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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.
#{
Examples:
i) 4 frame interleaver, 10 seconds, AWGN channel at (coded) Eb/No=3dB
octave:4> ofdm_ldpc_tx('awgn_ebno_3dB_700d.raw', "700D", 4, 10, 3);
ii) 4 frame interleaver, 10 seconds, HF channel at (coded) Eb/No=6dB
ofdm_ldpc_tx('hf_ebno_6dB_700d.raw', "700D", 4, 10, 6, 'hf');
#}
#{
TODO:
[ ] measure and report raw and coded BER
[ ] maybe 10s worth of frames, sync up to any one automatically
+ or start with maybe 10 frames
+ measure BER match on each one
[ ] model clipping/PA compression
[ ] sample clock offsets
[ ] compare with same SNR from pathsim
[ ] How to pack arbitrary frames into ofdm frame and codec 2 frames
+ integer number of ofdm frames?
+ how to sync at other end
#}
% Set up LDPC code and voice codec to "codeword" packing
function ofdm_ldpc_tx(filename, mode="700D", interleave_frames = 1, Nsec, EbNodB=100, channel='awgn', freq_offset_Hz=0)
ofdm_lib;
ldpc;
gp_interleaver;
% init modem
bps = 2; Ns = 8; Tcp = 0.002;
[bps Rs Tcp Ns Nc] = ofdm_init_mode(mode, Ns);
states = ofdm_init(bps, Rs, Tcp, Ns, Nc);
ofdm_load_const;
% some constants used for assembling modem frames
[code_param Nbitspercodecframe Ncodecframespermodemframe] = codec_to_frame_packing(states, mode);
% Generate fixed test frame of tx bits and run OFDM modulator
Nrows = Nsec*Rs;
Nframes = floor((Nrows-1)/Ns);
% Adjust Nframes so we have an integer number of interleaver frames
% in simulation
Nframes = interleave_frames*round(Nframes/interleave_frames);
% OK generate a modem frame using random payload bits
if strcmp(mode, "700D")
codec_bits = round(ofdm_rand(code_param.data_bits_per_frame)/32767);
else
codec_bits = round(ofdm_rand(Ncodecframespermodemframe*Nbitspercodecframe)/32767);
end
[frame_bits bits_per_frame] = assemble_frame(states, code_param, mode, codec_bits, Ncodecframespermodemframe, Nbitspercodecframe);
% modulate to create symbols and interleave
tx_bits = tx_symbols = [];
for f=1:interleave_frames
tx_bits = [tx_bits codec_bits];
for b=1:2:bits_per_frame
tx_symbols = [tx_symbols qpsk_mod(frame_bits(b:b+1))];
end
end
tx_symbols = gp_interleave(tx_symbols);
% generate txt symbols
txt_bits = zeros(1,Ntxtbits);
txt_symbols = [];
for b=1:2:length(txt_bits)
txt_symbols = [txt_symbols qpsk_mod(txt_bits(b:b+1))];
end
% assemble interleaved modem frames that include UW and txt symbols
atx = [];
for f=1:interleave_frames
st = (f-1)*bits_per_frame/bps+1; en = st + bits_per_frame/bps-1;
modem_frame = assemble_modem_frame_symbols(states, tx_symbols(st:en), txt_symbols);
atx = [atx ofdm_txframe(states, modem_frame) ];
end
tx = [];
for f=1:Nframes/interleave_frames
tx = [tx atx];
end
Nsam = length(tx);
% channel simulation
EsNo = rate * bps * (10 .^ (EbNodB/10));
variance = 1/(M*EsNo/2);
woffset = 2*pi*freq_offset_Hz/Fs;
SNRdB = EbNodB + 10*log10(Nc*bps*Rs*rate/3000);
printf("EbNo: %3.1f dB SNR(3k) est: %3.1f dB foff: %3.1fHz inter_frms: %d ",
EbNodB, SNRdB, freq_offset_Hz, interleave_frames);
% set up HF model ---------------------------------------------------------------
if strcmp(channel, 'hf') || strcmp(channel, 'hfgood')
randn('seed',1);
% ITUT "poor" or "moderate" channels
if strcmp(channel, 'hf')
dopplerSpreadHz = 1; path_delay_ms = 1;
else
% "hfgood"
dopplerSpreadHz = 0.1; path_delay_ms = 0.5;
end
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
rx = tx;
if strcmp(channel, 'hf') || strcmp(channel, 'hfgood')
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
rx = rx .* exp(j*woffset*(1:Nsam));
% note variance/2 as we are using real() operator, mumble,
% reflection of -ve freq to +ve, mumble, hand wave
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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