Methods and user equipment for mobile communication

A User Equipment (UE) including a wireless transceiver and a controller is provided. The wireless transceiver performs wireless transmission and reception to and from a mobile communication network. The controller determines whether a trigger condition is met. In response to the trigger condition being met, the controller reports a first value representing a first Rank Indicator (RI) or a second value representing the UE's capability of Multiple-Input-Multiple-Output (MIMO) layers to the mobile communication network via the wireless transceiver. In particular, the first value is less than a third value representing a second RI derived from a measurement by the UE, or the second value is less than a fourth value representing a number of maximum MIMO layers supported by the UE.