When to use synchronous or asynchronous HARQ in 4G LTE?

Hello Experts,
How does an UE knows whether eNB is going to use a synchronous or an asynchronous HARQ mechanism in 4G LTE?
Can anyone please suggest?

In DL it is asynchronous and eNodeB inform HARQ process id in DCI message.
In UL, it is synchromous.

In LTE eBN uses asynchronous HARQ and UE synchronous HARQ and eNB explicitly tells HARQ process information using DCI.

Is it hard coded in the specification?
Any spec reference?

36.321 you can refer.

Thanks for the information.