Paging in Lte

There are a number of myths about paging. Let’s clear them all in one go. Paging is not only used to bring a UE from Idle to connected mode. It is also used in RRC Connected mode. It is used for the following use cases.

1. Initiate a mobile terminating packet Switch (PS) data connection
2.  Initiate a mobile terminating SMS connection
3. Initiate a mobile terminating CS fallback connection
4. Trigger a UE to re-acquire system information
5. Provide an Earthquake and Tsunami Warning System (ETWS) notification
6. Provide a Commercial Mobile Alert Service (CMAS) notification

Paging in LTE

From the picture above. There are two types of paging with respect to which node will initiate paging i.e., eNodeB initiated Paging and MME initiated paging.

eNodeB Initiated Paging

This paging is used to inform a UE about :

• Change in system information
• Emergency notifications
• Earthquake or tsunami warning notifications
• Etc.

MME Initiated Paging

This paging is used to inform the end user about

• Incoming calls
• SMS messages
• Any other service, you as a consumer may have subscribed to with the operator
• Etc.

Okay, all this is great. How does paging take place? 

UEs in RRC Idle mode use Discontinuous Reception (DRX) also known as paging cycle to reduce power consumption. This DRX cycle determines how frequently UE check for paging messages. The default DRX cycle is broadcast within System Information Block 2  (SIB 2). The range for paging cycle varies as 32, 64, 128 and up to 256 radio frames.

The UE can also propose its own DRX cycle length within the Attach Request and Tracking Area Update Request message.  The range of allowed values is the same as those used in SIB 2.

If a UE proposes its own DRX cycle to be used, the smaller of the two DRX cycles ( eNodeB proposed DRX cycle and UE proposed DRX cycle) is used.  In others words, the minimum of the default DRX cycle and UE specific DRX cycle is used.

Paging Cycle 

Well, for paging to take place. Operators set up paging cycle.  Paging cycle is measured in Radio frames.   Commonly used value for Paging cycles is 128.

It means 128 radio frames.

That means

128 radio frames

One radio frame = 10 msec

128 = 128 radio frames = 1280 msec

1280 msec = 1.28 seconds

Therefore, if an operator has setup default paging cycle as 128. It means UE will wake up after every 1.28 seconds even in Idle Mode to see if there is Paging information for the UE or not.

After waking up. If it does not find paging information related to the UE it will go back to sleep.

UE waking up every 1.28 seconds for paging cycle 128 rf

Paging Frame and Paging Occasion

The radio frame in which UE wakes up is called Paging Frame (PF).

Within the radio frame, there are 10 subframes. UE does not remain awake in all 10 subframes. Instead, it wakes up in a specific subframe either subframe 0, 4, 5 or 9 within a radio frame. These specific subframes within a Paging Frame when UE wakes up are called as Paging Occasions (POs).

Paging Frame (PF) and Paging Occasion (PO) Elaborated

How does a UE know in which Radio frame to wake up? 

The radio frame in which UE has to wake up depends upon the following factors

1. Paging Cycle (abbreviated as T)
2. nB ( multiple of paging cycle) It can have values such as { 4T, 2T, T, T/2, T/4, T/8, T/16, T/32 } 
3. UE_ID = IMSI mod 1024

Paging Cycle and nB value arrive in SIB 2.

UE already knows its IMSI value. Based on IMSI value, UE identity is computed as

UE ID = IMSI mod 1024

In order to find out which paging frame the UE will wake up in. Paging frame index (PF index)  is computed as

PF index =  (T / N)*(UE_ID mod N)

where,

T = DRX cycle length in radio frames

N = Min ( T, nB)

nB = Broadcast in SIB 2

Based on PF index computed, UE finds out in which radio frame it has to wake up.

How does UE know in which Subframe of Paging Frame, it needs to wake up?

