WSJT-X/lib/fst4/fastosd240_74.f90

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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
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