shallow zip-file copy from codec2 e9d726bf20

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diff --git a/octave/hf_modem_curves.m b/octave/hf_modem_curves.m
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+++ b/octave/hf_modem_curves.m
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+% hf_modem_curves
+% David Rowe Feb 2017
+%
+% Ideal implementations of a bunch of different HF modems, used to
+% generate plots for a blog post.
+
+#{
+  [X] ideal AWGN/HF curves
+  [X] exp AWGN QPSK curves
+  [X] exp AWGN DQPSK curves
+  [X] exp HF channel model
+  [ ] diversity
+  [ ] COHPSK frames
+      + would require multiple carriers
+      + filtering or OFDM
+#}
+
+1;
+
+% Gray coded QPSK modulation function
+
+function symbol = qpsk_mod(two_bits)
+    two_bits_decimal = sum(two_bits .* [2 1]); 
+    switch(two_bits_decimal)
+        case (0) symbol =  1;
+        case (1) symbol =  j;
+        case (2) symbol = -j;
+        case (3) symbol = -1;
+    endswitch
+endfunction
+
+
+% Gray coded QPSK demodulation function
+
+function two_bits = qpsk_demod(symbol)
+    bit0 = real(symbol*exp(j*pi/4)) < 0;
+    bit1 = imag(symbol*exp(j*pi/4)) < 0;
+    two_bits = [bit1 bit0];
+endfunction
+
+
+% Rate Rs modem simulation model -------------------------------------------------------
+
+function sim_out = ber_test(sim_in)
+    bps     = 2;     % two bits/symbol for QPSK
+    Rs      = 50;    % symbol rate (needed for HF model)
+    
+    verbose = sim_in.verbose;
+    EbNovec = sim_in.EbNovec;
+    hf_en   = sim_in.hf_en;
+
+    % user can supply number of bits per point to get good results
+    % at high Eb/No
+
+    if length(sim_in.nbits) > 1
+      nbitsvec = sim_in.nbits;
+      nbitsvec += 100 - mod(nbitsvec,100);  % round up to nearest 100
+    else
+      nbitsvec(1:length(EbNovec)) = sim_in.nbits;
+    end
+
+    % init HF model
+
+    if hf_en
+
+      % some typical values
+
+      dopplerSpreadHz = 1.0; path_delay = 1E-3*Rs;
+
+      nsymb = max(nbitsvec)/2;
+      spread1 = doppler_spread(dopplerSpreadHz, Rs, nsymb);
+      spread2 = doppler_spread(dopplerSpreadHz, Rs, nsymb);
+      hf_gain = 1.0/sqrt(var(spread1)+var(spread2));
+      % printf("nsymb: %d lspread1: %d\n", nsymb, length(spread1));
+    end
+
+    for ne = 1:length(EbNovec)
+
+        % work out noise power -------------
+
+        EbNodB = EbNovec(ne);
+        EsNodB = EbNodB + 10*log10(bps);
+        EsNo = 10^(EsNodB/10);
+        variance = 1/EsNo;
+        nbits = nbitsvec(ne);
+        nsymb = nbits/bps;
+
+        % modulator ------------------------
+
+        tx_bits = rand(1,nbits) > 0.5;        
+        tx_symb = [];
+        prev_tx_symb = 1;
+        for s=1:nsymb
+          atx_symb = qpsk_mod(tx_bits(2*s-1:2*s));
+          if sim_in.dqpsk
+            atx_symb *= prev_tx_symb;
+            prev_tx_symb = atx_symb;
+          end
+          tx_symb = [tx_symb atx_symb];
+        end
+
+        % channel ---------------------------
+
+        rx_symb = tx_symb;
+
+        if hf_en
+
+          % simplified rate Rs simulation model that doesn't include
+          % ISI, just freq filtering.  We assume perfect phase estimation
+          % so it's just amplitude distortion.
+
+          hf_model1 = hf_model2 = zeros(1, nsymb);
+          for s=1:nsymb 
+            hf_model1(s) = hf_gain*(spread1(s) + exp(-j*path_delay)*spread2(s));
+            hf_model     = abs(hf_model1(s));
+
+            if sim_in.diversity
+              % include amplitude information from another frequency in channel model
+              w1 = 7*2*pi;
+              hf_model2(s) = hf_gain*(spread1(s) + exp(-j*w1*path_delay)*spread2(s));
+              hf_model     = 0.5*abs(hf_model1(s)) + 0.5*abs(hf_model2(s));
+            end
+
+            rx_symb(s) = rx_symb(s).*hf_model;
+          end
+        end
+
+        % variance is noise power, which is divided equally between real and
+        % imag components of noise
+
+        noise = sqrt(variance*0.5)*(randn(1,nsymb) + j*randn(1,nsymb));
+        rx_symb += noise;
+
+        % demodulator ------------------------------------------
+
+        % demodulate rx symbols to bits
+ 
+        rx_bits = [];
+        prev_rx_symb = 1;
+        for s=1:nsymb
+          arx_symb = rx_symb(s);
+          if sim_in.dqpsk
+            tmp = arx_symb;
+            arx_symb *= prev_rx_symb';
