Change in ...osmo-trx[master]: Add C/I computation

[pespin]

Hello tnt,

I'd like you to do a code review. Please visit

    https://gerrit.osmocom.org/c/osmo-trx/+/14674

to review the following change.


Change subject: Add C/I computation
......................................................................

Add C/I computation

Change-Id: Ib4ceec553f2e5f77bf3f6777724968456a180f5e
Signed-off-by: Sylvain Munaut <tnt at 246tNt.com>
---
M Transceiver52M/Transceiver.cpp
M Transceiver52M/sigProcLib.cpp
M Transceiver52M/sigProcLib.h
3 files changed, 34 insertions(+), 5 deletions(-)



  git pull ssh://gerrit.osmocom.org:29418/osmo-trx refs/changes/74/14674/1

diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index 32297b6..f90decd 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -621,7 +621,7 @@
   bi->modulation = (type == EDGE) ? MODULATION_8PSK : MODULATION_GMSK;
   bi->tss = 0; /* TODO: we only support tss 0 right now */
   bi->tsc = 0;
-  bi->ci = 0; /* TODO: calculate C/I and assign the value here, see branch tnt/ci. */
+  bi->ci = 0;
 
   /* Select the diversity channel with highest energy */
   for (size_t i = 0; i < radio_burst->chans(); i++) {
@@ -670,6 +670,7 @@
   type = (CorrType) rc;
   bi->toa = ebp.toa;
   bi->tsc = ebp.tsc;
+  bi->ci = (int16_t) ebp.ci;
   rxBurst = demodAnyBurst(*burst, mSPSRx, ebp.amp, ebp.toa, type);
 
   /* EDGE demodulator returns 444 (gSlotLen * 3) bits */
diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index c22009a..a65ea5e 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -1453,6 +1453,28 @@
   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
  *
@@ -1468,6 +1490,7 @@
 {
   const signalVector *corr_in;
   signalVector *dec = NULL;
+  complex xcorr;
 
   if (sps == 4) {
     dec = downsampleBurst(burst);
@@ -1495,15 +1518,18 @@
   if ((ebp->toa < 3 * sps) || (ebp->toa > len - 3 * sps))
     return 0;
 
-  /* Peak -to-average ratio */
+  /* Peak-to-average ratio */
   if (computePeakRatio(&corr, sps, ebp->toa, ebp->amp) < thresh)
     return 0;
 
-  /* 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;
@@ -1563,6 +1589,7 @@
   } 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 @@
         complex amp;
         float toa;
         uint8_t tsc;
+        float ci;
 };
 /**
         8-PSK/GMSK/RACH burst detector

-- 
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Gerrit-Project: osmo-trx
Gerrit-Branch: master
Gerrit-Change-Id: Ib4ceec553f2e5f77bf3f6777724968456a180f5e
Gerrit-Change-Number: 14674
Gerrit-PatchSet: 1
Gerrit-Owner: pespin <pespin at sysmocom.de>
Gerrit-Reviewer: tnt <tnt at 246tNt.com>
Gerrit-MessageType: newchange
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