Question: In a 3GPP-compliant FDD network, during a throughput measurement test for a User Equipment (UE), which of the following parameters are **ALL** directly observable by the Serving Station (SS) to determine Block Error Ratio (BLER)?
Option 1: Data bits received, ACK signals, NACK signals
Option 2: Number of bits in payload, ACK signals, NACK signals
Option 3: Successful TTIs, Unsuccessful TTIs (NACK), Regular DTX-TTIs (regDTX)
Option 4: ACK signals, NACK signals, Statistical DTX-TTIs (statDTX)
Explanation: The correct answer is **Option 4** because, according to the provided context: - **ACK and NACK signals** are directly signalled by the UE to the SS, indicating successful or failed receptions, respectively. - **Statistical DTX-TTIs (statDTX)**, although occurring statistically, are observable in the sense that their occurrence can be inferred from the absence of expected ACK or NACK responses when data was sent but not acknowledged. This inference is crucial for calculating BLER. In contrast: - **Data bits received** (Option 1) are not directly accessible to the SS; only the outcome (ACK/NACK) is. - **Number of bits in payload** (Option 2) is known to the SS from what was sent, but this isn't about direct observation for BLER calculation. - **Regular DTX-TTIs (regDTX)** (Option 3) are implicitly known and occur at fixed intervals within a test, making them less relevant for the dynamic calculation of BLER based on observable outcomes.