Random Access procedure is required for UE to get access to network. The most likely scenarios requiring random access are
- Initial random access by UE (RRC connection establishment)
- Cell Update
- Transfer of small PS data
Steps involved in random access procedure are given below:
. UE decodes the BCH to find out the available RACH sub-channels and their scrambling codes and signatures
. UE randomly selects one of the RACH sub-channels from the group its access class allows it to use. Furthermore, the signature is also selected randomly from among the available signatures.
. The downlink power level is measured and the initial RACH power level is set with the proper margin due to the open-loop inaccuracy
. A 1 ms RACH preamble is sent with the selected signature
. if no AICH is detected, the terminal increases the preamble transmission power by a step given by the base station, as multiples of 1 dB. The preamble is retransmitted in the next available access slot.
. When an AICH transmission is detected from the base station, the terminal transmits the 10ms or 20ms message part of the RACH transmission.
Following diagram show how RACH procedure works