=0
do i=1,ntau
if(e2(i).eq.1) then
ipat=ipat+ishft(1,ntau-i)
endif
enddo
index=fp(ipat)
if(lastpat.ne.ipat .and. index.gt.0) then ! return first set of indices
i1=indexes(index,1)
i2=indexes(index,2)
inext=np(index)
elseif(lastpat.eq.ipat .and. inext.gt.0) then
i1=indexes(inext,1)
i2=indexes(inext,2)
inext=np(inext)
else
i1=-1
i2=-1
inext=-1
endif
lastpat=ipat
return
end subroutine fetchit74