program msksim ! Simulate characteristics of a potential "MSK10" mode using LDPC (168,84) ! code, OQPDK modulation, and 30 s T/R sequences. ! Reception and Demodulation algorithm: ! 1. Compute coarse spectrum; find fc1 = approx carrier freq ! 2. Mix from fc1 to 0; LPF at +/- 0.75*R ! 3. Square, FFT; find peaks near -R/2 and +R/2 to get fc2 ! 4. Mix from fc2 to 0 ! 5. Fit cb13 (central part of csync) to c -> lag, phase ! 6. Fit complex ploynomial for channel equalization ! 7. Get soft bits from equalized data parameter (KK=84) !Information bits (72 + CRC12) parameter (ND=168) !Data symbols: LDPC (168,84), r=1/2 parameter (NS=65) !Sync symbols (2 x 26 + Barker 13) parameter (NR=3) !Ramp up/down parameter (NN=NR+NS+ND) !Total symbols (236) parameter (NSPS=16) !Samples per MSK symbol (16) parameter (N2=2*NSPS) !Samples per OQPSK symbol (32) parameter (N13=13*N2) !Samples in central sync vector (416) parameter (NZ=NSPS*NN) !Samples in baseband waveform (3760) parameter (NFFT1=4*NSPS,NH1=NFFT1/2) character*8 arg complex cbb(0:NZ-1) !Complex baseband waveform complex csync(0:NZ-1) !Sync symbols only, from cbb complex cb13(0:N13-1) !Barker 13 waveform complex c(0:NZ-1) !Complex waveform complex cs(0:NZ-1) !For computing spectrum complex c2(0:NFFT1-1) !Short spectra complex zz(NS+ND) !Complex symbol values (intermediate) complex z,z0 real s(-NH1+1:NH1) !Coarse spectrum real xnoise(0:NZ-1) !Generated random noise real ynoise(0:NZ-1) !Generated random noise real x(NS),yi(NS),yq(NS) !For complex polyfit real rxdata(ND),llr(ND) !Soft symbols real pp(2*NSPS) !Shaped pulse for OQPSK real a(5) !For twkfreq1 real aa(20),bb(20) !Fitted polyco's integer id(NS+ND) !NRZ values (+/-1) for Sync and Data integer icw(NN) integer*1 msgbits(KK),decoded(KK),apmask(ND),cw(ND) ! integer*1 codeword(ND) 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, & 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, & 1,1,0,1,0,1,1,0,0,0,0,0,1,0,0,0,0,0,1,0,1,0,1,0/ nargs=iargc() if(nargs.ne.6) then print*,'Usage: msksim f0(Hz) delay(ms) fspread(Hz) maxn iters snr(dB)' print*,'Example: msksim 20 0 0 5 10 -20' print*,'Set snr=0 to cycle through a range' go to 999 endif call getarg(1,arg) read(arg,*) f0 !Generated carrier frequency call getarg(2,arg) read(arg,*) delay !Delta_t (ms) for Watterson model call getarg(3,arg) read(arg,*) fspread !Fspread (Hz) for Watterson model call getarg(4,arg) read(arg,*) maxn !Max nterms for polyfit call getarg(5,arg) read(arg,*) iters !Iterations at each SNR call getarg(6,arg) read(arg,*) snrdb !Specified SNR_2500 twopi=8.0*atan(1.0) fs=12000.0/72.0 !Sample rate = 166.6666667 Hz dt=1.0/fs !Sample interval (s) tt=NSPS*dt !Duration of "itone" symbols (s) ts=2*NSPS*dt !Duration of OQPSK symbols (s) baud=1.0/tt !Keying rate for "itone" symbols (baud) txt=NZ*dt !Transmission length (s) bandwidth_ratio=2500.0/(fs/2.0) write(*,1000) f0,delay,fspread,maxn,iters,baud,1.5*baud,txt 