Here I write about what is main work of Acquisition Indicator Channel AICH in 3G

There is 2 types main work of AICH

  1. Carrying the Acquisition Indicators (AI), SF = 256
  2. There are 16 kinds of Signature to generate AI

AICH Channel in wcdma

The Acquisition Indicator Channel (AICH) is a common downlink channel used to control the uplink random accesses. It carries the Acquisition Indicators (AI), each corresponding to a signature on the PRACH (uplink). When the NodeB receives the random access from a mobile, it sends back the signature of the mobile to grant its access. If the NodeB receives multiple signatures, it can sent all these signatures back by adding the together. At reception, the UE can apply its signature to check if the NodeB sent an acknowledgement (taking advantage of the orthogonality of the signatures).

The AICH consists of a burst of data transmitted regularly every access slot frame. One access slot frame is formed of 15 access slots, and lasts 2 radio frames (20 ms). Each access slot consists of two parts, an acquisition indicator part of 32 real-valued symbols and a long part during which nothing is transmitted to avoid overlapping due to propagation delays.

s (with values 0, +1 and -1, corresponding to the answer from the network to a specific user) and the 32 chip long sequence <bs,j> is given by a predefined table. There are 16 sequences <bs,j>, each corresponding to one PRACH signatures. A maximum of 16 AIs can be sent in each access slot. The user can multiply the received multi-level signal by the signature it used to know if its access was granted. The SF used is always 256 and the OVSF code used by the cell is indicated in system information type 5.