Paging Study in LTE

Introduction

Paging is a procedure of transmitting paging messages to UEs in RRC_IDLE mode or informing all UEs in RRC_CONNECTED mode and RRC_IDLE mode about an SI message change .

         Triggering of Paging

                MME Triggering

                     •To initiate mobile terminated PS call
                     •To initiate mobile terminated CS fallback call

                 NodeB Triggering

         •To trigger LTE UE to re-acquire system information

Paging on the Uu Interface

•UEs in RRC_IDLE mode use DRX to receive paging messages in order to  reduce power consumption.

•UE in RRC IDLE mode checks for paging once every DRX cycle.

•Paging occasion within the paging frame defines specific subframe  during which an LTE UE checks for paging message.

•One PF is one radio frame which may contain one or multiple POs.

•One PO is a subframe where the Paging Radio Network Temporary Identifier (P-RNTI) is contained. 

•The PO is transmitted over the PDCCH. According to 3GPP specifications, the

 P-RNTI value is fixed. UEs read paging messages over the Physical Downlink Shared Channel (PDSCH) according to the P-RNTI.

Notes

Ø  MME is responsible for the initiation of LTE paging procedure. MME does this by forwarding S1AP paging message to one or more eNodeB.

Ø  The LTE paging procedure is applicable to UE in ECM IDLE State. UE in this state are in RRC IDLE mode and do not have S1 connectivity with MME. 

Ø  UE in idle state known to MME by its TAC . Paging message is forwarded to TAC or  TAL(Tracking area List) which UE is located.

Ø  Paging message can include multiple paging records to page multiple UEs

Paging Description over Uu interface

·         UE searches for P-RNTI within PDCCH of subframe belong to paging occasion. P-RNTI  indicates that UE may have a paging message on PDSCH.

·         IF UE found its P-RNTI on the PDCCH , then UE listen  to PDSCH RB where in paging message has been sent.

Note 1

·         There is paging information sent over PDCCHàlike P-RNTI

·         There is paging message sent over PCCH(logical)àDL-SCH(Transport)à PDSCH(physical) à paging message contains UE ID (UE ID may be IMSI or S-TMSI for the paged UEs .

Note 2

P-RNTI(paging-Radio network temporary identifier )

·         The P-RNTI is the 4G complement of the paging indicator.

·         It does not refer to a particular UE, but to a group of UEs.

·         UE decodes PDSCH RBs and checks UE identity in all the records.

1-       If UE do not find its identity in paging record then it will return back to sleeping mode and then it will  check PDCCH for P-RNTI at next paging occasions. 

2-       If the UE find its identity, it will trigger random access procedure to establish RRC connection.

·         UE sends RRC connection request message and eNodeB responds with RRC connection setup message.

Ø  If the LTE paging procedure is for PS data call, UE includes service request NAS message within RRC connection setup complete message.

Ø  If the paging procedure is for CS fallback call, UE includes extended service request NAS message within RRC connection setup complete message.

Paging Discard per eNodeB

•When the CPU usage of the main control board or baseband board is equal to or greater than threshold for certain continuous time, the eNodeB starts flow control based on service priorities . The eNodeB preferentially discards PS paging messages in paging message flow control