195 lines
7.4 KiB
Fortran
195 lines
7.4 KiB
Fortran
program msksim
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! Simulate characteristics of a potential "MSK10" mode using LDPC (168,84)
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! code, OQPDK modulation, and 30 s T/R sequences.
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! Reception and Demodulation algorithm:
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! 1. Compute coarse spectrum; find fc1 = approx carrier freq
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! 2. Mix from fc1 to 0; LPF at +/- 0.75*R
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! 3. Square, FFT; find peaks near -R/2 and +R/2 to get fc2
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! 4. Mix from fc2 to 0
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! 5. Fit cb13 (central part of csync) to c -> lag, phase
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! 6. Fit complex ploynomial for channel equalization
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! 7. Get soft bits from equalized data
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parameter (KK=84) !Information bits (72 + CRC12)
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parameter (ND=168) !Data symbols: LDPC (168,84), r=1/2
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parameter (NS=65) !Sync symbols (2 x 26 + Barker 13)
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parameter (NR=3) !Ramp up/down
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parameter (NN=NR+NS+ND) !Total symbols (236)
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parameter (NSPS=1152/72) !Samples per MSK symbol (16)
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parameter (N2=2*NSPS) !Samples per OQPSK symbol (32)
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parameter (N13=13*N2) !Samples in central sync vector (416)
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parameter (NZ=NSPS*NN) !Samples in baseband waveform (3776)
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parameter (NFFT1=4*NSPS,NH1=NFFT1/2)
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character*8 arg
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complex cbb(0:NZ-1) !Complex baseband waveform
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complex csync(0:NZ-1) !Sync symbols only, from cbb
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complex cb13(0:N13-1) !Barker 13 waveform
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complex c(0:NZ-1) !Complex waveform
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complex c0(0:NZ-1) !Complex waveform
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complex zz(NS+ND) !Complex symbol values (intermediate)
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complex z
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real xnoise(0:NZ-1) !Generated random noise
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real ynoise(0:NZ-1) !Generated random noise
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real rxdata(ND),llr(ND) !Soft symbols
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real pp(2*NSPS) !Shaped pulse for OQPSK
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real a(5) !For twkfreq1
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real aa(20),bb(20) !Fitted polyco's
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integer id(NS+ND) !NRZ values (+/-1) for Sync and Data
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integer ierror(NS+ND)
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integer icw(NN)
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integer*1 msgbits(KK),decoded(KK),apmask(ND),cw(ND)
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! integer*1 codeword(ND)
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data msgbits/0,0,1,0,0,1,1,1,1,0,0,1,0,0,0,0,0,0,0,0,1,0,0,0,1,1,0,0,0,1, &
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1,1,1,0,1,1,1,1,1,1,1,0,0,1,0,0,1,1,0,1,0,1,1,1,0,1,1,0,1,1, &
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1,1,0,1,0,1,1,0,0,0,0,0,1,0,0,0,0,0,1,0,1,0,1,0/
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nargs=iargc()
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if(nargs.ne.6) then
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print*,'Usage: mskhfsim f0(Hz) delay(ms) fspread(Hz) maxn iters snr(dB)'
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print*,'Example: mskhfsim 0 0 0 5 10 -20'
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print*,'Set snr=0 to cycle through a range'
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go to 999
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endif
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call getarg(1,arg)
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read(arg,*) f0 !Generated carrier frequency
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call getarg(2,arg)
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read(arg,*) delay !Delta_t (ms) for Watterson model
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call getarg(3,arg)
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read(arg,*) fspread !Fspread (Hz) for Watterson model
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call getarg(4,arg)
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read(arg,*) maxn !Max nterms for polyfit
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call getarg(5,arg)
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read(arg,*) iters !Iterations at each SNR
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call getarg(6,arg)
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read(arg,*) snrdb !Specified SNR_2500
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twopi=8.0*atan(1.0)
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fs=12000.0/72.0 !Sample rate = 166.6666667 Hz
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dt=1.0/fs !Sample interval (s)
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tt=NSPS*dt !Duration of "itone" symbols (s)
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ts=2*NSPS*dt !Duration of OQPSK symbols (s)
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baud=1.0/tt !Keying rate for "itone" symbols (baud)
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txt=NZ*dt !Transmission length (s)
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bandwidth_ratio=2500.0/(fs/2.0)
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write(*,1000) f0,delay,fspread,maxn,iters,baud,3*baud,txt
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1000 format('f0:',f5.1,' Delay:',f4.1,' fSpread:',f5.2,' maxn:',i3, &
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' Iters:',i6/'Baud:',f7.3,' BW:',f5.1,' TxT:',f5.1,f5.2/)
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write(*,1004)
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1004 format(/' SNR err ber fer fsigma'/37('-'))
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do i=1,N2 !Half-sine pulse shape
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pp(i)=sin(0.5*(i-1)*twopi/(2*NSPS))
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enddo
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call genmskhf(msgbits,id,icw,cbb,csync)!Generate baseband waveform and csync
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cb13=csync(1680:2095) !Copy the Barker 13 waveform
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a=0.
