huawei wcdma paging procedure and problem analysis

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OWJ200104 WCDMA Paging Procedure and

Problem Analysis

ISSUE 1.0


HUAWEI TECHNOLOGIES CO., LTD. Page 1All rights reserved

Paging is one of the most important

processes in WCDMA, which is also an

important KPI.

This course includes basic paging

process. The paging parameters,

signaling, performance indicators and

analysis methods are also mentioned,

which is very helpful for WCDMA

optimization engineers.

Different from fixed communication system, the location of the terminal in

mobile communication system is not fixed. To establish a call, the CN

sends a paging message to the UTRAN via the Iu interface, and the

UTRAN forwards the message to the UE through the paging procedure on

the Uu interface, so that the UE being paged initiates the signaling

connection setup procedure with the CN.



� Upon completion of this course, you will be

able to:

�Paging procedure in UMTS

�Paging message

�Paging procedure performance analysis



Chapter 1 Paging Procedure in UMTS Chapter 1 Paging Procedure in UMTS

Chapter 2 Paging MessagesChapter 2 Paging Messages

Chapter 3 Paging Procedure Performance AnalysisChapter 3 Paging Procedure Performance Analysis

Chapter 4 Paging Problem Analysis ProcessChapter 4 Paging Problem Analysis Process




Section 1 Paging TypeSection 1 Paging Type

Section 2 DRX Process of UESection 2 DRX Process of UE



Paging Type

� CN initiated paging

� UTRAN initiated paging

Core network (CN) can initiate paging. For example, establishing a

signaling connection for a call.

UTRAN can also initiate a paging request to trigger the state transition

procedure for UE which is in CELL_PCH or URA_PCH state.

In addition, UTRAN may initiate paging to ask UE which is in idle mode,

CELL_PCH and URA_PCH state to read the updated system information.

In the existing network, most of pagings are initiated by CN.



Paging Type 1CN RNC1 RNC2 NODEB1.1 NODEB2.1 UERANAPRANAPRANAP RANAP PCCH: PAGING TYPE 1PAGINGPAGING

PCCH: PAGING TYPE 1Paging type 1:

� If UE is in CELL-PCH,URA-PCH or IDLE state， the paging

message will be transmitted on PCCH with paging type 1

� The above figure shows the paging message from CN is

transmitted by two RNCs.

� The message is transmitted in one LA or RA according to LAI or

RAI.

� After calculating the paging time, the paging message will be

transmitted at that time



Paging Type 2CN SRNC UERANAPRANAP PAGINGRRCRRC DCCH: PAGING TYPE 2

Paging type 2:

� If UE is in Cell_DCH or Cell_FACH state，the paging message will

be transmitted on DCCH with paging type 2

� The message will be only transmitted in a cell



Typical Call Flow of UE

UE NAS UE AS NSS MSC

paging

AUTHENTICATION REQUEST

AUTHENTICATION RESPONSE

RR_SECURITY_CONTROL_REQ

(IK CK)

Security mode control

SETUP

CALL CONFIRM

ALERT

CONNECT

CONNECT ACKNOW LEDGE

RAB setup process

paging

RR_EST_REQ (PAGING RESPONSE)

RR_PAING_IND

IN ITIAL_DIRECT_TRANSFER

(PAGING RESPONSE)

RANAPRANAP

RRC setup process

Paging will be initiated on network side in the following cases:

� UE is called

� Cell system information is updated

� UE state transition

Many problems will cause the target UE cannot receive the paging

message properly

� Power setting of paging channel is unreasonable.

� Unreasonable paging strategies will result in paging channel

congestion, which can cause paging message loss.

� Paging parameter is unreasonable

� Equipment fault




Section 1 Paging Type Section 1 Paging Type

Section 2 DRX Process of UESection 2 DRX Process of UE



DRX Procedure

� UE receives the paging indicator on PICH periodically, that

is the Discontinuous Reception (DRX), which can reduce

the power consumption.

� The value for the DRX paging cycle length is determined as

follows: :

DRX Cycle Length ＝ (2^K)×PBP frames

In idle mode, the UE can monitor the paging in two modes: one is to

decode SCCPCH directly every 10ms, the other is to decode the PICH

periodically. The second one is the DRX, which is Discontinuous Reception

Mechanism.

The paging period formula:

DRX Cycle Length ＝ (2^K)*PBP frames

� K is the “CN domain specific DRX cycle length coefficient”, which

is in SIB1. The typical value is 8 or 6.

� PBP is paging block period, which is 1 for FDD mode

� The paging period cloud be 2.56s or 640ms



DRX Procedure

� Through DRX, UE only listens to PICH at certain predefined

time. And UE will read the paging information on SCCPCH if

the paging indicator is 1.

