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Diffstat (limited to 'octave/ofdm_helper.m')
| -rw-r--r-- | octave/ofdm_helper.m | 272 |
1 files changed, 272 insertions, 0 deletions
diff --git a/octave/ofdm_helper.m b/octave/ofdm_helper.m new file mode 100644 index 0000000..b34ec23 --- /dev/null +++ b/octave/ofdm_helper.m @@ -0,0 +1,272 @@ +% ofdm_helper.m +% +% Misc functions that are used to support OFDM modem development, that +% aren't required for modem operation + +1; + +%------------------------------------------------------------------------------ +% print_config - utility function to use ascii-art to describe the modem frame +%------------------------------------------------------------------------------ + +function print_config(states) + ofdm_load_const; + + % ASCII-art packet visualisation + s=1; u=1; Nuwsyms=length(uw_ind_sym); + cr = 1:Nc+2; + for f=1:Np + for r=1:Ns + for c=cr + if r == 1 + if (c==1) && states.edge_pilots + sym="P"; + elseif (c==Nc+1) && states.edge_pilots + sym="P"; + elseif c>1 && c <=(Nc+1) + sym="P"; + else + sym=" "; + end + elseif c>1 && c <=(Nc+1) + sym="."; + if (u <= Nuwsyms) && (s == uw_ind_sym(u)) sym="U"; u++; end + s++; + else + sym=" "; + end + printf("%s",sym); + end + printf("\n"); + end + end + + printf("Nc=%d Ts=%4.3f Tcp=%4.3f Ns: %d Np: %d\n", Nc, 1/Rs, Tcp, Ns, Np); + printf("Nsymperframe: %d Nbitsperpacket: %d Nsamperframe: %d Ntxtbits: %d Nuwbits: %d Nuwframes: %d\n", + Ns*Nc, Nbitsperpacket, Nsamperframe, Ntxtbits, Nuwbits, Nuwframes); + printf("uncoded bits/s: %4.1f\n", Nbitsperpacket*Fs/(Np*Nsamperframe)); +end + +%----------------------------------------------------------------------- +% create_ldpc_test_frame - generate a test frame of bits +%----------------------------------------------------------------------- + +function [tx_bits payload_data_bits codeword] = create_ldpc_test_frame(states, coded_frame=1) + ofdm_load_const; + ldpc; + gp_interleaver; + + if coded_frame + % Set up LDPC code + + mod_order = 4; bps = 2; modulation = 'QPSK'; mapping = 'gray'; + + init_cml(); % TODO: make this path sensible and portable + load HRA_112_112.txt + [code_param framesize rate] = ldpc_init_user(HRA_112_112, modulation, mod_order, mapping); + assert(Nbitsperframe == (code_param.coded_bits_per_frame + Nuwbits + Ntxtbits)); + + payload_data_bits = round(ofdm_rand(code_param.data_bits_per_frame)/32767); + codeword = LdpcEncode(payload_data_bits, code_param.H_rows, code_param.P_matrix); + Nsymbolsperframe = length(codeword)/bps; + + % need all these steps to get actual raw codeword bits at demod .. + + tx_symbols = []; + for s=1:Nsymbolsperframe + tx_symbols = [tx_symbols qpsk_mod( codeword(2*(s-1)+1:2*s) )]; + end + + tx_symbols = gp_interleave(tx_symbols); + + codeword_raw = []; + for s=1:Nsymbolsperframe + codeword_raw = [codeword_raw qpsk_demod(tx_symbols(s))]; + end + else + codeword_raw = round(ofdm_rand(Nbitsperpacket-(Nuwbits+Ntxtbits))/32767); + end + + % insert UW and txt bits + + tx_bits = assemble_modem_packet(states, codeword_raw, zeros(1,Ntxtbits)); + assert(Nbitsperpacket == length(tx_bits)); + +endfunction + +% automated test + +function test_assemble_disassemble(states) + ofdm_load_const; + + Nsymsperpacket = Nbitsperpacket/bps; + Ndatabitsperpacket = Nbitsperpacket-(Nuwbits+Ntxtbits); + Ndatasymsperpacket = Ndatabitsperpacket/bps; + codeword_bits = round(ofdm_rand(Ndatabitsperpacket)/32767); + tx_bits = assemble_modem_packet(states, codeword_bits, zeros(1,Ntxtbits)); + + tx_syms = zeros(1,Nsymsperpacket); + for s=1:Nsymsperpacket + if bps == 2 + tx_syms(s) = qpsk_mod(tx_bits(bps*(s-1)+1:bps*s)); + elseif bps == 4 + tx_syms(s) = qam16_mod(states.qam16,tx_bits(bps*(s-1)+1:bps*s)); + end + end + codeword_syms = zeros(1,Ndatasymsperpacket); + for s=1:Ndatasymsperpacket + if bps == 2 + codeword_syms(s) = qpsk_mod(codeword_bits(bps*(s-1)+1:bps*s)); + elseif bps == 4 + codeword_syms(s) = qam16_mod(states.qam16,codeword_bits(bps*(s-1)+1:bps*s)); + end + end + + [rx_uw rx_codeword_syms payload_amps txt_bits] = disassemble_modem_packet(states, tx_syms, ones(1,Nsymsperpacket)); + assert(rx_uw == states.tx_uw); + Ndatasymsperframe = (Nbitsperpacket-(Nuwbits+Ntxtbits))/bps; + assert(codeword_syms == rx_codeword_syms); +endfunction + +% test function, kind of like a CRC for QPSK symbols, to compare two vectors + +function acc = test_acc(v) + sre = 0; sim = 0; + for i=1:length(v) + x = v(i); + re = round(real(x)); im = round(imag(x)); + sre += re; sim += im; + %printf("%d %10f %10f %10f %10f\n", i, re, im, sre, sim); + end + acc = sre + j*sim; +end + + +% Save test bits frame to a text file in the form of a C array +% +% usage: +% ofdm_lib; test_bits_ofdm_file +% + +function test_bits_ofdm_file + Ts = 0.018; Tcp = 0.002; Rs = 1/Ts; bps = 2; Nc = 17; Ns = 8; + states = ofdm_init(bps, Rs, Tcp, Ns, Nc); + [test_bits_ofdm payload_data_bits codeword] = create_ldpc_test_frame(states); + printf("%d test bits\n", length(test_bits_ofdm)); + + f=fopen("../src/test_bits_ofdm.h","wt"); + fprintf(f,"/* Generated by test_bits_ofdm_file() Octave function */\n\n"); + fprintf(f,"const int test_bits_ofdm[]={\n"); + for m=1:length(test_bits_ofdm)-1 + fprintf(f," %d,\n",test_bits_ofdm(m)); + endfor + fprintf(f," %d\n};\n",test_bits_ofdm(end)); + + fprintf(f,"\nconst int payload_data_bits[]={\n"); + for m=1:length(payload_data_bits)-1 + fprintf(f," %d,\n",payload_data_bits(m)); + endfor + fprintf(f," %d\n};\n",payload_data_bits(end)); + + fprintf(f,"\nconst int test_codeword[]={\n"); + for m=1:length(codeword)-1 + fprintf(f," %d,\n",codeword(m)); + endfor + fprintf(f," %d\n};\n",codeword(end)); + + fclose(f); + +endfunction + + +% Get rid of nasty unfiltered stuff either side of OFDM signal +% This may need to be tweaked, or better yet made a function of Nc, if Nc changes +% +% usage: +% ofdm_lib; make_ofdm_bpf(1); + +function bpf_coeff = make_ofdm_bpf(write_c_header_file) + filt_n = 100; + Fs = 8000; + + bpf_coeff = fir2(filt_n,[0 900 1000 2000 2100 4000]/(Fs/2),[0.001 0.001 1 1 0.001 0.001]); + + if write_c_header_file + figure(1) + clf; + h = freqz(bpf_coeff,1,Fs/2); + plot(20*log10(abs(h))) + grid minor + + % save coeffs to a C header file + + f=fopen("../src/ofdm_bpf_coeff.h","wt"); + fprintf(f,"/* 1000 - 2000 Hz FIR filter coeffs */\n"); + fprintf(f,"/* Generated by make_ofdm_bpf() in ofdm_lib.m */\n"); + + fprintf(f,"\n#define OFDM_BPF_N %d\n\n", filt_n); + + fprintf(f,"float ofdm_bpf_coeff[]={\n"); + for r=1:filt_n + if r < filt_n + fprintf(f, " %f,\n", bpf_coeff(r)); + else + fprintf(f, " %f\n};", bpf_coeff(r)); + end + end + fclose(f); + end + +endfunction + +% Helper function to help design UW error thresholds, in particular for raw +% data modes. See also https://www.rowetel.com/wordpress/?p=7467 +function ofdm_determine_bad_uw_errors(Nuw) + figure(1); clf; + + % Ideally the 10% and 50% BER curves are a long way apart + + plot(0:Nuw, binocdf(0:Nuw,Nuw,0.1),';BER=0.1;'); hold on; + plot(binocdf(0:Nuw,Nuw,0.5),';BER=0.5;'); + + % Suggested threshold for raw data modes is the 5% probability + % level for the 50% BER curve. The pre/post-amble has a low chance + % of failure. If it does make an error, then we will have random + % bits presented as the UW (50% BER in UW). This threshold means + % there is only a 5% case of random bits being accepted as a valid UW + + bad_uw_errors = max(find(binocdf(0:Nuw,Nuw,0.5) <= 0.05))+1; + plot([bad_uw_errors bad_uw_errors],[0 1],';bad uw errors;'); hold off; grid + + xlabel('bits'); + printf("for Nuw = %d, suggest bad_uw_errors = %d\n", Nuw, bad_uw_errors); +end + +% Returns level threshold such that threshold_cdf of the tx magnitudes are +% beneath that level. Helper function that can be used to design +% the clipper level. See also https://www.rowetel.com/?p=7596 +function threshold_level = ofdm_determine_clip_threshold(tx, threshold_cdf) + Nsteps = 25; + mx = max(abs(tx)); + cdf = empirical_cdf(mx*(1:Nsteps)/Nsteps,abs(tx)); + threshold_level = find(cdf >= threshold_cdf)(1)*mx/25; + printf("threshold_cdf: %f threshold_level: %f\n", threshold_cdf, threshold_level); + figure(1); clf; [hh nn] = hist(abs(tx),Nsteps,1); + plotyy(nn,hh,mx*(1:Nsteps)/Nsteps,cdf); title('PDF and CDF Estimates'); grid; +end + + +% helper function that adds channel simulation and ensures we don't saturate int16 output samples +function [rx_real rx] = ofdm_channel(states, tx, SNR3kdB, channel, freq_offset_Hz) + [rx_real rx sigma] = channel_simulate(states.Fs, SNR3kdB, freq_offset_Hz, channel, tx, states.verbose); + + % multipath models can lead to clipping of int16 samples + num_clipped = length(find(abs(rx_real>32767))); + while num_clipped/length(rx_real) > 0.001 + rx_real /= 2; + num_clipped = length(find(abs(rx_real>32767))); + printf("WARNING: output samples clipped, reducing level\n") + end +endfunction + + |