Question: In a TDD (Time Division Duplex) network, when an RRC (Radio Resource Control) parameter `Treselection` is set to 0, which of the following metrics is used for cell re-selection from a Cell-FACH state to **another TDD cell**?
Option 1: CPICH Ec/Io (Common Pilot Channel Ec/Io)
Option 2: GSM Carrier RSSI (Received Signal Strength Indicator)
Option 3: P-CCPCH RSCP (Primary Common Control Physical Channel Received Signal Code Power)
Option 4: UE-transmitted SIR (Signal-to-Interference Ratio)
Explanation: According to the specified requirements for radio resource management in TDD mode, when transitioning from a Cell-FACH state to another TDD cell and `Treselection` is set to 0, the metric used for cell re-selection is specifically the P-CCPCH RSCP. This is because P-CCPCH RSCP provides an accurate measure of the signal strength from the primary common control channel, which is crucial for reliable communication in a TDD environment. The other options are either used for different types of handovers (e.g., to FDD or GSM cells) or not directly relevant to this specific re-selection scenario.