Method for uplink communication by terminal in mmwave communication system and terminal

A user equipment supportive of an mmWave communication system, the user equipment comprising:

• a transmitter;
• a receiver;
• and
• a processor configured to operate by being connected to the transmitter and the receiver,
• wherein the processor is further configured to:
• control the receiver to receive data via an mmWave downlink from an mmWave base station;
• control the transmitter to transmit an ACKnowledgement/Negative ACK (ACK/NACK) response for the data and an mmWave uplink reference signal to the mmWave base station via an mmWave uplink;
• control the receiver to receive an indicator indicating to use a legacy uplink instead of the mmWave uplink from the mmWave base station;
• shorten a length of a legacy Transmission Time Interval (TTI) to match up with a length of an mmWave TTI according to the indicator;
• and
• control the transmitter and the receiver to perform communication with the mmWave base station via the legacy uplink using the shortened legacy TTI.