From 147051f1a1e47e7b0391c2cdc7033b840cebdca6 Mon Sep 17 00:00:00 2001
From: Alexander Chemeris <Alexander.Chemeris@gmail.com>
Date: Sat, 14 Jul 2018 21:02:29 +0200
Subject: coding: Fix (E)GPRS BER calculation to correctly account for
 puncturing.

Previously we didn't take into account puncturing and BER was always around
30% for GPRS/EDGE bursts because of they use puncturing coding unlike
"classical" GSM bursts.

Change-Id: I9da22e7051522d06d923fcec3b63cbed8db93910
---
 src/coding/gsm0503_coding.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

(limited to 'src/coding')

diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 215cc6dd..1d06adb5 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -528,16 +528,18 @@ const struct gsm0503_mcs_code gsm0503_mcs_dl_codes[EGPRS_NUM_MCS] = {
 	},
 };
 
-/*! Convolutional Decode + compute BER
+/*! Convolutional Decode + compute BER for punctured codes
  *  \param[in] code Description of Convolutional Code
  *  \param[in] input Input soft-bits (-127...127)
  *  \param[out] output bits
  *  \param[out] n_errors Number of bit-errors
  *  \param[out] n_bits_total Number of bits
+ *  \param[in] data_punc Puncturing mask array. Can be NULL.
  */
-static int osmo_conv_decode_ber(const struct osmo_conv_code *code,
+static int osmo_conv_decode_ber_punctured(const struct osmo_conv_code *code,
 	const sbit_t *input, ubit_t *output,
-	int *n_errors, int *n_bits_total)
+	int *n_errors, int *n_bits_total,
+	const uint8_t *data_punc)
 {
 	int res, i, coded_len;
 	ubit_t recoded[EGPRS_DATA_C_MAX];
@@ -553,7 +555,8 @@ static int osmo_conv_decode_ber(const struct osmo_conv_code *code,
 	if (n_errors) {
 		*n_errors = 0;
 		for (i = 0; i < coded_len; i++) {
-			if (!((recoded[i] && input[i] < 0) ||
+			if (((!data_punc) || (data_punc && !data_punc[i])) &&
+				!((recoded[i] && input[i] < 0) ||
 					(!recoded[i] && input[i] > 0)) )
 						*n_errors += 1;
 		}
@@ -565,6 +568,21 @@ static int osmo_conv_decode_ber(const struct osmo_conv_code *code,
 	return res;
 }
 
+/*! Convolutional Decode + compute BER for non-punctured codes
+ *  \param[in] code Description of Convolutional Code
+ *  \param[in] input Input soft-bits (-127...127)
+ *  \param[out] output bits
+ *  \param[out] n_errors Number of bit-errors
+ *  \param[out] n_bits_total Number of bits
+ */
+static int osmo_conv_decode_ber(const struct osmo_conv_code *code,
+	const sbit_t *input, ubit_t *output,
+	int *n_errors, int *n_bits_total)
+{
+	return osmo_conv_decode_ber_punctured(code, input, output,
+		n_errors, n_bits_total, NULL);
+}
+
 /*! convenience wrapper for decoding coded bits
  *  \param[out] l2_data caller-allocated buffer for L2 Frame
  *  \param[in] cB 456 coded (soft) bits as per TS 05.03 4.1.3
@@ -884,7 +902,8 @@ static int egprs_decode_data(uint8_t *l2_data, const sbit_t *c,
 			C[i] = 0;
 	}
 
-	osmo_conv_decode_ber(code->data_conv, C, u, n_errors, n_bits_total);
+	osmo_conv_decode_ber_punctured(code->data_conv, C, u,
+		n_errors, n_bits_total, code->data_punc[p]);
 	rc = osmo_crc16gen_check_bits(&gsm0503_mcs_crc12, u,
 		data_len, u + data_len);
 	if (rc)
-- 
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