What is the reason behind UL grant applicable after 4SF from received DCI0 SF?

Question

Why cant we use the UL grant in the very next SF after DCI0? Why do we need 4SF?

Answer 1

Hi,

This timing concept is mentioned in PHY layer spec 36.213, but as far as i can assume the reason of 4 SF is something like this:
Let me start with this,In LTE, in FDD, RTT is 8 msec. The question arises why 8 HARQ processes? The answer is- consider the case of downlink, when eNodeB sends data, UE takes 3msec as processing time, therefore, it sends ACK/NACK 4 msec later, similarly after receiving ACK/NACK, eNodeB takes 3msec processing time and sends a new transmission/ re-transmission in a total of 8 msec

So suppose if eNB has send DCI at SF0,(1ms), UE will take 3ms processing time, at send UL after 4SF.

I don't know whether i am correct or not, but i can conclude some thing like that. Do post if you have correct answer.

Happy Learning