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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 SNR3k=3dB
octave:4> ofdm_tx("awgn_snr_3dB_700d.raw", "700D", 10, 3)
ii) 10 seconds, multipath poor channel at SNR=6dB
octave:5> ofdm_tx("hf_snr_6dB_700d.raw", "700D", 10, 6, "mpp")
iii) Data mode example, three bursts of one packet each, SNR=100dB:
octave:6> ofdm_tx("test_datac0.raw","datac0",1,100,"awgn","bursts",3)
#}
function ofdm_tx(filename, mode="700D", N, SNR3kdB=100, channel='awgn', varargin)
ofdm_lib;
channel_lib;
randn('seed',1);
pkg load signal;
tx_clip_en = 0; freq_offset_Hz = 0.0; burst_mode = 0; Nbursts = 1;
i = 1;
while i<=length(varargin)
if strcmp(varargin{i},"txclip")
tx_clip_en = 1;
elseif strcmp(varargin{i},"bursts")
burst_mode = 1;
Nbursts = varargin{i+1}; i++;
else
printf("\nERROR unknown argument: [%d] %s \n", i ,varargin{i});
return;
end
i++;
end
% init modem
config = ofdm_init_mode(mode);
states = ofdm_init(config);
print_config(states);
ofdm_load_const;
if burst_mode
% burst mode: treat N as Npackets
Npackets = N;
else
% streaming mode: treat N as Nseconds
Npackets = round(N/states.Tpacket);
end
% Generate fixed test frame of tx bits and concatenate packets
tx_bits = create_ldpc_test_frame(states, coded_frame=0);
atx = ofdm_mod(states, tx_bits);
tx = [];
for f=1:Npackets
tx = [tx atx];
end
if length(states.data_mode)
% note postamble provides a "column" of pilots at the end of the burst
tx = [states.tx_preamble tx states.tx_postamble];
end
% if burst mode concatenate multiple bursts with spaces
if burst_mode
atx = tx; tx = zeros(1,states.Fs); on_time = 0; off_time = states.Fs;
for b=1:Nbursts
tx = [tx atx zeros(1,states.Fs)];
on_time += length(atx);
off_time += states.Fs;
end
% adjust channel simulator SNR setpoint given (burst on length)/(total length including silence) ratio
mark_space_SNR_offset = 10*log10(on_time/(on_time+off_time));
SNRdB_setpoint = SNR3kdB + mark_space_SNR_offset;
printf("SNR3kdB: %4.2f Burst offset: %4.2f SNRdB_setpoint: %4.2f\n", SNR3kdB, mark_space_SNR_offset, SNRdB_setpoint)
else
SNRdB_setpoint = SNR3kdB; % no adjustment to SNR in streaming mode
end
printf("Npackets: %d Nbursts: %d ", Npackets, Nbursts);
states.verbose=1;
tx = ofdm_hilbert_clipper(states, tx, tx_clip_en);
rx_real = ofdm_channel(states, tx, SNRdB_setpoint, channel, freq_offset_Hz);
frx = fopen(filename,"wb"); fwrite(frx, rx_real, "short"); fclose(frx);
endfunction
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