osmo-bts/src/common/power_control.c

/* MS Power Control Loop L1 */

/* (C) 2014 by Holger Hans Peter Freyther
 * Contributions 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/>.
 *
 */

#include <stdint.h>
#include <unistd.h>
#include <errno.h>
#include <inttypes.h>

#include <osmo-bts/logging.h>
#include <osmo-bts/bts.h>
#include <osmo-bts/gsm_data.h>
#include <osmo-bts/measurement.h>
#include <osmo-bts/bts_model.h>
#include <osmo-bts/l1sap.h>
#include <osmo-bts/power_control.h>

/* We don't want to deal with floating point, so we scale up */
#define EWMA_SCALE_FACTOR 100

/* Base Low-Pass Single-Pole IIR Filter (EWMA) formula:
 *
 *   Avg[n] = a * Pwr[n] + (1 - a) * Avg[n - 1]
 *
 * where parameter 'a' determines how much weight of the latest UL RSSI measurement
 * result 'Pwr[n]' carries vs the weight of the average 'Avg[n - 1]'.  The value of
 * 'a' is usually a float in range 0 .. 1, so:
 *
 *  - value 0.5 gives equal weight to both 'Pwr[n]' and 'Avg[n - 1]';
 *  - value 1.0 means no filtering at all (pass through);
 *  - value 0.0 makes no sense.
 *
 * Further optimization:
 *
 *   Avg[n] = a * Pwr[n] + Avg[n - 1] - a * Avg[n - 1]
 *   ^^^^^^                ^^^^^^^^^^
 *
 * a) this can be implemented in C using '+=' operator:
 *
 *   Avg += a * Pwr - a * Avg
 *   Avg += a * (Pwr - Avg)
 *
 * b) everything is scaled up by 100 to avoid floating point stuff:
 *
 *   Avg100 += A * (Pwr - Avg)
 *
 * where 'Avg100' is 'Avg * 100' and 'A' is 'a * 100'.
 *
 * For more details, see:
 *
 *   https://en.wikipedia.org/wiki/Moving_average
 *   https://en.wikipedia.org/wiki/Low-pass_filter#Simple_infinite_impulse_response_filter
 *   https://tomroelandts.com/articles/low-pass-single-pole-iir-filter
 */
static int8_t do_pf_ewma(const struct bts_power_ctrl_params *params,
			 struct lchan_power_ctrl_state *state,
			 const int8_t Pwr)
{
	const uint8_t A = params->pf.ewma.alpha;
	int *Avg100 = &state->avg100_rxlev_dbm;

	/* We don't have 'Avg[n - 1]' if this is the first run */
	if (*Avg100 == 0) {
		*Avg100 = Pwr * EWMA_SCALE_FACTOR;
		return Pwr;
	}

	*Avg100 += A * (Pwr - *Avg100 / EWMA_SCALE_FACTOR);
	return *Avg100 / EWMA_SCALE_FACTOR;
}

/* Calculate a 'delta' value (for the given MS/BS power control state and parameters)
 * to be applied to the current Tx power level to approach the target level. */
static int calc_delta(const struct bts_power_ctrl_params *params,
		      struct lchan_power_ctrl_state *state,
		      const int rxlev_dbm)
{
	int rxlev_dbm_avg;
	int delta;

	/* Filter input value(s) to reduce unnecessary Tx power oscillations */
	switch (params->pf_algo) {
	case BTS_PF_ALGO_EWMA:
		rxlev_dbm_avg = do_pf_ewma(params, state, rxlev_dbm);
		break;
	case BTS_PF_ALGO_NONE:
	default:
		/* No filtering (pass through) */
		rxlev_dbm_avg = rxlev_dbm;
	}

	/* How many dBs measured power should be increased (+) or decreased (-)
	 * to reach expected power. */
	delta = params->target_dbm - rxlev_dbm_avg;

	/* Tolerate small deviations from 'rx-target' */
	if (abs(delta) <= params->hysteresis_db)
		return 0;

