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UE triggers system select, system reselect, MPSR under Idle status, while system redirect under Active status.

System Select:

UE is in LTE Idle status, later it moves out of LTE coverage(system lost), it will select other system. 

System Reselect:

UE is in LTE Idle status, but it detects LTE signal falls below a threshold, and other system signal goes above a threshold, it will reselect other system. 

MPSR(MORE PREFERRED SYSTEM RESELECTION):

If UE is in other RAT(Radio Access Technologies) Idle status, then two kinds of system reselection may occur:

a) It detects current signal falls below a threshold, and LTE signal goes above a threshold, it will reselect LTE.

b) Current signal is same as LTE signal, due to LTE has higher priority than other RAT, UE will reselect LTE after a while using MPSR procedure.

System Redirect:

If UE is in LTE Active status, it detects LTE signal falls below a threshold, and other system signal goes above a threshold, it will send measurement reports to eNB, and eNB will redirect UE to other system.