7.2 Timing adjustment and Time of Arrival monitoring on Iub/Iur interfaces

25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS

A receiving window is configured in Node B at Transport bearer Setup and Reconfiguration for DL frames (TOAWS and TOAWE). The purpose is to make it possible to supervise whether data frames are received in the window or not. When a frame is received outside that window, a response is sent to RNC by means of a TIMING ADJUSTMENT control frame containing the Time of Arrival information (TOA) (see Figure 10 and Figure 11). This allow the L1 to indicate to L2 (through the L1-MAC primitive carried by the TIMING ADJUSTMENT control frame) the necessity to adjust the timing of the DL transmission, in order to control and minimise the transmission delay and the buffering time for the transmission on the air interface (i.e. to ensure that the TBS does not arrive too much in advance respect to the transmission time).

Figure 10: Illustration of TOAWS, TOAWE, LTOA and TOA

The window could be defined to have a margin before LTOA (TOAWE >0). This is to indicate to RNC that data frames are a bit late but they are still processed by Node B. In this case, data frames are received after TOAWE but before LTOA.

Using this window definition and supervising method, it is possible to determine the correct timing for sending data frames from the RNC over Iur/ Iub.

The window size and position is chosen with respect to expected data frame delay variation and different macro‑diversity leg delays.

Figure 11: Timing Adjustment Procedure

In order to monitor the TOA when no DL DATA FRAMES are sent, a synchronisation procedure is defined in the Iub/Iur frame protocols (TS 25.435 [4], TS 25.427 [5]). This procedure makes use of UL and DL SYNCHRONISATION control frames (see Figure 12 and Figure 13). The SRNC sends DL SYNCHRONISATION control frame containing the CFN in which the control frame should be received by the Node B. When the Node B receives the DL SYNCHRONISATION control frame, it always replies with an UL SYNCHRONISATION control frame containing the TOA, even if the DL SYNCHRONISATION control frame is received within the receiving window as in Figure 12.

Figure 12: TOA monitoring through Frame Protocol Synchronisation Procedure (TOA > 0)

Figure 13: TOA monitoring through Frame Protocol Synchronisation Procedure (TOA < 0)

In case of macro-diversity with recombining in the DRNC, the DL SYNCHRONISATION control frame is duplicated in the DRNC on the different links, while the UL SYNCHRONISATION control frames received from all the Node B’s are forwarded transparently to the SRNC (see Figure 14).

Figure 14: [FDD – TOA monitoring through FP Synchronisation Procedure during soft handover with selection/recombining in the DRNC]

Once the SRNC receives the two UL SYNCHRONISATION control frames containing TOA1 and TOA2, it may consider either TOA1 or TOA2 to advance or delay DL transmission (see Table 1).

Table 1

Relation between TOA1 and TOA2	TAO considered and action performed by the SRNC
TOA1 < TOA2 < 0	TOA1 may be considered to advance DL transmission
TOA2 < TOA1 < 0	TOA2 may be considered to advance DL transmission
TOA1 < 0, TOA2 > 0	TOA1 may be considered to advance DL transmission
TOA2 < 0, TOA1 > 0	TOA2 may be considered to advance DL transmission
TOA1 > TOA2 > 0	TOA2 may be considered to delay DL transmission
TOA2 > TOA1 > 0	TOA1 may be considered to delay DL transmission