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Use matched filter for AIS GMSK demodulation demo.

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Less than optimal for plain FSK, but flexible enough to also support GFSK/GMSK. Not at all optimized for SIMD instructions, so may not perform adequately at Bluetooth/GSM rates.
This commit is contained in:
Jared Boone
2015-09-28 11:02:39 -07:00
parent a9c190b6bb
commit 0bc8f551de
2 changed files with 36 additions and 12 deletions
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22
firmware/baseband/proc_fsk.cpp
View File

@@ -43,7 +43,7 @@ FSKProcessor::~FSKProcessor() {
} }
void FSKProcessor::configure(const FSKConfiguration new_configuration) { void FSKProcessor::configure(const FSKConfiguration new_configuration) {
demod.configure(sampling_rate, 2 * new_configuration.symbol_rate); // TODO: Matched filter characteristics are hard-coded for the moment. YUCK!
clock_recovery.configure(sampling_rate / 4, new_configuration.symbol_rate); clock_recovery.configure(sampling_rate / 4, new_configuration.symbol_rate);
access_code_correlator.configure( access_code_correlator.configure(
new_configuration.access_code, new_configuration.access_code,
@@ -84,16 +84,18 @@ void FSKProcessor::execute(buffer_c8_t buffer) {
// 76.8k // 76.8k
const buffer_s16_t work_demod_buffer {
(int16_t*)decimator_out.p,
decimator_out.count * sizeof(*decimator_out.p) / sizeof(int16_t)
};
auto demodulated = demod.execute(channel, work_demod_buffer);
// TODO: Factor out this hidden decimation magic. // TODO: Factor out this hidden decimation magic.
for(size_t i=0; i<demodulated.count; i+=4) { for(size_t i=0; i<channel.count; i+=4) {
clock_recovery(demodulated.p[i] / 32768.0f); std::complex<float> sample { channel.p[i].real(), channel.p[i].imag() };
mf_0.execute_once(sample);
if( mf_1.execute_once(sample) ) {
const auto value_0 = mf_0.get_output();
const float mag_0 = std::sqrt(value_0.real() * value_0.real() + value_0.imag() * value_0.imag());
const auto value_1 = mf_1.get_output();
const float mag_1 = std::sqrt(value_1.real() * value_1.real() + value_1.imag() * value_1.imag());
const float diff = mag_1 - mag_0;
clock_recovery(diff);
}
} }
i2s::i2s0::tx_mute(); i2s::i2s0::tx_mute();

26
firmware/baseband/proc_fsk.hpp
View File

@@ -26,7 +26,7 @@
#include "channel_decimator.hpp" #include "channel_decimator.hpp"
#include "dsp_decimate.hpp" #include "dsp_decimate.hpp"
#include "dsp_demodulate.hpp" #include "matched_filter.hpp"
#include "dsp_fir_taps.hpp" #include "dsp_fir_taps.hpp"
#include "clock_recovery.hpp" #include "clock_recovery.hpp"
@@ -40,6 +40,20 @@
#include <cstddef> #include <cstddef>
#include <bitset> #include <bitset>
constexpr std::array<std::complex<float>, 8> ais_taps_n { {
{ 0.00533687f, 0.00000000f }, { -0.00667109f, -0.00667109f },
{ -0.00000000f, -0.01334218f }, { -0.05145006f, 0.05145006f },
{ -0.14292666f, 0.00000000f }, { -0.05145006f, -0.05145006f },
{ 0.00000000f, 0.01334218f }, { -0.00667109f, 0.00667109f },
} };
constexpr std::array<std::complex<float>, 8> ais_taps_p { {
{ 0.00533687f, 0.00000000f }, { -0.00667109f, 0.00667109f },
{ -0.00000000f, 0.01334218f }, { -0.05145006f, -0.05145006f },
{ -0.14292666f, -0.00000000f }, { -0.05145006f, 0.05145006f },
{ 0.00000000f, -0.01334218f }, { -0.00667109f, -0.00667109f },
} };
class FSKProcessor : public BasebandProcessor { class FSKProcessor : public BasebandProcessor {
public: public:
FSKProcessor(MessageHandlerMap& message_handlers); FSKProcessor(MessageHandlerMap& message_handlers);
@@ -55,7 +69,15 @@ private:
ChannelDecimator decimator { ChannelDecimator::DecimationFactor::By16 }; ChannelDecimator decimator { ChannelDecimator::DecimationFactor::By16 };
const fir_taps_real<64>& channel_filter_taps = taps_64_lp_031_070_tfilter; const fir_taps_real<64>& channel_filter_taps = taps_64_lp_031_070_tfilter;
dsp::decimate::FIRAndDecimateBy2Complex<64> channel_filter { channel_filter_taps.taps }; dsp::decimate::FIRAndDecimateBy2Complex<64> channel_filter { channel_filter_taps.taps };
dsp::demodulate::FM demod { sampling_rate, 9600 * 2 };
dsp::matched_filter::MatchedFilter mf_0 {
ais_taps_n,
1
};
dsp::matched_filter::MatchedFilter mf_1 {
ais_taps_p,
1
};
clock_recovery::ClockRecovery clock_recovery { clock_recovery::ClockRecovery clock_recovery {
sampling_rate / 4, sampling_rate / 4,