� The value of the Paging Occasion is determined as follows:

Paging Occasion (CELL SFN) =

{(IMSI mod M) mod (DRX cycle length div PBP)} * PBP

+ n * DRX cycle length + Frame Offset

Paging SFN formula:

Paging Occasion (CELL SFN) = {(IMSI mod M) mod (DRX cycle length

div PBP)} *PBP + n *DRX cycle length + Frame Offset

� n =0, 1, 2……and the requirement is the calculated CELL SFN

must be below its maximum value 4096

� Frame Offset is 0 for FDD mode

� M is the number of SCCPCH which carries PCH, and the typical

value is 1

� The formula cloud be simplified as:

SFN = IMSI mod (2^K) + n * (2^K)



DRX Procedure

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P I P I P I P I¡ £¡ £¡ £¡ £¡ £¡ £

0 1 q NP-1

One DRX cycle

DRX Cycle Length ＝ (2^k)*10ms

UE needs to monitor the PI on these PICH frames. As shown in this figure ,

the UE needs to monitor the frames (paging occasions) indicated by the red

dots, and then decode the qth PI of this frame.



PICH Structure

b1b0

288 bits for paging indication12 bits (transmission

off)

One radio frame (10 ms)

b287 b288 b299

The Page Indication Channel (PICH) is a fixed rate (SF=256) physical

channel used to carry the paging indicators which are used to

alert UE of a incoming page sent on S-CCPCH.

Each radio frame for PICH is 10ms, and consists of 300 bits.

The first 288 bits are used to carry the Paging Indicators (PIs), and the rest

12bits are undefined.

The number of PI in one frame is Np. In WCDMA system, Np value is 18,

36, 72 and 144. That is to say, each PI has 288/Np bits.

UE can get Np value from system information.



Paging Formula

� UE must calculate q to know which PI to monitor in one

frame of PICH.

� The q value is achieved by the following formula :

( )( )( ) NpNp

SFNSFNSFNSFNPIq mod144

144mod512/64/8/18

×+++×+=

PI = (IMSI div 8192) mod NP

Here, SFN is the paging time of the UE



Time Offset between PICH and S-CCPCH

τPICH

Associated S-CCPCH frame

PICH frame containing paging indicator

The timing relationship between PICH and S-CCPCH is defined by the

above figure, and the interval is 3 slots duration (2ms, 7680 chips).



Example for DRX

� In system information, the parameters about paging:

� K=8, the CN domain specific DRX cycle length coefficient

� Np=36, the number of PI per frame

� After UE received the information, the paging time, PI and q can be calculated

� Assume the IMSI of a UE is 448835805669362, then

� DRX cycle length=2^8=256, the paging interval is 2.56s

� Cell SFN ＝ “448835805669362” mod 256+ n×256 = 242 + n×256 (n = 0,1,2,...)

� PI= (448835805669362 div 8192) mod 36 = 14

� q={14+[(18×(242+[242/8]+[242/64]+[242/512])) mod 144)×0.25]} mod 36 = 27

Conclusion:

Each PICH radio frame contains 36 PIs, and each PI has 8bits (288/36).

UE has to monitor bit 216 (27*8)～bit 223 of each frame, if all of these 8

bits become “1111 1111”, UE will know there is a paging for it, and receive

the paging information in SCCPCH.



System Information

The value of “K” is transmitted in SIB1



System Information

The detailed parameters about PCH and PICH are transmitted in SIB5



Paging From CN

Paging from CN contains the following information:

CN Domain Indicator

� The domain where the paging comes from

Permanent NAS UE Identity

� The UE IMSI

DRX Cycle Length Coefficient

� K value

Temporary UE Identity

� Which is optional, TMSI or P-TMSI

Paging Area

� LAI if the paging is from CS domain

� RAI if the paging is from PS domain

Paging Cause



Paging Type1

Paging type1 contains the following information element:

Paging record list

� If paging is from CN, the CN identification, UE NAS identification,

paging cause should be included.

� If paging is from UTRAN, the AS identification URNTI should be

included.

� The protocol defines at most 8 UE can be paged in one paging

time

BCCH modification info

� It is optional, and value tag is used to show the change of system

information. If this IE exist, UE will neglect paging record list, and

read system information directly



Paging Type2

� For the UE in CELL_DCH or CELL_FACH state, the

UTRAN sends the PAGING TYPE 2 message with AM RLC

on DCCH channel to initiate the paging procedure

UTRAN should set the information element (IE) "Paging cause" to the

cause for paging received from upper layers. If no cause for paging is

received from upper layers, UTRAN should set the value "Terminating -

cause unknown".



