Change in osmo-bts[master]: sched_lchan_tchh: fix frame number and fill FACCH gap

laforge gerrit-no-reply at lists.osmocom.org
Sat Mar 27 18:37:55 UTC 2021


laforge has submitted this change. ( https://gerrit.osmocom.org/c/osmo-bts/+/21329 )

Change subject: sched_lchan_tchh: fix frame number and fill FACCH gap
......................................................................

sched_lchan_tchh: fix frame number and fill FACCH gap

The measurement results / TCH indications that are handed when decoding
the TCH/H are off by two bursts. Since a measurement result / TCH block
is expected every two bursts anyway the problem can only be noticed when
a FACCH transmission is going on and the frame numbers of the BFI TCH
blocks appear to be missaligned towards the FACCH block.

The reason is that the incoming bursts are shifted into a 6 burst wide
buffer. The decoding functions always look at the beginning of that
buffer while the bursts are shifted into the buffer from the end. A
facch will always fit exactly in that buffer but TCH/H blocks are only
4 bursts wide and thereofre they need two additional bursts until they
reach the coverage of the decoding function. Lets fix this by putting
the correct frame number (from two bursts before) into the remap
functions in order to get the correct beginning of the block.

Since the FACCH transmission uses six blocks it takes out two TCH
blocks. This means that if we count the FACCH block we end up with a gap
of one TCH voice block. Lets generate a dummy measurement to compensate
the gap. This will also match the behavior of the osmo-bts-sysmo phy.

Change-Id: I1ad9fa3815feb2b4da608ab7df716a87ba1f2f91
Related: OS#4799
---
M include/osmo-bts/scheduler.h
M src/osmo-bts-trx/sched_lchan_tchh.c
2 files changed, 51 insertions(+), 8 deletions(-)

Approvals:
  laforge: Looks good to me, approved
  Jenkins Builder: Verified



diff --git a/include/osmo-bts/scheduler.h b/include/osmo-bts/scheduler.h
index 6f05756..b1e4294 100644
--- a/include/osmo-bts/scheduler.h
+++ b/include/osmo-bts/scheduler.h
@@ -115,6 +115,8 @@
 	/* TCH/H */
 	uint8_t			dl_ongoing_facch; /* FACCH/H on downlink */
 	uint8_t			ul_ongoing_facch; /* FACCH/H on uplink */
+	struct l1sched_meas_set meas_avg_facch;   /* measurement results for last FACCH */
+	uint16_t		ber10k_facch;	  /* bit error rate for last FACCH */
 
 	/* encryption */
 	int			ul_encr_algo;	/* A5/x encry algo downlink */
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index c7dac70..378d47a 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -68,9 +68,11 @@
 	enum sched_meas_avg_mode meas_avg_mode = SCHED_MEAS_AVG_M_QUAD;
 	struct l1sched_meas_set meas_avg;
 	unsigned int fn_begin;
+	unsigned int fn_tch_end;
 	uint16_t ber10k;
 	uint8_t is_sub = 0;
 	uint8_t ft;
+	bool mask_stolen_tch_block = false;
 
 	/* If handover RACH detection is turned on, treat this burst as an Access Burst.
 	 * Handle NOPE.ind as usually to ensure proper Uplink measurement reporting. */
@@ -128,6 +130,10 @@
 		 * was decoded (see below), now send the second one. */
 		ber10k = 0;
 		memset(&meas_avg, 0, sizeof(meas_avg));
+		/* In order to provide an even stream of measurement reports
+		 * we ask the code below to mask the missing TCH/H block
+		 * measurement report with the FACCH measurement results. */
+		mask_stolen_tch_block = true;
 		goto bfi;
 	}
 
@@ -267,6 +273,18 @@
 			meas_avg.rssi, meas_avg.toa256,
 			meas_avg.ci_cb, ber10k,
 			PRES_INFO_UNKNOWN);
+
+		/* Keep a copy of the measurement results of the last FACCH
+		 * transmission in order to be able to create a replacement
+		 * measurement result for the one missing TCH block
+		 * measurement */
+		memcpy(&chan_state->meas_avg_facch, &meas_avg, sizeof(meas_avg));
+		chan_state->ber10k_facch = ber10k;
+
+		/* Invalidate the current measurement result to prevent the
+		 * code below from handing up the current measurement a second
+		 * time. */
+		memset(&meas_avg, 0, sizeof(meas_avg));
 bfi:
 		/* A FACCH/H frame replaces two speech frames, so we need to send two BFIs.
 		 * One is sent here, another will be sent two bursts later (see above). */
@@ -317,15 +335,38 @@
 
 compose_l1sap:
 	/* TCH or BFI */
-	/* Note on FN 19 or 20: If we received the last burst of a frame,
-	 * it actually starts at FN 8 or 9. A burst starting there, overlaps
-	 * with the slot 12, so an extra FN must be subtracted to get correct
-	 * start of frame.
-	 */
-	if (lchan->nr == 0)
-		fn_begin = gsm0502_fn_remap(bi->fn, FN_REMAP_TCH_H0);
+
+	/* The input to gsm0502_fn_remap() needs to get the frame number we
+	 * got two bursts ago. The reason for this is that the burst shift
+	 * buffer we use for decoding is 6 bursts wide (one SACCH block) but
+	 * TCH/H blocks are only 4 bursts wide. The decoder functions look
+	 * at the beginning of the buffer while we shift into it at the end,
+	 * this means that TCH/H blocks always decode delayed by two frame
+	 * number positions late. To calculatue the ending frame number of
+	 * the TCH/H we need to subtract 4 or 5 frames if there was a SACCH
+	 * in between. (Note: this is TCH/H, 4 frames ==> 2 bursts) */
+	if (bi->fn % 13 < 4)
+		fn_tch_end = GSM_TDMA_FN_SUB(bi->fn, 5);
 	else
-		fn_begin = gsm0502_fn_remap(bi->fn, FN_REMAP_TCH_H1);
+		fn_tch_end = GSM_TDMA_FN_SUB(bi->fn, 4);
+	
+	if (lchan->nr == 0)
+		fn_begin = gsm0502_fn_remap(fn_tch_end, FN_REMAP_TCH_H0);
+	else
+		fn_begin = gsm0502_fn_remap(fn_tch_end, FN_REMAP_TCH_H1);
+
+	/* A FACCH/H transmission takes out two TCH/H voice blocks and the
+	 * related measurement results. The first measurement result is handed
+	 * up directly with the FACCH (see above), the second one needs to be
+	 * compensated by filling the gap with the measurement result we got
+	 * from the FACCH transmission. */
+	if (mask_stolen_tch_block) {
+		memcpy(&meas_avg, &chan_state->meas_avg_facch, sizeof(meas_avg));
+		ber10k = chan_state->ber10k_facch;
+		memset(&chan_state->meas_avg_facch, 0, sizeof(meas_avg));
+		chan_state->ber10k_facch = 0;
+	}
+
 	return _sched_compose_tch_ind(l1t, bi->tn, fn_begin, chan, tch_data, rc,
 				      /* FIXME: what should we use for BFI here? */
 				      bfi_flag ? bi->toa256 : meas_avg.toa256, ber10k,

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Gerrit-Project: osmo-bts
Gerrit-Branch: master
Gerrit-Change-Id: I1ad9fa3815feb2b4da608ab7df716a87ba1f2f91
Gerrit-Change-Number: 21329
Gerrit-PatchSet: 4
Gerrit-Owner: dexter <pmaier at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: fixeria <vyanitskiy at sysmocom.de>
Gerrit-Reviewer: laforge <laforge at osmocom.org>
Gerrit-MessageType: merged
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