What is AICH (Acquisition Indicator Channel)?

AICH is an Acquisition Indicator channel. It is a common downlink channel used to control the uplink random accesses. AICH carries AI which is known as the Acquisition indicator which carries a signature on the PRACH (Physical Random access channel) and uplink channel. The AICH consists of a burst of data transmitted regularly every access slot frame. AI consists of two parts: an Acquisition indicator part of 32 real-valued symbols and a long part during which nothing is transmitted, this is to avoid overlapping due to the propagation delays. AICH is a fixed rate physical channel with a spreading factor of 256 which is equal to 32 kbps.