Add C/I computation
Change-Id: Ib4ceec553f2e5f77bf3f6777724968456a180f5e
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index fcda5fa..1ea1c54 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -1455,6 +1455,28 @@
return out;
};
+static float computeCI(const signalVector *burst, CorrelationSequence *sync,
+ float toa, int start, complex xcorr)
+{
+ float S, C;
+ int ps;
+
+ /* Integer position where the sequence starts */
+ ps = start + 1 - sync->sequence->size() + (int)roundf(toa);
+
+ /* Estimate Signal power */
+ S = 0.0f;
+ for (int i=0, j=ps; i<(int)sync->sequence->size(); i++,j++)
+ S += (*burst)[j].norm2();
+ S /= sync->sequence->size();
+
+ /* Esimate Carrier power */
+ C = xcorr.norm2() / ((sync->sequence->size() - 1) * sync->gain.abs());
+
+ /* Interference = Signal - Carrier, so C/I = C / (S - C) */
+ return 3.0103f * log2f(C / (S - C));
+}
+
/*
* Detect a burst based on correlation and peak-to-average ratio
*
@@ -1470,6 +1492,7 @@
{
const signalVector *corr_in;
signalVector *dec = NULL;
+ complex xcorr;
if (sps == 4) {
dec = downsampleBurst(burst);
@@ -1497,15 +1520,18 @@
if ((*toa < 3 * sps) || (*toa > len - 3 * sps))
return 0;
- /* Peak -to-average ratio */
+ /* Peak-to-average ratio */
if (computePeakRatio(&corr, sps, *toa, *amp) < thresh)
return 0;
- /* Compute peak-to-average ratio. Reject if we don't have enough values */
- *amp = peakDetect(corr, toa, NULL);
+ /* Refine TOA and correlation value */
+ xcorr = peakDetect(corr, toa, NULL);
+
+ /* Compute C/I */
+ float CI = computeCI(corr_in, sync, *toa, start, xcorr);
/* Normalize our channel gain */
- *amp = *amp / sync->gain;
+ *amp = xcorr / sync->gain;
/* Compensate for residuate time lag */
*toa = *toa - sync->toa;