LTE HARQ why 7ms retransmission???

[Zhouyou Gu]

Hello, all!

In LTE module Doc:

"According to the standard, the UL retransmissions are synchronous and therefore are allocated 7 ms after the original transmission. On the other hand, for the DL, they are asynchronous and therefore can be allocated in a more flexible way starting from 7 ms and it is a matter of the specific scheduler implementation. The HARQ processes behavior is depicted in Figure:ref:fig-harq-processes-scheme."

I do not quite understand why it is 7ms for the retransmission.

Which 3gpp standard number I may find this information?

Thanks a lot!!!