WSJT-X/lib/fst4/ldpcsim240_74.f90
Steven Franke 11b7aa9429
Implements decoding of FST4W messages as (240,50) crc-less codewords.
By cascading the full 24-bit crc generator matrix with the (240,74) LDPC code generator, create a (240,50) generator that is used to decode with approximately 1 dB better sensitivity than the (240,64) with 14-bit CRC approach that is normally used. This approach treats the CRC bits as additional parity bits and provides no means for identifying incorrect codewords. All codewords on the list generated by the OSD algorithm have CRCs that match the CRC of the message payload. Codewords are validated by unpacking the message and comparing the unpacked message with the list of stored callsign/grid pairs stored in the fst4w_calls.txt file.
2021-01-04 14:59:56 +00:00

126 lines
3.5 KiB
Fortran

program ldpcsim240_74
! End-to-end test of the (240,74)/crc24 encoder and decoders.
use packjt77
parameter(N=240, K=74, M=N-K)
character*8 arg
character*37 msg0,msg
character*77 c77
character*24 c24
integer*1 msgbits(74)
integer*1 apmask(240)
integer*1 cw(240)
integer*1 codeword(N),message74(74)
integer ncrc24
real rxdata(N),llr(N)
logical first,unpk77_success
data first/.true./
nargs=iargc()
if(nargs.ne.5 .and. nargs.ne.6) then
print*,'Usage: ldpcsim niter ndeep #trials s K [msg]'
print*,'e.g. ldpcsim240_74 20 5 1000 0.85 64 "K9AN K1JT FN20"'
print*,'s : if negative, then value is ignored and sigma is calculated from SNR.'
print*,'niter: is the number of BP iterations.'
print*,'ndeep: -1 is BP only, ndeep>=0 is OSD order'
print*,'K :is the number of message+CRC bits and must be in the range [50,74]'
print*,'WSPR-format message is optional'
return
endif
call getarg(1,arg)
read(arg,*) max_iterations
call getarg(2,arg)
read(arg,*) norder
call getarg(3,arg)
read(arg,*) ntrials
call getarg(4,arg)
read(arg,*) s
call getarg(5,arg)
read(arg,*) Keff
msg0='K9AN K1JT FN20 '
if(nargs.eq.6) call getarg(6,msg0)
call pack77(msg0,i3,n3,c77)
rate=real(Keff)/real(N)
write(*,*) "code rate: ",rate
write(*,*) "niter : ",max_iterations
write(*,*) "norder : ",norder
write(*,*) "s : ",s
write(*,*) "K : ",Keff
msgbits=0
read(c77,'(50i1)') msgbits(1:50)
write(*,*) 'message'
write(*,'(50i1)') msgbits(1:50)
call get_crc24(msgbits,74,ncrc24)
write(c24,'(b24.24)') ncrc24
read(c24,'(24i1)') msgbits(51:74)
write(*,'(24i1)') msgbits(51:74)
write(*,*) 'message with crc24'
write(*,'(74i1)') msgbits(1:74)
call encode240_74(msgbits,codeword)
call init_random_seed()
call sgran()
write(*,*) 'codeword'
write(*,'(77i1,1x,24i1,1x,73i1)') codeword
write(*,*) "Eb/N0 Es/N0 ngood nundetected sigma symbol error rate"
do idb = 8,-3,-1
db=idb/2.0-1.0
sigma=1/sqrt( 2*rate*(10**(db/10.0)) ) ! to make db represent Eb/No
! sigma=1/sqrt( 2*(10**(db/10.0)) ) ! db represents Es/No
ngood=0
nue=0
nberr=0
do itrial=1, ntrials
! Create a realization of a noisy received word
do i=1,N
rxdata(i) = 2.0*codeword(i)-1.0 + sigma*gran()
enddo
nerr=0
do i=1,N
if( rxdata(i)*(2*codeword(i)-1.0) .lt. 0 ) nerr=nerr+1
enddo
nberr=nberr+nerr
rxav=sum(rxdata)/N
rx2av=sum(rxdata*rxdata)/N
rxsig=sqrt(rx2av-rxav*rxav)
rxdata=rxdata/rxsig
if( s .lt. 0 ) then
ss=sigma
else
ss=s
endif
llr=2.0*rxdata/(ss*ss)
apmask=0
dmin=0.0
maxosd=2
call decode240_74(llr, Keff, maxosd, norder, apmask, message74, cw, ntype, nharderror, dmin)
if(nharderror.ge.0) then
n2err=0
do i=1,N
if( cw(i).ne.codeword(i) ) n2err=n2err+1
enddo
if(n2err.eq.0) then
ngood=ngood+1
else
nue=nue+1
endif
endif
enddo
! snr2500=db+10*log10(200.0/116.0/2500.0)
esn0=db+10*log10(rate)
pberr=real(nberr)/(real(ntrials*N))
write(*,"(f4.1,4x,f5.1,1x,i8,1x,i8,8x,e10.3)") db,esn0,ngood,nue,pberr
enddo
end program ldpcsim240_74