rx: improvements made during on-air testing

- limit frequency adjustment range
- pre-filter the baseband signal to remove out-of-band interference
- reset the frequency acquisition periodically (every 30s) if no preamble is found

impl/src/layer1/rx.c

@@ -10,11 +10,15 @@
 
 #include "rx.h"
 
+#include "utils.h"
+
 #define HEADER_SIZE_BYTES 4
 #define FREQ_EST_L 8
 
-#define AGC_BW_ACQUISITION 5e-2
-#define AGC_BW_TRACKING    1e-4
+#define AGC_BW_ACQUISITION 5e-2f
+#define AGC_BW_TRACKING    1e-4f
+
+#define MAX_COARSE_FREQ_OFFSET 0.20f
 
 #define SHOW_DEBUG_LOG 0
 
@@ -83,6 +87,15 @@ static bool acquire_preamble(layer1_rx_t *rx, const float complex sample)
 			float freq_adjustment = (freq_est / RRC_SPS) * 0.3f;
 			nco_crcf_adjust_frequency(rx->carrier_coarse_nco, freq_adjustment);
 
+			float actual_freq = nco_crcf_get_frequency(rx->carrier_coarse_nco);
+			if(actual_freq > MAX_COARSE_FREQ_OFFSET) {
+				fprintf(stderr, ">");
+				nco_crcf_set_frequency(rx->carrier_coarse_nco, MAX_COARSE_FREQ_OFFSET);
+			} else if(actual_freq < -MAX_COARSE_FREQ_OFFSET) {
+				fprintf(stderr, "<");
+				nco_crcf_set_frequency(rx->carrier_coarse_nco, -MAX_COARSE_FREQ_OFFSET);
+			}
+
 			DEBUG_LOG("Frequency adjustment: %.6f - carrier frequency: %.6f\n", freq_adjustment, nco_crcf_get_frequency(rx->carrier_coarse_nco));
 
 			freq_est_history_write_idx = 0;
@@ -101,12 +114,19 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 	static size_t symbol_counter = 0;
 	static uint8_t symbols_int[1 << 12];
 
+	float complex samples2dump[sample_count];
+	size_t nsamples2dump = 0;
+
+	// filter the input
+	float complex filtered_samples[sample_count];
+	firfilt_crcf_execute_block(rx->channel_filter, (float complex *)samples, sample_count, filtered_samples);
+
 	for(unsigned int i = 0; i < sample_count; i++) {
 		rx_state_t last_state = rx->state;
 
 		// Apply the AGC.
 		float complex agc_out;
-		agc_crcf_execute(rx->agc, samples[i], &agc_out);
+		agc_crcf_execute(rx->agc, filtered_samples[i], &agc_out);
 
 		// Mix the input signal with the carrier NCO, which oscillates at the
 		// frequency coarsly estimated so far.
@@ -114,6 +134,8 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 		nco_crcf_step(rx->carrier_coarse_nco);
 		nco_crcf_mix_down(rx->carrier_coarse_nco, agc_out, &mixed_sample);
 
+		samples2dump[nsamples2dump++] = mixed_sample;
+
 		// run the timing synchronizer (works even with shifted frequency)
 		unsigned int out_len;
 		float complex symsync_out;
@@ -124,6 +146,14 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 				// Try to acquire packets by synchronizing the frequency
 				// (symbol-independent search) and correlating the preamble.
 				case RX_STATE_ACQUISITION:
+					// reset the acquisition periodically if the preamble is not found.
+					symbol_counter++;
+					if(symbol_counter == 30 * SYMBOL_RATE) {
+						symbol_counter = 0;
+						nco_crcf_set_frequency(rx->carrier_coarse_nco, 0.0f);
+						nco_crcf_set_phase(rx->carrier_coarse_nco, 0.0f);
+					}
+
 					if(acquire_preamble(rx, symsync_out)) {
 						// Preamble found and frequency corrected!
 						rx->callback(RX_EVT_PREAMBLE_FOUND, NULL, 0);
@@ -250,6 +280,8 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 		}
 	}
 
+	dump_array_cf(samples2dump, nsamples2dump, 1.0f, "/tmp/rx_dbg.cpx64");
+
 	return OK;
 }
 
@@ -260,6 +292,9 @@ result_t layer1_rx_init(layer1_rx_t *rx, rx_callback_t callback)
 
 	rx->state = RX_STATE_ACQUISITION;
 
+	// create channel FIR filter to remove out-of-band interferers
+	rx->channel_filter = firfilt_crcf_create_kaiser(21, 0.28f * RRC_SPS / 4, 60.0f, 0.0f);
+
 	// create the AGC
 	rx->agc = agc_crcf_create();
 	agc_crcf_set_bandwidth(rx->agc, AGC_BW_ACQUISITION);
@@ -296,6 +331,8 @@ result_t layer1_rx_init(layer1_rx_t *rx, rx_callback_t callback)
 
 result_t layer1_rx_shutdown(layer1_rx_t *rx)
 {
+	firfilt_crcf_destroy(rx->channel_filter);
+
 	agc_crcf_destroy(rx->agc);
 
 	nco_crcf_destroy(rx->carrier_coarse_nco);

impl/src/layer1/rx.h

@@ -27,6 +27,9 @@ typedef void (*rx_callback_t)(rx_evt_t evt, uint8_t *packet_data, size_t packet_
 
 typedef struct
 {
+	// Input channel filter
+	firfilt_crcf channel_filter;
+
 	// AGC
 	agc_crcf agc;