According to an aspect, a network node reduces a probability of collision between HARQ process IDs used for acknowledging transmissions between the network node and a UE. The network node determines HARQ process IDs associated with dynamic grants of uplink resources and HARQ process IDs associated with semi-persistent scheduling (SPS). The HARQ process IDs are determined so as to reduce the probability of collision of the HARQ process IDs. The network node indicates at least the HARQ process IDs associated with the dynamic grants to the UE. The UE communicates HARQ messages using the HARQ process IDs.