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

4G LTE
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1

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?

2

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

3

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

4

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

5

36.321 you can refer.

6

Thanks for the information.