WCDMA AICH (Acquisition Indicator Channel)

The Acquisition Indicator Channel (AICH) is a downlink physical channel in WCDMA (UMTS) systems, used to facilitate the random access procedure between User Equipment (UE) and the network.

Function of AICH

When a UE initiates communication, it sends a preamble on the Physical Random Access Channel (PRACH). The NodeB (base station) monitors for these preambles and responds via the AICH with an Acquisition Indicator (AI):​

• Positive Acknowledgment (AI = +1): Indicates successful detection of the preamble. After this, the UE can proceed to transmit its message.
• Negative Acknowledgment (AI = -1): Indicates failure to detect the preamble. In this case, UE must retry after a random backoff period.

Acquisition Indicator

AIs correspond to the signature S of the PRACH.

• The AICH provides timely feedback to UEs, enabling efficient and controlled access to the network resources.

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