Use data whitening

impl/CMakeLists.txt

@@ -23,6 +23,8 @@ set(sources
 	src/correlator.c
 	src/freq_est.h
 	src/freq_est.c
+	src/whitening.h
+	src/whitening.c
 )
 
 include_directories(

impl/src/main.c

@@ -12,6 +12,7 @@
 #include "transmission.h"
 #include "correlator.h"
 #include "freq_est.h"
+#include "whitening.h"
 
 typedef enum {
 	RX_STATE_ACQUISITION,
@@ -286,6 +287,9 @@ int main(void)
 
 						fec_decode(payload_fec, payload_len_bytes, payload_enc, payload);
 
+						// de-whiten the data
+						whitening_apply_in_place(payload, payload_len_bytes);
+
 						int valid = crc_validate_message(PAYLOAD_CRC_SCHEME, payload, payload_len_bytes, payload_crc);
 
 						payload[payload_len_bytes] = '\0';

impl/src/packet_mod.c

@@ -1,6 +1,7 @@
 #include <liquid/liquid.h>
 #include <string.h>
 
+#include "whitening.h"
 #include "preamble.h"
 #include "config.h"
 
@@ -95,6 +96,10 @@ result_t packet_mod_encode(packet_mod_ctx_t *ctx)
 		return ERR_NO_MEM;
 	}
 
+	// whiten the raw data
+	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));
 
 	free(ctx->pkt_bytes);

impl/src/whitening.c

@@ -0,0 +1,25 @@
+#include <stdio.h>
+
+#include "whitening.h"
+
+void whitening_apply_in_place(uint8_t *data, size_t length)
+{
+	/* 9-bit LFSR according to CCITT data whitening procedure. */
+	uint16_t lfsr = 0x1FF;
+
+	for(size_t i = 0; i < length; i++) {
+		uint8_t byte = 0;
+
+		// advance the LFSR and collect the output in a byte.
+		for(int k = 0; k < 8; k++) {
+			byte <<= 1;
+			byte |= lfsr & 0x001;
+
+			uint16_t tmp = lfsr ^ (lfsr >> 5);
+			lfsr >>= 1;
+			lfsr |= (tmp & 0x001) << 8;
+		}
+
+		data[i] ^= byte;
+	}
+}

impl/src/whitening.h

@@ -0,0 +1,9 @@
+#ifndef WHITENING_H
+#define WHITENING_H
+
+#include <stdint.h>
+#include <stdlib.h>
+
+void whitening_apply_in_place(uint8_t *data, size_t length);
+
+#endif // WHITENING_H