falconia has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-bts/+/41049?usp=email )

Change subject: FR/HR/EFR: implement SID filter in TCH UL path ......................................................................

FR/HR/EFR: implement SID filter in TCH UL path

As a result of how FR/HR/EFR DTX interacts with block diagonal interleaving, at the beginning and end of each DTX pause a correctly functioning TCH receiver will always pick up an artifact consisting of 4 received bursts (2 for TCH/HS) and same number of omitted bursts. Standard channel decoding of this Rx artifact will produce a "half-block" in which half of the bits prior to convolutional decoding will come from a SID repetition whose Tx was notionally suppressed, while the other half will be garbage. As a result of convolutional decoding, the result will often appear as valid SID per classification rules - but passing it as such to the Rx DTX handler is wrong. Classic E1 BTS and GSM MS implementations include a kind of SID filter at this point, setting BFI on these received half-blocks, so that the Rx DTX handler will see an invalid SID condition. Invalid SID means that comfort noise generation is to be continued, but no updated CN parameters are available - which is the truth in half-block Rx situations. Implement the same filter.

Additional background info can be found here:

https://osmocom.org/projects/retro-gsm/wiki/DTXu_half-blocks

Change-Id: I46c62312316b04567bcadf6050597673f071247d --- M include/osmo-bts/lchan.h M src/common/l1sap.c 2 files changed, 55 insertions(+), 8 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-bts refs/changes/49/41049/1

diff --git a/include/osmo-bts/lchan.h b/include/osmo-bts/lchan.h index 5c34015..70ca5b9 100644 --- a/include/osmo-bts/lchan.h +++ b/include/osmo-bts/lchan.h @@ -292,6 +292,8 @@ bool dl_sid_transmitted; /* The current frame in the DL is taken up by FACCH */ bool dl_facch_stealing; + /* UL SID filter to catch DTXu half-blocks */ + bool ul_sid_filter; } dtx_fr_hr_efr; uint8_t last_cmr; uint32_t last_fn; diff --git a/src/common/l1sap.c b/src/common/l1sap.c index 8226648..e8453b8 100644 --- a/src/common/l1sap.c +++ b/src/common/l1sap.c @@ -2326,7 +2326,19 @@ { enum osmo_gsm631_sid_class sidc;

- /* If we got no payload (BFI without data), there is nothing + /* No matter what else is happening, even if we are about to bail out + * early because we received FACCH and thus BFI-no-data, if we are + * at a mandatory-Tx position for SID, we need to clear UL SID filter + * state so that the next valid SID frame will be allowed through, + * whether it occurs right now or in a later frame position because + * of FACCH. See the note in GSM 06.31 section 5.1.2: if the + * SACCH-aligned SID update position is stolen by FACCH, the next + * frame position shall carry the SID update as soon as FACCH stealing + * is over. */ + if (fr_hr_efr_sid_position(lchan, fn)) + lchan->tch.dtx_fr_hr_efr.ul_sid_filter = false; + + /* If we got no payload (BFI without data), there is nothing more * for us to do here. */ if (msg->len == 0) return; @@ -2367,11 +2379,24 @@ * is an accepted SID frame in the definition of GSM 06.31 * and its HR & EFR counterparts. * - * 2) Next patch in the series will introduce logic that sets BFI - * under certain conditions dependent on SID classification and - * previous state, in order to suppress false indications of - * "valid" SID to the Rx DTX handler on the RTP receiving end - * during "half-block" conditions. + * 2) As a result of how FR/HR/EFR DTX interacts with block diagonal + * interleaving, at the beginning and end of each DTX pause a + * correctly functioning TCH receiver will always pick up an + * artifact consisting of 4 received bursts (2 for TCH/HS) + * and same number of omitted bursts. Standard channel decoding + * of this Rx artifact will produce a "half-block" in which half + * of the bits prior to convolutional decoding will come from + * a SID repetition whose Tx was notionally suppressed, while + * the other half will be garbage. As a result of convolutional + * decoding, the result will often appear as valid SID per + * classification rules - but passing it as such to the Rx DTX + * handler is wrong. Classic E1 BTS and GSM MS implementations + * include a kind of SID filter at this point, setting BFI on + * these received half-blocks, so that the Rx DTX handler will + * see an invalid SID condition. Invalid SID means that comfort + * noise generation is to be continued, but no updated CN + * parameters are available - which is the truth in half-block + * Rx situations. We implement the same filter. * * 3) For HR codec only, RTP output format functions need to know * both BFI flag and SID classification. @@ -2380,12 +2405,32 @@ case OSMO_GSM631_SID_CLASS_SPEECH: /* Only a good speech frame, not an unusable frame, * using GSM 06.31 definitions, marks exit from a DTX pause. */ - if (!tch_ul_msg_bfi(msg)) + if (!tch_ul_msg_bfi(msg)) { lchan_set_marker(false, lchan); + lchan->tch.dtx_fr_hr_efr.ul_sid_filter = false; + } break; - case OSMO_GSM631_SID_CLASS_INVALID: case OSMO_GSM631_SID_CLASS_VALID: + /* Here comes the just-described SID filter. The logic is + * thus: a valid SID is allowed through to the Rx DTX handler + * if it follows speech (end of talkspurt), if it appears + * in the expected mandatory-Tx position for SID updates, + * or if it happens after that mandatory-Tx position due to + * FACCH stealing. Otherwise, valid SID is converted to + * invalid by setting BFI. + */ + if (lchan->tch.dtx_fr_hr_efr.ul_sid_filter) + tch_ul_msg_bfi(msg) = true; + /* fall through */ + case OSMO_GSM631_SID_CLASS_INVALID: + /* Whether we got valid or invalid SID, we know that the MS + * has entered or remains in a DTXu pause. (Invalid SID + * means that MS state is known to be DTXu, but no CN + * parameters are available.) Hence we indicate DTXu pause + * state for both RTP marker bit and SID filter mechanisms. + */ lchan_set_marker(true, lchan); + lchan->tch.dtx_fr_hr_efr.ul_sid_filter = true; break; default: /* There are only 3 possible SID classifications per