Systems, methods and devices for flexible retransmissions

A hybrid automatic repeat request (HARQ) process enables a retransmission to be sent on a carrier (or medium, set of frequencies, band, etc.) different from the carrier on which the previous transmissions (initial transmission and/or retransmissions) were sent. An enhanced HARQ process can improve system performance by aiding user throughput, system throughput, and delay performance by making retransmissions possible even when the unlicensed band is not available (e.g., when occupied by other RATs or the same RAT deployed by another operator). For example, a transmitter sends a subpacket in an unlicensed band. The receiver feeds back a NACK to the transmitter due to the packet decoding failure. If the medium is not idle, the transmitter sends the retransmission in another band or another channel that can be either a licensed band/channel or another unlicensed band/channel. The retransmission can be sent in multiple licensed and/or unlicensed bands/channels at the same time.