A terminal is disclosed including a processor that determines, when retransmission of a side channel in a communication between a plurality of terminals is performed, a modulation order for the retransmission of the side channel by using a MCS table based on side control information; a transmitter that transmits the side channel based on the determined modulation order, and a receiver that receives downlink control information including at least a UL-SCH indicator field, wherein the processor, if retransmission of an uplink shared channel is performed, determines a time density of a phase tracking reference signal (PTRS) based on whether a modulation and coding scheme (MCS) index is larger than a certain value, and wherein the transmitter transmits the PTRS. In other aspects, a radio communication method, and a system are also disclosed.