Once UE has computed the paging frame. It goes ahead and finds out in which subframe of the paging frame it will wake up. A UE does not need to wake up in all one 1 msec subframes within its Paging Frame. A UE only needs to check the subframe identified  by its Paging Occasion

The formula to compute paging occasion is extracted from a look-up table which is indexed using:

• Ns = Max (1, nB/T )
• i_s = Floor(UE_ID / N) mod Ns

Once Ns and i_s values are computed. UE makes a use of the look-up table. The intersection of i_s and Ns values in the look-up table shows in which subframes UE will wake up.

The look-up table for Paging Occasion is shown below.

Paging Occasion table for FDD

Cells Carrying Low and High Quantities of Traffic

The value of Ns is cell-specific so cells carrying low quantities of traffic can be configured with a value of (one) 1 to provide relatively low paging capacity, while cells carrying higher quantities of traffic can be configured with a value of 4 to provide relatively high paging capacity.

Low and High Capacity Configurations

3GPP selected subframes 4 and 9 for the lower capacity configurations because they are adjacent to subframes 5 and 0 which carry the Synchronisation Signals. This allows UE to check the synchronization Signals at the same time as ‘waking up’ to check for a paging message.

Subframes 5 and 0 were not selected for the lower capacity configurations (likely to have smaller channel bandwidth ) because subframe 0 already includes Masters Information Block (MIB) and subframe 5 already includes System Information Block 1 ( SIB 1).

For higher capacity configuration (likely to have a larger channel bandwidth ) there should be sufficient capacity to accommodate both paging messages and the MIB/SIB 1 within the same subframe. Selecting subframes 0 and 5 helps to minimise the impact upon multimedia broadcast multicast single frequency network (MBSFN) which cannot use the same subframes as paging messages nor can it use the same subframes as the MIB/SIB 1.

Paging Occasion for TDD

The equivalent look-up table of Paging occasion table for TDD is shown below.

Similar to FDD, the values of Ns can be configured for TDD to match the paging capacity.

Paging Occasion Table for TDD

Paging Capacity Configuration for TDD

3GPP selected subframes 0 and 5 for all capacity configurations because these subframes are always downlink subframes irrespective of the uplink-downlink subframe configuration. In this case, it is accepted that the paging subframes will clash with the MIB/SIB 1 subframes for both the low and high capacity configurations.

The high capacity configuration also uses subframes (one) 1 and 6. Subframe 1 (one)  is always a special subframe whereas subframe 6 can be either a special subframe or a downlink subframe (depending upon the uplink-downlink subframe configuration). Special subframes tend to be PDCCH capacity limited so these subframes were not used for the lower capacity configurations

EXAMPLE CALCULATION

 Let’s take an example.

Assume the following parameter values are broadcasted in SIB 2 for the UE.

• Paging Cycle = T = 64
• nB = 2T = 128
• IMSI = 0  (assume)

Let’s calculate the value of N, Ns , i_s and UE_ID. Given the value of the parameter. The following calculation reveals.

• UE_ID = IMSI mod (1024) = 0
• N = min (T,nB) = 64
• Ns = max(1,nB/T) = 2

Therefore, the Paging Frame (PF)  in which UE will wake up can be calculated using

PF index = (T / N)*(UE_ID mod N)

Plugging the values of T, N, and UE_ID, we find as follows

First Paging Frame

1st Paging Frame index = 0 .  UE will wake up for the first time in Radio Frame (System Frame number 0)

SFN 0

Second Paging Frame

In the second attempt, UE will wake up in Radio frame number

0 + 64  = 64  i.e., SFN 64

Third Paging Frame

In the third attempt, UE will wake up in Radio Frame number

0+64+64 = 128  i.e., SFN 128

Fourth Paging Frame

In the fourth attempt, UE will wake up in Radio Frame number

0+64+64+64 = 192 i.e., SFN 192

So on and so forth, until it reaches closer to system frame number (SFN) 1023 and then it will start to loop back again.

The computed paging frames for the example above are shown below in the picture.

Example representing the Paging frames computed above

What Paging Occasion (PO) UE will wake up in each computed Paging Frame?

Continuing the example above. In order to find out which subframe UE will wake up in each subframe.  Let’s compute i_s.

