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


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