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Test program to simulate MSK modulation and coherent reception.

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git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6478 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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Joe Taylor 2016-02-22 18:32:39 +00:00
parent 2994a924e5
commit b88affa2ee
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program mskber
! Generate an MSK waveform, pass it through an AWGN channel, apply coherent
! MSK receiver, and count number of errors at each Eb/No.
parameter (NSYM=100000) !Number of symbols to test
parameter (NSPS=6) !Samples per symbol
real ct(-NSPS:NSPS*NSYM-1) !cos(pi*t/2T)
real st(-NSPS:NSPS*NSYM-1) !sin(pi*t/2T)
real r(NSYM) !Random numbers to determine test bits
real ai(0:NSPS*(NSYM+1)-1) !Rectangular pulses for even symbols
real aq(0:NSPS*(NSYM+1)-1) !Rectangular pulses for odd symbols
real xe(0:NSPS*(NSYM+3)-1) !Temp array for received even symbols
real xo(0:NSPS*(NSYM+3)-1) !Temp array for received odd symbols
real xsym(0:NSYM-1) !Soft Rx symbols
complex xt(0:NSPS*(NSYM+1)-1) !Complex baseband Tx waveform
complex nt(0:NSPS*(NSYM+1)-1) !Generated AWGN channel noise
complex yt(0:NSPS*(NSYM+1)-1) !Received signal, yt = xt + fac*nt
integer sym0(0:NSYM-1) !Generated test bits
integer sym(0:NSYM-1) !Hard-copy received bits
pi=4.0*atan(1.0)
iz=NSPS*(NSYM+1)
do i=-NSPS,NSPS*NSYM-1 !Define ct, st arrays
t=i*pi/(2.0*NSPS)
ct(i)=cos(t)
st(i)=sin(t)
enddo
fac=1.0/sqrt(float(NSPS))
do iEbNo=0,10 !Loop over a range of Eb/No
sym0=0
call random_number(r)
where(r.gt.0.5) sym0=1 !Generate random data bits
call mskmod(sym0,NSYM,NSPS,ct,st,xt) !Generate Tx waveform at baseband
! NB: In WSJT-X, will mix xt upward from 0 to 1500 Hz.
do i=0,iz-1 !Generate Gaussian noise
xx=0.707*gran()
yy=0.707*gran()
nt(i)=cmplx(xx,yy)
enddo
fac_noise=10.0**(-iEbNo/20.0)
yt=xt + fac_noise*nt !Rx signal, with noise
call mskdemod(yt,NSYM,NSPS,ct,st,xsym) !MSK demodulator
sym=0
where(xsym.gt.0.0) sym=1
! Count the hard errors
nerr=count(sym(0:NSYM-1).ne.sym0(0:NSYM-1))
thber=0.5*erfc(10.0**(iEbNo/20.0))
write(*,1000) iEbNo,thber,float(nerr)/NSYM
1000 format(i3,2f10.6)
enddo
end program mskber
subroutine mskmod(sym,nsym,nsps,ct,st,xt)
! Generate MSK Tx waveform at baseband.
integer sym(0:nsym-1) !Hard-copy received bits
complex xt(0:nsps*(nsym+1)-1) !Complex baseband Tx waveform
real ct(-nsps:nsps*nsym-1) !cos(pi*t/2T)
real st(-nsps:nsps*nsym-1) !sin(pi*t/2T)
real ai(0:nsps*(nsym+1)-1) !Rectangular pulses for even symbols
real aq(0:nsps*(nsym+1)-1) !Rectangular pulses for odd symbols
fac=1.0/sqrt(float(nsps))
do j=0,nsym-1,2
ia=j*nsps
ib=ia+2*nsps-1
ai(ia:ib)=2*sym(j)-1 !Even bits as rectangular pulses
aq(ia+nsps:ib+nsps)=2*sym(j+1)-1 !Odd bits as rectangular pulses
enddo
ai(ib+1:)=0 !Pad ai with zeros at end
aq(0:nsps-1)=0 !Pad aq with zeros at start
xt=fac*cmplx(ai*ct,aq*st) !Baseband Tx waveform
return
end subroutine mskmod
subroutine mskdemod(yt,nsym,nsps,ct,st,xsym)
! MSK demodulator
! Rx phase must be known and stable; symbol sync must be established.
complex yt(0:nsps*(nsym+1)-1) !Received signal
real ct(-nsps:nsps*nsym-1) !cos(pi*t/2T)
real st(-nsps:nsps*nsym-1) !sin(pi*t/2T)
real xe(0:nsps*(nsym+3)-1) !Temp array for received even symbols
real xo(0:nsps*(nsym+3)-1) !Temp array for received odd symbols
real xsym(0:nsym-1) !Soft Rx symbols
iz=nsps*nsym
xe(0:iz-1)=real(yt)*ct
xo(0:iz-1)=aimag(yt)*st
do j=0,nsym-1,2
ia=j*nsps
ib=ia+2*nsps-1
xsym(j)=sum(xe(ia:ib)) !Integrate over 2 successive symbols
xsym(j+1)=sum(xo(ia+6:ib+6))
enddo
return
end subroutine mskdemod