Paging in LTE

S-GW:
1. DL data arrives for the UE
2. Creates DL Data Notification message and forwards to MME

MME: 
1. When UE is in ECM-Idle state, the UE location is known to MME on a per TA basis. Therefore, MME has to page all eNB's within a group of TA.
2. MME starts timer 3413when:
    a. Paging is for PS data
    b. UE is addressed by S-TMSI
3. List of TAI: informs the eNB to broadcast the Paging messages in the mentioned TAI's
4. Sends DL Data Notification ACK message to S-GW

eNB:
1. Receives S1AP: Paging and constructs RRC: Paging message
2. RRC: Paging message may contain multiple PAGING RECORDS to page multiple UE's

UE:
1. Wakes up every Paging occasion and searches for P-RNTI within the PDCCH transmission
2. If UE finds the P-RNTI then it proceeds to decode the PDSCH information which is present in PDCCH
3. UE decodes RRC:Paging from the PDSCH Resource Block within which the PAGING message is sent
4. If UE doesn't find it's own UE identity then it returns to monitor the Paging Occasion.
5. When UE finds it's identity in the message it triggers the Random Access Procedure. followed by establishing the RRC Connection.
6. If PAGING is for PS domain then UE NAS layer triggers SERVICE REQUEST otherwise, if the PAGING is for CSFB then UE triggers Extended SERVICE REQUEST.
 


How does UE determine the Paging Frame and Paging Occasion?
OR
How does eNB know when to send the Paging message so that UE will be awake to receive the message?

During this specific occurrence (Frame and Occasion), UE will wake up to check if there is any PAGING intended to it. If there is no information, UE will be go back to sleep. This happens when UE is in IDLE mode after registration. The related parameters are sent through SIB2. Therefore,a synchronization exist between the UE and eNB and both of them know when to look for Paging(UE) and when to transmit the Paging (eNB).

Paging Frame calulation:

Formula for calculating Paging Frame:::  SFN mod T=  (T div N) * (UE_id mod N)

where, 

- T: DRX cycle of the UE. T is determined by the shortest of the UE specific DRX value, if allocated by upper layers, and a default DRX value broadcast in system information. If UE specific DRX is not configured by upper layers, the default value is applied.

- nB: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.

- N: min(T,nB)

- Ns: max(1,nB/T)

- UE_ID: IMSI mod 1024

Parameters broadcasted in SIB2:

Default Paging cycle=T; (value=rf128)

nB= Broadcasted in SIB2 (value=1T)

Example:

Calculation:

a.       IMSI of UE (in DECIMAL)= 262022008880715

b.      N=128 (Look for the formula above to determine the value of N)

c.       UE_id = 262022008880715 mod 1024= 587

d.      (UE_id mod N)= 587 mod 128=75

e.      (T div N)= 128 / 128 = 1

Therefore, SFN mod T =75

So, whenever SFN mod T will be 75, UE will wake to read if there is a Paging message addressed to it. 

From this particular SFN the frame is derived. Now, the frame is identified but we also need to know that on which sub frame the Paging information is present. To determine, this Paging Ocassion is used.

Paging Ocassion calculation:

Formula for calculating the PO::: i_s = floor(UE_ID/N) mod Ns

a.       Ns=1 (Look for the formula above to determine the value of N)

b.      floor (587/128)= 4

c.       i_s = 0

Index i_s points to PO from the sub-frame pattern defined in the below table will be derived from following calculation:

 Table taken from 3GPP for FDD:

Ns	PO when i_s=0	PO when i_s=1	PO when i_s=2	PO when i_s=3
1	9	N/A	N/A	N/A
2	4	9	N/A	N/A
4	0	4	5	9

The Ns is 1 and PO would be 9 since i_s =0 in this example. Also, Paging Frame is derived as 75.

Therefore, from this example, whenever SFN mod T will be equal to 75, UE will look for P-RNTI in sub-frame 9 of that particular Radio Frame.