1. A user apparatus configured to perform communication with a base station in a mobile communication system that supports downlink spatial multiplexing, comprising:a transmitter that transmits capability information of the user apparatus to the base station, wherein the capability information comprises a user equipment category and a number of supported MIMO layers for a specific transmission mode;
a receiver that receives configuration information from the base station, wherein the configuration information comprises:
a parameter indicating a transmission mode corresponding to the downlink spatial multiplexing and
predetermined additional information; and
a processor that determines a bit width of a rank indicator based on the predetermined additional information,
wherein before determining the bit width of the rank indicator, the user apparatus determines whether the predetermined additional information is a maximum number of MIMO layers transmitted from the base station as information to be used for determining a bit width of the rank indicator.