145 lines
4.7 KiB
Matlab
145 lines
4.7 KiB
Matlab
% ofdm_ldpc_tx.m
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% David Rowe April 2017
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%
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% File based ofdm tx with LDPC encoding and interleaver. Generates a
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% file of ofdm samples, including optional channel simulation.
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#{
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1. 10 seconds, AWGN channel at SNR3k=3dB
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octave:4> ofdm_ldpc_tx("test_700d.raw", "700D", 10, 3)
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2. 10 seconds, multipath poor channel at SNR=6dB
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octave:5> ofdm_ldpc_tx("test_700d.raw", "700D", 10, 6, "mpp")
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3. Data mode example, three bursts of one packet each, SNR=100dB:
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octave:6> ofdm_ldpc_tx("test_datac0.raw","datac0",1,100,"awgn","bursts",3)
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4. Data mode example, three bursts of one packet each, SNR=100dB, with CRC
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to enable demodulation by freedv_data_raw_rx:
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octave:6> ofdm_ldpc_tx("test_datac0.raw","datac0",1,100,"awgn","bursts",3, "crc")
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#}
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function ofdm_ldpc_tx(filename, mode="700D", N, SNR3kdB=100, channel='awgn', varargin)
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ofdm_lib;
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ldpc;
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gp_interleaver;
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channel_lib;
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pkg load signal;
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randn('seed',1);
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more off;
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tx_clip_en = 0; freq_offset_Hz = 0.0; burst_mode = 0; Nbursts = 1;
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crc_mode = 0;
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i = 1;
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while i<=length(varargin)
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if strcmp(varargin{i},"txclip")
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tx_clip_en = 1;
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elseif strcmp(varargin{i},"bursts")
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burst_mode = 1;
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Nbursts = varargin{i+1}; i++;
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elseif strcmp(varargin{i},"crc")
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crc_mode = 1;
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else
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printf("\nERROR unknown argument: %s\n", varargin{i});
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return;
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end
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i++;
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end
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% init modem
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config = ofdm_init_mode(mode);
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states = ofdm_init(config);
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print_config(states);
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ofdm_load_const;
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if burst_mode
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% burst mode: treat N as Npackets
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Npackets = N;
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else
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% streaming mode: treat N as Nseconds
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Npackets = round(N/states.Tpacket);
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end
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% some constants used for assembling modem frames
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[code_param Nbitspercodecframe Ncodecframespermodemframe] = codec_to_frame_packing(states, mode);
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% OK generate a modem frame using random payload bits
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if strcmp(mode, "2020")
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payload_bits = round(ofdm_rand(Ncodecframespermodemframe*Nbitspercodecframe)/32767);
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else
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payload_bits = round(ofdm_rand(code_param.data_bits_per_frame)/32767);
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if crc_mode
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unpacked_crc16 = crc16_unpacked(payload_bits(1:end-16));
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payload_bits(end-15:end) = unpacked_crc16;
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end
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end
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[packet_bits bits_per_packet] = fec_encode(states, code_param, mode, payload_bits);
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% modulate to create symbols and interleave
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tx_symbols = [];
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for b=1:bps:bits_per_packet
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if bps == 2 tx_symbols = [tx_symbols qpsk_mod(packet_bits(b:b+bps-1))]; end
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if bps == 4 tx_symbols = [tx_symbols qam16_mod(states.qam16, packet_bits(b:b+bps-1))]; end
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end
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assert(gp_deinterleave(gp_interleave(tx_symbols)) == tx_symbols);
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tx_symbols = gp_interleave(tx_symbols);
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% generate txt (non FEC protected) symbols
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txt_bits = zeros(1,Ntxtbits);
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txt_symbols = [];
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for b=1:bps:length(txt_bits)
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if bps == 2 txt_symbols = [txt_symbols qpsk_mod(txt_bits(b:b+bps-1))]; end
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if bps == 4 txt_symbols = [txt_symbols qam16_mod(states.qam16,txt_bits(b:b+bps-1))]; end
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end
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% assemble interleaved modem packet that include UW and txt symbols
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modem_packet = assemble_modem_packet_symbols(states, tx_symbols, txt_symbols);
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% sanity check
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[rx_uw rx_codeword_syms payload_amps txt_bits] = disassemble_modem_packet(states, modem_packet, ones(1,length(modem_packet)));
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assert(rx_uw == states.tx_uw);
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% create a burst of concatenated packets
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atx = ofdm_txframe(states, modem_packet); tx = [];
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for f=1:Npackets
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tx = [tx atx];
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end
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if length(states.data_mode)
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% note for burst mode postamble provides a "column" of pilots at the end of the burst
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tx = [states.tx_preamble tx states.tx_postamble];
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end
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% if burst mode concatenate multiple bursts with spaces
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if burst_mode
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atx = tx; tx = zeros(1,states.Fs); on_time = 0; off_time = states.Fs;
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for b=1:Nbursts
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tx = [tx atx zeros(1,states.Fs)];
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on_time += length(atx);
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off_time += states.Fs;
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end
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% adjust channel simulator SNR setpoint given (burst on length)/(total length including silence) ratio
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mark_space_SNR_offset = 10*log10(on_time/(on_time+off_time));
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SNRdB_setpoint = SNR3kdB + mark_space_SNR_offset;
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printf("SNR3kdB: %4.2f Burst offset: %4.2f SNRdB_setpoint: %4.2f\n", SNR3kdB, mark_space_SNR_offset, SNRdB_setpoint)
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else
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SNRdB_setpoint = SNR3kdB; % no adjustment to SNR in streaming mode
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end
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printf("Npackets: %d Nbursts: %d ", Npackets, Nbursts);
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states.verbose=1;
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tx = ofdm_hilbert_clipper(states, tx, tx_clip_en);
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[rx_real rx] = ofdm_channel(states, tx, SNRdB_setpoint, channel, freq_offset_Hz);
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frx = fopen(filename,"wb"); fwrite(frx, rx_real, "short"); fclose(frx);
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if length(rx) >= states.Fs
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figure(1); clf; plot(20*log10(abs(fft(rx(1:states.Fs)/16384))));
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axis([1 states.Fs -20 60])
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end
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endfunction
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