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