Plugging values into the formula

• i_s = floor(UE_ID/N) mod Ns
• i_s = floor( 0/64) mod 2
• i_s = 0

Already computed Ns value is 2

Intercepting the two values from the table below. UE will wake up in sub-frame 4 and 9 respectively during paging occasion

Paging Occasion (Sub-frame for UE to wake up in )

Example Conclusion

From the example computation, we learned that with the parameters of paging cycle = 64 and nB = 2 T , and UE_ID  = 0 .

UE will wake up in SFN 0 , SFN 64, SFN 128, …..all they way upto SFN 960 and repeat (loop back) .
With each Paging Frame it will wake up in subframe 4 and 9.

Now that we have gone through computation of Paging Frame and Paging Occasion. You should have a good understanding of how to computer PF and PO, given parameters from SIB 2.

—————————————————————————

For readers, who are interested in finding out some more detailed theoretical information on Paging in LTE as how MME and eNodeB, play its role in conveying a paging message up to the end user. Continue reading below.

MME Initiated Paging Procedure

MME starts the procedure by sending an S1 Application Protocol (S1AP) : Paging message is sent to each eNodeB  with cells belonging to the relevant Tracking Area(s).  The location of UE in EPS Mobility Managed (EMM) registered  , EPS Connection Management (ECM) Idle state is known by the MME at Tracking Area level.

MME broadcasts the paging message across multiple Tracking Areas for UEs which are registered in more than a single Tracking Area.

Normally a single tracking area includes multiple eNode Bs so the paging message is broadcast by multiple cells belonging to multiple eNodeBs.

High Level Paging Procedure initiated by the MME

For establishing a PS data connection and if is being addressed using S-TMSI ( System -TMSI)  then the MME starts timer T3413 when sending the S1AP : Paging Message.

Timer T3413 is NOT used when :

• Paging a UE for CS fallback connection
• Paging an IMSI attached UE for an incoming SMS
• When addressing a UE by its IMSI

Usually registered UEs are addressed using S-TMSI. A UE can be paged by its IMSI during a network error recovery situation.

What is T3413?

T3413 is a supervision timer for the paging procedure. Its expiry time is implementation dependent and is not specified in 3GPP. The MME can re-attempt the paging procedure if T3413 expires before a response is received.

So far you have read about the procedure and high-level information is shown in the diagram above as how an S1AP message comes in for the UE during a paging procedure. What contents are included in the paging message is still a question for a lot of you. By contents, I mean the information elements.

The following information elements arrive in S1AP: Paging message

Contents of S1AP: Paging Message

RRC Portion of the Paging Message : 

As depicted in high-level paging procedure diagram above. The eNode B receives the S1AP: Paging message from the MME and constructs the RRC part of the paging message . A single RRC paging message can include information from multiple S1AP Paging messages.

Focus on RRC portion of the paging message coming in the above diagram

Paging message can include multiple paging records to page multiple UEs. The structure of an RRC paging message and its contents are shown below

Content of an RRC: Paging message

How is Paging Message Transferred within the eNodeB over Air Interface channels? 

Paging message is transferred using the

• Paging Control Channel (PCCH) Logical channel.
• Paging Channel (PCH) Transport Channel
• Physical Downlink Shared Channel (PDSCH)  Physical Channel

Logical , Transport and Physical channels carrying paging information highlighted

Frequently Asked Questions 

What is a Paging Record? How many paging records can be included within a paging message?

A paging record specifies the UE which is being paged within an RRC paging messages. It specifies the UE identity and the core network domain i.e., forwarding the content of the S1-AP: Paging message. A maximum of 16 paging records can be included in the paging record list.

When eNodeB initiated paging takes place? 

