Random Access in Carrier Aggregation

Could you Please explain when Random Access procedure is used in Carrier Aggregation in LTE?
Is Scell PDCCH used for System Information of Scell after activation.( As some texts suggest Common Search space lies in PDCCH of PCell only).

As far as I remember, RA may occur as for Pcell and for SCell (at least for the purpose of UL synch with a new carrier).
As a second option, it could be that info about UL sync with SCell could be transmitted to UE from PCell. Not 100% sure.
Also about SCell PDCCH. It seems to me that SCell provide only PDSCH for UE and act only as a kind of data pipe. All system info goes via PCell PDCCH/PDSCH.

1 Like

Random access is used in case of UL CA

SCell typically has both PDCCH and PDSCH

SCell for one UE is a PCell for different UE

1 Like

One more question/Doubt,
in Cross carrier scheduling for a component Carrier, the concerned CC uses different CC for PDCCH(for Both Common and UE Specific search space ) Then why do we need pdcch on Scell. We should have all Symbols available for PDSCH.

All the System Info etc is not broadcast for Scell. It is sent to UE as a dedicated information at the time of addition (RRC Connection reconfiguration). if any change in System info happens SCell is deactivated 1st and then added again with updated Information.(Not 100% sure though)

CA is a UE-based concept
If you have 2 cells: Cell A and Cell B
For some UEs, cell A is a Pcell and cell B is an SCell but for some other UEs, cell B is a Pcell and cell A is an SCell
So from cell perspective, each cell is expected to behave as a normal PCell ( even if it serves as SCell for some UEs) with PDCCH and SIBs
There is an exception if the cell is configured as a supplementry DL (without UL). In this case, there is no need to broadcast SI. However, it can still have PDCCH because cross carrier scheduling (which is a UE-based feature as well) may not be supported by all UEs which is the practical case nowadays

1 Like

Thankyou very very Much.
Should have thought of it Myself.
Thanks gain.