mirror of https://github.com/saitohirga/WSJT-X.git
Significantly more efficient and slightly more sensitive implementation of the ordered statistics decoder.
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@7677 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
This commit is contained in:
Steven Franke
parent
f7e8c81357
commit
669aa55457
|
|
@ -1,18 +1,14 @@
|
|||
subroutine osd300(llr,norder,decoded,niterations,cw)
|
||||
!
|
||||
! An ordered-statistics decoder based on ideas from:
|
||||
! "Soft-decision decoding of linear block codes based on ordered statistics,"
|
||||
! by Marc P. C. Fossorier and Shu Lin,
|
||||
! IEEE Trans Inf Theory, Vol 41, No 5, Sep 1995
|
||||
!
|
||||
|
||||
! An ordered-statistics decoder for the (300,60) code.
|
||||
!
|
||||
include "ldpc_300_60_params.f90"
|
||||
|
||||
integer*1 gen(K,N)
|
||||
integer*1 genmrb(K,N)
|
||||
integer*1 temp(K),m0(K),me(0:K)
|
||||
integer indices(N)
|
||||
integer*1 codeword(N),cw(N),hdec(N)
|
||||
integer*1 genmrb(K,N),g2(N,K)
|
||||
integer*1 temp(K),m0(K),me(K),mi(K)
|
||||
integer indices(N),nxor(N)
|
||||
integer*1 cw(N),ce(N),c0(N),hdec(N)
|
||||
integer*1 decoded(K)
|
||||
integer indx(N)
|
||||
real llr(N),rx(N),absrx(N)
|
||||
|
|
@ -83,58 +79,100 @@ do id=1,K ! diagonal element indices
|
|||
enddo
|
||||
enddo
|
||||
|
||||
g2=transpose(genmrb)
|
||||
!do i=1,N
|
||||
! g2(i,1:K)=genmrb(1:K,i)
|
||||
!enddo
|
||||
|
||||
! 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. Test all such codewords against the received word to decide which
|
||||
! codeword is most likely to be correct.
|
||||
hdec=hdec(indices)
|
||||
m0=hdec(1:K)
|
||||
|
||||
nhardmin=N
|
||||
j0=0
|
||||
j1=0
|
||||
j2=0
|
||||
j3=0
|
||||
if( norder.ge.4 ) j0=K
|
||||
if( norder.ge.3 ) j1=K
|
||||
if( norder.ge.2 ) j2=K
|
||||
if( norder.ge.1 ) j3=K
|
||||
! me(0) is a dummy position --- only me(1:K) are actually used. This is done
|
||||
! to avoid "if" statements within the inner loop.
|
||||
do i1=0,j0
|
||||
do i2=i1,j1
|
||||
do i3=i2,j2
|
||||
do i4=i3,j3
|
||||
me(1:K)=m0
|
||||
me(i1)=1-me(i1)
|
||||
me(i2)=1-me(i2)
|
||||
me(i3)=1-me(i3)
|
||||
me(i4)=1-me(i4)
|
||||
hdec=hdec(indices) ! hard decisions from received symbols
|
||||
m0=hdec(1:K) ! zero'th order message
|
||||
absrx=absrx(indices)
|
||||
rx=rx(indices)
|
||||
|
||||
! me is the m0 + error pattern. encode this message using genmrb to
|
||||
! produce a codeword. test the codeword against the received vector
|
||||
! and save it if it's the best that we've seen so far.
|
||||
do i=1,N
|
||||
nsum=sum(iand(me(1:K),genmrb(1:K,i)))
|
||||
codeword(i)=mod(nsum,2)
|
||||
enddo
|
||||
nhard=count(codeword .ne. hdec)
|
||||
if( nhard .lt. nhardmin ) then
|
||||
cw=codeword
|
||||
nhardmin=nhard
|
||||
i1min=i1
|
||||
i2min=i2
|
||||
i3min=i3
|
||||
i4min=i4
|
||||
endif
|
||||
enddo
|
||||
enddo
|
||||
enddo
|
||||
s1=sum(absrx(1:K))
|
||||
s2=sum(absrx(K+1:N))
|
||||
xlam=5.0
|
||||
rho=s1/(s1+xlam*s2)
|
||||
call mrbencode(m0,c0,g2,N,K)
|
||||
nxor=ieor(c0,hdec)
|
||||
nhardmin=sum(nxor)
|
||||
dmin=sum(nxor*absrx)
|
||||
thresh=rho*dmin
|
||||
|
||||
cw=c0
|
||||
nt=0
|
||||
nrejected=0
|
||||
do iorder=1,norder
|
||||
mi(1:K-iorder)=0
|
||||
mi(K-iorder+1:K)=1
|
||||
iflag=0
|
||||
do while(iflag .ge. 0 )
|
||||
dpat=sum(mi*absrx(1:K))
|
||||
nt=nt+1
|
||||
if( dpat .lt. thresh ) then ! reject unlikely error patterns
|
||||
me=ieor(m0,mi)
|
||||
call mrbencode(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)
|
||||
thresh=rho*dmin
|
||||
endif
|
||||
else
|
||||
nrejected=nrejected+1
|
||||
endif
|
||||
! get the next test error pattern, iflag will go negative
|
||||
! when the last pattern with weight iorder has been generated
|
||||
call nextpat(mi,k,iorder,iflag)
|
||||
enddo
|
||||
enddo
|
||||
|
||||
!write(*,*) 'nhardmin ',nhardmin
|
||||
!write(*,*) 'total patterns ',nt,' number rejected ',nrejected
|
||||
|
||||
! re-order the codeword to place message bits at the end
|
||||
cw(indices)=cw
|
||||
decoded=cw(M+1:N)
|
||||
niterations=1
|
||||
return
|
||||
end subroutine osd300
|
||||
|
||||
subroutine mrbencode(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 mrbencode
|
||||
|
||||
subroutine nextpat(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
|
||||
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
|
||||
iflag=ind
|
||||
return
|
||||
end subroutine nextpat
|
||||
|
|
|
|||
Loading…
Reference in New Issue