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ldpcsim.f90 is a simulator that creates valid JT-mode packed 72-bit messages and implements an 8-bit hash check for testing the (N,80) codes.
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6638 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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Steven Franke
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lib/ldpcsim.f90
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181
lib/ldpcsim.f90
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program ldpcsim
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use, intrinsic :: iso_c_binding
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use hashing
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use packjt
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! To change to a new code, edit the following line and the filenames
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! that contain the parity check and generator matrices.
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! parameter (N=128, K=80) ! M and N are global variables on the C side.
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character*22 msg,msgsent
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integer*4 i4Msg6BitWords(12)
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integer*1, target:: i1Msg8BitBytes(10) ! 72 bit msg + 8 bit hash
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integer*1, target:: i1Dec8BitBytes(10) ! 72 bit msg + 8 bit hash
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integer*1 i1hashdec
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character*80 prefix
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character*85 pchk_file,gen_file
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character*8 arg
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integer*1, allocatable :: codeword(:), decoded(:), message(:)
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real*8, allocatable :: lratio(:), rxdata(:)
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integer*1 i1hash(4),i1
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equivalence (ihash,i1hash)
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nargs=iargc()
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if(nargs.ne.7) then
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print*,'Usage: ldpcsim pchk/gen file prefix N K niter ndither #trials s '
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print*,'eg: ldpcsim "128-80-peg-reg3" 128 80 10 1 1000 0.75'
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return
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endif
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call getarg(1,prefix)
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call getarg(2,arg)
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read(arg,*) N
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call getarg(3,arg)
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read(arg,*) K
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call getarg(4,arg)
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read(arg,*) max_iterations
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call getarg(5,arg)
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read(arg,*) max_dither
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call getarg(6,arg)
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read(arg,*) ntrials
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call getarg(7,arg)
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read(arg,*) s
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pchk_file=trim(prefix)//".pchk"
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gen_file=trim(prefix)//".gen"
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!rate=real(K)/real(N)
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! don't count hash bits as data bits
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rate=72.0/real(N)
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write(*,*) "pchk file: ",pchk_file
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write(*,*) "niter= ",max_iterations," ndither= ",max_dither," s= ",s
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allocate ( codeword(N), decoded(K), message(K) )
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allocate ( lratio(N), rxdata(N) )
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call init_ldpc(trim(pchk_file)//char(0),trim(gen_file)//char(0))
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msg="K1JT K9AN EN50"
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call fmtmsg(msg,iz)
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call packmsg(msg,i4Msg6BitWords,itype)
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call unpackmsg(i4Msg6BitWords,msgsent)
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write(*,*) "Message: ",msgsent
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! Convert from 12 6-bit words to 10 8-bit words
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i4=0
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ik=0
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im=0
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do i=1,12
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nn=i4Msg6BitWords(i)
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do j=1, 6
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ik=ik+1
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i4=i4+i4+iand(1,ishft(nn,j-6))
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i4=iand(i4,255)
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if(ik.eq.8) then
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im=im+1
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! if(i4.gt.127) i4=i4-256
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i1Msg8BitBytes(im)=i4
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ik=0
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endif
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enddo
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enddo
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ihash=nhash(c_loc(i1Msg8BitBytes),int(9,c_size_t),146)
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ihash=2*iand(ihash,255)
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i1Msg8BitBytes(10)=i1hash(1)
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mbit=0
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do i=1, 10
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i1=i1Msg8BitBytes(i)
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do ibit=1,8
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mbit=mbit+1
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message(mbit)=iand(1,ishft(i1,ibit-8))
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enddo
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enddo
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call ldpc_encode(message,codeword)
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write(*,*) "Eb/N0 ngood nundetected nbadhash"
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do idb = 0, 11
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db=idb/2.0-0.5
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sigma=1/sqrt( 2*rate*(10**(db/10.0)) )
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ngood=0
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nue=0
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nbadhash=0
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do itrial=1, ntrials
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! create a realization of a noisy received word
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do i=1,N
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rxdata(i) = 2.0*(codeword(i)-0.5) + sigma*gran()
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enddo
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! correct signal normalization is important for this decoder.
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rxav=sum(rxdata)/N
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rx2av=sum(rxdata*rxdata)/N
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rxsig=sqrt(rx2av-rxav*rxav)
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rxdata=rxdata/rxsig
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! To match the metric to the channel, s should be set to the noise standard deviation.
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! For now we just set s to the value that optimizes decode probability near threshold.
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! The s parameter can be tuned to trade a few tenth's dB of threshold for an order of
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! magnitude in UER
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do i=1,N
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if( s .le. 0 ) then
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ss=sigma
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else
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ss=s
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endif
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lratio(i)=exp(2.0*rxdata(i)/(ss*ss))
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enddo
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! call interface to Radford Neal implementation of binary belief propagation.
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! max_iterations is max number of belief propagation iterations
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! max_dither is the number of tries - try number 2 and beyond start with dithered likelihood ratios.
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call ldpc_decode(lratio, decoded, max_iterations, niterations, max_dither, ndither)
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! if the decoder finds a valid codeword, niterations will be .ge. 0
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if( niterations .ge. 0 ) then
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nueflag=0
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nhashflag=0
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! the decoder produced a codeword --- compare hash part of message (byte 10) with computed hash
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! first collapse 80 decoded bits to 10 bytes
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! the first 9 bytes are the message, 10'th byte is the hash.
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do ibyte=1,10
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itmp=0
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do ibit=1,8
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itmp=ishft(itmp,1)+iand(1,decoded((ibyte-1)*8+ibit))
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enddo
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i1Dec8BitBytes(ibyte)=itmp
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enddo
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! calculate the hash using the first 9 bytes
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ihashdec=nhash(c_loc(i1Dec8BitBytes),int(9,c_size_t),146)
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ihashdec=2*iand(ihashdec,255)
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! compare calculated hash with received byte 10 - if they agree, keep the message
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i1hashdec=ihashdec
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if( i1hashdec .ne. i1Dec8BitBytes(10) ) then
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nbadhash=nbadhash+1
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nhashflag=1
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endif
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! check the message plus hash against what was sent
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do i=1,K
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if( message(i) .ne. decoded(i) ) then
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nueflag=1
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endif
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enddo
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if( nhashflag .eq. 0 .and. nueflag .eq. 0 ) then
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ngood=ngood+1
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else if( nhashflag .eq. 0 .and. nueflag .eq. 1 ) then
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nue=nue+1;
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endif
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endif
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enddo
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write(*,"(f4.1,1x,i8,1x,i8,1x,i8)") db,ngood,nue,nbadhash
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enddo
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end program ldpcsim
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