building up DF simulation

git-svn-id: https://svn.code.sf.net/p/freetel/code@2479 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/df_mixer.m b/codec2-dev/octave/df_mixer.m
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+% df_mixer.m
+%
+% David Rowe October 2015
+%
+% Experimental direction fing using a mixer to FDM signals from two
+% antennas.  This simulation tests estimation of phase and freq of 
+% mixer LO.
+%
+% [ ] any block size, dft size, Fs
+% [ ] stats for repeated tests
+% [ ] add modulation
+
+randn('state',1);
+
+f     = 48E3;   % signal frequency
+flo   = 32E3;   % LO frequency
+m     = 0.1;    % modulation index, how far each sideband is down wrt carrier (0.1 == 20dB)
+phi   = pi/4;   % LO phase
+Fs    = 96E3;   % sample rate
+CNodB = 40;     % C/No of carrier, which dominates power
+                % Around C/No = 50dB is min for FM (12dB-ish SINAD)
+N     = Fs;     % processing block size
+
+% BW is Fs Hz.  No=(total noise power Nt)/Fs. N=noise power=variance of noise source
+% C = carrier power = 1
+% CNo = C/No = 1/(Nt/Fs), therefore var = Nt = Fs/CNo
+
+CNo = 10^(CNodB/10);
+var = Fs/CNo;
+
+t=0:N-1;
+
+% tx signal
+
+tx = exp(j*t*2*pi*f/Fs); 
+
+% simulate rx signals at antenna 1 and 2 by adding noise.  Note signal
+% at antenna 2 is phase shifted
+
+ant1 = tx + sqrt(var/2)*randn(1,N) + j*sqrt(var/2)*randn(1,N);
+ant2 = tx*exp(j*phi) + sqrt(var/2)*randn(1,N) + j*sqrt(var/2)*randn(1,N);
+
+% ant2 passed through mixer, that attenuates signal quite a bit and
+% has poor (0dB) carrier supression at UHF
+
+ant2_mix = m*(ant2 .* (1 + 2*cos(t*2*pi*flo/Fs + phi)));
+
+% sum the two signals, to get the overal signal that the SDR samples
+
+rx = ant1 + ant2_mix;
+
+% Turns out a carrier with two sidebands looks at lot like AM.  That
+% was a lucky break! So lets simply envelope detect it.
+
+d = abs(rx);
+D = (1/N)*fft(d);
+
+figure(1);
+clf;
+plot((1:N)*Fs/N, 20*log10(abs((1/N)*fft(rx))));
+axis([1 Fs -60 0])
+title('Rx signal at SDR input');
+
+figure(2);
+clf;
+subplot(211)
+plot((1:N)*Fs/N, 20*log10(abs(D)))
+axis([1 Fs -60 0])
+title('AM demodulated');
+subplot(212)
+plot(d(1:10*Fs/flo))
+
+
+% search for maxima due to LO/sidebands.  We need a search range to
+% avoid big DC component
+
+st = round(0.9*N*flo/Fs);
+en = round(1.1*N*flo/Fs);
+[mx mx_ind] = max(abs(D(st:en)));
+phi_est = angle(D(st-1+mx_ind));
+st
+en
+mx_ind
+mx
+abs(D(st-1+mx_ind))
+D(st-1+mx_ind-1:st-1+mx_ind+1)
+printf("N: %d C/No (dB): %2.0f dB C/No (lin): %3.1f var: %f\n", N, CNodB, CNo, var);
+printf("phi: %4.3f phi: %4.3f\n", phi, phi_est);
+
+