Initial Commit
This commit is contained in:
Jordan Sherer
@@ -0,0 +1,138 @@
|
||||
subroutine fano232(symbol,nbits,mettab,ndelta,maxcycles,dat, &
|
||||
ncycles,metric,ierr)
|
||||
|
||||
! Sequential decoder for K=32, r=1/2 convolutional code using
|
||||
! the Fano algorithm. Translated from C routine for same purpose
|
||||
! written by Phil Karn, KA9Q.
|
||||
|
||||
parameter (MAXBITS=103)
|
||||
parameter (MAXBYTES=(MAXBITS+7)/8)
|
||||
integer*1 symbol(0:2*MAXBITS-1) !Soft symbols (as unsigned i*1)
|
||||
integer*1 dat(MAXBYTES) !Decoded user data, 8 bits per byte
|
||||
integer mettab(-128:127,0:1) !Metric table
|
||||
|
||||
! These were the "node" structure in Karn's C code:
|
||||
integer nstate(0:MAXBITS) !Encoder state of next node
|
||||
integer gamma(0:MAXBITS) !Cumulative metric to this node
|
||||
integer metrics(0:3,0:MAXBITS) !Metrics indexed by all possible Tx syms
|
||||
integer tm(0:1,0:MAXBITS) !Sorted metrics for current hypotheses
|
||||
integer ii(0:MAXBITS) !Current branch being tested
|
||||
|
||||
logical noback
|
||||
include 'conv232.f90' !Polynomials defined here
|
||||
|
||||
ntail=nbits-31
|
||||
|
||||
! Compute all possible branch metrics for each symbol pair.
|
||||
! This is the only place we actually look at the raw input symbols
|
||||
i4a=0
|
||||
i4b=0
|
||||
do np=0,nbits-1
|
||||
j=2*np
|
||||
i4a=symbol(j)
|
||||
i4b=symbol(j+1)
|
||||
metrics(0,np) = mettab(i4a,0) + mettab(i4b,0)
|
||||
metrics(1,np) = mettab(i4a,0) + mettab(i4b,1)
|
||||
metrics(2,np) = mettab(i4a,1) + mettab(i4b,0)
|
||||
metrics(3,np) = mettab(i4a,1) + mettab(i4b,1)
|
||||
enddo
|
||||
|
||||
np=0
|
||||
nstate(np)=0
|
||||
|
||||
n=iand(nstate(np),npoly1) !Compute and sort branch metrics
|
||||
n=ieor(n,ishft(n,-16)) !from the root node
|
||||
lsym=partab(iand(ieor(n,ishft(n,-8)),255))
|
||||
n=iand(nstate(np),npoly2)
|
||||
n=ieor(n,ishft(n,-16))
|
||||
lsym=lsym+lsym+partab(iand(ieor(n,ishft(n,-8)),255))
|
||||
m0=metrics(lsym,np)
|
||||
m1=metrics(ieor(3,lsym),np)
|
||||
if(m0.gt.m1) then
|
||||
tm(0,np)=m0 !0-branch has better metric
|
||||
tm(1,np)=m1
|
||||
else
|
||||
tm(0,np)=m1 !1-branch is better
|
||||
tm(1,np)=m0
|
||||
nstate(np)=nstate(np) + 1 !Set low bit
|
||||
endif
|
||||
|
||||
ii(np)=0 !Start with best branch
|
||||
gamma(np)=0
|
||||
nt=0
|
||||
|
||||
do i=1,nbits*maxcycles !Start the Fano decoder
|
||||
ngamma=gamma(np) + tm(ii(np),np) !Look forward
|
||||
if(ngamma.ge.nt) then
|
||||
! Node is acceptable. If first time visiting this node, tighten threshold:
|
||||
if(gamma(np).lt.(nt+ndelta)) nt=nt + ndelta * ((ngamma-nt)/ndelta)
|
||||
gamma(np+1)=ngamma !Move forward
|
||||
nstate(np+1)=ishft(nstate(np),1)
|
||||
np=np+1
|
||||
if(np.eq.nbits) go to 100 !We're done!
|
||||
|
||||
n=iand(nstate(np),npoly1)
|
||||
n=ieor(n,ishft(n,-16))
|
||||
lsym=partab(iand(ieor(n,ishft(n,-8)),255))
|
||||
n=iand(nstate(np),npoly2)
|
||||
n=ieor(n,ishft(n,-16))
|
||||
lsym=lsym+lsym+partab(iand(ieor(n,ishft(n,-8)),255))
|
||||
|
||||
if(np.ge.ntail) then
|
||||
tm(0,np)=metrics(lsym,np) !We're in the tail, now all zeros
|
||||
else
|
||||
m0=metrics(lsym,np)
|
||||
m1=metrics(ieor(3,lsym),np)
|
||||
if(m0.gt.m1) then
|
||||
tm(0,np)=m0 !0-branch has better metric
|
||||
tm(1,np)=m1
|
||||
else
|
||||
tm(0,np)=m1 !1-branch is better
|
||||
tm(1,np)=m0
|
||||
nstate(np)=nstate(np) + 1 !Set low bit
|
||||
endif
|
||||
endif
|
||||
ii(np)=0 !Start with best branch
|
||||
else
|
||||
do while(.true.)
|
||||
noback=.false. !Threshold violated, can't go forward
|
||||
if(np.eq.0) noback=.true.
|
||||
if(np.gt.0) then
|
||||
if(gamma(np-1).lt.nt) noback=.true.
|
||||
endif
|
||||
|
||||
if(noback) then !Can't back up, either
|
||||
nt=nt-ndelta !Relax threshold and look forward again
|
||||
if(ii(np).ne.0) then
|
||||
ii(np)=0
|
||||
nstate(np)=ieor(nstate(np),1)
|
||||
endif
|
||||
exit
|
||||
endif
|
||||
|
||||
np=np-1 !Back up
|
||||
if(np.lt.ntail .and. ii(np).ne.1) then
|
||||
ii(np)=ii(np)+1 !Search the next best branch
|
||||
nstate(np)=ieor(nstate(np),1)
|
||||
exit
|
||||
endif
|
||||
enddo
|
||||
endif
|
||||
enddo
|
||||
i=nbits*maxcycles
|
||||
|
||||
100 metric=gamma(np) !Final path metric
|
||||
nbytes=(nbits+7)/8 !Copy decoded data to user's buffer
|
||||
np=7
|
||||
do j=1,nbytes-1
|
||||
i4a=nstate(np)
|
||||
dat(j)=i4a
|
||||
np=np+8
|
||||
enddo
|
||||
dat(nbytes)=0
|
||||
ncycles=i+1
|
||||
ierr=0
|
||||
if(i.ge.maxcycles*nbits) ierr=-1
|
||||
|
||||
return
|
||||
end subroutine fano232
|
||||
Reference in New Issue
Block a user