1. wcdma rno paging problem analysis guidance-20041101-a-1.0

Product name Confidentiality level

WCDMA RNP For internal use only

Product version Total 29 pages

V100R001

WCDMA RNO Paging Problem

Analysis Guidance

(For internal use only)

Prepared by: URNP -SANA Date: 2003-12-27

Reviewed by: Date:

Reviewed by: Date:

Approved by: Date:

Huawei Technologies Co., Ltd.

All rights reserved

WCDMA RNO Paging Problem Analysis Guidance For Internal Use Only

2004-07-09 Confidential of 29

Revision Record

Date Revision version

Description Author

2003-12-27 1.00 Initial transmittal Jiao Anqiang



Table of Contents

1 Overview .................................................................................................................................................... 7

2 Paging Problem Analysis Process .............................................................................................................. 9

2.1 Problem Analysis Flow ......................................................................................................................... 9

2.2 Network Information Collection ......................................................................................................... 10

2.2.1 Traffic Statistics ............................................................................................................................ 10

2.2.2 Alarm ............................................................................................................................................ 13

2.2.3 Subscriber Complaints .................................................................................................................. 13

2.2.4 History Record of Network Planning Optimization ..................................................................... 14

2.2.5 Radio Parameter Configuration .................................................................................................... 14

2.3 Determining Optimization Goal .......................................................................................................... 15

2.4 Locating Paging Problems .................................................................................................................. 15

2.4.1 Determining Basic Location Direction ......................................................................................... 15

2.4.2 Direct Causes of Paging Loss ....................................................................................................... 16

2.4.3 Further Analysis on Paging Loss Cause ....................................................................................... 17

2.4.4 Other Cause Analysis.................................................................................................................... 17

2.5 Paging Problems Optimization ............................................................................................................ 17

2.6 Optimization Verification ................................................................................................................... 17

3 Typical Paging Problem Analysis ............................................................................................................ 18

3.1 Too Large Paging Area Planned ......................................................................................................... 18

3.1.1 Problem Analysis .......................................................................................................................... 18

3.1.2 Optimization Measures ................................................................................................................. 19

3.2 Improper Setting of CN Paging retransmission Times and Interval ................................................... 19



3.3 Improper Setting of UTRAN Paging Retransmission Times and Interval .......................................... 21



3.4 CN Adopts Whole-Network Paging .................................................................................................... 21



3.5 Improper Setting of DRX Paging Cycle Coefficient ........................................................................... 22



3.6 Improper setting of Np value ............................................................................................................... 23



3.7 UE Identifier Utilized by CN Paging .................................................................................................. 24



3.8 UTRAN Should Activate IMSI ATTACH and DETACH Functions ................................................. 25



3.9 Too Low Power allocation of Paging Channels .................................................................................. 26



3.10 Existence of Coverage Dead Zones .............................................................................................. 27

3.10.1 Problem Analysis ................................................................................................................... 27

3.10.2 Optimization Measures ........................................................................................................... 27

3.11 UE Performance Problems ............................................................................................................ 27

3.11.1 Problem Analysis ................................................................................................................... 27

3.11.2 Optimization Measures ........................................................................................................... 27

4 Outstanding Problems .............................................................................................................................. 27



List of Figures

Figure 1 Typical called flow of UE ...................................................................................................... 8

Figure 2 Paging Problem Analysis Flow .......................................................................................... 10

Figure 3 System Message 1 Resolution .......................................................................................... 26



List of Tables

Table 1 UMSC Paging Traffic Statistics Indexes ................................................................................ 12

Table 2 SGSN Paging Traffic Statistics .............................................................................................. 12

Table 3 Subscriber complaint information .......................................................................................... 14

Table 4 Calculaiton Result of Paging Area When Cn Id Adopting IMSI ........................................... 19

Table 5 IMSI ATTACH and DETACH Identifier ............................................................................... 25



WCDMA RNO Paging Problem Analysis Guidance

Key words: Paging, paging area, paging retransmission

Abstract: This document describes the general flow of solving paging problems, analyzes typical

problems during paging in details and provides optimization measures.

