5.2 UL synchronisation
25.2243GPPPhysical layer procedures (TDD)TS
5.2.1 General description
Support of UL synchronization is mandatory for the UE.
5.2.1.1 Preparation of uplink synchronization (downlink synchronization)
When a UE is powered on, it first needs to establish the downlink synchronisation with the cell. Only after the UE has established the downlink synchronisation, it shall start the uplink synchronisation procedure.
5.2.1.2 Establishment of uplink synchronization
The establishment of uplink synchronization is done during the random access procedure and involves the UpPCH and the PRACH or the E-RUCCH.
Although the UE can receive the downlink signal from the Node B, the distance to Node B is still uncertain. This would lead to unsynchronised uplink transmission. Therefore, the first transmission in the uplink direction is performed in a special time-slot UpPTS or other uplink access position indicated by UTRAN to reduce interference in the traffic time-slots.
After the detection of the SYNC-UL sequence in the searching window, the Node B will evaluate the timing, and reply by sending the adjustment information to the UE to modify its timing for next transmission. This is done with the FPACH within the following 4 sub-frames. After sending the PRACH or the E-RUCCH, the uplink synchronization is established. The uplink synchronisation procedure shall also be used for the re-establishment of the uplink synchronisation when uplink is out of synchronisation.
5.2.1.3 Maintenance of uplink synchronisation
Uplink synchronization is maintained in 1.28 Mcps TDD by sending the uplink advanced in time with respect to the timing of the received downlink.
For the maintenance of the uplink synchronization, the midamble field of each uplink burst can be used.
In each uplink time slot the midamble for each UE is different. The Node B may estimate the timing by evaluating the channel impulse response of each UE in the same time slot. Then, in the next available downlink time slot, the Node B will signal Synchronisation Shift (SS) commands to enable the UE to properly adjust its Tx timing.
5.2.2 UpPCH
Open loop uplink synchronisation control is used for UpPCH.
The UE may estimate the propagation delay tp based upon the path loss using the received P-CCPCH and/or DwPCH power.
The UpPCH is sent to the Node B advanced in time according to the timing of the received DwPCH. The time of the beginning of the UpPCH TTX‑UpPCH is given by:
TTX-UpPCH = TRX-DwPCH -2tp +12*16 TC＋nUpPCHShift*16Tc
nUpPCHShift =0..127，nUpPCHShift is indicated by higher layers
in multiple of 1/8 chips, where
TTX-UpPCH is the beginning time of UpPCH transmission with the UE’s timing,
TRX-DwPCH is the received beginning time of DwPCH with the UE’s timing,
2tp is the timing advance of the UpPCH default value is 48Tc.
5.2.3 PRACH
The Node B shall measure the received SYNC-UL timing deviation UpPCHPOS. UpPCHPOS is sent in the FPACH and is represented as a 13 bit number (0-8191) being the multiple of 1/8 chips which is nearest to received position of the UpPCH.
Time of the beginning of the PRACH TTX‑PRACH is given by:
TTX-PRACH = TRX-PRACH –(UpPCHADV + UpPCHPOS – 8*16 TC)
in multiple of 1/8 chips, where
TTX-PRACH is the beginning time of PRACH transmission with the UE’s timing,
TRX-PRACH is the beginning time of PRACH reception with the UE’s timing if the PRACH was a DL channel.
5.2.4 DPCH and PUSCH
The closed loop uplink synchronisation control uses layer 1 symbols (SS commands) for DPCH and PUSCH. After establishment of the uplink synchronisation, NodeB and UE start to use the closed loop UL synchronisation control procedure. This procedure is continuous during connected mode.
The Node B will continuously measure the timing of the UE and send the necessary synchronisation shift commands in each sub-frame. The UE shall derive a single SS command separately for each controlled uplink timeslot by combining all received SS commands that are related to the controlled time slot (cf. [8]) and that are received within the last up to M sub-frames. The value of the "Uplink synchronisation frequency" M (1..8) is configured by higher layers.
