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