List of abbreviations:

Abbreviations

Full spelling

DRX Discontinuous Reception

LA Location Area

PCH Paging Channel

PI Paging Indication

PICH Page Indication Channel

RA Route Area

RAN Radio Access Network

RNC Radio Network Controller

RNO Radio Network Optimization

WCDMA Wideband Code Division Multiple Access



1 Overview

If it is necessary for network side to contact with UE which is in idle mode or in

CELL_PCH or URA_PCH state, a paging flow will be initiated. Paging is an

important way for network to contact with UE. Compared with other flows, the paging

flow has the characteristics of high frequency, heavy traffic and strong outburst in

radio network. Paging performance is closely related to the performances of the

whole radio network. Therefore, the study of paging problems is of practical

significance for radio network performances.

From the aspect that UE receives paging messages, paging messages consist

of PAGING TYPE1 and PAGING TYPE2. Which type of paging messages will be

sent to UE is decided by UTRAN. PAGING TYPE1 is used to page UEs in IDLE,

CELL_PCH and URA_PCH states through PCCH logical channel. PAGING TYPE2

is used to page UEs in CELL_FACH or CELL_DCH states through DCCH. This

document mainly discusses PAGING TYPE1, while PAGING TYPE2 can be

regarded as the common RRC signaling to process and will not be discussed in this

document.

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

When UE is called: In order to set up a call, the core network (CN) sends

paging message to UTRAN through Iu interface, and UTRAN sends the

paging message from CN to UE through the paging process at Uu interface,

which will have UE initiate a signaling connection setup process with CN.

Cell system message update: When system messages change, UTRAN will

trigger paging process in order to inform UE in idle, CELL_PCH and

URA_PCH state to carry out the system message update. Therefore, UE

can read the updated system message. For UE in CELL_FACH state, in

order to inform it to carry out corresponding system update, it is necessary

to send SYSTEM INFORMATION CHANGE INDICATION message to it

through BCCH (V1.2 version does not support the message, so UE in

CELL_FACH state will not be processed).

UE state migration: In order to trigger UE in CELL_PCH and URA_PCH

state to carry out state migration (for example, migration to CELL_FACH

state), UTRAN will perform a paging process. Meanwhile, UE will initiate a

cell update or URA update, as a reply to the paging.

A typical called flow caused by paging is shown in Figure 1 : CN sends a paging

message to UTRAN, UTRAN calculates the paging time and obtains the target cell



after receiving the paging message, and delivers the paging message at the air

interface upon the paging time. If CN receives the paging response message from

UE, the UE paging is successful. The whole process includes paging delivery and

UE access. For UE access, refer to Access Process Guidance. There are probably

various problems in the paging process, which will cause that the target UE cannot

receive the paging message properly. For instance, during short messages

multi-casting and full-text paging in network, unreasonable paging strategies will

result in paging channel congestion, so large amount of messages will be lost, in

more serious case, the system will be caused over-loaded for long time and the

paging channel power allocation is too low, which will lead to low paging success

rate. This document will make a further discussion on these problems leading to

paging abnormality and present corresponding solutions.

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 ACKNOWLEDGE

RAB setup process

paging

RR_EST_REQ (PAGING RESPONSE)

RR_PAING_IND

INITIAL_DIRECT_TRANSFER

(PAGING RESPONSE)

RANAPRANAP

RRC setup process

Figure 1 Typical called flow of UE

The general structure of this document is as follows:

1. Overview: It briefly introduces the paging time, paging flow and general

paging problems;

2. Paging problem analysis process: It describes the problem analysis flow of

paging, how to analyze paging problems step by step and what to do in each step.



3. Typical problem analysis of paging: It analyzes typical problems of paging,

including the phenomenon, principles and optimization methods of the problems.

4. Outstanding problems: They refer to problems which cannot be solved in this

document.