When the combined SS command is judged as ‘down’, the UE transmit timing for the controlled UL timeslot shall be delayed by one timing adjustment step of k/8 chips. When the command is judged as ‘up’, the UE transmit timing for the controlled UL timeslot shall be advanced by one timing adjustment step of k/8 chips. When the command is judged as ‘do nothing’, the timing shall not be changed. The value of the "Uplink synchronisation step size" k (1..8) is configured by higher layers.
The timing adjustment shall take place in each sub-frame satisfying the following equation:
where
SFN’ is the system frame number counting the sub-frames. The system frame number of the radio frames (SFN) can be derived from SFN’ by
SFN=SFN’ div 2, where div is the remainder free division operation.
During a 1.28 Mcps TDD to 1.28 Mcps TDD hand-over the UE shall transmit in the new cell with timing advance TA adjusted by the relative timing difference t between the new and the old cell if indicated by higher layers:
TAnew = TAold + 2t.
5.2.4.1 Out of synchronization handling
Same as that of 3,84 Mcps TDD, cf.[ 4.2.2.3.2 Out of synchronisation handling.]
5.2.5 HS-SICH
If there is associated uplink DPCH or non-scheduled E-PUCH, the initial transmit timing for the HS-SICH shall be taken from that of the associated uplink DPCH or non-scheduled E-PUCH. Otherwise, the initial transmit timing for the HS-SICH shall be taken from that of the last uplink transmission except UpPCH transmission. The UE shall then adjust the timing of the HS-SICH according to SS commands transmitted to it on the HS-SCCH or HS-PDSCH. The step size for these commands shall be signalled to the UE by higher layers. The UE shall ignore the SS command transmitted on an HS-SCCH order which is an uplink synchronization establishment command. In the case that there is a gap of one or more subframes during which no SS commands are received by the UE, the UE shall adjust the timing of the HS-SICH according to the associated uplink DPCH or non-scheduled E-PUCH or other uplink channel except UpPCH until such time as another HS-SCCH transmission or HS-PDSCH with SS command is received.
5.2.6 E-PUCH
Uplink synchronization control procedure for E-PUCH remains the same as that used for DPCH, cf. 5.2.4 DPCH and PUSCH, using SS commands carried on E-AGCH or on E-HICH. When receipt of SS commands on E-AGCH and on E-HICH is in the same sub-frame, UE shall treat one single SS command if they are identical and discard them if they are different.
If configured with DPCH, the timing of E-PUCH can be directly set to the timing of the DPCH. If configured with non-scheduled transmission, the timing of scheduled E-PUCH can be set to the timing of the non-scheduled E-PUCH. If neither DPCH nor non-scheduled transmission is configured, the timing of E-PUCH can be set to the timing of other uplink channels except UpPCH.
For multi-carrier E-DCH transmission, in one TTI, the timing advance of the E-PUCHs of all carriers of one UE is the same. The UE shall combine all the SS commands on each E-AGCH and E-HICH in the same TTI into one UL timing advance value.
5.2.7 E-RUCCH
The Node B shall measure the received SYNC-UL timing deviation UpPCHPOS. UpPCHPOS is sent in the FPACH and is represented as an 13 bit number (0-8191) being the multiple of 1/8 chips which is nearest to received position of the UpPCH.
Time of the beginning of the E-RUCCH TTX‑E-RUCCH is given by:
TTX‑E-RUCCH = TRX-E-RUCCH –(UpPCHADV + UpPCHPOS – 8*16 TC)
in multiple of 1/8 chips, where
TTX- E-RUCCH is the beginning time of E-RUCCH transmission with the UE’s timing,
TRX- E-RUCCH is the beginning time of E-RUCCH reception with the UE’s timing if the E-RUCCH was a DL channel,
UpPCHADV is the timing advance of the UpPCH.
5.2.8 Standalone midamble channel
The transmit timing for the standalone midamble channel shall be taken from that of the last uplink transmission.