Systems and methods are disclosed for performing hybrid automatic repeat request (HARQ) for grant-free uplink transmissions. Some of the systems and methods disclosed herein may address problems such as how to perform acknowledgement (ACK) and/or negative acknowledgement (NACK), how to determine and signal retransmission timing, how to determine the transmission/retransmission attempt and the redundancy version (RV), and/or how to perform the HARQ combining.