layer1: implement per-packet modulation/coding

This allows a higher layer to adjust data rate based on the current
channel conditions. For example, the SNR could be measured and if it
becomes too low for the current modulation/code, a lower-order
modulation or stronger code could be used.

impl/CMakeLists.txt

@@ -29,6 +29,8 @@ set(sources
 	src/layer1/rx.h
 	src/layer1/whitening.c
 	src/layer1/whitening.h
+	src/layer1/modcod.c
+	src/layer1/modcod.h
 )
 
 include_directories(

impl/src/layer1/modcod.c

@@ -0,0 +1,34 @@
+#include <stddef.h>
+
+#include "modcod.h"
+
+modem modcod_create_modem(modcod_t modcod)
+{
+	static const modulation_scheme mod_mapping[MODCOD_LIST_SIZE] = {
+		LIQUID_MODEM_QAM16,
+		LIQUID_MODEM_QPSK,
+	};
+
+	if(modcod < MODCOD_LIST_SIZE) {
+		return modem_create(mod_mapping[modcod]);
+	} else {
+		return NULL;
+	}
+}
+
+
+fec modcod_create_fec(modcod_t modcod)
+{
+	static const fec_scheme fec_mapping[MODCOD_LIST_SIZE] = {
+		LIQUID_FEC_CONV_V27P34,
+		LIQUID_FEC_CONV_V27P34,
+	};
+
+	if(modcod < MODCOD_LIST_SIZE) {
+		return fec_create(fec_mapping[modcod], NULL);
+	} else {
+		return NULL;
+	}
+}
+
+

impl/src/layer1/modcod.h

@@ -0,0 +1,23 @@
+#ifndef LAYER1_MODCOD_H
+#define LAYER1_MODCOD_H
+
+#include <liquid/liquid.h>
+
+// up to 32 ModCods are supported. Do not reuse indices, as that will break
+// backwards compatibility! Only append new ModCods at the end of the list!
+typedef enum
+{
+	MODCOD_16QAM_CONV_V27P34,
+	MODCOD_QPSK_CONV_V27P34,
+
+	MODCOD_LIST_SIZE
+} modcod_t;
+
+#define MODCOD_DEFAULT MODCOD_16QAM_CONV_V27P34
+
+modem modcod_create_modem(modcod_t modcod);
+fec   modcod_create_fec(modcod_t modcod);
+
+#define MODCOD_IS_VALID(modcod) ((modcod) < MODCOD_LIST_SIZE)
+
+#endif // LAYER1_MODCOD_H

impl/src/layer1/packet_mod.c

@@ -14,12 +14,11 @@ result_t packet_mod_init(packet_mod_ctx_t *ctx)
 
 	ctx->length = 0;
 
-	ctx->fec   = fec_create(PAYLOAD_CHANNEL_CODE, NULL);
-	ctx->modem = modem_create(PAYLOAD_MODULATION);
-
 	ctx->hdr_fec   = fec_create(HEADER_CHANNEL_CODE, NULL);
 	ctx->hdr_modem = modem_create(HEADER_MODULATION);
 
+	ctx->modcod = MODCOD_DEFAULT;
+
 	ctx->state = NOT_STARTED;
 
 	return OK;
@@ -50,9 +49,6 @@ result_t packet_mod_free(packet_mod_ctx_t *ctx)
 	ctx->state = NOT_STARTED;
 	ctx->length = 0;
 
-	fec_destroy(ctx->fec);
-	modem_destroy(ctx->modem);
-
 	fec_destroy(ctx->hdr_fec);
 	modem_destroy(ctx->hdr_modem);
 
@@ -60,6 +56,18 @@ result_t packet_mod_free(packet_mod_ctx_t *ctx)
 }
 
 
+result_t packet_mod_set_modcod(packet_mod_ctx_t *ctx, modcod_t modcod)
+{
+	if(ctx->state != NOT_STARTED) {
+		return ERR_INVALID_STATE;
+	}
+
+	ctx->modcod = modcod;
+
+	return OK;
+}
+
+
 result_t packet_mod_set_data(packet_mod_ctx_t *ctx, const unsigned char *data, size_t length)
 {
 	if(ctx->state != NOT_STARTED) {
@@ -100,7 +108,9 @@ result_t packet_mod_encode(packet_mod_ctx_t *ctx)
 	whitening_apply_in_place(ctx->pkt_bytes, ctx->length);
 
