A simple hard-decision bit-flip decoder for LDPC codes.

lib/fsk4hf/bitflip128_90.f90

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+subroutine bitflip128_90(llr,message77,cw,nharderror)
+!
+! A hard-decision bit flipping decoder for the (128,90) code.
+! 
+ 
+  use iso_c_binding, only: c_loc,c_size_t
+  use crc
+  integer, parameter:: N=128, K=90, M=N-K
+  integer*1 cw(N),apmask(N)
+  integer*1 decoded(K)
+  integer*1 message77(77)
+  integer Nm(11,M)   
+  integer Mn(3,N) 
+  integer nrw(M)
+  integer synd(M)
+  integer nuns(N)
+  real zn(N)
+  real llr(N)
+
+  include "ldpc_128_90_reordered_parity.f90"
+
+  decoded=0
+  zn=llr
+
+  do iter=0,0
+
+! Check to see if we have a codeword (check before we do any iteration).
+    cw=0
+    where( zn .gt. 0. ) cw=1
+    ncheck=0
+    nuns=0
+    do i=1,M
+      synd(i)=sum(cw(Nm(1:nrw(i),i)))
+      if( mod(synd(i),2) .ne. 0 ) then
+        ncheck=ncheck+1
+        do j=1,nrw(i)
+          nuns(Nm(j,i))=nuns(Nm(j,i))+1
+        enddo
+      endif 
+    enddo
+    if( ncheck .eq. 0 ) then ! we have a codeword - reorder the columns and return it
+      decoded=cw(1:K)
+      call chkcrc13a(decoded,nbadcrc)
+      if(nbadcrc.eq.0) then
+        message77=decoded(1:77)
+        nharderror=count( (2*cw-1)*llr .lt. 0.0 )
+        return
+      endif
+    endif
+! flip the sign on the symbols that show up in the largest number
+! of un-satisfied parity checks
+    where( nuns .eq. maxval(nuns) ) zn=-zn
+
+  enddo
+  llr=zn
+  nharderror=-1
+  return
+
+end subroutine bitflip128_90