% hf_sim.m
% David Rowe March 2014
%
% Two path CCIR poor HF channel simulation, with apaologies to PathSim

% Init HF channel model from stored sample files of spreading signal ----------------------------------

global spread;
global spread_2ms;
global hf_gain;

% 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.  Because I'm too
% lazy to generate my own spreading signals
    
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));

% This function simulates the HF channel at 8kHz for real signals.  A
% good use case is passing a vector of speech samples through it to
% simulate SSB over HF.  There's a really good reason for the 300 -
% 3000 Hz filter that escapes me right now :-)

function [sim_out snr3kHz_measured ] = hf_sim_real(sim_in, snr3kHz)

    % 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);
    snr3kHz_measured = 10*log10(var(real(combined))/var(real(filtered_noise)));
endfunction