Paging in LTE

Paging in LTE : 

The main purpose of a Paging message is to page UEs in RRC_IDLE mode for a mobile terminated call. Also the  Paging message can be used to inform UEs, in RRC IDLE as well as

in RRC CONNECTED modes.

Paging messages originating from the MME to notify the terminal about incoming connection requests is used in the RRC_IDLE state described further below) when the terminal is not connected to a particular cell. Indication of system-information update is another use of the Paging mechanism, as is public warning systems.

Paging in RRC connected state

Indeed, the UE can be paged for ETWS/CMAS(KPAS/EU Alert etc.), however this is not really about reaching a specific UE but to inform about the fact the ETWS/CMAS notifications are being broadcast. UE can be paged for System info modification.

Paging in LTE RRC Idle state

UE Monitors a Paging channel to detect incoming calls, system information change, and for ETWS capable UEs, ETWS notification.

For a mobile terminated call, MSC/VLR sends a Paging message via SGs interface to the correct MME based on the location update information.

Paging in LTE : The main purpose of a Paging message is to page UEs in RRC_IDLE mode for a mobile terminated call. Also the Paging message can be used to inform UEs, in RRC_IDLE as well as in RRC_CONNECTED modes.

Paging messages originating from the MME to notify the terminal about incoming connection requests is used in the RRC_IDLE state described further below) when the terminal is not connected to a particular cell. Indication of system-information update is another use of the Paging mechanism, as is public warning systems.

Pagings in RRC connected state:

Indeed, the UE can be paged for ETWS/CMAS(KPAS/EU Alert etc.), however this is not really about reaching a specific UE but to inform about the fact the ETWS/CMAS notifications are being broadcast. UE can be paged for System info modification.

Paging in LTE RRC Idle state:

UE Monitors a Paging channel to detect incoming calls, system information change, and for ETWS capable UEs, ETWS notification. For a mobile terminated call, MSC/VLR sends a Paging message via SGs interface to the correct MME based on the location update information

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 3413 when:

• Paging is for PS data.
• 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.

Thought ->  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 subframe: The Paging procedure is the same for LTE TDD and FDD; 

For FDD, the subframes 0, 4, 5 and 9 can be used for Paging. 
For TDD, the subframes 0, 1, 5 and 6 can be used for Paging.

Paging happens on different situation:

Paging messages are sent in the Paging occasions(PO), where the UE can pick them up (it is simplest for the UE to check its own Paging occasion similarly as in RRC idle mode to see if Paging message is being transmitted by the network. For ETWS and CMAS (etc. warning systems), the network will send Paging messages in all valid Paging occasions defined for the cell, because the network does not even know all the UE’s that are camped on the cell (in RRC idle mode) and possibly monitoring Paging messages for ETWS/CMAS notifications.

The UE, which receives Paging message with ETWS-indication while it has been trying to acquire SIB11 according to previously received schedulingInfoList (SIB1), will try to acquire SIB1 first before continue to acquire the rest of SIB11 segments.

UE can be paged by either S-TMSI or IMSI, as stated in the Spec, when UE receives a Paging with IMSI , it should locally deactivate any EPS bearer and locally detach from EPS, that is UE will turn into EMM-DEREGISTERED state.(  IMEI Paging is not considered in Rel8.)

For the CS fallback, if UE had registered to a CS core network and received a temporary ID from the CS core network, CS Paging can be delivered to UE with a corresponding core network indicator (i.e. CS or PS). In this case, the UE should deduce the correct CS UE identity based on the PS UE identity included in the Paging message and include the CS UE identity in the Paging response once it is moved to another RAT.

UE_ID is “UE Identity Index value” parameter of Paging message initiated by MME-S1AP . Since network node MME knows about UE’s IMSI, it will calculate UE_ID and send it to eNB S1AP as part of Paging message

The Paging Control Channel (PCCH) is used for Paging of terminals whose location on a cell level is not known to the network. The Paging message therefore needs to be transmitted in multiple cells.

