basic fsk modem simulation

git-svn-id: https://svn.code.sf.net/p/freetel/code@2881 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/fsk_basic.m b/codec2-dev/octave/fsk_basic.m
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+++ b/codec2-dev/octave/fsk_basic.m
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+% fsk_basic.m
+% David Rowe 30 sep 2016
+%
+% Basic non-coherence FSK modem simulation to illustrate principles
+% and compare to ideal
+
+rand('seed',1);
+randn('seed',1);
+
+Fs = 9600;     % sample rate
+f1 = 1200;
+f2 = 2400;
+Rs = 1200;     % symbol rate
+Ts = Fs/Rs;    % length of each symbol in samples
+Nbits = 10000;
+EbNodB = 9;    
+
+tx_bits = round(rand(1,Nbits));
+
+% continuous phase FSK modulator
+
+w1 = 2*pi*f1/Fs;
+w2 = 2*pi*f2/Fs;
+tx_phase = 0;
+tx = zeros(1,Ts*Nbits);
+
+for i=1:Nbits
+  for k=1:Ts
+    if tx_bits(i)
+      tx_phase += w2;
+    else
+      tx_phase += w1;
+    end
+    tx((i-1)*Ts+k) = exp(j*tx_phase);
+  end
+end
+
+% AWGN channel noise
+
+EbNo = 10^(EbNodB/10);
+variance = Fs/(Rs*EbNo);
+noise = sqrt(variance/2)*(randn(1,Nbits*Ts) + j*randn(1,Nbits*Ts));
+rx = tx + noise;
+
+% integrate and dump demodulator
+
+rx_bits = zeros(1,Nbits);
+for i=1:Nbits
+  arx_symb = rx((i-1)*Ts + (1:Ts));
+  filt1 = sum(exp(-j*w1*(1:Ts)) .* arx_symb);
+  filt2 = sum(exp(-j*w2*(1:Ts)) .* arx_symb);
+  rx_bits(i) = filt2 > filt1;
+end
+
+Nerrors = sum(xor(tx_bits, rx_bits));
+ber = Nerrors/Nbits;
+printf("EbNodB: %4.1f  Nerrors: %d BER: %1.3f\n", EbNodB, Nerrors, ber);
+
+
+