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
This commit is contained in:
Steven Franke 2016-04-24 01:12:38 +00:00
parent 8cf1b33b59
commit 9d72f280ad

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