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M17 demodulation improvement 

Add logging in syncword sweep, fix unsigned underflow bug,
fixed symbol average computation for quantization.
Do syncword sweep whenever available.

TG-81
This commit is contained in:
Niccolò Izzo 2022-05-20 12:22:57 +02:00 committed by Silvano Seva
parent d17d683b2d
commit 29ad0830f0
2 changed files with 92 additions and 55 deletions
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18
openrtx/include/protocols/M17/M17Demodulator.h
View File

@ -37,6 +37,7 @@
#include <stdio.h>
#include <M17/M17Datatypes.h>
#include <M17/M17Constants.h>
#include <cmath>
namespace M17
{
@ -116,11 +117,14 @@ private:
*/
static constexpr size_t M17_RX_SAMPLE_RATE = 24000;
static constexpr size_t M17_SAMPLES_PER_SYMBOL = M17_RX_SAMPLE_RATE / M17_SYMBOL_RATE;
static constexpr size_t M17_FRAME_SAMPLES = M17_FRAME_SYMBOLS * M17_SAMPLES_PER_SYMBOL;
static constexpr size_t M17_SAMPLE_BUF_SIZE = M17_FRAME_SAMPLES / 2;
static constexpr size_t M17_SYNCWORD_SAMPLES = M17_SAMPLES_PER_SYMBOL * M17_SYNCWORD_SYMBOLS;
static constexpr size_t M17_BRIDGE_SIZE = M17_SYNCWORD_SAMPLES;
static constexpr int8_t SYNC_SWEEP_WIDTH = 10;
static constexpr int8_t SYNC_SWEEP_OFFSET = ceil(SYNC_SWEEP_WIDTH / M17_SAMPLES_PER_SYMBOL);
static constexpr size_t M17_BRIDGE_SIZE = M17_SYNCWORD_SAMPLES + 2 * SYNC_SWEEP_WIDTH;
static constexpr float CONV_STATS_ALPHA = 0.005f;
static constexpr float CONV_THRESHOLD_FACTOR = 3.40;
@ -146,7 +150,7 @@ private:
bool locked; ///< A syncword was correctly demodulated.
bool newFrame; ///< A new frame has been fully decoded.
int16_t basebandBridge[M17_BRIDGE_SIZE] = { 0 }; ///< Bridge buffer
uint16_t phase; ///< Phase of the signal w.r.t. sampling
int16_t phase; ///< Phase of the signal w.r.t. sampling
/*
* State variables
@ -161,10 +165,12 @@ private:
/*
* Quantization statistics computation
*/
std::deque<int16_t> qnt_pos_fifo;
std::deque<int16_t> qnt_neg_fifo;
float qnt_pos_avg = 0.0f; ///< Rolling average of positive samples
float qnt_neg_avg = 0.0f; ///< Rolling average of negative samples
int8_t qnt_pos_cnt; ///< Number of received positive samples
int8_t qnt_neg_cnt; ///< Number of received negative samples
int32_t qnt_pos_acc; ///< Accumulator for quantization average
int32_t qnt_neg_acc; ///< Accumulator for quantization average
float qnt_pos_avg = 0.0f; ///< Rolling average of positive samples
float qnt_neg_avg = 0.0f; ///< Rolling average of negative samples
/*
* DSP filter state

