108 lines
2.7 KiB
Plaintext
108 lines
2.7 KiB
Plaintext
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subroutine msk144decodeframe(c,softbits,msgreceived,nsuccess,recent_calls,nrecent)
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! use timer_module, only: timer
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parameter (NSPM=864)
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character*22 msgreceived
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character*12 recent_calls(nrecent)
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complex cb(42)
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complex cfac,cca,ccb
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complex c(NSPM)
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integer*1 decoded(80)
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integer s8(8),hardbits(144)
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real*8 dt, fs, pi, twopi
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real cbi(42),cbq(42)
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real pp(12)
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real softbits(144)
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real llr(128)
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logical first
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data first/.true./
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data s8/0,1,1,1,0,0,1,0/
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save first,cb,fs,pi,twopi,dt,s8,pp
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if(first) then
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! define half-sine pulse and raised-cosine edge window
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pi=4d0*datan(1d0)
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twopi=8d0*datan(1d0)
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fs=12000.0
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dt=1.0/fs
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do i=1,12
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angle=(i-1)*pi/12.0
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pp(i)=sin(angle)
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enddo
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! define the sync word waveforms
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s8=2*s8-1
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cbq(1:6)=pp(7:12)*s8(1)
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cbq(7:18)=pp*s8(3)
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cbq(19:30)=pp*s8(5)
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cbq(31:42)=pp*s8(7)
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cbi(1:12)=pp*s8(2)
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cbi(13:24)=pp*s8(4)
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cbi(25:36)=pp*s8(6)
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cbi(37:42)=pp(1:6)*s8(8)
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cb=cmplx(cbi,cbq)
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first=.false.
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endif
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nsuccess=0
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msgreceived=' '
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! Estimate carrier phase.
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cca=sum(c(1:1+41)*conjg(cb))
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ccb=sum(c(1+56*6:1+56*6+41)*conjg(cb))
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cfac=ccb*conjg(cca)
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phase0=atan2(imag(cca+ccb),real(cca+ccb))
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! Remove phase error - want constellation rotated so that sample points lie on I/Q axes
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cfac=cmplx(cos(phase0),sin(phase0))
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c=c*conjg(cfac)
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! matched filter -
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softbits(1)=sum(imag(c(1:6))*pp(7:12))+sum(imag(c(864-5:864))*pp(1:6))
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softbits(2)=sum(real(c(1:12))*pp)
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do i=2,72
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softbits(2*i-1)=sum(imag(c(1+(i-1)*12-6:1+(i-1)*12+5))*pp)
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softbits(2*i)=sum(real(c(7+(i-1)*12-6:7+(i-1)*12+5))*pp)
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enddo
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! sync word hard error weight is used as a discriminator for
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! frames that have reasonable probability of decoding
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hardbits=0
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do i=1,144
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if( softbits(i) .ge. 0.0 ) then
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hardbits(i)=1
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endif
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enddo
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nbadsync1=(8-sum( (2*hardbits(1:8)-1)*s8 ) )/2
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nbadsync2=(8-sum( (2*hardbits(1+56:8+56)-1)*s8 ) )/2
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nbadsync=nbadsync1+nbadsync2
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if( nbadsync .gt. 4 ) then
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return
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endif
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! normalize the softsymbols before submitting to decoder
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sav=sum(softbits)/144
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s2av=sum(softbits*softbits)/144
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ssig=sqrt(s2av-sav*sav)
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softbits=softbits/ssig
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sigma=0.75
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llr(1:48)=softbits(9:9+47)
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llr(49:128)=softbits(65:65+80-1)
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llr=2.0*llr/(sigma*sigma)
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max_iterations=10
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! call timer('bpdec144 ',0)
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call bpdecode144(llr,max_iterations,decoded,niterations)
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! call timer('bpdec144 ',1)
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if( niterations .ge. 0.0 ) then
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call extractmessage144(decoded,msgreceived,nhashflag,recent_calls,nrecent)
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if( nhashflag .gt. 0 ) then ! CRCs match, so print it
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nsuccess=1
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endif
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endif
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return
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end subroutine msk144decodeframe
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