A UE can perform first uplink transmission on the basis of a first scheduling message without NDI, receive a second scheduling message including an NDI field, and perform i) retransmission for the first uplink transmission or ii) new uplink transmission on the basis of the NDI field in the second scheduling message. Performing the retransmission for the first uplink transmission or the new uplink transmission can comprise: determining, in a state in which an NDI value for the first scheduling message is considered a specific value, whether the value of the NDI field is toggled in comparison to the specific value; performing the retransmission for the first uplink transmission on the basis of the non-toggling of the value of the NDI field in comparison to the specific value; and performing the new uplink transmission on the basis of the toggling of the value of the NDI field in comparison to the specific value.