129
openrtx/src/protocols/M17/M17Demodulator.cpp
View File

@ -198,24 +198,31 @@ void M17Demodulator::resetQuantizationStats()
void M17Demodulator::updateQuantizationStats(int32_t frame_index,
int32_t symbol_index)
{
int16_t sample = baseband.data[symbol_index];
if (sample > 0)
qnt_pos_fifo.push_front(sample);
int16_t sample = 0;
// When we are at negative indices use bridge buffer
if (symbol_index < 0)
sample = basebandBridge[M17_BRIDGE_SIZE + symbol_index];
else
qnt_neg_fifo.push_front(sample);
sample = baseband.data[symbol_index];
if (sample > 0)
{
qnt_pos_acc += sample;
qnt_pos_cnt++;
}
else
{
qnt_neg_acc += sample;
qnt_neg_cnt++;
}
// If we reached end of the syncword, compute average and reset queue
if(frame_index == M17_SYNCWORD_SYMBOLS - 1)
{
int32_t acc = 0;
for(auto e : qnt_pos_fifo)
acc += e;
qnt_pos_avg = acc / static_cast<float>(qnt_pos_fifo.size());
acc = 0;
for(auto e : qnt_neg_fifo)
acc += e;
qnt_neg_avg = acc / static_cast<float>(qnt_neg_fifo.size());
qnt_pos_fifo.clear();
qnt_neg_fifo.clear();
qnt_pos_avg = qnt_pos_acc / static_cast<float>(qnt_pos_cnt);
qnt_neg_avg = qnt_neg_acc / static_cast<float>(qnt_neg_cnt);
qnt_pos_acc = 0;
qnt_neg_acc = 0;
qnt_pos_cnt = 0;
qnt_neg_cnt = 0;
}
}
@ -247,7 +254,7 @@ sync_t M17Demodulator::nextFrameSync(int32_t offset)
// Leverage the fact LSF syncword is the opposite of the frame syncword
// to detect both syncwords at once. Stop early because convolution needs
// access samples ahead of the starting offset.
int32_t maxLen = static_cast < int32_t >(baseband.len - M17_BRIDGE_SIZE);
int32_t maxLen = static_cast < int32_t >(baseband.len - M17_SYNCWORD_SAMPLES);
for(int32_t i = offset; (syncword.index == -1) && (i < maxLen); i++)
{
int32_t conv = convolution(i, stream_syncword, M17_SYNCWORD_SYMBOLS);
@ -291,9 +298,9 @@ int8_t M17Demodulator::quantize(int32_t offset)
sample = basebandBridge[M17_BRIDGE_SIZE + offset];
else // Otherwise use regular data buffer
sample = baseband.data[offset];
if (sample > static_cast< int16_t >(qnt_pos_avg / 2.0))
if (sample > static_cast< int16_t >(qnt_pos_avg / 1.5f))
return +3;
else if (sample < static_cast< int16_t >(qnt_neg_avg / 2.0))
else if (sample < static_cast< int16_t >(qnt_neg_avg / 1.5f))
return -3;
else if (sample > 0)
return +1;
@ -308,7 +315,7 @@ const frame_t& M17Demodulator::getFrame()
return *activeFrame;
}
bool M17::M17Demodulator::isLocked()
bool M17Demodulator::isLocked()
{
return locked;
}
@ -317,12 +324,30 @@ int32_t M17Demodulator::syncwordSweep(int32_t offset)
{
int32_t max_conv = 0, max_index = 0;
// Start from 5 samples behind, end 5 samples after
for(int i = -5; i <= 5; i++)
for(int i = -SYNC_SWEEP_WIDTH; i <= SYNC_SWEEP_WIDTH; i++)
{
// TODO: Extend for LSF and BER syncwords
int32_t conv = convolution(offset + i,
stream_syncword,
M17_SYNCWORD_SYMBOLS);
#ifdef ENABLE_DEMOD_LOG
int16_t sample;
if (offset + i < 0)
sample = basebandBridge[M17_BRIDGE_SIZE + offset + i];
else
sample = baseband.data[offset + i];
log_entry_t log;
log =
{
sample,
conv,
0.0,
offset + i,
0.0,0.0,0,0
};
logBuf.push(log, false);
#endif
if (conv > max_conv)
{
max_conv = conv;
@ -341,11 +366,11 @@ bool M17Demodulator::update()
// Read samples from the ADC
