PRACH Physical Random Access Channel work in WCDMA

  • Carrying uplink signaling and data, consist of two parts:
  • One or several preambles: 16 kinds of available preambles
  • 10 or 20ms message part

PRACH in wcdmaThe Physical Random Access Channel (PRACH) is used by the UE to access the network and to carry small data packets. It carries the RACH transport channel. The PRACH is an open loop power control channel, with contention resolution mechanisms (ALOHA approach) to enable a random access from several users.

The PRACH is composed of two different parts: the preamble part and the message part that carries the RACH message. The preamble is an identifier which consists of 256 repetitions of a 16 chip long signature (total of 4096 chips). There are 16 possible signatures, basically, the UE randomly selects one of the 16 possible preambles and transmits it at increasing power until it gets a response from the network (on the AICH).

That preamble is scrambled before being sent. That is a sign that the power level is high enough and that the UE is authorized to transmit, which it will do after acknowledgment from the network. If the UE doesn’t get a response from the network, it has to select a new signature to transmit.

The message part is 10 or 20 ms long (split into 15 or 30 time slots) and is made of the RACH data and the layer 1 control information.