Synchronous HARQ (used in UPLINK):
1. Re-transmissions are scheduled at fixed time intervals
2. Generates lower over-head as it doesn't need to include HARQ process Id in the outgoing data
3. Always works in cycle even if no resources are allocated during a specific sub frame; which means that the 1st process will repeat itself after every 8 ms.
Asynchronous HARQ (used in DOWNLINK):
1. eNB can use any of the HARQ process (out of 1-8 SAW process) in the DL
2. eNB provides instructions to the UE regarding which HARQ process to use during each sub-frame for which resources are allocated. (The HARQ process identity is included within the PDCCH transmission)
3. Asynchronous HARQ increases signalling overhead because it includes the HARQ process Identity within the DCI.
4. Asynchronous HARQ increases flexibility because re-transmissions doesn't have to be scheduled during every sub-frame.