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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

RachProcedure

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