Implement time-based carrier sensing

In the following events, the modem cannot switch to transmit mode for the
given time:

- after a preamble has been received, for 500 ms
- after a packet has been successfully decoded, for 1 ms
- after receive mode was initially enabled, for 500 ms

Note that the times are that long to work around the RX/TX switching of
the hackrf and can be hopefully reduced to <10 milliseconds in a later
version.

impl/CMakeLists.txt

@@ -53,6 +53,7 @@ target_link_libraries(
 	${CMAKE_PROJECT_NAME}
 	libliquid.a
 	m
+	rt
 	fftw3f
 	fec
 	SoapySDR

impl/src/config.h

@@ -3,6 +3,12 @@
 
 #include <liquid/liquid.h>
 
+/*** TIMING CONFIG ***/
+
+#define TX_SWITCH_BACKOFF_PREAMBLE_MS          500   // only relevant if packet cannot be decoded
+#define TX_SWITCH_BACKOFF_END_OF_PACKET_MS       1
+#define TX_SWITCH_BACKOFF_AFTER_RX_ON          500   // time the transceiver must stay in RX mode
+
 /*** LAYER 1 CONFIG ***/
 
 #define SYMBOL_RATE   100e3f

impl/src/main.c

@@ -36,6 +36,14 @@
 static int  m_tunfd = -1;
 static bool m_running = true;
 
+static double next_tx_switch_time = 0.0;
+
+
+static void block_tx_for(unsigned offset_ms)
+{
+	next_tx_switch_time = get_hires_time() + (double)offset_ms * 0.001;
+}
+
 void print_complex_array(const char *varname, float complex const *array, size_t len)
 {
 	fprintf(stderr, "%s=np.array([%f%+fj", varname, crealf(array[0]), cimagf(array[0]));
@@ -101,9 +109,12 @@ void cb_rx(rx_evt_t evt, uint8_t *packet_data, size_t packet_len)
 			break;
 
 		case RX_EVT_PACKET_RECEIVED:
-			fprintf(stderr, "=== DECODED PAYLOAD (%4zu bytes) ===\n", packet_len);
-			hexdump(packet_data, packet_len);
-			fprintf(stderr, "====================================\n");
+			fprintf(stderr, "A message of %zu bytes was decoded successfully.\n", packet_len);
+			//fprintf(stderr, "=== DECODED PAYLOAD (%4zu bytes) ===\n", packet_len);
+			//hexdump(packet_data, packet_len < 64 ? packet_len : 64);
+			//fprintf(stderr, "====================================\n");
+
+			block_tx_for(TX_SWITCH_BACKOFF_END_OF_PACKET_MS);
 
 			ret = write(m_tunfd, packet_data, packet_len);
 			if(ret < 0) {
@@ -113,6 +124,7 @@ void cb_rx(rx_evt_t evt, uint8_t *packet_data, size_t packet_len)
 
 		case RX_EVT_PREAMBLE_FOUND:
 			fprintf(stderr, "Found preamble!\n");
+			block_tx_for(TX_SWITCH_BACKOFF_PREAMBLE_MS);
 			break;
 	}
 }
@@ -207,7 +219,9 @@ int main(void)
 	unsigned rx_retries = 0;
 
 	while(m_running) {
-		if(on_air || !layer1_rx_is_busy(&rx)) {
+		double now = get_hires_time();
+
+		if((now > next_tx_switch_time) && (on_air || !layer1_rx_is_busy(&rx))) {
 			int ret = poll(&pfd, 1, 0);
 			if(ret < 0) {
 				perror("poll");
@@ -252,6 +266,8 @@ int main(void)
 
 				RESULT_CHECK(sdr_start_rx(&sdr));
 				on_air = false;
+
+				block_tx_for(TX_SWITCH_BACKOFF_AFTER_RX_ON);
 			}
 		}
 

impl/src/utils.c

@@ -1,5 +1,8 @@
 #include <stdio.h>
 
+#include <errno.h>
+#include <time.h>
+
 #include "utils.h"
 
 bool dump_array_cf(const float complex *data, size_t n, float T, const char *filename)
@@ -64,3 +67,33 @@ err_close:
 	fclose(f);
 	return false;
 }
+
+double get_hires_time(void)
+{
+	struct timespec clk;
+	clock_gettime(CLOCK_MONOTONIC, &clk);
+	return clk.tv_sec + 1e-9 * clk.tv_nsec;
+}
+
+void fsleep(double d)
+{
+	struct timespec ts;
+
+	ts.tv_sec = (time_t)d;
+	ts.tv_nsec = (long)(1e9 * (d - (long)d));
+
+	nanosleep(&ts, NULL);
+}
+
+void sleep_until(double hires_time)
+{
+	struct timespec tv;
+	int ret;
+
+	tv.tv_sec = hires_time;
+	tv.tv_nsec = (uint64_t)(1e9 * hires_time) % 1000000000;
+	do {
+		ret = clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &tv, NULL);
+	} while(ret == EINTR);
+}
+

impl/src/utils.h

@@ -24,4 +24,8 @@ bool dump_array_cf(const float complex *data, size_t n, float T, const char *fil
  */
 bool dump_array_f(const float *data, size_t n, float T, const char *filename);
 
+void sleep_until(double hires_time);
+void fsleep(double d);
+double get_hires_time(void);
+
 #endif // UTILS_H