rx: implement squelch + symsync reset

Whenever the squelch opens, the symsync is reset to prevent lock-up in
that module due to noise.

impl/src/layer1/rx.c

@@ -148,6 +148,49 @@ static bool acquire_preamble(layer1_rx_t *rx, const float complex sample, bool d
 }
 
 
+typedef enum squelch_state_t {
+	SQUELCH_CLOSED,
+	SQUELCH_OPEN,
+	SQUELCH_JUST_OPENED,
+};
+
+static enum squelch_state_t update_and_check_squelch(layer1_rx_t *rx, unsigned int sample_idx)
+{
+	float level = agc_crcf_get_rssi(rx->agc);
+	enum squelch_state_t result = SQUELCH_CLOSED;
+
+	if((sample_idx & 0xFF) == 0) { // every 256 samples
+		if(level < rx->noise_floor_level) {
+			rx->noise_floor_level = level;
+		} else {
+			level += 1e-3; // slowly increase the measured noise floor level to compensate for drift
+		}
+		agc_crcf_squelch_set_threshold(rx->agc, rx->noise_floor_level + 3.0f); // in dB
+	}
+
+	switch(agc_crcf_squelch_get_status(rx->agc)) {
+		case LIQUID_AGC_SQUELCH_RISE:
+			DEBUG_LOG("Squelch disabled at RSSI = %.3f dB\n", level);
+			result = SQUELCH_JUST_OPENED;
+			break;
+
+		case LIQUID_AGC_SQUELCH_SIGNALHI:
+			result = SQUELCH_OPEN;
+			break;
+
+		case LIQUID_AGC_SQUELCH_FALL:
+			DEBUG_LOG("Squelch enabled at RSSI = %.3f dB\n", level);
+		case LIQUID_AGC_SQUELCH_SIGNALLO:
+		case LIQUID_AGC_SQUELCH_ENABLED:
+		case LIQUID_AGC_SQUELCH_TIMEOUT:
+			result = SQUELCH_CLOSED;
+			break;
+	}
+
+	return result;
+}
+
+
 result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t sample_count)
 {
 	static size_t symbol_counter = 0;
@@ -174,6 +217,20 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 		float complex agc_out;
 		agc_crcf_execute(rx->agc, filtered_samples[i], &agc_out);
 
+		switch(update_and_check_squelch(rx, i)) {
+			case SQUELCH_CLOSED:
+				// ignore this sample
+				continue;
+
+			case SQUELCH_JUST_OPENED:
+				symsync_crcf_reset(rx->symsync);
+				// fall through
+			case SQUELCH_OPEN:
+				// sample processing continues
+				break;
+		}
+
+
 		// Mix the input signal with the carrier NCO, which oscillates at the
 		// frequency coarsly estimated so far.
 		float complex mixed_sample;
@@ -396,6 +453,11 @@ result_t layer1_rx_init(layer1_rx_t *rx, rx_callback_t callback)
 	rx->agc = agc_crcf_create();
 	agc_crcf_set_bandwidth(rx->agc, AGC_BW_ACQUISITION);
 
+	// set up squelch
+	rx->noise_floor_level = 0.0f;
+	agc_crcf_squelch_set_threshold(rx->agc, rx->noise_floor_level + 3.0f); // in dB
+	agc_crcf_squelch_enable(rx->agc);
+
 	// create NCOs for carrier frequency compensation
 	rx->carrier_coarse_nco = nco_crcf_create(LIQUID_NCO);
 	nco_crcf_set_frequency(rx->carrier_coarse_nco, 0.00f);

impl/src/layer1/rx.h

@@ -34,6 +34,8 @@ typedef struct
 	// AGC
 	agc_crcf agc;
 
+	float noise_floor_level;
+
 	// NCOs for carrier frequency offset correction
 	nco_crcf carrier_coarse_nco;
 	nco_crcf carrier_fine_nco;