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Add SNR calculation in oversample mode.

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This commit is contained in:
Oliver Jowett 2014-09-27 16:47:50 +01:00
parent e118668925
commit ca372ed105
1 changed files with 54 additions and 2 deletions
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56
mode_s.c
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@ -2074,6 +2074,8 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
int msglen, scanlen;
uint16_t *pPtr;
uint8_t theByte, theErrs;
uint32_t sigLevel, noiseLevel;
uint16_t snr;
// Look for a message starting at around sample 0 with phase offset 3..7
@ -2098,30 +2100,40 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
preamble[10] < preamble[11]) { // 11-12
// peaks at 1,3,9,11-12: phase 3
high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4;
sigLevel = preamble[1] + preamble[3] + preamble[9];
noiseLevel = preamble[5] + preamble[6] + preamble[7];
} else if (preamble[1] > preamble[2] && // 1
preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
preamble[11] < preamble[12]) { // 12
// peaks at 1,3,9,12: phase 4
high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4;
sigLevel = preamble[1] + preamble[3] + preamble[9] + preamble[12];
noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
} else if (preamble[1] > preamble[2] && // 1
preamble[2] < preamble[3] && preamble[4] > preamble[5] && // 3-4
preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10
preamble[11] < preamble[12]) { // 12
// peaks at 1,3-4,9-10,12: phase 5
high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4;
sigLevel = preamble[1] + preamble[12];
noiseLevel = preamble[6] + preamble[7];
} else if (preamble[1] > preamble[2] && // 1
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
preamble[11] < preamble[12]) { // 12
// peaks at 1,4,10,12: phase 6
high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4;
sigLevel = preamble[1] + preamble[4] + preamble[10] + preamble[12];
noiseLevel = preamble[5] + preamble[6] + preamble[7] + preamble[8];
} else if (preamble[2] > preamble[3] && // 1-2
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
preamble[11] < preamble[12]) { // 12
// peaks at 1-2,4,10,12: phase 7
high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4;
sigLevel = preamble[4] + preamble[10] + preamble[12];
noiseLevel = preamble[6] + preamble[7] + preamble[8];
} else {
// no suitable peaks
continue;
@ -2196,6 +2208,35 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
case 4:
test = correlate_phase4(pPtr);
// A phase-4 bit exactly straddles a sample boundary.
// Here's what a 1-0 bit with phase 4 looks like:
//
// |SYM 1|
// xxx| | |xxx
// |SYM 2|
//
// 012340123401234012340 <-- sample phase
// | 0 | 1 | 2 | 3 | <-- sample boundaries
//
// Samples 1 and 2 only have power from symbols 1 and 2.
// So we can use this to extract signal/noise values
// as one of the two symbols is high (signal) and the
// other is low (noise)
//
// This also gives us an equal number of signal and noise
// samples, which is convenient. Using the first half of
// a phase 0 bit, or the second half of a phase 3 bit, would
// also work, but we have no guarantees about how many signal
// or noise bits we'd see in those phases.
if (test < 0) { // 0 1
noiseLevel += pPtr[1];
sigLevel += pPtr[2];
} else { // 1 0
sigLevel += pPtr[1];
noiseLevel += pPtr[2];
}
phase = 1;
pPtr += 3;
break;
@ -2290,13 +2331,24 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
}
}
// snr = 5 * 20log10(sigLevel / noiseLevel) (in units of 0.2dB)
// = 100log10(sigLevel) - 100log10(noiseLevel)
while (sigLevel > 65535 || noiseLevel > 65535) {
sigLevel >>= 1;
noiseLevel >>= 1;
}
snr = Modes.log10lut[sigLevel] - Modes.log10lut[noiseLevel];
// When we reach this point, if error is small, and the signal strength is large enough
// we may have a Mode S message on our hands. It may still be broken and the CRC may not
// be correct, but this can be handled by the next layer.
if ( (msglen > 0) && (errors <= MODES_MSG_ENCODER_ERRS) ) {
if ( (msglen)
// && ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
// Set initial mm structure details
mm.timestampMsg = Modes.timestampBlk + (j*5) + initial_phase;
mm.signalLevel = 0;
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
mm.phase_corrected = 0;
//dumpRawMessage("decoded with oversampling", msg, m, j);