Transceiver: Compansate for frequency error.
diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index b9902c2..753f7f1 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -1041,11 +1041,41 @@
return sqrt(rms/vec.size());
}
+bool vectorLinearFit(const Vector<float> &y, float &slope, float &interceptor)
+{
+ int len_y = y.size();
+ float numerator = 0.0;
+ float denominator = 0.0;
+ float avg_x = len_y/2.0;
+ float avg_y = 0.0;
+
+ if (len_y==0)
+ return false;
+
+ for (int i=0; i<len_y; i++)
+ avg_y += y[i];
+ avg_y /= len_y;
+
+ for (int i=0; i<len_y; i++) {
+ numerator += (i - avg_x) * (y[i] - avg_y);
+ denominator += (i - avg_x) * (i - avg_x);
+ }
+
+ if (denominator == 0.0)
+ return false;
+
+ slope = numerator/denominator;
+ interceptor = avg_y - avg_x*slope;
+
+ return true;
+}
+
void Transceiver::estimateBurstQuality(const BitVector &wBits, signalVector *received,
const GSM::Time &wTime, size_t chan,
Transceiver::BurstQuality &qual)
{
signalVector *burst;
+ float slope, interceptor;
// this code supports only 4 SPS modulation
// we also assume that received vector is 1 SPS
@@ -1065,11 +1095,24 @@
// calculate phase error for each bit
for (size_t i=0; i<qual.phase_err.size(); i++) {
float rx_phase = (*received)[i].arg();
+ // modulated data is 4SPS and is 1/4 bit shifted
float mod_phase = (*burst)[1+(2+i)*4].arg();
qual.phase_err[i] = wrapAnglePi(rx_phase - mod_phase);
qual.phase_err_deg[i] = rad2deg(qual.phase_err[i]);
}
+ // compensate for frequency error
+ if (vectorLinearFit(qual.phase_err, slope, interceptor)) {
+ for (size_t i=0; i<qual.phase_err.size(); i++) {
+ qual.phase_err[i] -= i*slope + interceptor;
+ }
+ }
+
+ // convert to degrees
+ for (size_t i=0; i<qual.phase_err.size(); i++) {
+ qual.phase_err_deg[i] = rad2deg(qual.phase_err[i]);
+ }
+
// calculate Peak and RMS values
qual.phase_err_max_idx = vectorMaxAbs(qual.phase_err);
qual.phase_err_max = qual.phase_err[qual.phase_err_max_idx];