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Use a lookup table for SNR calculation.

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This commit is contained in:
Oliver Jowett 2014-09-22 14:53:06 +01:00
parent 2e45a59986
commit 76c958b03e
4 changed files with 30 additions and 12 deletions
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19
mode_s.c
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@ -1575,6 +1575,7 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
uint8_t theByte, theErrs;
int msglen, scanlen;
uint32_t sigLevel, noiseLevel;
uint16_t snr;
pPreamble = &m[j];
pPayload = &m[j+MODES_PREAMBLE_SAMPLES];
@ -1779,22 +1780,24 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
}
}
// adjust for magnitude zero offset
sigLevel += 365*56;
noiseLevel += 365*56;
// 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)
&& ((sigLevel * 10) > (noiseLevel * MODES_MSG_SQUELCH_FACTOR)) // (sigLevel/noiseLevel) > (MODES_MSG_SQUELCH_FACTOR/10)
&& ((2 * snr) > (int) (MODES_MSG_SQUELCH_DB * 10))
&& (errors <= MODES_MSG_ENCODER_ERRS) ) {
float snr;
// Set initial mm structure details
mm.timestampMsg = Modes.timestampBlk + (j*6);
snr = 5.0 * 20.0 * log10f( (float)sigLevel / noiseLevel ); // sig/noise levels are amplitudes, so square them when computing SNR
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)round(snr));
mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
mm.phase_corrected = use_correction;
// Decode the received message