LTE Paging Explained

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How does paging in LTE work? What are the different situations when LTE UE receives paging from network?

Paging in LTE network is used to inform and notify UE about various events. The propose of paging procedure is:

  • To transmit paging information to a UE in RRC_IDLE state.
  • To inform UEs in RRC_IDLE and UEs in RRC_CONNECTED state about a system information change.
  • To inform UE about ETWS primary notification and ETWS secondary notification.
  • To inform CMAS notification.

When RRC layer receives paging it notify upper layer about this. Upper layer may request RRC to establish RRC connection as a response of paging.

How Paging Channel (PCH) is selected in LTE?

System information block type 5 or 5bis (SIB 5 or SIB 5bis) defines all common channels used in idle mode. In one cell one or several PCH channels can be established.

Each SCCPCH (Secondary Common Control Physical Channel) indicated to the UE in system information can carry on PCH. For each defined PCH there is also a uniquely associated PICH.

If more than one PCH,PICH combination is defined in SIB 5 or SIB 5bis, UE should choose the correct one according the following formula:

UE shall select a SCCPCH from the ones listed in SIB 5 or SIB 5bis based on IMSI as follows:

“Index of selected SCCPCH” = IMSI mod K,

Where:

  • K: The number of listed SCCPCHs which carry a PCH. These SCCPCHs shall be indexed in the order of their occurrence in SIB 5 or SIB 5bis from 0 to K-1.
  • IMSI (GSM-MAP)” is given as sequence of digits of type Integer(0..9), IMSI shall in the formula above be interpreted as a decimal integer number, where the first digit given in the sequence represents the highest order digit.
  • IMSI (DS-41)” is given as octet string, IMSI shall in the formulae above correspond to the decoded decimal representation of the IMSI-S part included in the octet string.

If the UE has no IMSI, for instance when making an emergency call without USIM, the UE shall use as default number IMSI = 0.

Actions by UE after receiving paging message

  • If UE receives paging in RRC_IDLE state if UE Identity included in the paging record matches with UE identities assigned by upper layer then UE RRC (Radio Resource Control) needs to forward ue-Identity and cn-Identity to upper layer.
  • If UE receives paging with request to modify system information (systemInfoModification) then reacquire the required system information.
  • If UE is ETWS capable and received etws-Indication in paging message, UE needs to re-acquire SIB 1 immediately. If schedulingInfoLost IE indicates that SIB 10 is present then RRC will acquire SIB 10 otherwise if SIB 11 is present then RRC will read SIB 11.
  • Similarly when smas-Indication is included in paging message, RRC will re-acquire SIB 1 immediately. If schedulingInfoList indicates that SIB 12 is present then RRC layer will acquire SIB 12.

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