2 Paging Problem Analysis Process

2.1 Problem Analysis Flow

The paging problem analysis flow is as shown in Figure 2 . Similar to the

general analysis methods of network problems, there are four steps of paging

problem analysis, which are detailed as follows:

Network information collection: Collect traffic statistics information 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 optimizing and adjusting

measures according to location results;

Optimization verification: Verify whether the KPI meets the requirements and

whether other information about paging is normal after optimization.



Collect network infomration

Identify optimization goal

Locate paging problem

Optimize paging problem

Optimization verification

Optimization goal achieved?

END

YES

NO

Figure 2 Paging Problem Analysis Flow

2.2 Network Information Collection

Network information collection is the first step of paging problem analysis. The

optimization personnel should obtain the paging-related information to-be-optimized,

such as traffic statistics, subscriber complaints, alarms, optimization history record of

network planning and radio parameters configuration, so as to prepare for the

subsequent analysis.

2.2.1 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.

RNC traffic statistics corresponds to a RNC area, UMSC traffic statistics

corresponds to a location area and SGSN traffic statistics corresponds to a RA. In

the actual traffic statistics analysis process, the traffic statistics of CN is mainly

analyzed, with combination of the traffic statistics data of RNC and CN. If there is no

paging-related traffic statistics on the traffic statistics console, it is necessary for

equipment users to establish the paging traffic statistics tasks. The paging-related

traffic statistics of RNC, UMSC and SGSN are discussed below based on the paging

traffic statistics of our company.



The traffic statistics of RNC is shown in Table 1 . Attention should be paid to

CN_PAGE_IDLE_UE_SUCC_RATE (paging success rate of UE in idle state initiated

by CN) and UTRAN_PAGE1_SUCC_RATE (The success rate of Paging Type 1

initiated by UTRAN). These two indexes indicate the page success rate of the

corresponding paging area of RNC. CN_PAGE_IDLE_UE_SUCC_RATE indicates

the page success rate from the aspect of CN, while UTRAN_PAGE1_SUCC_RATE

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

state migration (the paging information of UE is cell updating). The two indexes can

be used to analyze the paging performance of an RNC area which includes one or

more location areas.

Table 1 RNC Paging Traffic Statistics Index

Name Description Standard measuring point

CN_PAGE_REQ Count paging times at IU interface

Receive the PAGING messages initiated by CN

CN_PAGE_IDLE_UE_REQ Count paging times at IU interface of idle subscribers

Receive the PAGING messages initiated by CN when UE is in idle status

CN_PAGE_IDLE_UE_SUCC Count paging success times of idle subscribers

Receive RRC connection request message of UE and the reason belongs to the called type, such as “Terminating Conversational Call”

UTRAN_PAGE1_REQ

Count times of initiating PAGING TYPE 1 initiated from UTRAN side

PAGING TYPE 1 message initiated from UTRAN side

UTRAN_PAGE1_SUCC

Count PAGING TYPE 1 message initiated from UTRAN side and the times of UE success receive response received

UE paging response message received at UTRAN side .

CN_PAGE_IDLE_UE_SUCC_RATE

Count paging success rate of idle UE initiated by CN

The calculation index, obtained by the formula of [CN_PAGE_IDLE_UE_SUCC]/ [CN_PAGE_IDLE_UE_REQ]

UTRAN_PAGE1_SUCC_RATE

Count the paging success rate of Page Type 1 initiated by UTRAN

The calculation index, obtained by the formula of [UTRAN_PAGE1_SUCC]/[UTRAN_PAGE1_REQ]

The relevant traffic statistics of UMSC paging are all based on a location area,

as shown in Table 1. Generally, the location area will not be configured cross RNC

and BSC, so we can carry out statistics on the paging success rate, the first paging

success rate and non-first paging success rate of a location area. The location area

paging success rate focuses on the paging status of a location area, not on paging



repeat times, while first paging success rate and non-first paging success rate focus

on the influence of paging repeat times on paging success rate.

ion times transmisspagingFirst

messages response-no delivered ofnumber ion times transmisspagingFirst rate success pagingLA