Paging Channel Setup

� PICH and PCH

channels are

established in cell

setup procedure

RNC will send information about PCH and PICH to NodeB through the Iub

message “Common Transport Channel Setup Request”

From the above figure, we can get the following information:

2 transport block format (0×240,1×240) of PCH

Power offset (2dB) of PCH

Np value (36) of PICH

Power offset (−3dB) of PICH



Paging Scheduling

� The paging scheduling is performed in the MAC-C

� After the MAC-C receives the paging message from the upper layer, it

will calculate the paging time, and then stores the paging record at the

corresponding location. For the MAC-C, a maximum of 8 paging records

can be stored at one paging time, and the excessive ones will be

discarded.

� When each paging time comes, the MAC-C codes the paging records

corresponding to this paging time, and delivers them to the NodeB, and

then clears them. NodeB delivers the paging records through the cell

paging channel

If the paging is caused by system message update, the MAC-C will

immediately code them and deliver them, and clears all the paging records.

At present, the transport block of the PCH supported by the MAC-C is 240

bits. The coded paging message supported by each frame cannot exceed

240 bits. According to the signaling structure of PAGING TYPE 1 and

ASN.1 PER coding rule, if the UE ID of the paging message is IMSI, a

maximum of 3 UEs can be paged at each paging time. If the UE ID is TMSI

or PTMSI, a maximum of 5 UEs is supported.

Therefore, with the current coding mode of PCH, the number of UEs paged

at one paging frame can’t reach the protocol value 8. If the number of UEs

paged at the same paging time exceed the system capacity, it will lead to

paging loss, and consequently call loss.


In the configuration of IMSI, the detailed planning should be made

according to the calculation formula for paging time, IMSI mod 2^k+n*2^k,

so as to ensure the paging occasion of every UE are evenly distributed on

the frames of a paging cycle.

On the other hand, the network can improve the paging success rate by

means of re-transmission. After the paging timer is expired, the CN will re-

transmit the paging message at the Iu interface, and the UTRAN can also

re-transmit paging through the scheduling of the MAC-C, but in different

paging cycles.



Paging Parameters Analysis

� DRX Paging Cycle Coefficient

� Np Value

� Paging Retransmission Times and Interval

(1) The higher K value, the longer the DRX cycle, and the lower the UE

power consumption.

This value can be modified by the command MOD CNDOMAIIN. The

modified value will be transmitted to UE by paging type 1.

(2) Np is the number of PIs per frame, which is in the value range of (18,

36, 72, 144). After UE gets PI, it will demodulate the subsequent S-CCPCH

frame.

All the UEs will be divided into Np groups, and all the UEs in the same

group share the same PI.

If Np is small, more UE will share one PI, then UE will be waken more

times; If Np is big, less UE will share one PI, then UE will be waken less

times.

More Np means the higher PICH demodulation requirement for UE

because each PI has less bits.


(3) To increase the paging success rate, the CN and RNC will re-transmit

the paging message.

Paging re-transmission leads to some negative effect, such as paging traffic

increase.

The CN re-transmission times and interval should be considered together

with the re-transmission of the UTRAN

The default configuration is the Paging type 1 information is transmitted

twice by RNC. And the interval is one paging period. CN can support four

retransmissions.



Traffic Statistics for Paging

When the RRC connection request

message of UE is received, with the

request cause being a called type

cause, for example, Terminating

Conversational Call.

Measures the times of paging

idle UEs successfullyCN_PAGE_IDLE_UE_SUCC

When the PAGING message initiated

by the CN is terminated by an idle UE.


idle UEs at the Iu interfaceCN_PAGE_IDLE_UE_REQ

When the PAGING message initiated

by the CN is terminated


at the Iu interfaceCN_PAGE_REQ

Statistics point of timeMeaningTraffic statistics item name

This table shows the traffic statistics item for paging in the RNC. Among

them, the two main items CN_PAGE_IDLE_UE_SUCC_RATE and

UTRAN_PAGE1_SUCC_RATE greatly indicate the actual paging

performance.

The paging success rate, the first paging success rate and second paging

success rate at the CN side can also be measured.



