S RACH frame' in a PRACH transmission consisting of transmitting the preamble and a message part (see Figure 2.41). The preamble PRACH a repetition of a specific signature which maps the channelisation code used for the PRACH data part. There are 16 different signatures and the mobile randomly picks up one of those. In case no acquisition Indication Channel (AICH) is detected it is re-transmitted with a different signature and with higher power. The preamble is 4096 chips long and is made by 256 repetitions of the signature, which is 16 chips long. The preamble PRACH is then scrambled with a PRACH scrambling code related to the scrambling code of the serving cell. For each primary scrambling code (downlink) there are 16 RACH scrambling codes (uplink) associated with it. The scrambling code use for the message part of the RACH is the same as the one for the preamble. The channelisation code is related to the signature used for the preamble while the SF is decided by the mobile. The signature specifies one of the 16 nodes in the code tree, which corresponds to channelisation with SF 16. The message part can be one or at a maximum two frames long. Because the message is short there is no power control. Acquisition Indication Channel (AICH) The AICH is a downlink physical channel with SF 256 in which an echo of the preamble RACH is sent from the WBTS (WCDMA BTS). The WBTS knows that there will be a message part coming and starts to listen to the channellisation code indicated by the signature. At this point the WBTS does not have any information regarding the user. Dedicated Physical Control Channel (DPCCH) The DPCCH has a constant bit rate and carries all information needed to keep a physical connection running. On the DPCCH the reference symbols (pilots) are sent for a channel estimation in coherent detection and for signal-to-interference ratio (SIR) estimation in fast power control. The power control signalling bits (transmission power control, or TPC) are also sent. The information of how data are coded is carri