mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-29 07:39:43 -05:00
293 lines
8.0 KiB
Fortran
293 lines
8.0 KiB
Fortran
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
|
|
|