Main work of SCCPCH Secondary Common Control Physical Channel for wcdma is as below.
- Carrying FACH and PCH, SF = 256 – 4
- Pilot: used for demodulation
- TFCI: Transport Format Control Indication, used for describe data Format
The Secondary Common Control Physical Channel (S-CCPCH) is used to carry the FACH and PCH transport channels. Unlike the P-CCPCH, it is not broadcasted continuously. It is only transmitted when there is a PCH or FACH information to transmit. At the mobile side, the mobile only decodes the S-CCPCH when it expects a useful message on the PCH or FACH.
A UE will expect a message on the PCH after indication from the PICH (page indicator channel), and it will expect a message on the FACH after it has transmitted something on the RACH.
The FACH and the PCH can be mapped on the same or on separate S-CCPCHs. If they are mapped on the same S-CCPCH, TFCI bits have to be sent to support multiple transport formats
The figure above shows the frame structure of the S-CCPCH. There are 18 different slot formats determining the exact number of data, pilot and TFCI bits. The data bits correspond to the PCH and/or FACH bits coming from the transport sub-layer. Pilot bit are typically used when beamforming techniques are used.
The SF ranges from 4 to 256. The channelization code is assigned by the RRC layer as is the scrambling code, and they are fixed during the communication. They are sent on the BCCH so that every UE can decode the channel.
As said before, FACH can be used to carry user data. The difference with the dedicated channel is that it cannot use fast power control, or soft handover. The advantage is that it is a fast access channel.