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gsm: Improve the TCH/H2.4 coding routines 

The spec isn't super clear, but basically the conv coding is done
in two blocks of 72 bits.

The way it's currently implemented isn't "wrong" in the sense it will
produce correct output given no bit errors, but it's better to do the
decoding in two blocks because this then it makes use of the fact we
know the state of the encoder after the 72 bits, which improves the
corrective ability of the code.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: Id2551ffe2a0ebfd0a6df0e1d288a6f0af7e1eda7
This commit is contained in:
Sylvain Munaut 2023-07-07 15:57:32 +02:00
parent c27a8ae3d9
commit c87c6a3a73
4 changed files with 39 additions and 14 deletions
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26
src/coding/gsm0503_coding.c
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@ -3480,15 +3480,10 @@ int gsm0503_tch_hr48_decode(ubit_t *data, const sbit_t *bursts,
int gsm0503_tch_hr24_encode(ubit_t *bursts, const ubit_t *data)
{
ubit_t iB[22 * 114], cB[4 * 114];
ubit_t conv[2 * 72 + 8];
/* 3.7.2 Block code: d1(72) + pad(4) + d2(72) + pad(4) */
memset(&conv[0], 0, sizeof(conv));
memcpy(&conv[0], &data[0], 72);
memcpy(&conv[76], &data[72], 72);
/* 3.7.3 Convolutional encoder: as specified for the TCH/F4.8 in subclause 3.4.3 */
osmo_conv_encode(&gsm0503_tch_f48, &conv[0], &cB[0]);
/* 3.7.{1-3} Block code and Convolutional encoder */
osmo_conv_encode(&gsm0503_tch_h24, &data[ 0], &cB[ 0]);
osmo_conv_encode(&gsm0503_tch_h24, &data[72], &cB[228]);
/* 3.7.4 Interleaving: as specified for the TCH/F9.6 in subclause 3.3.4 */
memset(&iB[0], 0, sizeof(iB));
@ -3511,8 +3506,8 @@ int gsm0503_tch_hr24_encode(ubit_t *bursts, const ubit_t *data)
int gsm0503_tch_hr24_decode(ubit_t *data, const sbit_t *bursts,
int *n_errors, int *n_bits_total)
{
int n_errors_l[2], n_bits_total_l[2];
sbit_t iB[22 * 114], cB[4 * 114];
ubit_t conv[120 + 32];
/* 3.7.5 Mapping on a burst: as specified for TCH/FS in subclause 3.1.4 */
for (unsigned int i = 0; i < 22; i++) {
@ -3523,12 +3518,15 @@ int gsm0503_tch_hr24_decode(ubit_t *data, const sbit_t *bursts,
/* 3.7.4 Interleaving: as specified for the TCH/F9.6 in subclause 3.3.4 */
gsm0503_tch_f96_deinterleave(&cB[0], &iB[0]);
/* 3.7.3 Convolutional encoder: as specified for the TCH/F4.8 in subclause 3.4.3 */
osmo_conv_decode_ber(&gsm0503_tch_f48, &cB[0], &conv[0], n_errors, n_bits_total);
/* 3.7.{1-3} Block code and Convolutional encoder */
osmo_conv_decode_ber(&gsm0503_tch_h24, &cB[ 0], &data[ 0], &n_errors_l[0], &n_bits_total_l[0]);
osmo_conv_decode_ber(&gsm0503_tch_h24, &cB[228], &data[72], &n_errors_l[1], &n_bits_total_l[1]);
/* 3.7.2 Block code: d1(72) + pad(4) + d2(72) + pad(4) */
memcpy(&data[0], &conv[0], 72);
memcpy(&data[72], &conv[76], 72);
if (n_errors)
*n_errors = n_errors_l[0] + n_errors_l[1];
if (n_bits_total)
*n_bits_total = n_bits_total_l[0] + n_bits_total_l[1];
return 2 * 72;
}

1
src/gsm/libosmogsm.map
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@ -123,6 +123,7 @@ gsm0503_cs2_np;
gsm0503_cs3_np;
gsm0503_tch_fr;
gsm0503_tch_f24;
gsm0503_tch_h24;
gsm0503_tch_f48;
gsm0503_tch_f96;
gsm0503_tch_f144;

8
tests/conv/conv_gsm0503_test.ok
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@ -14,6 +14,14 @@
[..] Encoding / Decoding cycle : OK
[..] Encoding / Decoding cycle : OK
[+] Testing: gsm0503_tch_h24
[.] Input length : ret = 72 exp = 72 -> OK
[.] Output length : ret = 228 exp = 228 -> OK
[.] Random vector checks:
[..] Encoding / Decoding cycle : OK
[..] Encoding / Decoding cycle : OK
[..] Encoding / Decoding cycle : OK
[+] Testing: gsm0503_tch_f48
[.] Input length : ret = 148 exp = 148 -> OK
[.] Output length : ret = 456 exp = 456 -> OK

18
utils/conv_codes_gsm.py
View File

@ -66,6 +66,24 @@ conv_codes = [
]
),
# TCH/H2.4 definition
ConvolutionalCode(
72,
[
(G1, 1),
(G2, 1),
(G3, 1),
],
name = "tch_h24",
description = [
"TCH/H2.4 convolutional code:",
"72 bits blocks, rate 1/3, k = 5",
"G1 = 1 + D + D3 + D4",
"G2 = 1 + D2 + D4",
"G3 = 1 + D + D2 + D3 + D4",
]
),
# TCH/F4.8 definition
ConvolutionalCode(
148,