Traffic Statistics for Paging

A calculation index, calculated with the

formula of

[UTRAN_PAGE1_SUCC]/[UTRAN_PAGE1_R

EQ]

Measures the success rate

of PAGING TYPE 1UTRAN_PAGE1_SUCC_RATE

A calculation index, calculated with the

formula of

[CN_PAGE_IDLE_UE_SUCC]/

[CN_PAGE_IDLE_UE_REQ]

Measures the success rate

of paging to idle UEs

initiated by the CN

CN_PAGE_IDLE_UE_SUCC_RATE

When the UTRAN receives the paging

response message from the UE

Measures the times of

successful response from

the UE after the UTRAN

initiates the PAGING TYPE

1 message

UTRAN_PAGE1_SUCC

When the UTRAN initiates a PAGING TYPE 1

message

Measures the number of

PAGING TYPE 1 messages

initiated by the UTRAN

UTRAN_PAGE1_REQ

Success of paging is indicated by “paging response” in the initial DT sent by

the called user after his RRC connection is setup.



Paging Abnormity Handling

� To ensure the stability of the system and avoid impact of burst

message storm, the RNC performs flow control on the

messages (including paging messages) with high processing

frequency.

System AbnormitySystem Abnormity

When the RNC receives the paging message of the CN, it will judge

whether the system is overloaded. If so, it will discard the paging message,

and record the number of discarded paging message. If the loss proportion

reaches a specific threshold, the RNC will send the message “Overload” to

the CN, then the CN will control the message transmission flow by

decreasing the flow based on a certain step . Thereafter, if the CN does not

receive “Overload” within a certain period of time, the traffic of the Iu

interface will increase to the normal status step by step.

In addition, if some failure occurs in the system, for example, the Iub

interface transmission layer failure or cell in abnormal state, it may cause

paging loss.

NodeB also will not process the paging message packet when it is

overloaded.

To minimize the call failure caused by paging loss, the network adopts the

method of paging message re-transmission.



Paging Abnormity Handling

� Due to the restriction of the PCH capacity, the number of UEs that can be

paged at the same paging time is limited. If the number of UEs exceeds this

limit, paging message will be discarded. This will occur in two cases:

�The number of UEs paged at the same paging time exceeds eight (the

protocol value)

�The transport block of each frame after coding is 240 bits at the

maximum

Paging Channel Capacity RestrictionPaging Channel Capacity Restriction

Decided by the

buffer of MAC-C

Decided by the

buffer of MAC-C

Decided by the

IMSI or TMSI

Decided by the

IMSI or TMSI

(1) If the number of UEs paged at the same time is more than eight (the

protocol value), it will exceed the processing capability of the MAC-C

(The MAC-C applies a memory for eight paging records at each paging

time.).

(2) As the transport block of each frame after coding is 240 bits at the

maximum, even if the number of UEs of each paging time is smaller

than 8, paging loss cannot be avoided. If the UE ID of the paging record

is IMSI, the paging message of each frame can contain the paging

records of only 3 UEs. If the UE ID of the paging record is TMSI or

PTMSI, the paging message of each frame can contain the paging

records of only 5 UEs. If this limit is exceeded, the excessive paging

records will be discarded.




Section 1 Problem Analysis ProcessSection 1 Problem Analysis Process

Section 2 Network Information CollectionSection 2 Network Information Collection

Section 3 Troubleshooting Paging ProblemsSection 3 Troubleshooting Paging Problems

Section 4 Optimization Verification Section 4 Optimization Verification



Problem Analysis Flow

Collect network information

Identify optimization goal

Locate paging problem

Optimize paging problem

Optimization verification

Optimization goal achieved?

END

YES

NO

Network information collection:

� Traffic statistics related to network and paging,

� Alarm information, complaints of subscribers,

� Network planning information, optimization records and configuration of

network parameters

Determination of optimization goal:

� Determine KPI of paging problem optimization

Paging problems location:

� Locate causes of paging problems

Paging problems optimization:

� Take corresponding optimization methods according to the specific

problems

Optimization verification:

� Verify whether the KPI meets the requirements and whether other

information about paging is normal after optimization




Section 1 Problem Analysis Process Section 1 Problem Analysis Process






Network Information Collection

� Network information collection is the first step of paging

problem analysis

�Traffic statistics

�Subscriber complaints

�Alarms

�Optimization history record of network planning

�Radio parameters configuration



Traffic Statistics

� The paging-related traffic statistics can be observed

respectively on the traffic statistics console of RNC, UMSC and

SGSN according to different paging areas

CN_PAGE_IDLE_UE_SUCC_RATE is paging success rate of UE in idle

state initiated by CN

UTRAN_PAGE1_SUCC_RATE is the success rate of Paging Type 1

initiated by UTRAN

These two items indicate the paging success rate of the corresponding

paging area of RNC. CN_PAGE_IDLE_UE_SUCC_RATE indicates the

paging success rate from CN, while UTRAN_PAGE1_SUCC_RATE

includes not only CN paging, but also UTRAN system information update

and UE state transition, which will request UE for cell update.



