Discontinuous reception (DRx) is a process of turning off a radio receiver when it does not expect to receive incoming messages. For DRx to operate, the system must coordinate with the mobile radio for the grouping of messages. The mobile device will wake up during scheduled periods to look for its messages. This reduces the power consumption that extends battery life. This is sometimes called sleep mode.

The WCDMA system divides the paging channel into sub-channel groups to provide for DRx capability. The number of sub-channel groups is determined by the system. Each 10 frames contain a paging channel frame. To inform the mobile device of the sleep periods, a paging indicator channel (PICH) is used. A paging indicator (PI) message is sent at the beginning of the paging channel frame to identify the paging channel group. This allows the mobile device to quickly determine if it must keep its receiver on during the paging group or if it can turn off its receiver and wait for the next paging channel group.

The number of the paging sub-channel is determined by the last digits of the mobile telephone's international mobile service identity (IMSI). The system parameter information sent on the BCCH identifies the grouping of paging sub-channels. The broadcast control channel (BCCH) identifies which multiframes contain paging and access messages and which contain sub-paging classes.

Mobile telephones only need to wakeup for multiframes that are part of its paging sub-channel. During multiframes that are not part...