1000 format('f0:',f5.1,' Delay:',f4.1,' fSpread:',f5.2,' maxn:',i3, & ' Iters:',i6/'Baud:',f7.3,' BW:',f5.1,' TxT:',f5.1,f5.2/) write(*,1004) 1004 format(/' SNR err ber fer fsigma'/37('-')) do i=1,N2 !Half-sine pulse shape pp(i)=sin(0.5*(i-1)*twopi/(2*NSPS)) enddo call genmskhf(msgbits,id,icw,cbb,csync) !Generate baseband waveform and csync cb13=csync(1680:2095) !Copy the Barker 13 waveform a=0. a(1)=f0 call twkfreq1(cbb,NZ,fs,a,cbb) !Mix to specified frequency isna=-10 isnb=-30 if(snrdb.ne.0.0) then isna=nint(snrdb) isnb=isna endif do isnr=isna,isnb,-1 !Loop over SNR range snrdb=isnr sig=sqrt(bandwidth_ratio) * 10.0**(0.05*snrdb) if(snrdb.gt.90.0) sig=1.0 nhard=0 nhardsync=0 nfe=0 sqf=0. do iter=1,iters !Loop over requested iterations nhard0=0 nhardsync0=0 c=cbb if(delay.ne.0.0 .or. fspread.ne.0.0) call watterson(c,fs,delay,fspread) c=sig*c !Scale to requested SNR if(snrdb.lt.90) then do i=0,NZ-1 !Generate gaussian noise xnoise(i)=gran() ynoise(i)=gran() enddo c=c + cmplx(xnoise,ynoise) !Add AWGN noise endif !----------------------------------------------------------------- fc1 ! First attempt at finding carrier frequency, fc1: low-resolution power spectra nspec=NZ/NFFT1 df1=fs/NFFT1 s=0. do k=1,nspec ia=(k-1)*N2 ib=ia+N2-1 c2(0:N2-1)=c(ia:ib) c2(N2:)=0. call four2a(c2,NFFT1,1,-1,1) do i=0,NFFT1-1 j=i if(j.gt.NH1) j=j-NFFT1 s(j)=s(j) + real(c2(i))**2 + aimag(c2(i))**2 enddo enddo ! call smo121(s,NFFT1) smax=0. ipk=0 fc1=0. ia=nint(40.0/df1) do i=-ia,ia f=i*df1 if(s(i).gt.smax) then smax=s(i) ipk=i fc1=f endif ! write(51,3001) f,s(i),db(s(i)) ! 3001 format(f10.3,e12.3,f10.3) enddo ! The following is for testing SNR calibration: ! sp3n=(s(ipk-1)+s(ipk)+s(ipk+1)) !Sig + 3*noise ! base=(sum(s)-sp3n)/(NFFT1-3.0) !Noise per bin ! psig=sp3n-3*base !Sig only ! pnoise=(2500.0/df1)*base !Noise in 2500 Hz ! xsnrdb=db(psig/pnoise) a(1)=-fc1 a(2:5)=0. call twkfreq1(c,NZ,fs,a,cs) !Mix down by fc1 !----------------------------------------------------------------- fc2 ! Filter, square, then FFT to get refined carrier frequency fc2. call four2a(cs,NZ,1,-1,1) !To freq domain df=fs/NZ ia=nint(0.75*baud/df) cs(ia:NZ-1-ia)=0. !Save only freqs around fc1 call four2a(cs,NZ,1,1,1) !Back to time domain cs=cs/NZ cs=cs*cs !Square the data call four2a(cs,NZ,1,-1,1) !Compute squared spectrum ! Find two peaks separated by baud pmax=0. fc2=0. ic=nint(baud/df) ja=nint(0.5*baud/df) do j=-ja,ja f2=j*df ia=nint((f2-0.5*baud)/df) if(ia.lt.0) ia=ia+NZ ib=nint((f2+0.5*baud)/df) p=real(cs(ia))**2 + aimag(cs(ia))**2 + & real(cs(ib))**2 + aimag(cs(ib))**2 if(p.gt.pmax) then pmax=p fc2=0.5*f2 endif ! write(52,1200) f2,p,db(p) !1200 format(f10.3,2f15.3) enddo sqf=sqf + (fc1+fc2-f0)**2 a(1)=-(fc1+fc2) a(2:5)=0. call twkfreq1(c,NZ,fs,a,c) !Mix c down by fc1+fc2 ! z=sum(c(1680:2095)*cb13)/208.0 !Get phase from Barker 13 vector ! z0=z/abs(z) ! c=c*conjg(z0) !