Network-Assisted Cell Change (NACC) improves the performance of network-controlled cell re-selection operation and autonomous cell re-selection.

When a mobile station (MS) performs an autonomous cell re-selection, or is commanded to perform a cell change, it has to stop the data transmission in the serving cell. The serving cell is the cell containing the serving BTS. After that the MS has to acquire certain system information from the target cell. The target cell is the neighbour cell that has been determined by the MS or the network during the cell change. After that the MS has to restart the data transmission in the new cell. This leads to a certain delay because the MS has to synchronise with the system information broadcast cycle, and collect a consistent set of system information (SI) messages of the target cell.

NACC reduces this service outage time for an MS that is in packet transfer mode

1. By giving the network the means to assist MSS before and during the cell change: the network assists MSS by sending neighbour cell system information on a packet associated control channel (PACCH) to the MS in packet transfer mode while it is still camping on the serving cell.

2. By the introduction of PACKET SI STATUS procedure: Neighbour cell system information messages that the network sends to the MS contain a set of SI messages needed in performing a packet access in the new cell. When all the required messages are sent to the MS, and the PCU supports the PACKET SI STATUS procedure in the new cell, the MS is able to perform packet access and use PACKET SI STATUS procedures for the acquisition of SI messages.

NACC functions with and without Network-Controlled Cell Re-selection (NCCR).

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