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a(1)=f0
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call twkfreq1(cbb,NZ,fs,a,cbb) !Mix to specified frequency
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isna=-10
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isnb=-30
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if(snrdb.ne.0.0) then
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isna=nint(snrdb)
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isnb=isna
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endif
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do isnr=isna,isnb,-1 !Loop over SNR range
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snrdb=isnr
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sig=sqrt(bandwidth_ratio) * 10.0**(0.05*snrdb)
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if(snrdb.gt.90.0) sig=1.0
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nhard=0
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nhardsync=0
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nfe=0
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sqf=0.
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do iter=1,iters !Loop over requested iterations
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c=cbb
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if(delay.ne.0.0 .or. fspread.ne.0.0) then
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call watterson(c,NZ,fs,delay,fspread)
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endif
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c=sig*c !Scale to requested SNR
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if(snrdb.lt.90) then
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do i=0,NZ-1 !Generate gaussian noise
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xnoise(i)=gran()
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ynoise(i)=gran()
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enddo
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c=c + cmplx(xnoise,ynoise) !Add AWGN noise
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endif
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call getfc1(c,fc1) !First approx for freq
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call getfc2(c,csync,fc1,fc2,fc3) !Refined freq
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sqf=sqf + (fc1+fc2-f0)**2
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!NB: Measured performance is about equally good using fc2 or fc3 here:
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a(1)=-(fc1+fc2)
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a(2:5)=0.
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call twkfreq1(c,NZ,fs,a,c) !Mix c down by fc1+fc2
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! The following may not be necessary?
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! z=sum(c(1680:2095)*cb13)/208.0 !Get phase from Barker 13 vector
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! z0=z/abs(z)
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! c=c*conjg(z0)
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!---------------------------------------------------------------- DT
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! Not presently used:
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amax=0.
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jpk=0
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do j=-20*NSPS,20*NSPS !Get jpk
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z=sum(c(1680+j:2095+j)*cb13)/208.0
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if(abs(z).gt.amax) then
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amax=abs(z)
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jpk=j
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endif
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enddo
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xdt=jpk/fs
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nterms=maxn
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c0=c
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do itry=1,10
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idf=itry/2
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if(mod(itry,2).eq.0) idf=-idf
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nhard0=0
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nhardsync0=0
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ifer=1
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a(1)=idf*0.01
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a(2:5)=0.
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call twkfreq1(c0,NZ,fs,a,c) !Mix c0 into c
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call cpolyfit(c,pp,id,maxn,aa,bb,zz,nhs)
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call msksoftsym(zz,aa,bb,id,nterms,ierror,rxdata,nhard0,nhardsync0)
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if(nhardsync0.gt.12) cycle
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rxav=sum(rxdata)/ND
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rx2av=sum(rxdata*rxdata)/ND
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rxsig=sqrt(rx2av-rxav*rxav)
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rxdata=rxdata/rxsig
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ss=0.84
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llr=2.0*rxdata/(ss*ss)
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apmask=0
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max_iterations=40
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ifer=0
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call bpdecode168(llr,apmask,max_iterations,decoded,niterations,cw)
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nbadcrc=0
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if(niterations.ge.0) call chkcrc12(decoded,nbadcrc)
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if(niterations.lt.0 .or. count(msgbits.ne.decoded).gt.0 .or. &
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nbadcrc.ne.0) ifer=1
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! if(ifer.eq.0) write(67,1301) snrdb,itry,idf,niterations, &
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! nhardsync0,nhard0
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!1301 format(f6.1,5i6)
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if(ifer.eq.0) exit
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enddo !Freq dither loop
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nhard=nhard+nhard0
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nhardsync=nharsdync+nhardsync0
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nfe=nfe+ifer
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enddo
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fsigma=sqrt(sqf/iters)
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ber=float(nhard)/((NS+ND)*iters)
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fer=float(nfe)/iters
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write(*,1050) snrdb,nhard,ber,fer,fsigma
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! write(60,1050) snrdb,nhard,ber,fer,fsigma
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1050 format(f6.1,i7,f8.4,f7.3,f8.2)
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enddo
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999 end program msksim
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