Question: In a TDD system with FDD neighbors, what triggers a UE to evaluate an inter-frequency cell for potential reselection within NcarrierFDD * TevaluateFDD, given Treselection timer is set to zero?
Option 1: When the serving cell's CPICH RSCP drops below a threshold.
Option 2: When an identified inter-frequency cell becomes at least 3 dB better than the current serving cell.
Option 3: When an identified inter-frequency cell becomes at least 5 dB better than the current serving cell.
Option 4: When a non-identified inter-frequency cell's CPICH Ec/Io surpasses the serving cell's by any margin.
Explanation: According to the specifications for radio resource management in TDD systems with FDD neighbors, a UE is required to evaluate an already identified inter-frequency cell for potential reselection within NcarrierFDD * TevaluateFDD from the moment this cell becomes at least 5 dB better than the serving cell, provided the Treselection timer is set to zero. This ensures timely handovers to superior cells while preventing unnecessary reselections due to minor signal fluctuations.