	/* Don't ever change more than PWR_{LOWER,RAISE}_MAX_DBM during one loop
	 * iteration, i.e. reduce the speed at which the MS transmit power can
	 * change. A higher value means a lower level (and vice versa) */
	if (delta > params->raise_step_max_db)
		delta = params->raise_step_max_db;
	else if (delta < -params->lower_step_max_db)
		delta = -params->lower_step_max_db;

	return delta;
}

/*! compute the new MS POWER LEVEL 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_power_lvl MS Power Level received from Uplink L1 SACCH Header in SACCH block.
 *  \param[in] ul_rssi_dbm Signal level of the received SACCH block, in dBm.
 */
int lchan_ms_pwr_ctrl(struct gsm_lchan *lchan,
		      const uint8_t ms_power_lvl,
		      const int8_t ul_rssi_dbm)
{
	struct gsm_bts_trx *trx = lchan->ts->trx;
	struct gsm_bts *bts = trx->bts;
	enum gsm_band band = bts->band;
	int8_t new_power_lvl; /* TS 05.05 power level */
	int8_t ms_dbm, new_dbm, current_dbm, bsc_max_dbm;

	const struct bts_power_ctrl_params *params = &bts->ul_power_ctrl;
	struct lchan_power_ctrl_state *state = &lchan->ms_power_ctrl;

	if (!trx_ms_pwr_ctrl_is_osmo(trx))
		return 0;
	if (state->fixed)
		return 0;

	ms_dbm = ms_pwr_dbm(band, ms_power_lvl);
	if (ms_dbm < 0) {
		LOGPLCHAN(lchan, DLOOP, LOGL_NOTICE,
			  "Failed to calculate dBm for power ctl level %" PRIu8 " on band %s\n",
			  ms_power_lvl, gsm_band_name(band));
		return 0;
	}
	bsc_max_dbm = ms_pwr_dbm(band, state->max);
	if (bsc_max_dbm < 0) {
		LOGPLCHAN(lchan, DLOOP, LOGL_NOTICE,
			  "Failed to calculate dBm for power ctl level %" PRIu8 " on band %s\n",
			  state->max, gsm_band_name(band));
		return 0;
	}

	/* Calculate the new Tx power value (in dBm) */
	new_dbm = ms_dbm + calc_delta(params, state, ul_rssi_dbm);

	/* Make sure new_dbm is never negative. ms_pwr_ctl_lvl() can later on
	   cope with any unsigned dbm value, regardless of band minimal value. */
	if (new_dbm < 0)
		new_dbm = 0;

	/* Don't ask for smaller ms power level than the one set by BSC upon RSL CHAN ACT */
	if (new_dbm > bsc_max_dbm)
		new_dbm = bsc_max_dbm;

	new_power_lvl = ms_pwr_ctl_lvl(band, new_dbm);
	if (new_power_lvl < 0) {
		LOGPLCHAN(lchan, DLOOP, LOGL_NOTICE,
			  "Failed to retrieve power level for %" PRId8 " dBm on band %d\n",
			  new_dbm, band);
		return 0;
	}

	if (state->current == new_power_lvl) {
		LOGPLCHAN(lchan, DLOOP, LOGL_INFO, "Keeping MS power at control level %d, %d dBm "
			  "(rx-ms-pwr-lvl %" PRIu8 ", max-ms-pwr-lvl %" PRIu8 ", rx-current %d dBm, rx-target %d dBm)\n",
			  new_power_lvl, new_dbm, ms_power_lvl, state->max,
			  ul_rssi_dbm, params->target_dbm);
		return 0;
	}

	current_dbm = ms_pwr_dbm(band, state->current);
	LOGPLCHAN(lchan, DLOOP, LOGL_INFO, "%s MS power from control level %d (%d dBm) to %d, %d dBm "
		  "(rx-ms-pwr-lvl %" PRIu8 ", max-ms-pwr-lvl %" PRIu8 ", rx-current %d dBm, rx-target %d dBm)\n",
		  (new_dbm > current_dbm) ? "Raising" : "Lowering",
		  state->current, current_dbm, new_power_lvl, new_dbm,
		  ms_power_lvl, state->max, ul_rssi_dbm, params->target_dbm);

