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% ofdm_ldpc_tx.m
% David Rowe April 2017
%
% File based ofdm tx with LDPC encoding and interleaver. Generates a
% file of ofdm samples, including optional channel simulation.
#{
1. 10 seconds, AWGN channel at SNR3k=3dB
octave:4> ofdm_ldpc_tx("test_700d.raw", "700D", 10, 3)
2. 10 seconds, multipath poor channel at SNR=6dB
octave:5> ofdm_ldpc_tx("test_700d.raw", "700D", 10, 6, "mpp")
3. Data mode example, three bursts of one packet each, SNR=100dB:
octave:6> ofdm_ldpc_tx("test_datac0.raw","datac0",1,100,"awgn","bursts",3)
4. Data mode example, three bursts of one packet each, SNR=100dB, with CRC
to enable demodulation by freedv_data_raw_rx:
octave:6> ofdm_ldpc_tx("test_datac0.raw","datac0",1,100,"awgn","bursts",3, "crc")
#}
function ofdm_ldpc_tx(filename, mode="700D", N, SNR3kdB=100, channel='awgn', varargin)
ofdm_lib;
ldpc;
gp_interleaver;
channel_lib;
pkg load signal;
randn('seed',1);
more off;
tx_clip_en = 0; freq_offset_Hz = 0.0; burst_mode = 0; Nbursts = 1;
crc_mode = 0;
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++;
elseif strcmp(varargin{i},"crc")
crc_mode = 1;
else
printf("\nERROR unknown argument: %s\n", 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
% some constants used for assembling modem frames
[code_param Nbitspercodecframe Ncodecframespermodemframe] = codec_to_frame_packing(states, mode);
% OK generate a modem frame using random payload bits
if strcmp(mode, "2020")
payload_bits = round(ofdm_rand(Ncodecframespermodemframe*Nbitspercodecframe)/32767);
else
payload_bits = round(ofdm_rand(code_param.data_bits_per_frame)/32767);
if crc_mode
unpacked_crc16 = crc16_unpacked(payload_bits(1:end-16));
payload_bits(end-15:end) = unpacked_crc16;
end
end
[packet_bits bits_per_packet] = fec_encode(states, code_param, mode, payload_bits, Ncodecframespermodemframe, Nbitspercodecframe);
% modulate to create symbols and interleave
tx_symbols = [];
for b=1:bps:bits_per_packet
if bps == 2 tx_symbols = [tx_symbols qpsk_mod(packet_bits(b:b+bps-1))]; end
if bps == 4 tx_symbols = [tx_symbols qam16_mod(states.qam16, packet_bits(b:b+bps-1))]; end
end
assert(gp_deinterleave(gp_interleave(tx_symbols)) == tx_symbols);
tx_symbols = gp_interleave(tx_symbols);
% generate txt (non FEC protected) symbols
txt_bits = zeros(1,Ntxtbits);
txt_symbols = [];
for b=1:bps:length(txt_bits)
if bps == 2 txt_symbols = [txt_symbols qpsk_mod(txt_bits(b:b+bps-1))]; end
if bps == 4 txt_symbols = [txt_symbols qam16_mod(states.qam16,txt_bits(b:b+bps-1))]; end
end
% assemble interleaved modem packet that include UW and txt symbols
modem_packet = assemble_modem_packet_symbols(states, tx_symbols, txt_symbols);
% sanity check
[rx_uw rx_codeword_syms payload_amps txt_bits] = disassemble_modem_packet(states, modem_packet, ones(1,length(modem_packet)));
assert(rx_uw == states.tx_uw);
% create a burst of concatenated packets
atx = ofdm_txframe(states, modem_packet); tx = [];
for f=1:Npackets
tx = [tx atx];
end
if length(states.data_mode)
% note for burst mode 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 rx] = ofdm_channel(states, tx, SNRdB_setpoint, channel, freq_offset_Hz);
frx = fopen(filename,"wb"); fwrite(frx, rx_real, "short"); fclose(frx);
if length(rx) >= states.Fs
figure(1); clf; plot(20*log10(abs(fft(rx(1:states.Fs)/16384))));
axis([1 states.Fs -20 60])
end
endfunction
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