Question: In the context of Acquisition Indicator (AI) receiver characteristics for UTRA FDD User Equipment (UE), what do the probabilities Pfa and Pd represent, respectively?
Option 1: - Pfa: Probability of AI signature transmission
  - Pd: Probability of incorrect AI signature detection
Option 2: - Pfa: Conditional probability of AI signature detection when not transmitted
  - Pd: Overall probability of AI signature transmission success
Option 3: - Pfa: Probability of correct AI signature detection
  - Pd: Conditional probability of AI signature non-detection when transmitted
Option 4: - Pfa: Conditional probability of false AI signature detection (when not transmitted)
  - Pd: Conditional probability of correct AI signature detection (when transmitted)
Explanation: The correct answer, Option 4, accurately defines the probabilities in the context of Acquisition Indicator (AI) receiver characteristics. 
  - **Pfa (Probability of False Alarm)** correctly refers to the likelihood of detecting an AI signature when none was actually transmitted.
  - **Pd (Probability of Detection)** correctly refers to the likelihood of correctly detecting an AI signature when one is indeed transmitted.
  These definitions are crucial for understanding and optimizing the performance of UTRA FDD UE's receiver in detecting Acquisition Indicators, impacting overall network efficiency and reliability.