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% ofdm_tx.m
% David Rowe March 2018
%
% File based, uncoded OFDM tx. Generates a file of ofdm samples,
% including optional channel simulation. See also ofdm_ldpc_tx.m, and
% ofdm_mod.c
#{
Examples:
i) 10 seconds, AWGN channel at Eb/No=3dB
octave:4> ofdm_tx('awgn_ebno_3dB_700d.raw', "700D", 10, 3);
ii) 10 seconds, HF channel at Eb/No=6dB
ofdm_tx('hf_ebno_6dB_700d.raw', "700D", 10, 6, 'hf');
iii) 10 seconds, 2200 waveform, AWGN channel, Eb/No=100dB (effectively noise free)
ofdm_tx('hf_ebno_6dB_700d.raw', "2200", 10);
#}
% Note EbNodB is for payload data bits, so will be 10log10(rate) higher than
% raw EbNodB used in ofdm_tx() at uncoded bit rate
function ofdm_tx(filename, mode="700D", Nsec, EbNodB=100, channel='awgn', freq_offset_Hz=0, dfoff_hz_per_sec = 0, initial_noise_sams=0, tx_filter=0)
ofdm_lib;
% init modem
[bps Rs Tcp Ns Nc] = ofdm_init_mode(mode)
states = ofdm_init(bps, Rs, Tcp, Ns, Nc);
print_config(states);
ofdm_load_const;
% Generate fixed test frame of tx bits and run OFDM modulator
Nrows = Nsec*Rs;
Nframes = floor((Nrows-1)/Ns);
tx_bits = create_ldpc_test_frame(states, coded_frame=0);
tx = [];
for f=1:Nframes
tx = [tx ofdm_mod(states, tx_bits)];
end
Nsam = length(tx);
% channel simulation
EsNo = rate * bps * (10 .^ (EbNodB/10));
variance = 1/(M*EsNo/2);
woffset = 2*pi*freq_offset_Hz/Fs;
dwoffset = 2*pi*dfoff_hz_per_sec/(Fs*Fs);
SNRdB = EbNodB + 10*log10(Nc*bps*Rs/3000);
printf("EbNo: %3.1f dB SNR(3k) est: %3.1f dB foff: %3.1fHz ", EbNodB, SNRdB, freq_offset_Hz);
% set up HF model ---------------------------------------------------------------
if strcmp(channel, 'hf')
randn('seed',1);
% some typical values, or replace with user supplied
dopplerSpreadHz = 1; path_delay_ms = 1;
path_delay_samples = path_delay_ms*Fs/1000;
printf("Doppler Spread: %3.2f Hz Path Delay: %3.2f ms %d samples\n",
dopplerSpreadHz, path_delay_ms, path_delay_samples);
% generate same fading pattern for every run
randn('seed',1);
spread1 = doppler_spread(dopplerSpreadHz, Fs, (Nsec*(M+Ncp)/M)*Fs*1.1);
spread2 = doppler_spread(dopplerSpreadHz, Fs, (Nsec*(M+Ncp)/M)*Fs*1.1);
% sometimes doppler_spread() doesn't return exactly the number of samples we need
assert(length(spread1) >= Nsam, "not enough doppler spreading samples");
assert(length(spread2) >= Nsam, "not enough doppler spreading samples");
end
% experimental coarse amplitude quantisation
quant_tx = 0;
if quant_tx
tx_re = real(tx); tx_im = imag(tx);
tx_re = min(tx_re,0.5); tx_re = max(tx_re,-0.5);
tx_im = min(tx_im,0.5); tx_im = max(tx_im,-0.5);
step = 0.05/4;
tx_re = step*round(tx_re/step);
tx_im = step*round(tx_im/step);
tx = tx_re + j*tx_im;
figure(1); clf; subplot(211); plot(real(tx(1:100)),'+-'); subplot(212); plot(imag(tx(1:100)),'+-');
end
rx = tx;
if tx_filter
bpf_coeff = make_ofdm_bpf(write_c_header_file=0);
rx = filter(bpf_coeff,1,tx);
figure(1); clf;
subplot(211);
plot((1:length(tx))*8000/length(tx), 20*log10(abs(fft(tx))))
axis([1 4000 0 60])
subplot(212);
plot((1:length(tx))*8000/length(tx), 20*log10(abs(fft(rx))))
axis([1 4000 0 60])
end
if strcmp(channel, 'hf')
rx = tx(1:Nsam) .* spread1(1:Nsam);
rx += [zeros(1,path_delay_samples) tx(1:Nsam-path_delay_samples)] .* spread2(1:Nsam);
% normalise rx power to same as tx
nom_rx_pwr = 2/(Ns*(M*M)) + Nc/(M*M);
rx_pwr = var(rx);
rx *= sqrt(nom_rx_pwr/rx_pwr);
end
phase_offset = woffset*(1:Nsam) + 0.5*dwoffset*((1:Nsam).^2);
rx = rx .* exp(j*phase_offset);
rx = [zeros(1,initial_noise_sams) rx];
Nsam = length(rx);
% note variance/2 as we are using real() operator, mumble,
% reflection of -ve freq to +ve, mumble, hand wave
randn('seed',1);
noise = sqrt(variance/2)*0.5*randn(1,Nsam);
rx = real(rx) + noise;
printf("measured SNR: %3.2f dB\n", 10*log10(var(real(tx))/var(noise))+10*log10(4000) - 10*log10(3000));
% adjusted by experiment to match rms power of early test signals
frx=fopen(filename,"wb"); fwrite(frx, states.amp_scale*rx, "short"); fclose(frx);
endfunction
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