---------------------------------------------------------------- DT amax=0. jpk=0 do j=-20*NSPS,20*NSPS !Get jpk z=sum(c(1680+j:2095+j)*cb13)/208.0 if(abs(z).gt.amax) then amax=abs(z) jpk=j endif ! write(53,1220) j,j*dt,z !1220 format(i6,3f10.4) enddo xdt=jpk/fs !------------------------------------------------------------------ cpolyfit ib=NSPS-1 ib2=N2-1 n=0 do j=1,117 !First-pass demodulation ia=ib+1 ib=ia+N2-1 zz(j)=sum(pp*c(ia:ib))/NSPS if(abs(id(j)).eq.2) then !Save all sync symbols n=n+1 x(n)=float(ia+ib)/NZ - 1.0 yi(n)=real(zz(j))*0.5*id(j) yq(n)=aimag(zz(j))*0.5*id(j) ! write(54,1225) n,x(n),yi(n),yq(n) !1225 format(i5,3f12.4) endif if(j.le.116) then zz(j+117)=sum(pp*c(ia+NSPS:ib+NSPS))/NSPS endif enddo aa=0. bb=0. nterms=0 if(maxn.gt.0) then ! Fit sync info with a complex polynomial npts=n mode=0 chisqa0=1.e30 chisqb0=1.e30 do nterms=1,maxn call polyfit4(x,yi,yi,npts,nterms,mode,aa,chisqa) call polyfit4(x,yq,yq,npts,nterms,mode,bb,chisqb) if(chisqa/chisqa0.ge.0.98 .and. chisqb/chisqb0.ge.0.98) exit chisqa0=chisqa chisqb0=chisqb enddo endif !-------------------------------------------------------------- Soft Symbols n=0 do j=1,117 xx=j*2.0/117.0 - 1.0 yii=1. yqq=0. if(nterms.gt.0) then yii=aa(1) yqq=bb(1) do i=2,nterms yii=yii + aa(i)*xx**(i-1) yqq=yqq + bb(i)*xx**(i-1) enddo endif z0=cmplx(yii,yqq) z=zz(j)*conjg(z0) if(abs(id(j)).eq.2) then if(real(z)*id(j).lt.0) then nhardsync=nhardsync+1 nhardsync0=nhardsync0+1 endif ! write(55,2002) j,id(j)/2,xx,z*id(j)/2 !Sync bit !2002 format(2i5,3f10.3) else p=real(z) !Data bit n=n+1 rxdata(n)=p ierr=0 if(id(j)*p.lt.0) ierr=1 nhard0=nhard0+ierr nhard=nhard+ierr ! write(56,2003) j,id(j),n,ierr,nhard,xx,p*id(j),z !2003 format(5i6,4f10.3) endif enddo do j=118,233 xx=(j-116.5)*2.0/117.0 - 1.0 yii=1. yqq=0. if(nterms.gt.0) then yii=aa(1) yqq=bb(1) do i=2,nterms yii=yii + aa(i)*xx**(i-1) yqq=yqq + bb(i)*xx**(i-1) enddo endif z0=cmplx(yii,yqq) z=zz(j)*conjg(z0) p=aimag(z) n=n+1 rxdata(n)=p ierr=0 if(id(j)*p.lt.0) ierr=1 nhard=nhard+ierr enddo rxav=sum(rxdata)/ND rx2av=sum(rxdata*rxdata)/ND rxsig=sqrt(rx2av-rxav*rxav) rxdata=rxdata/rxsig ss=0.84 llr=2.0*rxdata/(ss*ss) apmask=0 max_iterations=40 call bpdecode168(llr,apmask,max_iterations,decoded,niterations,cw) nbadcrc=0 ifer=0 if(niterations.ge.0) call chkcrc12(decoded,nbadcrc) if(niterations.lt.0 .or. count(msgbits.ne.decoded).gt.0 .or. & nbadcrc.ne.0) ifer=1 nfe=nfe+ifer write(58,1045) snrdb,nhard0,nhardsync0,niterations,nbadcrc,ifer, & nterms,fc1+fc2-f0,xdt if(ifer.eq.1) write(59,1045) snrdb,nhard0,nhardsync0,niterations, & nbadcrc,ifer,nterms,fc1+fc2-f0,xdt 1045 format(f6.1,6i5,2f8.3) enddo fsigma=sqrt(sqf/iters) ber=float(nhard)/((NS+ND)*iters) fer=float(nfe)/iters write(*,1050) snrdb,nhard,ber,fer,fsigma write(60,1050) snrdb,nhard,ber,fer,fsigma 1050 format(f6.1,i7,f8.4,f7.3,f8.2) enddo 999 end program msksim