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108 lines
2.3 KiB
108 lines
2.3 KiB
6 years ago
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% fsk_basic_alt.m
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% David Rowe April 2018
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%
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% Modification of fsk_basic.m, to explore idea of alternating tone frequencies
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% for multipath channels.
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#{
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So alternate tone spacings at Rs/2 worked quite well, no implementation loss
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but odd spectrum.
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But when multipath added (e.g 2ms delay on 2.5ms Rs), performance the same
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as for non-stepped tones. Multipath echo must be interfering with stepped
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tone.
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When 2nd step of tones are Rs spaced - then multipath perf much better,
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as expected.
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There is also the impact of freq selective fading as well as ISI, FSK
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tone level will be pushed down and we may choose wrong tone.
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Also be interesting to test FSK with FEC on HF, combined response.
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#}
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% make sure we get same results from random number generators on each run
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rand('seed',1);
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randn('seed',1);
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% constants -----------------------------------------------------------
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Fs = 8000; % sample rate
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f1 = 1000;
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f2 = 1400;
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f3 = 1200;
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f4 = 1600;
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Rs = 400; % symbol rate
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Ts = Fs/Rs; % length of each symbol in samples
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% simulation parameters -----------------------------------------------
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Nbits = 10000;
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EbNodB = 9;
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multipath = 1;
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stepped = 1;
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% start simulation ----------------------------------------------------
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tx_bits = round(rand(1,Nbits));
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% continuous phase FSK modulator
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w1 = 2*pi*f1/Fs;
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w2 = 2*pi*f2/Fs;
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w3 = 2*pi*f3/Fs;
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w4 = 2*pi*f4/Fs;
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% choose tones here, e.g. offset on every 2nd symbol
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if stepped
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w = [w1 w2; w3 w4];
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else
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w = [w1 w2; w1 w2];
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end
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tx_phase = 0;
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tx = zeros(1,Ts*Nbits);
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for i=1:Nbits
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for k=1:Ts
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tx_phase += w(1+mod(i-1,2),1+tx_bits(i));
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tx((i-1)*Ts+k) = exp(j*tx_phase);
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end
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end
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% AWGN channel noise
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EbNo = 10^(EbNodB/10);
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variance = Fs/(Rs*EbNo);
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noise = sqrt(variance/2)*(randn(1,Nbits*Ts) + j*randn(1,Nbits*Ts));
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rx = tx + noise;
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if multipath
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d = 0.002*Fs;
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tx_delayed = [zeros(1,d) tx(1:end-d)];
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rx += 0.5*tx_delayed;
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end
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figure(1); clf;
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Tx = abs(fft(tx(1:Fs)));
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plot(Tx);
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% integrate and dump demodulator
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rx_bits = zeros(1,Nbits);
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for i=1:Nbits
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arx_symb = rx((i-1)*Ts + (1:Ts));
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wr = 1+mod(i-1,2);
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filt1 = sum(exp(-j*w(wr,1)*(1:Ts)) .* arx_symb);
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filt2 = sum(exp(-j*w(wr,2)*(1:Ts)) .* arx_symb);
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rx_bits(i) = filt2 > filt1;
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end
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Nerrors = sum(xor(tx_bits, rx_bits));
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ber = Nerrors/Nbits;
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printf("EbNodB: %4.1f Nerrors: %d BER: %1.3f\n", EbNodB, Nerrors, ber);
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