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129 lines
3.3 KiB
C++
129 lines
3.3 KiB
C++
/*
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Schmidl & Cox correlator
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Copyright 2021 Ahmet Inan <inan@aicodix.de>
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*/
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#pragma once
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#include "fft.hh"
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#include "sma.hh"
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#include "phasor.hh"
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#include "trigger.hh"
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template<typename value, typename cmplx, int search_pos, int symbol_len, int guard_len>
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class SchmidlCox {
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typedef DSP::Const<value> Const;
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static const int match_len = guard_len | 1;
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static const int match_del = (match_len - 1) / 2;
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DSP::FastFourierTransform<symbol_len, cmplx, -1> fwd;
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DSP::FastFourierTransform<symbol_len, cmplx, 1> bwd;
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DSP::SMA4<cmplx, value, symbol_len, false> cor;
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DSP::SMA4<value, value, 2 * symbol_len, false> pwr;
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DSP::SMA4<value, value, match_len, false> match;
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DSP::Delay<value, match_del> align;
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DSP::SchmittTrigger<value> threshold;
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DSP::FallingEdgeTrigger falling;
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cmplx tmp0[symbol_len], tmp1[symbol_len];
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cmplx kern[symbol_len];
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value timing_max = 0;
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value phase_max = 0;
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int index_max = 0;
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static int bin(int carrier) {
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return (carrier + symbol_len) % symbol_len;
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}
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static cmplx demod_or_erase(cmplx curr, cmplx prev, value pwr) {
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if (norm(curr) > pwr && norm(prev) > pwr) {
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cmplx cons = curr / prev;
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if (norm(cons) < value(4))
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return cons;
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}
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return 0;
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}
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public:
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int symbol_pos = 0;
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value cfo_rad = 0;
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value frac_cfo = 0;
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SchmidlCox(const cmplx *sequence) : threshold(value(0.17 * match_len), value(0.19 * match_len)) {
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fwd(kern, sequence);
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for (int i = 0; i < symbol_len; ++i)
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kern[i] = conj(kern[i]) / value(symbol_len);
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}
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bool operator()(const cmplx *samples) {
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cmplx P = cor(samples[search_pos + symbol_len] * conj(samples[search_pos + 2 * symbol_len]));
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value R = value(0.5) * pwr(norm(samples[search_pos + 2 * symbol_len]));
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value min_R = 0.00001 * symbol_len;
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R = std::max(R, min_R);
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value timing = match(norm(P) / (R * R));
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value phase = align(arg(P));
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bool collect = threshold(timing);
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bool process = falling(collect);
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if (!collect && !process)
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return false;
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if (timing_max < timing) {
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timing_max = timing;
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phase_max = phase;
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index_max = match_del;
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} else if (index_max < symbol_len + guard_len + match_del) {
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++index_max;
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}
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if (!process)
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return false;
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frac_cfo = phase_max / value(symbol_len);
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DSP::Phasor<cmplx> osc;
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osc.omega(frac_cfo);
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symbol_pos = search_pos - index_max;
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index_max = 0;
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timing_max = 0;
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for (int i = 0; i < symbol_len; ++i)
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tmp1[i] = samples[i + symbol_pos + symbol_len] * osc();
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fwd(tmp0, tmp1);
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value min_pwr = 0;
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for (int i = 0; i < symbol_len; ++i)
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min_pwr += norm(tmp0[i]);
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min_pwr /= symbol_len;
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for (int i = 0; i < symbol_len; ++i)
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tmp1[i] = demod_or_erase(tmp0[i], tmp0[bin(i - 1)], min_pwr);
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fwd(tmp0, tmp1);
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for (int i = 0; i < symbol_len; ++i)
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tmp0[i] *= kern[i];
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bwd(tmp1, tmp0);
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int shift = 0;
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value peak = 0;
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value next = 0;
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for (int i = 0; i < symbol_len; ++i) {
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value power = norm(tmp1[i]);
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if (power > peak) {
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next = peak;
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peak = power;
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shift = i;
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} else if (power > next) {
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next = power;
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}
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}
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if (peak <= next * 4)
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return false;
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int pos_err = std::nearbyint(arg(tmp1[shift]) * symbol_len / Const::TwoPi());
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if (abs(pos_err) > guard_len / 2)
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return false;
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symbol_pos -= pos_err;
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cfo_rad = shift * (Const::TwoPi() / symbol_len) - frac_cfo;
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if (cfo_rad >= Const::Pi())
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cfo_rad -= Const::TwoPi();
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return true;
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}
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};
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