Transceiver52M: Add FCCH based frequency correction

Enable frequency detection and correction by buffering the previous
frame to allow FCCH measurement and compensation after frame timing is
locked using the SCH. When the SCH is detected and symbol timing
matched, measure the FCCH burst from one frame prior and compensate by
baseband tuning the DDC on the device. Avoid appying frequency
corrections to the RF portion due to possible tuning delays, which is
not an issue with DDC tuning.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

1 files changed, 37 insertions, 3 deletions

diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index c3446f8..60be8d2 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -45,10 +45,11 @@ using namespace GSM;
 
 /* Number of running values use in noise average */
 #define NOISE_CNT			20
+#define FREQ_CNT			20
 
 TransceiverState::TransceiverState()
-  : mRetrans(false), mNoiseLev(0.0), mNoises(NOISE_CNT),
-    mode(Transceiver::TRX_MODE_OFF)
+  : mRetrans(false), mNoiseLev(0.0),
+    mNoises(NOISE_CNT), mFreqOffsets(FREQ_CNT), mode(Transceiver::TRX_MODE_OFF)
 {
   for (int i = 0; i < 8; i++) {
     chanType[i] = Transceiver::NONE;
@@ -56,6 +57,7 @@ TransceiverState::TransceiverState()
     chanResponse[i] = NULL;
     DFEForward[i] = NULL;
     DFEFeedback[i] = NULL;
+    prevFrame[i] = NULL;
 
     for (int n = 0; n < 102; n++)
       fillerTable[n][i] = NULL;
@@ -452,6 +454,34 @@ bool Transceiver::decodeSCH(SoftVector *burst, GSM::Time *time)
   return true;
 }
 
+#define FCCH_OFFSET_LIMIT	2e3
+#define FCCH_ADJUST_LIMIT	20.0
+
+/* Apply FCCH frequency correction */
+bool Transceiver::correctFCCH(TransceiverState *state, signalVector *burst)
+{
+  double offset, avg;
+
+  if (!burst)
+    return false;
+
+  offset = gsm_fcch_offset((float *) burst->begin(), burst->size());
+  if (offset > FCCH_OFFSET_LIMIT)
+    return false;
+
+  state->mFreqOffsets.insert(offset);
+  avg = state->mFreqOffsets.avg();
+
+  if (state->mFreqOffsets.full())
+    std::cout << "FCCH: Frequency offset  " << avg << " Hz" << std::endl;
+
+  if (state->mFreqOffsets.full() && (fabs(avg) > FCCH_ADJUST_LIMIT)) {
+    mRadioInterface->tuneRxOffset(-avg);
+    state->mFreqOffsets.reset();
+  }
+
+  return true;
+}
 
 /*
  * Detect normal burst training sequence midamble. Update equalization
@@ -614,6 +644,7 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime, int &RSSI,
   /* MS: Decode SCH and adjust GSM clock */
   if ((state->mode == TRX_MODE_MS_ACQUIRE) ||
       (state->mode == TRX_MODE_MS_TRACK)) {
+    correctFCCH(state, state->prevFrame[burst_time.TN()]->getVector());
 
     if (decodeSCH(bits, &sch_time)) {
       if (state->mode == TRX_MODE_MS_ACQUIRE) {
@@ -635,11 +666,14 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime, int &RSSI,
   RSSI = (int) floor(20.0 * log10(rxFullScale / avg));
   timingOffset = (int) round(toa * 256.0 / mSPSRx);
 
-  delete radio_burst;
+  delete state->prevFrame[burst_time.TN()];
+  state->prevFrame[burst_time.TN()] = radio_burst;
 
   return bits;
 
 release:
+  delete state->prevFrame[burst_time.TN()];
+  state->prevFrame[burst_time.TN()] = radio_burst;
   delete bits;
   return NULL;
 }