My understanding is that eNB decides the downlink (DL) frame start based on some time reference e.g. GPS.
The UE locks to this DL frame (arriving after radio delay). The UE then generates its uplink stream (such as Rach).
I am not concerned about Rach and Timing advance concepts in this thread but rather about UE at zero delay case.
The eNB decides the uplink frame timing independent of UEs signals which eventually have to join the eNB uplink.
So it must be some sort of UL offset that the eNB uses to decide on UL frame start. I imagine this offset have to be calibrated for UE at zero delay.
Can anyone shed some light at this UL offset and how does eNB calibrates it for various radios.
Thanks