scheduler: fix handling of PTCCH/U and PTCCH/D logical channels

According to 3GPP TS 45.010, section 5.6.2, for packet-switched
channels the BTS shall monitor the delay of the Access Bursts
sent by the MS on PTCCH and respond with timing advance values
for all MS performing the procedure on that PDCH.

According to 3GPP TS 45.002, section 3.3.4.2, PTCCH (Packet Timing
advance control channel) is a packet dedicated channel, that is
used for continuous Timing Advance control (mentioned above).

There are two sub-types of that logical channel:

  - PTCCH/U (Uplink): used to transmit random Access Bursts
    to allow estimation of the Timing Advance for one MS in
    packet transfer mode.

  - PTCCH/D (Downlink): used by the network to transmit
    Timing Advance updates for several MS.

As per 3GPP TS 45.003, section 5.2, the coding scheme used for
PTCCH/U is the same as for PRACH as specified in subclause 5.3,
while the coding scheme used for PTCCH/D is the same as for
CS-1 as specified in subclause 5.1.1.

The way we used to handle both PTCCH/U and PTCCH/D is absolutely
wrong - they have nothing to do with xCCH coding. Instead, we
need to use tx_pdtch_fn() for Downlink and rx_rach_fn() for Uplink.

In l1sap_ph_rach_ind() we need to check if an Access Burst was
received on PTCCH/U and forward it to OsmoPCU with proper SAPI
value (PCU_IF_SAPI_PTCCH). To be able to specify a SAPI, a new
parameter is introduced to pcu_tx_rach_ind().

Change-Id: I232e5f514fbad2c51daaa59ff516004aba97c8a3
Related: OS#4102

1 files changed, 16 insertions, 2 deletions

diff --git a/src/common/l1sap.c b/src/common/l1sap.c
index a77b53ab..2874c812 100644
--- a/src/common/l1sap.c
+++ b/src/common/l1sap.c
@@ -1313,6 +1313,20 @@ static int l1sap_ph_rach_ind(struct gsm_bts_trx *trx,
 
 	DEBUGPFN(DL1P, rach_ind->fn, "Rx PH-RA.ind\n");
 
+	/* PTCCH/UL (Packet Timing Advance Control Channel) */
+	if (L1SAP_IS_CHAN_PDCH(rach_ind->chan_nr) && L1SAP_IS_PTCCH(rach_ind->fn)) {
+		LOGPFN(DL1P, LOGL_DEBUG, rach_ind->fn,
+		       /* TODO: calculate and print Timing Advance Index */
+		       "Access Burst for continuous Timing Advance control (toa256=%d)\n",
+		       rach_ind->acc_delay_256bits);
+
+		/* QTA: Timing Advance in units of 1/4 of a symbol */
+		pcu_tx_rach_ind(bts, rach_ind->acc_delay_256bits >> 6,
+				rach_ind->ra, rach_ind->fn, rach_ind->is_11bit,
+				rach_ind->burst_type, PCU_IF_SAPI_PTCCH);
+		return 0;
+	}
+
 	/* check for handover access burst on dedicated channels */
 	if (!L1SAP_IS_CHAN_RACH(rach_ind->chan_nr)) {
 		rate_ctr_inc2(trx->bts->ctrs, BTS_CTR_RACH_HO);
@@ -1349,8 +1363,8 @@ static int l1sap_ph_rach_ind(struct gsm_bts_trx *trx,
 			rach_ind->acc_delay, rach_ind->ra);
 
 		pcu_tx_rach_ind(bts, rach_ind->acc_delay << 2,
-			rach_ind->ra, rach_ind->fn,
-			rach_ind->is_11bit, rach_ind->burst_type);
+			rach_ind->ra, rach_ind->fn, rach_ind->is_11bit,
+			rach_ind->burst_type, PCU_IF_SAPI_RACH);
 		return 0;
 	}