Paging Channel (PCH) is used for transmission of Paging information from the PCCH logical channel. The PCH supports discontinuous reception (DRX) to allow the terminal to save battery power by waking up to receive the PCH only at predefined time instants. The Physical Downlink Shared Channel (PDSCH) is also used for transmission of Paging information.

Paging Frame calculation/ Paging Occasion:

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

Example to calculate Paging Occasion and PF(Paging Frame) :

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:

In SIB2:

nB  = T / 4

Default PagingCycle (T) =  64

From S1- Paging Message:

PagingDRX : 128

UeIndex (S1 Paging message) : UE_ID = 0x0115 = (0000 0001 0001 0101 ) after Removing 6 LSB( UE_ID(% 1024 ) which makes (0000 0001 00 = 4)

(The above UeIndex is in UeIdentity % 1024 which means, results only  occupies 10 Bits Only. So, above we Removed 6 LS bits.)

T = Min ( PagingDRXCycle, Default PagingDrxCycle) = Min ( 128,64) = 64

nB = T/4 = 64/4 = 16

N = min(T,nB) = Min(64, 16) = 16

UE_ID = 4 (From above Steps derived from S1 Paging Message)

Calculation of PF(#Paging Frame)

SFN mod T= (T div N)*(UE_ID mod N)

RHS => (T Div N) * (UE_ID mod N) = ( 64 Div 16) * ( 4 % 16) = 4 * 4 = 16

LHS => SFN Mod T = SFN Mod 64 = 16 

So,

PF(Paging Frame) values could be anything  where SFN =  (64 * i ) + 16  (I = 0 to N But SFN <= 1024)

i.e Values PF can  be any of (80, 144, 208, 272, 336, 400, 464, 528, 592, 656, 720, 784, 848, 912, 976, 1040, 1104, 1168).

But here PF(Paging Frame)= 144

Calculation of Paging Occasion:

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

Ns: max(1,nB/T) = max (1, 16/64) = 1

i_s = floor(UE_ID/N) mod Ns

i_s = floor(4 / 16) mod 1 = 0

From the below table when i_s == 0 and Ns = 1 => P0 = 9

So considering current SFN is 100 : for the above UE PF which is mentioned in the series is 144. i.e Paging message can go in SF=144 and SFN=9

Ref : 3GPP 36.304 ,Section 7.1 and 7.2

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

The Ns is 1 and PO would be 9 since i_s =0 in this example. Also, #Paging Frame is derived as 144.

Therefore, from this example, whenever SFN mod T will be equal to 144, UE will look for P-RNTI in sub-frame 9 of that particular Radio Frame.

Paging Message flow

UE wakes up during On/active time to check PDCCH(allocation information on the PDCCH)  and Paging is provided in the PDSCH.

PDCCH Decoding Result

DCI Format  :1C

Search space  :common

Aggregation level :4

Decode status  :P_RNTI

PDSCH Decoding result:

Sub-frame Number = 9

System Frame Number  = 144

PDSCH RNTIl ID  = 65534

PDSCH RNTI Type = P-RNTI

Number of Tx Antennas (M) = 2 antennas

Number of Rx Antennas (N) = 2 antennas

Transmission Mode   = Transmit diversity

value PCCH-Message ::=

{

message c1 : Paging :

 {

 PagingRecordList

 {

 {

 ue-Identity s-TMSI :

 {

mmec ‘00000001’B,

m-TMSI ‘00000100 00000000 00000000 11110111’B

 },

cn-Domain ps

 }

 }

 }

}

 value UL-CCCH-Message ::=

{

message c1 : rrcConnectionRequest :

 {

criticalExtensions rrcConnectionRequest-r8 :

 {

ue-Identity s-TMSI :

 {

mmec ‘00000001’B,

 m-TMSI ‘00000100 00000000 00000000 11110111’B

 },

establishmentCause mt-Access,

 spare ‘0’B

 }

 }