9 Usage of Synchronisation Counters and Parameters to support Transport Channel and Radio Interface Synchronisation

25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS

9.1 General

This subclause describes how the different synchronisation parameters and counters are computed and used in order to obtain Transport Channel (L2) and Radio Interface (L1) Synchronisation.

The parameters that need to be determined by the UE are CFN, OFF [FDD – and Tm].

The parameter that need to be determined by the UTRAN are [FDD – DOFF_FDD], [TDD – DOFF_TDD], Frame Offset and [FDD – Chip Offset].

Figure 17 summarises how these parameters are computed. A detailed description of the actions in each state is given in the sections 9.2 – 9.4, while some examples of corrections applied to synchronisation counters during UE state transitions are shown in section 9.5.

Figure 17: Calculations performed by UE and UTRAN

Figure 18 describes what offset parameters are signalled and used in the different nodes at Initial RL setup and at Handover (HO) in FDD. The rounding to closest 256 chip boundary is done in Node B. The rounded Frame Offset and Chip Offset control the DL DPCH air-interface timing or the F-DPCH air-interface timing. The 256 chip boundary is to maintain DL orthogonality in the cell (the rounding to the closest 256 chip boundary is done in Node B to facilitate the initial UL chip synchronisation process in Node B). As this figure is also applicable to F-DPCH, "DL DPCH" has to be replaced by "F-DPCH" everywhere in the figure in the context of F-DPCH.

Figure18: [FDD – Usage of Offset values at initial RL and at HO]

Figure 19 describes what offset parameters are signalled and used in the different nodes at Initial RL setup and at Handover (HO) in TDD.

Note that in some cases the parameter OFF_target cannot be measured by the UE before handover (e.g. in case of inter frequency handover or inter-mode handover). In these cases a value as defined in [FDD – 13] [TDD – 14] shall be reported by the UE.

Figure 19: [TDD- Usage of Offset values at initial RL and at HO]

9.2 Calculations performed in the UTRAN

9.2.1 UE in CELL_FACH/PCH state

In CELL_FACH/PCH state the Frame Offset is set to 0 (for all common and shared channels).

9.2.2 UE changes from CELL_FACH/PCH state to CELL_DCH state: 1 RL

[FDD – Based on the received parameters from the UE and the DOFF_FDD value generated in the SRNC, the SRNC calculates the Frame Offset and the Chip Offset from formula (9.1) or formula (9.1.a):

Frame Offset*38400 +Chip Offset = DOFF_FDD*512 (9.1)

Frame Offset*38400 +Chip Offset = DOFF_FDD*512 + 256 (9.1.a)

Frame Offset and Chip Offset are then signalled to the Node B controlling the serving cell.]

[TDD – Based on the DOFF_TDD value generated in the SRNC, the SRNC calculates the Frame Offset = DOFF_TDD.

Frame Offset is then signalled to the Node B controlling the serving cell.]

[TDD – Note that for all common and shared channels Frame Offset is set to 0 even during CELL_DCH state.]

9.2.3 [FDD – UE changes from CELL_FACH/PCH state to CELL_DCH state: several RL’s]

Based on the received parameters from the UE for each cell_k (OFF_k and Tm_k) and the DOFF_FDD value generated in the SRNC, the SRNC calculates the Frame Offset_k and the Chip Offset_k. The Frame Offset_k and the Chip Offset_k are calculated from formula (9.2) or formula (9.2.a):

Frame Offset_k*38400 + Chip Offset_k = DOFF_FDD*512 + OFF_k*38400 + Tm_k (9.2)

Frame Offset_k*38400 + Chip Offset_k = DOFF_FDD*512 + OFF_k*38400 + Tm_k + 256 (9.2.a)

NOTE: Formula (9.2) is covering formula (9.1) since in the case described in section 9.2.2, OFF_k and Tm_k are both equal to zero.

Each Frame Offset_k and Chip Offset_k are then signalled to the Node B controlling the cell_k.

9.2.4 UE in CELL_DCH state: addition of a new RL or handover to a new cell

[FDD – Based on the received parameters from the UE or already known by the UTRAN (OFF_target, Tm_target), the SRNC calculates the Frame Offset_target and the Chip Offset_target with formula (9.3):

Frame Offset_target*38400 + Chip Offset _target= OFF_target*38400 + Tm_target (9.3)

During hard handover in case the parameter OFF_target cannot be measured by the UE and it is not already known by the UTRAN, than the SRNC calculates the Frame Offset_target and the Chip Offset_target with formula (9.1).

Frame Offset_target and Chip Offset_target are then signalled to the Node B controlling the target cell.]

[TDD – Based on the parameter OFF_target received from the UE or already known by the UTRAN, the SRNC calculates the Frame Offset_target = OFF_target.

In case the parameter OFF_target cannot be measured by the UE and it is not already known by the UTRAN, than the SRNC calculates the Frame Offset_target = DOFF_TDD.

It is signalled to the Node B controlling the target cell.]

9.2.5 Handover from other RAN to UMTS

[FDD – Based on the definitions for OFF and Tm formula (9.1) can also be used when the UE enters the UTRAN from another CN and establishes one dedicated RL. The same is true for formula (9.2) when establishing one or more dedicated RL’s.]

[TDD – When the UE enters the UTRAN from another CN and establishes one dedicated RL, OFF is 0.]

9.3 Calculations performed in the UE

9.3.A UE in CELL_FACH/PCH state

In CELL_FACH/PCH state the CFN is initialised with the values CFN = SFN for PCH and CFN = SFN mod 256 for all other common and shared channels. The CFN for all common and shared channels in the CRNC is increased (mod 256) by 1 every frame, except PCH, which CFN has the same range of the SFN.