+            prev_rx_symb = tmp;
+          end
+          two_bits = qpsk_demod(arx_symb);
+          rx_bits = [rx_bits two_bits];
+        end
+        
+        % count errors -----------------------------------------
+
+        error_pattern = xor(tx_bits, rx_bits);
+        nerrors = sum(error_pattern);
+        bervec(ne) = nerrors/nbits;
+        if verbose
+          printf("EbNodB: % 3.1f nbits: %5d nerrors: %5d ber: %4.3f\n", EbNodB, nbits, nerrors, bervec(ne));
+          if verbose == 2
+            figure(2); clf;
+            plot(rx_symb*exp(j*pi/4),'+','markersize', 10);
+            mx = max(abs(rx_symb));
+            axis([-mx mx -mx mx]);
+            if sim_in.diversity && sim_in.hf_en 
+              figure(3);
+              plot(1:nsymb, abs(hf_model1), 1:nsymb, abs(hf_model2), 'linewidth', 2);
+            end
+          end
+        end
+    end
+
+    sim_out.bervec = bervec;
+endfunction
+
+
+% -------------------------------------------------------------
+
+
+function run_single
+    sim_in.verbose   = 2;
+    sim_in.nbits     = 1000;
+    sim_in.EbNovec   = 4;
+    sim_in.dqpsk     = 0;
+    sim_in.hf_en     = 0;
+    sim_in.diversity = 0;
+
+    sim_qpsk = ber_test(sim_in);
+endfunction
+
+
+function run_curves
+    max_nbits = 1E5;
+    sim_in.verbose = 1;
+    sim_in.EbNovec = 0:10;
+    sim_in.dqpsk   = 0;
+    sim_in.hf_en   = 0;
+    sim_in.diversity = 0;
+
+    % AWGN -----------------------------
+
+    ber_awgn_theory = 0.5*erfc(sqrt(10.^(sim_in.EbNovec/10)));
+    sim_in.nbits    = min(max_nbits, floor(500 ./ ber_awgn_theory));
+
+    sim_qpsk = ber_test(sim_in);
+    sim_in.dqpsk = 1;
+    sim_dqpsk = ber_test(sim_in);
+       
+    % HF -----------------------------
+
+    hf_sim_in = sim_in; hf_sim_in.dqpsk = 0; hf_sim_in.hf_en = 1;
+    hf_sim_in.EbNovec = 0:16;
+
+    EbNoLin = 10.^(hf_sim_in.EbNovec/10);
+    ber_hf_theory = 0.5.*(1-sqrt(EbNoLin./(EbNoLin+1)));
+
+    hf_sim_in.nbits = min(max_nbits, floor(500 ./ ber_hf_theory));
+    sim_qpsk_hf = ber_test(hf_sim_in);
+
+    hf_sim_in.dqpsk = 1; 
+    sim_dqpsk_hf = ber_test(hf_sim_in);
+
+    hf_sim_in.dqpsk = 0; 
+    hf_sim_in.diversity = 1;
+    sim_qpsk_hf_div = ber_test(hf_sim_in);
+
+    % Plot results --------------------
+
+    close all;
+    figure (1, 'position', [100, 10, 600, 400]); clf;
+
+    semilogy(sim_in.EbNovec, ber_awgn_theory,'r+-;QPSK AWGN theory;', 'linewidth', 2)
+    xlabel('Eb/No (dB)')
+    ylabel('BER')
+    grid("minor")
+    axis([min(sim_in.EbNovec) max(sim_in.EbNovec) 1E-3 1])
+    hold on;
+    
+    semilogy([0 4 4], [ber_awgn_theory(5) ber_awgn_theory(5) 1E-3],'k--', 'linewidth', 2);
+    hold off;
+
+    figure (2, 'position', [300, 10, 600, 400]); clf;
+    semilogy(sim_in.EbNovec, ber_awgn_theory,'r+-;QPSK AWGN theory;','markersize', 10, 'linewidth', 2)
+    hold on;
+    semilogy(sim_in.EbNovec, sim_qpsk.bervec,'g+-;QPSK AWGN simulated;','markersize', 10, 'linewidth', 2)
+    semilogy(sim_in.EbNovec, sim_dqpsk.bervec,'b+-;DQPSK AWGN simulated;','markersize', 10, 'linewidth', 2)
+    xlabel('Eb/No (dB)')
+    ylabel('BER')
+    grid("minor")
+    axis([min(sim_in.EbNovec) max(sim_in.EbNovec) 1E-3 1])
+
+    figure (3, 'position', [400, 10, 600, 400]); clf;
+    semilogy(sim_in.EbNovec, ber_awgn_theory,'r+-;QPSK AWGN theory;','markersize', 10, 'linewidth', 2)
+    hold on;
+    semilogy(sim_in.EbNovec, sim_qpsk.bervec,'g+-;QPSK AWGN simulated;','markersize', 10, 'linewidth', 2)
+    semilogy(sim_in.EbNovec, sim_dqpsk.bervec,'b+-;DQPSK AWGN simulated;','markersize', 10, 'linewidth', 2)
+    semilogy(hf_sim_in.EbNovec, ber_hf_theory,'r+-;QPSK HF theory;','markersize', 10, 'linewidth', 2)
+    semilogy(hf_sim_in.EbNovec, sim_dqpsk_hf.bervec,'b+-;DQPSK HF simulated;','markersize', 10, 'linewidth', 2)
+    semilogy(hf_sim_in.EbNovec, sim_qpsk_hf.bervec,'g+-;QPSK HF simulated;','markersize', 10, 'linewidth', 2)
+    semilogy(hf_sim_in.EbNovec, sim_qpsk_hf_div.bervec,'c+-;QPSK Diversity HF simulated;','markersize', 10, 'linewidth', 2)
+    hold off;
+    xlabel('Eb/No (dB)')
+    ylabel('BER')
+    grid("minor")
+    axis([min(hf_sim_in.EbNovec) max(hf_sim_in.EbNovec) 1E-3 1])
+
+endfunction
+
+% -------------------------------------------------------------
+
+more off;
+rand('seed',1); randn('seed', 1);
+run_curves
+#run_single