Alarm

� Paging alarm

�Paging flow control alarm in RNC

�Paging overload in CN

� Boards alarm

Based on the current realization of Huawei, there is only “RNC overload”

alarm related to paging, alarm occurs when CN receives the overload

message of RNC.

The paging-related alarm of RNC is “flow control alarm”. When RNC is in

paging flow control state, the paging message will be discarded . When the

CPU occupation or the message packet occupation of a board sub-system

exceeds the threshold, the system will switch to the flow control state, and

the flow control alarm will occur. When the system returns to normal

operation state from the flow control state, the “flow control recovery” alarm

will occur. However, it is not absolute that paging will be discarded when

“flow control alarm” occurs, because RNC owns different flow control

thresholds for different flow control objects (paging, serial port printing and

message tracing), as long as there is flow control, alarm will occur; but

paging loss will occur only when paging loss occurs.



Subscriber Complaints

� In subscriber complaints, the following information should be

concerned

�Complaints occur no matter the network is busy or not. If

paging failure occurs at the busy time, paging congestion

analysis should be emphasized. If not, other factors should

be analyzed

� If the terminated UEs are of the same type. Problems

probably exist in UE itself

� If the complaints are centralized in a certain place; it is

possibly caused by poor coverage

Subscribers’ complaints are very valuable to locate specific network fault,

we should find the rules and solve problems as soon as possible



Network Planning Optimization Record

� Obtain the network planning report, and pay attention to the

division of the paging areas ------LA and RA

� For the network that has been put into service, it may have

experienced several optimization processes before. Emphasis

should be put on the former optimization records about

coverage hole, system overload, paging loss and paging

channel power allocation.

We should notice the difference between the LA and RA.



Radio Parameter Configuration

� CN paging retransmission times and paging interval

� UTRAN paging retransmission times and paging interval

� DRX paging cycle coefficient k (DRX paging cycle = 2^k)

� Paging indication quantity Np included in a PICH frame

� Channel power allocation of PICH and PCH

� Whether CN adopts global paging

� UE ID used by CN paging (IMSI,TMSI or PTMSI)

In practice, we should pay more attention to the first, second one and the

fifth one.



Determining Optimization KPI Goal

� Two KPIs, “location area paging success rate” and “RA paging

success rate” , should satisfy the optimization requirements.

The paging success rate is recommended to be higher than 86%



Position the problems

� Paging problems can be divided into three categories

�Paging message is not transmitted at the air interface at all

�Paging message has been transmitted, but UE does not

receive the message or receive wrong message

�UE fails to respond after receiving paging messages.

� Paging message is not transmitted at the air interface at all

� If paging message is not transmitted at the air interface at

all, it is most likely that paging is discarded.

� Paging message has been transmitted, but UE does not receive

the message or receive the wrong message

� The power allocation for PICH or PCH is too low

� Coverage problems

� UE fails to respond after receiving paging messages.



Direct Causes of Paging Loss

� RNC is in paging flow control state

� PCH capacity restriction

� Other causes

(1) If RNC detects that CPU occupation or message queuing occupation

reaches the preset threshold, paging flow control will be triggered and

paging message will be discarded.

(2) Based on the structure of PCH, if IMSI is used for paging, only three

UEs can be paged at one paging period; and if TMSI or PTMSI paging

is used, only five UEs can be paged at one paging period .

(3) Transmission faults of Iub interface and equipments faults. Probability

of those faults is small and can be viewed from the alarm console.



Further analysis on paging loss cause

� The following problems could be the main causes for paging loss

�Too large paging area

� Improper setting of CN paging retransmission times and interval

� Improper setting of UTRAN paging retransmission times and

interval

�CN adopts whole-network paging

� Improper setting of DRX paging cycle coefficient

� Improper setting of Np value

�CN adopts improper UE identifier

Other Cause Analysis

�Too low power allocation of paging channels

�Existence of coverage dead zones

�UE performance problems



Optimization Verification

� After network is optimized and adjusted, the optimization results

should be verified

� Traffic statistics

� Alarm

� Subscriber complaint

� Whether there is any subscriber complaint in a certain period

� Call test

� Test call success rate when traffic is at the peak and in the place

where subscriber complaint occurs

�Traffic statistics

� Check mainly whether “location area paging success rate” and “RA

paging success rate” achieve the optimization goal 86% specified in

advance

�Alarm

� Check whether there is any “RNC overload” alarm in CN or whether

there is any flow control alarm in RNC

huawei wcdma paging procedure and problem analysis

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