147 lines
4.6 KiB
Plaintext
147 lines
4.6 KiB
Plaintext
program mskber
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! Generate an MSK waveform, pass it through an AWGN channel, apply coherent
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! MSK receiver, and count number of errors at each Eb/No.
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parameter (MAXSYM=1000*1000)
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parameter (NSPS=5) !Samples per symbol
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real ct(-NSPS:NSPS*MAXSYM-1) !cos(pi*t/2T)
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real st(-NSPS:NSPS*MAXSYM-1) !sin(pi*t/2T)
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real r(0:MAXSYM-1) !Random numbers to set test bits
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real xsym(0:MAXSYM-1) !Soft Rx symbols
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complex xt(-NSPS:NSPS*MAXSYM-1) !Complex baseband Tx waveform
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complex nt(-NSPS:NSPS*MAXSYM-1) !Generated AWGN channel noise
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complex yt(-NSPS:NSPS*MAXSYM-1) !Received signal, yt = xt + fac*nt
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complex cwave(-NSPS:NSPS*MAXSYM-1) !Audio waveform, Tx real part
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complex z
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integer sym0(0:MAXSYM-1) !Generated test bits
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integer sym(0:MAXSYM-1) !Hard-copy received bits
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integer sym1(0:7)
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character*12 arg
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data sym1/1,1,0,0,0,1,1,1/
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nargs=iargc()
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if(nargs.ne.2) then
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print*,'Usage: mskber nsym EbNo'
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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,*) nsym
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call getarg(2,arg)
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read(arg,*) EbNo
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pi=4.0*atan(1.0)
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do i=-NSPS,NSPS*nsym-1 !Define ct, st arrays
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t=i*pi/(2.0*NSPS)
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ct(i)=cos(t)
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st(i)=sin(t)
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enddo
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fac=1.0/sqrt(float(NSPS))
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do iEbNo=0,10 !Loop over a range of Eb/No
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sym0=0
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call random_number(r)
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where(r(0:nsym-1).gt.0.5) sym0(0:nsym-1)=1 !Generate random data bits
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if(nsym.eq.8) sym0(0:nsym-1)=sym1
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call mskmod(sym0,nsym,NSPS,ct,st,xt,cwave) !Generate Tx waveform
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do i=-NSPS,NSPS*nsym-1 !Generate Gaussian noise
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xx=0.707*gran()
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yy=0.707*gran()
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nt(i)=cmplx(xx,yy)
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enddo
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fac_noise=10.0**(-iEbNo/20.0)
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if(EbNo.ne.0.0) fac_noise=10.0**(-EbNo/20.0)
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yt=xt + fac_noise*nt !Rx signal, with noise
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call mskdemod(yt,nsym,NSPS,ct,st,xsym) !MSK demodulator
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sym=0
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where(xsym.gt.0.0) sym=1
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if(nsym.le.160 .and. EbNo.ne.0.0) then
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write(*,1012) sym0(0:nsym-1)
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if(nsym.gt.50) write(*,1012)
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write(*,1012) sym(0:nsym-1)
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1012 format(50i1)
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do i=-nsps,nsps*nsym-1
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phi=i*2.0*pi*1500/12000.0
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z=cwave(i)*cmplx(cos(phi),sin(phi)) !Mix back to baseband
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write(51,1014) float(i)/nsps,xt(i),abs(xt(i)),cwave(i),z
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1014 format(8f8.4)
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enddo
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endif
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! Count the hard errors
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nerr=count(sym(0:nsym-1).ne.sym0(0:nsym-1))
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thber=0.5*erfc(10.0**(iEbNo/20.0))
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xEbNo=iEbNo
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if(EbNo.ne.0.0) xEbNo=EbNo
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write(*,1000) xEbNo,thber,float(nerr)/nsym
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1000 format(f6.1,2f10.6)
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if(EbNo.ne.0.0) exit
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enddo
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999 end program mskber
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subroutine mskmod(sym,nsym,nsps,ct,st,xt,cwave)
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! Generate MSK Tx waveform at baseband.
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integer sym(0:nsym-1) !Hard-copy received bits
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complex xt(-nsps:nsps*nsym-1) !Complex baseband Tx waveform
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complex cwave(-nsps:nsps*nsym-1) !Audio waveform, fc=1500 Hz.
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real ct(-nsps:nsps*nsym-1) !cos(pi*t/2T)
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real st(-nsps:nsps*nsym-1) !sin(pi*t/2T)
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real ai(-nsps:nsps*nsym-1) !Rectangular pulses for even symbols
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real aq(-nsps:nsps*nsym-1) !Rectangular pulses for odd symbols
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ai=0.
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aq=0.
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fac=1.0/sqrt(float(nsps))
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do j=0,nsym-1,2
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ia=(j-1)*nsps
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ib=ia+2*nsps-1
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ai(ia:ib)=2*sym(j)-1 !Even bits as rectangular pulses
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aq(ia+nsps:ib+nsps)=2*sym(j+1)-1 !Odd bits as rectangular pulses
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enddo
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xt=fac*cmplx(ai*ct,aq*st) !Baseband Tx waveform
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twopi=8.0*atan(1.0)
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do i=-nsps,nsps*nsym-1
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phi=i*twopi*1500/12000.0
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cwave(i)=xt(i)*cmplx(cos(phi),-sin(phi))
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enddo
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return
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end subroutine mskmod
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subroutine mskdemod(yt,nsym,nsps,ct,st,xsym)
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! MSK demodulator
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! Rx phase must be known and stable; symbol sync must be established.
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complex yt(-nsps:nsps*nsym-1) !Received signal
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real ct(-nsps:nsps*nsym-1) !cos(pi*t/2T)
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real st(-nsps:nsps*nsym-1) !sin(pi*t/2T)
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real xe(-nsps:nsps*nsym-1) !Temp array for received even symbols
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real xo(-nsps:nsps*nsym-1) !Temp array for received odd symbols
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real xsym(0:nsym-1) !Soft Rx symbols
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iz=nsps*(nsym+1)
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xe(-nsps:nsps*nsym-1)=real(yt)*ct !Apply matched filters
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xo(-nsps:nsps*nsym-1)=aimag(yt)*st
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do j=0,nsym-1,2
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ia=(j-1)*nsps
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ib=ia+2*nsps-1
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xsym(j)=sum(xe(ia:ib)) !Integrate over 2 symbol lengths
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xsym(j+1)=sum(xo(ia+nsps:ib+nsps))
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enddo
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return
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end subroutine mskdemod
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