9.3.1 UE changes from CELL_FACH/PCH state to CELL_DCH state: 1 RL

[FDD – Based on the received DOFF_FDD and the SFN of the cell in which the UE is source, the UE can initialise the CFN with the value given by formula (9.4):

CFN = (SFN – ((DOFF_FDD * 512) div 38400)) mod 256 (9.4)

This formula gives the CFN of the downlink DPCH frame or of the F-DPCH frame which starts at the same time as or which starts during the PCCPCH frame with the given SFN.]

[TDD – Based on the received DOFF_TDD, the UE can initialised the CFN with the value given by formula (9.5):

CFN = (SFN- DOFF_TDD) mod 256 (9.5)]

After the initialisation, the CFN in the UE is increased (mod 256) by 1 every frame.

[TDD – Note that for all common and shared channels CFN = SFN mod 256 even during CELL_DCH state.]

9.3.1A [FDD – UE changes from CELL_FACH/PCH to CELL_DCH state: several RL’s]

The UE reports to the SRNC the parameters OFF_k and Tm_k for each cell_k measured respect to the reference cell_j determined by means of formula (9.6):

OFF_k + Tm_k= (SFN_k – CFN) mod 256 (9.6)

After having performed OFF_k and Tm_k measurements for all target cells, the UE initialises the CFN with the value given by formula (9.7), based on the received DOFF_FDD and the SFN_j of the reference cell:

CFN = (SFN_j – ((DOFF_FDD * 512) div 38400)) mod 256 (9.7)

This formula gives the CFN of the downlink DPCH frame or of the F-DPCH frame which starts at the same time as or which starts during the PCCPCH frame with the given SFN.

After the initialisation, the CFN in the UE is increased (mod 256) by 1 every frame.

9.3.2 UE in CELL_DCH state: addition of a new RL or handover to a new cell

The UE in CELL_DCH state may be requested by the UTRAN to report OFF_target by means of System Info broadcast in the source cell.

[FDD – In case the SFN_target can be measured, the target cell OFF_target is calculated using formula (9.8):

OFF_target + Tm_target= (SFN_target – CFN) mod 256 (9.8)

otherwise a value as defined in TS 25.215 [13] is reported. Tm_target is always reported, except for the case of FDD-TDD handover.]

[TDD – In case the SFN_target can be measured, the target cell OFF_target is calculated using formula (9.9):

OFF_target = (SFN_target -CFN) mod 256 (9.9)

otherwise a value as defined in TS 25.225 [14] is reported.]

Note that, regarding the CFN, two cases may occur:

a) the value of OFF_target is known by the UTRAN before handover execution:

a1) either because the SFN_target has been measured by the UE and reported to the UTRAN by means of the OFF_target before handover;

a2) or because the UTRAN already knows the difference between serving cell SFN_source and target cell SFN_target and derives OFF_target from OFF_source by applying the difference between SFN_target and SFN_source (this difference between SFNs may be known in the UTRAN from previous UE’s measurement reports);

a3) [TDD – or because cells involved in the handover are synchronised – and hence OFF_target equals OFF_source ].

b) the value of OFF_target is not known by the UTRAN before handover execution because the SFN_target cannot be measured by the UE before handover and the UTRAN does not know the difference between serving cell SFN and target cell SFN.

In case a) the UTRAN shall not signal to the UE any value of [FDD- DOFF_FDD] [TDD- DOFF_TDD] before handover in the RRC message PHYSICAL CHANNEL RECONFIGURATION, and the UE shall maintain the old CFN, i.e. no correction to CFN is needed during handover.

In case b) the UTRAN shall signal to the UE the new value of [FDD- DOFF_FDD] [TDD- DOFF_TDD] before handover by means of the RRC message PHYSICAL CHANNEL RECONFIGURATION. The CFN shall be re-initialised after handover (as soon as the UE reads the SFN_target) according to formula [FDD- (9.4)] [TDD- (9.5)].

Note that in cases a2) and a3) the UTRAN may not request the UE to report OFF_target, while in case b) the value of OFF_target reported by the UE is the one defined in [FDD – 13], [TDD – 14] for this case.

9.4 Synchronisation of L1 configuration changes

When a synchronised L1 configuration change shall be made, the SRNC commands the related Node B’s to prepare for the change. When preparations are completed and SRNC informed, serving RNC decides appropriate change time (see TS 25.212 [24], subclause 4.2.14 and TS 25.222 [25], subclause 4.2.13). SRNC tells the CFN for the change by a suitable RRC message. The Node B’s are informed the CFN by RNSAP and NBAP Synchronised Radio Link Reconfiguration procedures.

At indicated switch time UE and Node B’s change the L1 configuration.

9.5 Examples of synchronisation counters during state transitions

The example of Figure 20 shows the corrections applied to UTRAN synchronisation counters during multiple transitions from CELL_FACH/PCH state to CELL_DCH state before and after handover, without SRNS relocation. In this example two handover cases described in 9.3.2 are considered.

Figure 20: Example 1

The example of Figure 21 shows the corrections applied to UTRAN synchronisation during multiple transitions from CELL_FACH/PCH state to CELL_DCH state after cell reselection, without SRNC relocation.

Figure 21: Example 2

The example of Figure 22 shows the corrections applied to UTRAN synchronisation counters during multiple transitions from CELL_FACH/PCH state to CELL_DCH state before and after handover and SRNS relocation (without UE involvement). In this example two handover cases described in 9.3.2 are considered.

Figure 22: Example 3