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

[Pritam.Nandy]

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?

[gkreddy]

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

[Vishakha]

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

[Pritam.Nandy]

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

[Vishakha]

36.321 you can refer.

[Pritam.Nandy]

Thanks for the information.