correlator: improved phase estimation

The phase is now calculated based on the preamble correlation directly instead
of the method of the frequency estimator.

impl/src/layer1/correlator.c

@@ -97,6 +97,11 @@ float complex correlator_step(correlator_ctx_t *ctx, float complex sample)
 		result += ctx->search_sequence[m] * ctx->input_history[nm];
 	}
 
+	// the current mean phase can be basically calculated for free here, as the
+	// sum of rotated symbols is already available, so we do so and cache that
+	// value.
+	ctx->phase_deviation = cargf(result);
+
 	return result;
 }
 
@@ -118,19 +123,10 @@ const float complex* correlator_get_conj_search_sequence(correlator_ctx_t *ctx)
 }
 
 
-float correlator_get_mean_phase_deviation(correlator_ctx_t *ctx, size_t L)
+float correlator_get_mean_phase_deviation(correlator_ctx_t *ctx)
 {
-	float complex mean_unrotated_symbol = 0;
-
-	size_t n = (ctx->history_ptr - ctx->search_sequence_len + 1) & ctx->buffer_size_mask;
-
-	for(size_t m = ctx->search_sequence_len - L; m < ctx->search_sequence_len; m++) {
-		size_t nm = (n + m) & ctx->buffer_size_mask;
-		mean_unrotated_symbol += ctx->search_sequence[m] * ctx->input_history[nm];
-	}
-
-	// no need to divide by ctx->search_sequence_len because division does not change the angle
-	return cargf(mean_unrotated_symbol);
+	// return cached value from last call to correlator_step().
+	return ctx->phase_deviation;
 }
 
 
@@ -147,5 +143,12 @@ float correlator_get_mean_frequency_deviation(correlator_ctx_t *ctx, size_t L, f
 		z[m - m0] = conjf(ctx->search_sequence[m]) * ctx->input_history[nm];
 	}
 
-	return freq_est_data_free(z, L, phase_offset);
+	float freq = freq_est_data_free(z, L, NULL);
+
+	// we calculate the final phase based on the phase estimated from the
+	// preamble and not the phase from the frequency estimator because this
+	// method is more reliable.
+	*phase_offset = ctx->phase_deviation + (ctx->search_sequence_len+1)/2.0f * freq;
+
+	return freq;
 }

impl/src/layer1/correlator.h

@@ -24,6 +24,8 @@ typedef struct
 	float complex *search_sequence;
 	float complex *input_history;
 
+	float phase_deviation;
+
 	/*!
 	 * \brief The history pointer.
 	 *
@@ -120,10 +122,9 @@ const float complex* correlator_get_conj_search_sequence(correlator_ctx_t *ctx);
  * found.
  *
  * \param ctx          The correlator context to use.
- * \param L            Number of symbols to use (at the end of the sequence)
  * \returns            The mean phase deviation in radians.
  */
-float correlator_get_mean_phase_deviation(correlator_ctx_t *ctx, size_t L);
+float correlator_get_mean_phase_deviation(correlator_ctx_t *ctx);
 
 /*!
  * \brief Calculate mean frequency deviation between current input and search sequence.