Question: For a UE (User Equipment) with HSDPA capabilities, specifically those categorized under types 15-20, what is the primary condition that leads to the declaration of event Em for assessing HS-SCCH detection performance?
Option 1: Successful decoding of data on HS-SCCH-2 without errors.
Option 2: Receipt of a positive acknowledgement (ACK) on the HS-DPCCH (High-Speed Dedicated Physical Control Channel).
Option 3: Observation of a transmission gap (DTX) in the uplink direction during an ongoing call.
Option 4: Signaling on HS-SCCH-1 accompanied by DTX observed in the corresponding HS-DPCCH/NACK field.
Explanation: The correct answer, Option 4, directly aligns with the provided context. Event Em is specifically declared when two conditions are met: (1) the UE is signaled on HS-SCCH-1, and (2) DTX (Discontinuous Transmission), indicating no transmission, is observed in the corresponding HS-DPCCH/NACK field. This scenario assesses the UE's ability to correctly detect and respond to signaling on the HS-SCCH, which is crucial for managing high-speed downlink packet access efficiently. The other options do not accurately describe the conditions leading to the declaration of event Em as per the given context.