Question: In an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), under what condition does a User Equipment (UE) in RRC_IDLE mode perform measurements on High Rate Packet Data (HRPD) cells?
Option 1: When the UE is experiencing high data throughput on the E-UTRAN serving cell.
Option 2: When the signal quality from the E-UTRAN serving cell falls below a given threshold.
Option 3: Immediately after transitioning from RRC_CONNECTED to RRC_IDLE mode.
Option 4: Only when explicitly commanded by the E-UTRAN network via a dedicated signaling message.
Explanation: This is because, according to idle mode measurement control guidelines, the UE initiates measurements on HRPD cells specifically when the signal strength or quality of its current E-UTRAN serving cell degrades below a predefined threshold, ensuring seamless handover or cell re-selection for optimal connectivity.