The eNodeB initiated paging can takes places in the following scenarios

1. When system information gets changed. The system information modification flag is generated by the eNode B rather than extracted from S1AP: Paging message. This flag is used to trigger UE to re-acquire the set of  System Information Blocks (SIBs)
2. The Earthquake and Tsunami Warning System (ETWS) flag is also generated by the eNode B rather than extracted from the S1AP: paging message. This flag is only applicable to ETWS capable UE and can be used to trigger those UE to re-acquire SIB 1. The content of SIB 1 directs the UE to the ETWS content with SIB 10 and SIB 11.
3. The commercial mobile alert service (CMAS) flag is also generated by the eNode B rather than extracted from S1AP: Paging message. This flag is only applicable to CMAS capable UE and can be used to trigger those UE to re-acquire SIB 1. The content of SIB 1 directs the UE to the CMAS content within SIB 12

UE support for ETWS and CMAS is optional so only UE which support these services react to these flags within the paging message.

How often does a UE check for paging message in Idle Mode? 

UE in RRC Idle mode check for paging messages once every Discontinuous receive (DRX) cycle. The paging occasion (PO) within the paging frame (PF) defines the specific subframe during which a UE checks for a paging message

Does UE have Paging Occasions in RRC Connected mode? 

UE in RRC Connected mode do not have specific Paging Occasions during which UE need to check for paging messages. However, UEs are expected to check for paging messages at a similar rate to UEs in RRC Idle mode. This allows them to receive paging messages which trigger re-acquisition of the system information or trigger the reception of ETWS or CMAS information. These paging messages are not directed towards individual UE. They are intended to be received by all UE. The eNode B will broadcast them during multiple subframes to allow sufficient time for their reception by all UE.

What is P-RNTI? How UE utilizes P-RNTI during paging?

UE in RRC Idle mode search for P-RNTI within the PDCCH of the subframe belonging to the Paging Occasion. The P-RNTI has a single fixed value of FFFE and serves as a flag to indicate that the UE may have a paging message on the PDSCH. UE in RRC Connected mode also searches for the P-RNTI within the PDCCH when checking for paging messages.

If a UE finds the P-RNTI within the PDCCH then it proceeds to decode the resource allocation information from within the PDCCH. This information directs the UE to the PDSCH Resource Blocks within which the paging message is sent.

P-RNTI use in Paging Identification

The UE decodes the RRC: Paging message from the PDSCH Resource Blocks and checks the UE identity within each of the paging records. If the UE does not find its identify within a paging record then it returns to checking the PDCCH for the P-RNTI at each paging occasion.

If the UE finds its identity within a paging record then it triggers the Random Access procedure to establish an RRC connection. The UE sends an RRC connection Request message while the eNode B responds with an RRC Connection Setup message.

Is Paging DRX Optional?

Yes, Paging DRX is optional. It is used by the eNodeB when calculating the Paging Frame if the UE is using a UE specific DRX cycle length.  The UE can specify a DRX cycle length within the Attach Request or Tracking Area Update messages.

How Core Network specifies if paging is PS or CS paging?

There are two values specified for paging from Core Network domain perspective. The Core Network domain indicates Packet Switched (PS) if the paging message is for data transfer or an incoming SMS.

The Core Network domain indicates Circuit Switched (CS) value if the paging message is for CS fallback services, e.g., an incoming CS speech call.

What information Tracking Area Identity (TAI) contains for the eNode B ?

The list of TAIs iinforms the eNodeB  of which Tracking Areas (TA) the paging message should be broadcast to. As a  UE can be registered with multiple TA so it may be necessary to broadcast across more than a single TA.

How to Page for Closed Subscriber Group UEs ?

If a UE is part of (subscribed) to Closed Subscriber Group. While paging , the set of closed subscriber group (CSG) identities to which the UE is subscribed can be included in paging. This helps eNodeB to avoid paging the UE within the CSG cells with which the UE is not subscribed.

What is Paging Priority ?

Paging priority is an optional field within the paging message. It was introduced within the release 10 version of the 3GPP specifications. It was introduced for the purpose of Multimedia Priority Services (MPS) which are intended to prioritise specific connections during periods of congestion. For example , to provide the emergency services with priority information during an emergency situation. Both MME and eNodeB can use the priority information during periods of congestion. (Refer to 3GPP TS 22.153)