Handover Redirection Cell Selection Cell Re-Selection.
When the user moves out of coverage of one cell to another cell. For smooth operation and to avoid call drop.
HO happens only in case of connected user.
When UE changes its RAT, cell redirection happens. and in redirection UE goes from connected state to idle state.
and when a particular service and coverage is not available to one cell, the UE is redirected by the eNB to other cells for better coverage and respective service requested.
↔️ Cell Selection:
In Idle mode, when UE is not RRC connected to any particular cell, but it camps on a cell instead.
In idle mode, the procedure of camping on a cell is called cell selection.
When UE moves out of coverage of one cell to another cell in idle mode, UE has to camp from one camp to another cell.
This process of reselecting a new cell, while UE is already camping on an existing selected cell is called Cell Re-Selection.