	/* store the resulting new MS power level in the lchan */
	state->current = new_power_lvl;
	bts_model_adjst_ms_pwr(lchan);

	return 1;
}

/*! compute the new Downlink attenuation value for the given logical channel.
 *  \param lchan logical channel for which to compute (and in which to store) new power value.
 *  \param[in] gh pointer to the beginning of (presumably) a Measurement Report.
 */
int lchan_bs_pwr_ctrl(struct gsm_lchan *lchan,
		      const struct gsm48_hdr *gh)
{
	struct gsm_bts_trx *trx = lchan->ts->trx;
	struct gsm_bts *bts = trx->bts;
	uint8_t rxqual_full, rxqual_sub;
	uint8_t rxlev_full, rxlev_sub;
	uint8_t rxqual, rxlev;
	int delta, new;

	const struct bts_power_ctrl_params *params = &bts->dl_power_ctrl;
	struct lchan_power_ctrl_state *state = &lchan->bs_power_ctrl;

	/* Check if BS Power Control is enabled */
	if (state->fixed)
		return 0;
	/* Check if this is a Measurement Report */
	if (gh->proto_discr != GSM48_PDISC_RR)
		return 0;
	if (gh->msg_type != GSM48_MT_RR_MEAS_REP)
		return 0;

	/* Check if the measurement results are valid */
	if ((gh->data[1] & 0x40) == 0x40) {
		LOGPLCHAN(lchan, DLOOP, LOGL_DEBUG,
			  "The measurement results are not valid\n");
		return 0;
	}

	/* See 3GPP TS 44.018, section 10.5.2.20 */
	rxqual_full = (gh->data[2] >> 4) & 0x7;
	rxqual_sub = (gh->data[2] >> 1) & 0x7;

	rxlev_full = gh->data[0] & 0x3f;
	rxlev_sub = gh->data[1] & 0x3f;

	LOGPLCHAN(lchan, DLOOP, LOGL_DEBUG, "Rx DL Measurement Report: "
		  "RXLEV-FULL(%02u), RXQUAL-FULL(%u), "
		  "RXLEV-SUB(%02u), RXQUAL-SUB(%u), "
		  "DTx is %s => using %s\n",
		  rxlev_full, rxqual_full, rxlev_sub, rxqual_sub,
		  lchan->tch.dtx.dl_active ? "enabled" : "disabled",
		  lchan->tch.dtx.dl_active ? "SUB" : "FULL");

	/* If DTx is active on Downlink, use the '-SUB' */
	if (lchan->tch.dtx.dl_active) {
		rxqual = rxqual_sub;
		rxlev = rxlev_sub;
	} else { /* ... otherwise use the '-FULL' */
		rxqual = rxqual_full;
		rxlev = rxlev_full;
	}

	/* Bit Error Rate > 0 => reduce by 2 */
	if (rxqual > 0) {
		LOGPLCHAN(lchan, DLOOP, LOGL_INFO, "Reducing Downlink attenuation "
			  "by half: %u -> %u dB due to RXQUAL %u > 0\n",
			  state->current, state->current / 2, rxqual);
		state->current /= 2;
		return 1;
	}

	/* Calculate a 'delta' for the current attenuation level */
	delta = calc_delta(params, state, rxlev2dbm(rxlev));

	/* Basic signal transmission / reception formula:
	 *
	 *   RxLev = TxPwr - (PathLoss + TxAtt)
	 *
	 * Here we want to change RxLev at the MS side, so:
	 *
	 *   RxLev + Delta = TxPwr - (PathLoss + TxAtt) + Delta
	 *
	 * The only parameter we can change here is TxAtt, so:
	 *
	 *   RxLev + Delta = TxPwr - PathLoss -  TxAtt + Delta
	 *   RxLev + Delta = TxPwr - PathLoss - (TxAtt - Delta)
	 */
	new = state->current - delta;
	if (new > state->max)
		new = state->max;
	if (new < 0)
		new = 0;

	if (state->current != new) {
		LOGPLCHAN(lchan, DLOOP, LOGL_INFO, "Changing Downlink attenuation: "
			  "%u -> %u dB (maximum %u dB, target %d dBm, delta %d dB)\n",
			  state->current, new, state->max, params->target_dbm, delta);
		state->current = new;
		return 1;
	} else {
		LOGPLCHAN(lchan, DLOOP, LOGL_INFO, "Keeping Downlink attenuation "
			  "at %u dB (maximum %u dB, target %d dBm, delta %d dB)\n",
			  state->current, state->max, params->target_dbm, delta);
		return 0;
	}
}