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/* Loop control for Timing Advance */
/* (C) 2013 by Andreas Eversberg <jolly@eversberg.eu>
* (C) 2021 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
*
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
/* Related specs: 3GPP TS 45.010 sections 5.5, 5.6 */
#include <osmo-bts/gsm_data.h>
#include <osmo-bts/bts_trx.h>
#include <osmo-bts/logging.h>
/* 3GPP TS 45.010 sec 5.6.3 Delay assessment error:
* 75% of one bit duration in 1/256 symbols: 256*0.75 */
#define TOA256_THRESH 192
/* rqd_ta value range */
#define TA_MIN 0
#define TA_MAX 63
/* TODO: make configurable over osmo-bts VTY? Pass it BSC->BTS? */
#define TA_MAX_INC_STEP 2
#define TA_MAX_DEC_STEP 2
/*! compute the new "Ordered Timing Advance" communicated to the MS and store it in lchan.
* \param lchan logical channel for which to compute (and in which to store) new power value.
* \param[in] ms_tx_ta The TA used by the MS and reported in L1SACCH, see struct gsm_sacch_l1_hdr field "ta".
* \param[in] toa256 Time of Arrival (in 1/256th bits) computed at Rx side
*/
void lchan_ms_ta_ctrl(struct gsm_lchan *lchan, uint8_t ms_tx_ta, int16_t toa256)
{
int16_t new_ta;
/* Shall we skip current block based on configured interval? */
/* TA control interval: how many blocks do we skip? */
if (lchan->ta_ctrl.skip_block_num-- > 0)
return;
/* Reset the number of SACCH blocks to be skipped:
* ctrl_interval=0 => 0 blocks to skip,
* ctrl_interval=1 => 1 blocks to skip,
* ctrl_interval=2 => 3 blocks to skip,
* so basically ctrl_interval * 2 - 1. */
lchan->ta_ctrl.skip_block_num = lchan->ts->trx->ta_ctrl_interval * 2 - 1;
int16_t delta_ta = toa256/256;
if (toa256 >= 0) {
if ((toa256 - (256 * delta_ta)) > TOA256_THRESH)
delta_ta++;
if (delta_ta > TA_MAX_INC_STEP)
delta_ta = TA_MAX_INC_STEP;
} else {
if ((toa256 - (256 * delta_ta)) < -TOA256_THRESH)
delta_ta--;
if (delta_ta < -TA_MAX_DEC_STEP)
delta_ta = -TA_MAX_DEC_STEP;
}
new_ta = ms_tx_ta + delta_ta;
/* Make sure new_ta is never negative: */
if (new_ta < TA_MIN)
new_ta = TA_MIN;
/* Don't ask for out of range TA: */
if (new_ta > TA_MAX)
new_ta = TA_MAX;
if (lchan->ta_ctrl.current == (uint8_t)new_ta) {
LOGPLCHAN(lchan, DLOOP, LOGL_DEBUG,
"Keeping current TA at %u: TOA was %d\n",
lchan->ta_ctrl.current, toa256);
return;
}
LOGPLCHAN(lchan, DLOOP, LOGL_INFO,
"%s TA %u => %u: TOA was too %s (%d)\n",
(uint8_t)new_ta > lchan->ta_ctrl.current ? "Raising" : "Lowering",
lchan->ta_ctrl.current, (uint8_t)new_ta,
(uint8_t)new_ta > lchan->ta_ctrl.current ? "late" : "early",
toa256);
/* store the resulting new TA in the lchan */
lchan->ta_ctrl.current = (uint8_t)new_ta;
}
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