PI (Paging indication)
a short indicator that is transmitted on the PICH to tell the UE that there is a paging message on an associated paging channel carried by the SCCPCH.
The number of PIs per radio frame (Np) can be 18, 36, 72 or 144.
Once the UE knows paging occasion (SFN), then the UE needs to know which PI to look for in the SFN.
The equation is
PI = DRX index mod Np
DRX index = IMSI div 8192
If the PI bits are set in the paging occasion (SFN), the UE reads the paging message on the PCH transmitted on the associated SCCPCH.