 	// apply the FEC
-	ERR_CHECK_LIQUID(fec_encode(ctx->fec, ctx->length, ctx->pkt_bytes, enc_msg));
+	fec payload_fec = modcod_create_fec(ctx->modcod);
+	ERR_CHECK_LIQUID(fec_encode(payload_fec, ctx->length, ctx->pkt_bytes, enc_msg));
+	fec_destroy(payload_fec);
 
 	free(ctx->pkt_bytes);
 
@@ -119,22 +129,27 @@ result_t packet_mod_modulate(packet_mod_ctx_t *ctx)
 		return ERR_INVALID_STATE;
 	}
 
-	unsigned int bps = modem_get_bps(ctx->modem);
+	modem payload_modem = modcod_create_modem(ctx->modcod);
+
+	unsigned int bps = modem_get_bps(payload_modem);
 	unsigned int nsyms = (ctx->length * 8 + bps/2) / bps;
 
 	unsigned char msg_sym_idcs[nsyms];
 
 	float complex *msg_mod = malloc(sizeof(float complex) * nsyms);
 	if(!msg_mod) {
+		modem_destroy(payload_modem);
 		return ERR_NO_MEM;
 	}
 
 	ERR_CHECK_LIQUID(liquid_repack_bytes(ctx->pkt_bytes, 8, ctx->length, msg_sym_idcs, bps, nsyms, &nsyms));
 
 	for(size_t i = 0; i < nsyms; i++) {
-		ERR_CHECK_LIQUID(modem_modulate(ctx->modem, msg_sym_idcs[i], &msg_mod[i]));
+		ERR_CHECK_LIQUID(modem_modulate(payload_modem, msg_sym_idcs[i], &msg_mod[i]));
 	}
 
+	modem_destroy(payload_modem);
+
 	free(ctx->pkt_bytes);
 	ctx->pkt_bytes = NULL;
 
@@ -162,7 +177,8 @@ result_t packet_mod_add_header(packet_mod_ctx_t *ctx)
 	uint8_t header[4];
 
 	// set length
-	header[0] = (ctx->raw_data_len >> 8) & 0xFF;
+	header[0] = (ctx->raw_data_len >> 8) & 0x07;
+	header[0] |= ctx->modcod << 3;
 	header[1] = (ctx->raw_data_len >> 0) & 0xFF;
 	// set raw data CRC
 	header[2] = (ctx->raw_data_crc >> 8) & 0xFF;

impl/src/layer1/packet_mod.h

@@ -6,6 +6,8 @@
 
 #include <liquid/liquid.h>
 
+#include "modcod.h"
+
 #include "results.h"
 
 typedef enum
@@ -25,15 +27,14 @@ typedef struct
 
 	packet_mod_state_t state;
 
-	modem modem;
-	fec   fec;
-
 	modem hdr_modem;
 	fec   hdr_fec;
 
 	uint16_t raw_data_crc;
 	uint16_t raw_data_len;
 
+	uint8_t modcod;
+
 	size_t length;
 } packet_mod_ctx_t;
 
@@ -52,6 +53,16 @@ result_t packet_mod_init(packet_mod_ctx_t *ctx);
  */
 result_t packet_mod_free(packet_mod_ctx_t *ctx);
 
+/*!\brief Set the modulation and code to use for the next packet.
+ *
+ * Must be called before setting the raw data.
+ *
+ * \param[inout] ctx    The context to use for this operation.
+ * \param[in]    modcod The modulation/code to use.
+ * \returns             An result code (see results.h).
+ */
+result_t packet_mod_set_modcod(packet_mod_ctx_t *ctx, modcod_t modcod);
+
 /*!\brief Set the raw packet data.
  *
  * In this step, the CRC of the data is calculated.

