Paging is used to network initiated connection setup. Typically, paging procedure should allow UE to sleep with no receiver processing most of the time and briefly wakeup at predefined time interval to monitor paging information from the network.
The eNB will wait for UE inactivity timer to check if the UE has received and call or data. Once the timer has expired, eNB sends UE to idle mode by sending RRC Connection Release message.
Then eNB will send a paging to UE as specified in TS 36.304
When the paging message will be initiated?
Paging procedure will be used to inform the UE about
1. Incoming PS call
2. Incoming SMS
3. Re acquire system information
4. Earthquake and Tsunami Warning System (ETWS) indication
5. Initiate mobile terminating CS fall-back connection.
6. Provide a Commercial Mobile Alert Service notification.
There are 2 ways where a Paging message can be initiated.
Information Elements in PAGING message
CN-Domain: Indicates the origin of paging if it is PS or CS.
UE-Identity: Provides the NAS identity of the uE is being paged (S-TMSI or IMSI)
SystemInfoModification: This IE indicates a BCCH modification.
etws-Indication: It is ETWS primary notification.
cmas-Indication: This IE indicates a CMAS notification.
1. when the UE is in sleep mode, it will wake up and check for PDCCH message.
2. The wake up time interval will be defined in DRX cycle (Discontinuous Receive Cycle)
3. When the UE is in idle mode, it will not be having S1 connectivity with MME.
4.When the UE is in Idle mode, location of UE will be known to MMW as per tracking area.
5. MME will forward paging to multiple eNBs as UE can be registered with multiple tracking area. UE is addressed by S-TMSI.
6. T3413 timer will be started after sending S1AP paging message.
7. eNB receives S1AP paging message from MME and constructs RRC paging message.
8. UE will search for P-RNTI within PDCCH of subframe belong to paging
9. UE finds P-RNTU in PDCCH, it will decode resource allocation information.
This information is used to know where the paging message has been sent.
UE decodes RRC message from PDSCH and check for UE identity. If it did not find, then it will check PDCCH for P-RNTI of other paging occasions.
After UE finds its identity, RACH will be triggered.
If the paging procedure is for PS data call, then UE will include service request NSA message with in RRC connection setup complete message.
If the paging procedure is for CS fallback call, UE includes extended service request NAS message within RRC connection setup complete message.
eNB forwards NAS message to MME and it will stop T3413 timer.
Paging Occasion and Paging Frame
The sub frame of paging frame within which paging message message will be transmitted is called as PO.
Paging Frame is a one radio frame, which contain one or more paging occasion.
Downlink channels are: PCCH/PCH/PDSCH
MME will encapsulate Paging information in S1AP Protocol and send it to eNB.
P-RNTI is present within PDCCH of the subframe belonging to the paging occasion.
P-RNTI has a flag that will indicate that the UE may have paging message in PDSCH.
This is used by UE in RRC Connected mode also