WSJT-X/lib/msk144decodeframe.f90

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Fortran
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subroutine msk144decodeframe(c,softbits,msgreceived,nsuccess,recent_calls,nrecent)
! use timer_module, only: timer
use packjt77
parameter (NSPM=864)
character*37 msgreceived
character*12 recent_calls(nrecent)
character*77 c77
complex cb(42)
complex cfac,cca,ccb
complex c(NSPM)
integer*1 decoded77(77),apmask(128),cw(128)
integer s8(8),hardbits(144)
real*8 dt, fs, pi, twopi
real cbi(42),cbq(42)
real pp(12)
real softbits(144)
real llr(128)
logical first
data first/.true./
data s8/0,1,1,1,0,0,1,0/
save first,cb,fs,pi,twopi,dt,s8,pp
if(first) then
! define half-sine pulse and raised-cosine edge window
pi=4d0*datan(1d0)
twopi=8d0*datan(1d0)
fs=12000.0
dt=1.0/fs
do i=1,12
angle=(i-1)*pi/12.0
pp(i)=sin(angle)
enddo
! define the sync word waveforms
s8=2*s8-1
cbq(1:6)=pp(7:12)*s8(1)
cbq(7:18)=pp*s8(3)
cbq(19:30)=pp*s8(5)
cbq(31:42)=pp*s8(7)
cbi(1:12)=pp*s8(2)
cbi(13:24)=pp*s8(4)
cbi(25:36)=pp*s8(6)
cbi(37:42)=pp(1:6)*s8(8)
cb=cmplx(cbi,cbq)
first=.false.
endif
nsuccess=0
msgreceived=' '
! Estimate carrier phase.
cca=sum(c(1:1+41)*conjg(cb))
ccb=sum(c(1+56*6:1+56*6+41)*conjg(cb))
cfac=ccb*conjg(cca)
phase0=atan2(imag(cca+ccb),real(cca+ccb))
! Remove phase error - want constellation rotated so that sample points lie on I/Q axes
cfac=cmplx(cos(phase0),sin(phase0))
c=c*conjg(cfac)
! matched filter -
softbits(1)=sum(imag(c(1:6))*pp(7:12))+sum(imag(c(864-5:864))*pp(1:6))
softbits(2)=sum(real(c(1:12))*pp)
do i=2,72
softbits(2*i-1)=sum(imag(c(1+(i-1)*12-6:1+(i-1)*12+5))*pp)
softbits(2*i)=sum(real(c(7+(i-1)*12-6:7+(i-1)*12+5))*pp)
enddo
! sync word hard error weight is used as a discriminator for
! frames that have reasonable probability of decoding
hardbits=0
do i=1,144
if( softbits(i) .ge. 0.0 ) then
hardbits(i)=1
endif
enddo
nbadsync1=(8-sum( (2*hardbits(1:8)-1)*s8 ) )/2
nbadsync2=(8-sum( (2*hardbits(1+56:8+56)-1)*s8 ) )/2
nbadsync=nbadsync1+nbadsync2
if( nbadsync .gt. 4 ) then
return
endif
! normalize the softsymbols before submitting to decoder
sav=sum(softbits)/144
s2av=sum(softbits*softbits)/144
ssig=sqrt(s2av-sav*sav)
softbits=softbits/ssig
sigma=0.60
llr(1:48)=softbits(9:9+47)
llr(49:128)=softbits(65:65+80-1)
llr=2.0*llr/(sigma*sigma)
max_iterations=10
! call timer('bpdec128_90 ',0)
apmask=0
call bpdecode128_90(llr,apmask,max_iterations,decoded77,cw,nharderror,niterations)
! call timer('bpdec128_90 ',1)
if( nharderror .ge. 0 .and. nharderror .lt. 18 ) then
nsuccess=1
write(c77,'(77i1)') decoded77
call unpack77(c77,msgreceived)
endif
return
end subroutine msk144decodeframe