－

ion times transmisspagingFirst

timesresponse pagingFirst rate success pagingFirst

timesresponse repitetion paging Interface

timesrepitetion paging Interfacerate success pagingfirst -Non

Table 1 UMSC Paging Traffic Statistics Indexes

Name Description Standard measure point

The first paging times The first sending times of Paging Req by MSC

Carry out statistics when MSC sends PAGING message to RNC/BSC

The first paging response times

Successfully receiving response times after the first sending of Paging message by MSC

Carry out statistics when MSC receives PAGING RESPONSE after the first sending of Paging message

Interface paging repeat times

The non-first paging times of MSC

Carry out statistics when MSC sends PAGING message to RNC/BSC

Interface paging repeat response times

Successfully receiving response times after the non-first sending of Paging message by MSC

Carry out statistics when MSC receives PAGING RESPONSE after the non-first sending Paging message

The first paging times of Iu interface

The first sending Paging times of Iu interface

Carry out statistics when MSC sends PAGING message to RNC

Lu interface paging repeat times

The non-first sending Paging times of Iu interface

Carry out statistics when MSC sends PAGING message to RNC not at the first time

Delivery paging no-response times

Times of having no paging response messages received

Paging timer stay-over statistics

The relevant traffic statistics indexes of SGSN paging are all based on a RA, as

shown in Table 2, the paging success rate of a certain RA can be obtained.

questTimesingRAGroupPag

TimesingFailureRAGroupPagquesTimesingRAGroupPagccessRateRAPagingSu

Re

Re －＝

Table 2 SGSN Paging Traffic Statistics

Name Description Standard measure point

Group paging request times of each RA

It provides delivery group paging request times in the

SGSN sends Iu interface paging request message (PAGING), and its



specific RA, not including retransmission messages

CN Domain is PS.

Group paging failure times of each RA

It provides group paging failure times in the specific RA

Paging retransmission message times reach maximum.

During traffic statistics analysis, attention should be paid to “location area

paging success rate” and “ RA paging success rate”, which are KPI measuring

paging performance. The RNC paging traffic statistics

CN_PAGE_IDLE_UE_SUCC_RATE and UTRAN_PAGE1_SUCC_RATE can be

taken as reference analysis.

2.2.2 Alarm

Based on the current realization in our company, CN is only “RNC overload”

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

RNC. In order to guarantee the system operation stability and avoid the strike on

system by outburst messages, RNC flow-controls the paging-included message with

high processing frequency. When RNC receives paging messages from CN, the

judging system will discard the paging message and record the quantity of paging

message loss if it is in paging flow control state. If the loss proportion has reached a

certain threshold, RNC will send Overload message to CN, then CN will reduce the

message flow by certain step. If CN has not received Overload message within a

certain period, the message flow of IU will increase gradually till it get normal.

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

paging flow control state, the paging message will be lost unconditionally. When the

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

the threshold, the system will be switched into 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 lost 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.

2.2.3 Subscriber Complaints

If paging messages are lost, UE will not be called. The calling party will hear the

system prompt tone “the subscriber you dialed is out of service”. We can learn about

the subscriber complaints from 1860 Customer Service Center of pilot office or learn

about the cases of “out of service” by contacting with the complainer directly.



Attention should be paid to the cases of UE called failure, and information to be

collected is shown in Table 3, but not limited to it 【5】:

Table 3 Subscriber complaint information

Time Name

(complainer) UE No.

(complainer)

UE No. (the

called)

UE type (the

called)

Place Whether

it is often

Description

Sort out complaint information, seek for rules and observe whether the

information belongs to the following cases:

Whether complaints occurs when paging is busy in the daytime and idle at

night; if paging failure occurs at the peak traffic, paging congestion analysis

should be emphasized on; if not at the peak traffic, other factors should be

analyzed.

Whether types of the called UE are the same. Problems probably exist in

UE itself.

Whether the complaint place is centralized; it is possibly because of signal

coverage;

Find the rules and solve problems more quickly.

2.2.4 History Record of Network Planning Optimization

Obtain the network planning report, and pay attention to the division of the

paging area (location area and RA). Obtain network planning report: If it is the

optimization before network is put into service, the planning report can be obtained

from the planning manager responsible for the project; if it is the optimization after

network is put into service, the planning report can be obtained from the client. The

planning report mentioned above should include planning reports after each network

capacity expansion.

