mirror of https://github.com/saitohirga/WSJT-X.git
113 lines
4.0 KiB
Fortran
113 lines
4.0 KiB
Fortran
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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 (NSYM=100000) !Number of symbols to test
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parameter (NSPS=6) !Samples per symbol
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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 r(NSYM) !Random numbers to determine test bits
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real ai(0:NSPS*(NSYM+1)-1) !Rectangular pulses for even symbols
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real aq(0:NSPS*(NSYM+1)-1) !Rectangular pulses for odd symbols
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real xe(0:NSPS*(NSYM+3)-1) !Temp array for received even symbols
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real xo(0:NSPS*(NSYM+3)-1) !Temp array for received odd symbols
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real xsym(0:NSYM-1) !Soft Rx symbols
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complex xt(0:NSPS*(NSYM+1)-1) !Complex baseband Tx waveform
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complex nt(0:NSPS*(NSYM+1)-1) !Generated AWGN channel noise
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complex yt(0:NSPS*(NSYM+1)-1) !Received signal, yt = xt + fac*nt
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integer sym0(0:NSYM-1) !Generated test bits
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integer sym(0:NSYM-1) !Hard-copy received bits
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pi=4.0*atan(1.0)
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iz=NSPS*(NSYM+1)
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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.gt.0.5) sym0=1 !Generate random data bits
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call mskmod(sym0,NSYM,NSPS,ct,st,xt) !Generate Tx waveform at baseband
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! NB: In WSJT-X, will mix xt upward from 0 to 1500 Hz.
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do i=0,iz-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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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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! 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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write(*,1000) iEbNo,thber,float(nerr)/NSYM
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1000 format(i3,2f10.6)
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enddo
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end program mskber
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subroutine mskmod(sym,nsym,nsps,ct,st,xt)
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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(0:nsps*(nsym+1)-1) !Complex baseband Tx waveform
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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(0:nsps*(nsym+1)-1) !Rectangular pulses for even symbols
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real aq(0:nsps*(nsym+1)-1) !Rectangular pulses for odd symbols
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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*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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ai(ib+1:)=0 !Pad ai with zeros at end
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aq(0:nsps-1)=0 !Pad aq with zeros at start
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xt=fac*cmplx(ai*ct,aq*st) !Baseband Tx waveform
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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(0:nsps*(nsym+1)-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(0:nsps*(nsym+3)-1) !Temp array for received even symbols
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real xo(0:nsps*(nsym+3)-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
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xe(0:iz-1)=real(yt)*ct
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xo(0:iz-1)=aimag(yt)*st
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do j=0,nsym-1,2
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ia=j*nsps
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ib=ia+2*nsps-1
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xsym(j)=sum(xe(ia:ib)) !Integrate over 2 successive symbols
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xsym(j+1)=sum(xo(ia+6:ib+6))
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enddo
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return
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end subroutine mskdemod
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