Question: For FDD UEs supporting MIMO-only with single-stream restriction, what is the primary condition that leads to declaring the probability of event Em (P(Em)) in HS-SCCH detection performance testing?
Option 1: The UE is signalled on HS-SCCH-1 and receives data in the corresponding HS-DPCCH/ACK field.
Option 2: The UE is signalled on HS-SCCH-1, but DTX (Discontinuous Transmission) is observed in the corresponding HS-DPCCH/NACK field.
Option 3: The UE fails to detect any signal on HS-SCCH-1 for a single-stream MIMO transmission.
Option 4: The UE successfully decodes multi-stream data on both HS-SCCH-1 and HS-SCCH-2.
Explanation: The correct answer, Option 2, accurately describes the condition for declaring P(Em) as per the given context. This involves the UE being signalled on HS-SCCH-1 (indicating a potential transmission) but then observing DTX in the corresponding HS-DPCCH/NACK field (suggesting no actual data transmission or an error in transmission). This scenario directly relates to assessing the detection performance of HS-SCCH under specific MIMO conditions. The other options either describe successful transmission scenarios (Option 1), failure to detect any signal which isn't specific to P(Em) declaration (Option 3), or multi-stream decoding not relevant to single-stream restriction (Option 4).