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