baseband = inputStream_getData(basebandId);
// Apply DC removal filter
dsp_dcRemoval(&dsp_state, baseband.data, baseband.len);
if(baseband.data != NULL)
{
// Apply DC removal filter
dsp_dcRemoval(&dsp_state, baseband.data, baseband.len);
// Apply RRC on the baseband buffer
for(size_t i = 0; i < baseband.len; i++)
{
@ -354,9 +379,7 @@ bool M17Demodulator::update()
}
// Process the buffer
while((syncword.index != -1) &&
((static_cast< int32_t >(M17_SAMPLES_PER_SYMBOL * decoded_syms) +
offset + phase) < static_cast < int32_t >(baseband.len)))
while(syncword.index != -1)
{
// If we are not demodulating a syncword, search for one
@ -369,6 +392,7 @@ bool M17Demodulator::update()
offset = syncword.index + 1;
phase = 0;
frame_index = 0;
decoded_syms = 0;
}
}
// While we detected a syncword, demodulate available samples
@ -378,6 +402,8 @@ bool M17Demodulator::update()
int32_t symbol_index = offset
+ phase
+ (M17_SAMPLES_PER_SYMBOL * decoded_syms);
if (symbol_index >= static_cast<int32_t>(baseband.len))
break;
// Update quantization stats only on syncwords
if (frame_index < M17_SYNCWORD_SYMBOLS)
updateQuantizationStats(frame_index, symbol_index);
@ -393,9 +419,9 @@ bool M17Demodulator::update()
log_entry_t log =
{
baseband.data[symbol_index + i],
0,0.0,symbol_index + i,
qnt_pos_avg / 2.0f,
qnt_neg_avg / 2.0f,
phase,0.0,symbol_index + i,
qnt_pos_avg / 1.5f,
qnt_neg_avg / 1.5f,
symbol,
frame_index
};
@ -409,20 +435,6 @@ bool M17Demodulator::update()
decoded_syms++;
frame_index++;
// If the frame buffer is full switch active and idle frame
if (frame_index == M17_FRAME_SYMBOLS)
{
std::swap(activeFrame, idleFrame);
frame_index = 0;
newFrame = true;
// Locate syncword to correct clock skew between Tx and Rx
int32_t expected_sync =
offset + phase + M17_SAMPLES_PER_SYMBOL * decoded_syms;
int32_t new_sync = syncwordSweep(expected_sync);
phase += new_sync;
}
if (frame_index == M17_SYNCWORD_SYMBOLS)
{
// If syncword is not valid, lock is lost, accept 2 bit errors
@ -444,11 +456,33 @@ bool M17Demodulator::update()
std::swap(activeFrame, idleFrame);
frame_index = 0;
newFrame = true;
phase = 0;
}
// Correct syncword found
else
locked = true;
}
// Locate syncword to correct clock skew between Tx and Rx
if (frame_index == M17_SYNCWORD_SYMBOLS + SYNC_SWEEP_OFFSET)
{
// Find index (possibly negative) of the syncword
int32_t expected_sync =
offset + phase +
M17_SAMPLES_PER_SYMBOL * decoded_syms -
M17_SYNCWORD_SAMPLES -
SYNC_SWEEP_OFFSET * M17_SAMPLES_PER_SYMBOL;
int32_t sync_skew = syncwordSweep(expected_sync);
phase += sync_skew;
}
// If the frame buffer is full switch active and idle frame
if (frame_index == M17_FRAME_SYMBOLS)
{
std::swap(activeFrame, idleFrame);
frame_index = 0;
newFrame = true;
}
}
}
@ -458,13 +492,10 @@ bool M17Demodulator::update()
// Compute phase of next buffer
phase = (static_cast<int32_t> (phase) + offset + baseband.len) % M17_SAMPLES_PER_SYMBOL;
}
else
{
// Copy last N samples to bridge buffer
memcpy(basebandBridge,
baseband.data + (baseband.len - M17_BRIDGE_SIZE),
sizeof(int16_t) * M17_BRIDGE_SIZE);
}
// Copy last N samples to bridge buffer
memcpy(basebandBridge,
baseband.data + (baseband.len - M17_BRIDGE_SIZE),
sizeof(int16_t) * M17_BRIDGE_SIZE);
}
return newFrame;