Posts Tagged paging

What is PI?

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.


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DRX cycle coefficient value

DRX cycle coefficient is different depending on the state of the UE.

In idle, CN domain has a value for it.

In Cell_PCH, URA_PCH, UTRAN has a value for it.

UTRAN DRX cycle length coefficient: values 3-9: cycle length 80 ms – 5.12 sec

CN domain DRX cycle length coefficient: values 6-9: cycle length 640 ms – 5.12 sec

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What is DRX cycle?

DRX cycle


the periodicity of the DRX process.

The longer the DRX cycle, the longer the UE is in a sleep state, but the longer the delay before the UE can respond to a paging message.

DRX cycle length = 2^k frames for FDD mode

where k is DRX cycle length coefficient.

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What is paging?

Paging is to tell UE that there is an incoming call or session.

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When are paging messages sent ?

There are many reasons for sending paging messages in idle, CELL_PCH and URA_PCH:

1. network originated call establishment

2. session set-up

3. request to trigger a cell update procedure

4. change to CELL_FACH state

5. request to start the release of an RRC connection

6. request to read updated system information in the broadcast channel

Similar procedures for CELL_FACH and CELL_DCH

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