7.1 General

25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS

The Transport Channel (or L2) synchronisation provides a L2 common frame numbering between UTRAN and UE (frame synchronisation between the L2 entities). This frame number is the Connection Frame Number (CFN) and it is associated at L2 to every TBS and passed to L1: the same CFN is received on the peer side associated with the same TBS.

The CFN is not transmitted in the air interface for each TBS, but is mapped by L1 to the SFN of the first radio frame used for the transmission of the TBS (the SFN is broadcast at L1 in the BCH). The mapping is performed via the Frame Offset parameters (see Figure 8).

Figure 8: Transport Channel Synchronisation

This transport channel synchronisation mechanism is valid for all downlink transport channels.

In case of soft handover (i.e. only for DCHs), the Frame Offsets of the different radio links are selected in order to have a timed transmission of the diversity branches on the air interface (see Figure 9).

Figure 9: [FDD – Transport Channel Synchronisation during soft handover]