impl/src/layer1/rx.c

@@ -12,7 +12,7 @@
 #define HEADER_SIZE_BYTES 4
 #define FREQ_EST_L 8
 
-#define SHOW_DEBUG_LOG 0
+#define SHOW_DEBUG_LOG 1
 
 #if SHOW_DEBUG_LOG
 #	define DEBUG_LOG(...) fprintf(stderr, __VA_ARGS__)
@@ -150,10 +150,18 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 
 						fec_decode(rx->hdr_fec, sizeof(header), header_enc, header);
 
-						uint16_t payload_bps = modem_get_bps(rx->payload_demod);
-
 						rx->payload_len_bytes = (((uint16_t)header[0] << 8) | header[1]) & 0x07FF;
 						rx->payload_crc       = ((uint16_t)header[2] << 8) | header[3];
+						rx->modcod            = header[0] >> 3;
+
+						// check the received information
+						if(!MODCOD_IS_VALID(rx->modcod)) {
+							rx->state = RX_STATE_ACQUISITION;
+							break;
+						}
+
+						rx->payload_demod = modcod_create_modem(rx->modcod);
+						uint16_t payload_bps = modem_get_bps(rx->payload_demod);
 
 						rx->payload_len_enc_bytes = fec_get_enc_msg_length(PAYLOAD_CHANNEL_CODE, rx->payload_len_bytes);
 						rx->payload_len_symbols = (rx->payload_len_enc_bytes * 8 + payload_bps - 1) / payload_bps;
@@ -194,7 +202,9 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 									symbols_int, modem_get_bps(rx->payload_demod), rx->payload_len_symbols,
 									payload_enc, 8, sizeof(payload_enc), &nsyms));
 
-						fec_decode(rx->payload_fec, rx->payload_len_bytes, payload_enc, payload);
+						fec payload_fec = modcod_create_fec(rx->modcod);
+						fec_decode(payload_fec, rx->payload_len_bytes, payload_enc, payload);
+						fec_destroy(payload_fec);
 
 						// de-whiten the data
 						whitening_apply_in_place(payload, rx->payload_len_bytes);
@@ -209,6 +219,8 @@ result_t layer1_rx_process(layer1_rx_t *rx, const float complex *samples, size_t
 						} else {
 							rx->callback(RX_EVT_CHECKSUM_ERROR, payload, rx->payload_len_bytes);
 						}
+
+						modem_destroy(rx->payload_demod);
 						rx->state = RX_STATE_ACQUISITION;
 					}
 					break;
@@ -237,11 +249,9 @@ result_t layer1_rx_init(layer1_rx_t *rx, rx_callback_t callback)
 
 	// forward error correction objects
 	rx->hdr_fec     = fec_create(HEADER_CHANNEL_CODE, NULL);
-	rx->payload_fec = fec_create(PAYLOAD_CHANNEL_CODE, NULL);
 
 	// demodulators
 	rx->hdr_demod     = modem_create(HEADER_MODULATION);
-	rx->payload_demod = modem_create(PAYLOAD_MODULATION);
 
 	// symbol timing synchronizer
 	rx->symsync = symsync_crcf_create_rnyquist(LIQUID_FIRFILT_RRC, RRC_SPS, RRC_DELAY, RRC_BETA, 32);
@@ -264,10 +274,8 @@ result_t layer1_rx_shutdown(layer1_rx_t *rx)
 	nco_crcf_destroy(rx->carrier_fine_nco);
 
 	fec_destroy(rx->hdr_fec);
-	fec_destroy(rx->payload_fec);
 
 	modem_destroy(rx->hdr_demod);
-	modem_destroy(rx->payload_demod);
 
 	symsync_crcf_destroy(rx->symsync);
 

impl/src/layer1/rx.h

@@ -3,6 +3,7 @@
 
 #include <liquid/liquid.h>
 
+#include "modcod.h"
 #include "correlator.h"
 
 #include "results.h"
@@ -32,7 +33,6 @@ typedef struct
 
 	// Forward error correction objects
 	fec hdr_fec;
-	fec payload_fec;
 
 	// Demodulators
 	modem hdr_demod;
@@ -59,6 +59,7 @@ typedef struct
 	uint16_t payload_len_bytes;
 	uint16_t payload_crc;
 	uint16_t payload_len_enc_bytes;
+	modcod_t modcod;
 } layer1_rx_t;