first pass at a Octave HF channelsimulator

git-svn-id: https://svn.code.sf.net/p/freetel/code@1484 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/hf_sim.m b/codec2-dev/octave/hf_sim.m
new file mode 100644 (file)
index 0000000..9045828
--- /dev/null
+++ b/codec2-dev/octave/hf_sim.m
@@ -0,0 +1,74 @@
+% hf_sim.m
+% David Rowe March 2014
+%
+% HF channel simulation.
+
+function sim_out = hf_sim(sim_in, snr3kHz)
+
+    % Init HF channel model from stored sample files of spreading signal ----------------------------------
+
+    % convert "spreading" samples from 1kHz carrier at Fs to complex
+    % baseband, generated by passing a 1kHz sine wave through PathSim
+    % with the ccir-poor model, enabling one path at a time.
+    
+    Fc = 1000; Fs=8000;
+    fspread = fopen("../raw/sine1k_2Hz_spread.raw","rb");
+    spread1k = fread(fspread, "int16")/10000;
+    fclose(fspread);
+    fspread = fopen("../raw/sine1k_2ms_delay_2Hz_spread.raw","rb");
+    spread1k_2ms = fread(fspread, "int16")/10000;
+    fclose(fspread);
+
+    % down convert to complex baseband
+    spreadbb = spread1k.*exp(-j*(2*pi*Fc/Fs)*(1:length(spread1k))');
+    spreadbb_2ms = spread1k_2ms.*exp(-j*(2*pi*Fc/Fs)*(1:length(spread1k_2ms))');
+
+    % remove -2000 Hz image
+    b = fir1(50, 5/Fs);
+    spread = filter(b,1,spreadbb);
+    spread_2ms = filter(b,1,spreadbb_2ms);
+
+    % discard first 1000 samples as these were near 0, probably as
+    % PathSim states were ramping up
+
+    spread    = spread(1000:length(spread));
+    spread_2ms = spread_2ms(1000:length(spread_2ms));
+
+    hf_gain = 1.0/sqrt(var(spread)+var(spread_2ms));
+
+    % 300 - 3000 Hz filter
+
+    b = fir1(100,[300/4000, 3000/4000], 'pass');
+    
+    % det power of unit variance noise passed through this filter
+    
+    filter_var = var(filter(b,1,randn(1000,1)));
+    
+    % Start simulation
+
+    s = hilbert(filter(b,1,sim_in));
+    n1 = length(s); n2 = length(spread);
+    n = min(n1,n2);
+    path1 = s(1:n) .* spread(1:n);
+    path2 = s(1:n) .* spread_2ms(1:n);
+    delay = floor(0.002*Fs);
+
+    combined = path1(delay+1:n) + path2(1:n-delay);
+
+    snr = 10 .^ (snr3kHz/10);
+    variance = (combined'*combined)/(snr*n);
+    noise = sqrt(variance*0.5/filter_var)*(randn(n-delay,1) + j*randn(n-delay,1));
+    filtered_noise = filter(b,1,noise);
+ 
+    sim_out = real(combined+filtered_noise);
+    printf("measured SNR: %3.2fdB\n", 10*log10(var(real(combined))/var(real(filtered_noise))));
+
+    figure(1);
+    plot(s);
+    figure(2)
+    plot(real(combined))
+    figure(2)
+    plot(sim_out)
+
+endfunction
+