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-rw-r--r--Transceiver52M/sigProcLib.cpp63
-rw-r--r--Transceiver52M/sigProcLib.h1
2 files changed, 51 insertions, 13 deletions
diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index c22009a..860bbe4 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -1454,6 +1454,33 @@ static signalVector *downsampleBurst(const signalVector &burst)
};
/*
+ * Computes C/I (Carrier-to-Interference ratio) in dB (deciBels).
+ * It is computed from the training sequence of each received burst,
+ * by comparing the "ideal" training sequence with the actual one.
+ */
+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
*
* For one sampler-per-symbol, perform fast peak detection (no interpolation)
@@ -1468,6 +1495,8 @@ static int detectBurst(const signalVector &burst,
{
const signalVector *corr_in;
signalVector *dec = NULL;
+ complex xcorr;
+ int rc = 1;
if (sps == 4) {
dec = downsampleBurst(burst);
@@ -1480,35 +1509,42 @@ static int detectBurst(const signalVector &burst,
/* Correlate */
if (!convolve(corr_in, sync->sequence, &corr,
CUSTOM, start, len)) {
- delete dec;
- return -1;
+ rc = -1;
+ goto del_ret;
}
- delete dec;
-
/* Running at the downsampled rate at this point */
sps = 1;
/* Peak detection - place restrictions at correlation edges */
ebp->amp = fastPeakDetect(corr, &ebp->toa);
- if ((ebp->toa < 3 * sps) || (ebp->toa > len - 3 * sps))
- return 0;
+ if ((ebp->toa < 3 * sps) || (ebp->toa > len - 3 * sps)) {
+ rc = 0;
+ goto del_ret;
+ }
- /* Peak -to-average ratio */
- if (computePeakRatio(&corr, sps, ebp->toa, ebp->amp) < thresh)
- return 0;
+ /* Peak-to-average ratio */
+ if (computePeakRatio(&corr, sps, ebp->toa, ebp->amp) < thresh) {
+ rc = 0;
+ goto del_ret;
+ }
- /* Compute peak-to-average ratio. Reject if we don't have enough values */
- ebp->amp = peakDetect(corr, &ebp->toa, NULL);
+ /* Refine TOA and correlation value */
+ xcorr = peakDetect(corr, &ebp->toa, NULL);
+
+ /* Compute C/I */
+ ebp->ci = computeCI(corr_in, sync, ebp->toa, start, xcorr);
/* Normalize our channel gain */
- ebp->amp = ebp->amp / sync->gain;
+ ebp->amp = xcorr / sync->gain;
/* Compensate for residuate time lag */
ebp->toa = ebp->toa - sync->toa;
- return 1;
+del_ret:
+ delete dec;
+ return rc;
}
static float maxAmplitude(const signalVector &burst)
@@ -1563,6 +1599,7 @@ static int detectGeneralBurst(const signalVector &rxBurst, float thresh, int sps
} else if (!rc) {
ebp->amp = 0.0f;
ebp->toa = 0.0f;
+ ebp->ci = 0.0f;
return clipping?-SIGERR_CLIP:SIGERR_NONE;
}
diff --git a/Transceiver52M/sigProcLib.h b/Transceiver52M/sigProcLib.h
index 6d4ca9b..fd9a5f0 100644
--- a/Transceiver52M/sigProcLib.h
+++ b/Transceiver52M/sigProcLib.h
@@ -111,6 +111,7 @@ struct estim_burst_params {
complex amp;
float toa;
uint8_t tsc;
+ float ci;
};
/**
8-PSK/GMSK/RACH burst detector