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

optimization processes before this optimization. Then we should obtain the history

optimization records and learn about the network adjusting process and outstanding

problems. Emphasis should be put on optimization records about coverage absence,

system overload, paging loss and low paging channel powerallocation.

2.2.5 Radio Parameter Configuration

Parameters related to paging are as follows, which should be collected before

optimization:



CN paging retransmission times and paging interval;

UTRAN paging retransmission times and paging interval;

DRX paging cycle coefficient k (DRX paging cycle = 2k);

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)

2.3 Determining Optimization Goal

Two KPI of “location area paging success rate” and “RA paging success rate”

should meet the optimization requirements. The paging success rate is

recommended to be over 86% 【3】.

2.4 Locating Paging Problems

2.4.1 Determining Basic Location Direction

Paging problem optimization is intended to guarantee the paging KPI. Whether

UE can respond successfully is directly related to the paging KPI. From this aspect,

paging problems can be generally divided into three parts:

Paging message is not delivered at the air interface at all. If paging

message is not delivered at the air interface at all, it is most likely that

paging is lost. Paging loss is a frequently-seen problem during paging

process and it is the emphasis of analysis in this document. For detailed

paging loss analysis, please refer to Section 2.4.2 and Section 2.4.3.

However, the reason why paging has not been sent out is probably the

transmission faults of IUB interface or faults of other equipments, which can

be seen through viewing the alarm information.

Paging message has been delivered, but UE does not receive the message

or receives the wrong message. According to the specific subscriber

complaints, if problems exist on called UE only (prompt tone “the subscriber

you dialed is out of service”), because the reason is probably that the power

allocation of PICH and PCH is too low or there are problems with UE

performance; if problems exist in calling and called UE, the reason is

probably that signal coverage dead zones exist in the area.



UE fails in responding after receiving paging messages. As shown in Figure

1,The problem is related to access failure, for solutions, please refer to

WCDMA RNP Access Process Problem Analysis Guidance.

During paging problem location, problems can be located by analyzing paging

traffic statistics, alarms and subscriber complaints. Paging loss generally occurs in

the period when traffic is heavy. Check “location area paging success rate” and “RA

paging success rate” of CN traffic statistics, if these two indexes are low (lower than

86%) when traffic is heavy, and subscriber complaints are centralized in this period,

paging loss is serious and attention should be paid to analyzing it. Meanwhile, check

whether RNC overload alarm or RNC flow control alarm occurs in CN. If these

alarms exist, the probability of paging loss is big.

If events that two indexes are low are evenly distributed on the time, and

subscriber complaints are regional, check other reasons except “paging loss”:

paging channel power allocation, signal coverage and UE performance.

It is necessary to make dialing test analysis on the site for further analysis on

paging problems. Select the period when traffic is heavy and the area where

subscriber complaints are centralized, trace paging message delivery and UE paging

response process on the CN and RNC maintenance consoles during the dialing test

process.

2.4.2 Direct Causes of Paging Loss

Paging will be lost in the following cases:

1. RNC system is in paging flow control state. Because paging message is

sent frequently and RNC flow-controls paging. 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 lost unconditionally.

2. PCH capacity restriction; Based on the current PCH coding mode, one

TTI can only transmit 240bits. If IMSI paging is utilized, only three UEs

can be paged at the same paging period; and if TMSI and PTMSI

paging are utilized, only five UEs can be paged at the same paging

period 【2】. If the quantity of UEs paged at the same paging period is

beyond the system processing capability, paging loss will occur.



3. Other causes: Transmission faults of IUB interface and equipments

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

alarm console.

2.4.3 Further Analysis on Paging Loss Cause

The direct causes of paging loss are system overload and paging channel

overload, after further analysis of its causes, it is found that CN and RNC have

adopted inappropriate paging strategies, for detailed analysis, please refer to

hyperlinks as follows:

Too large paging area planned

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

2.4.4 Other Cause Analysis

Too low power allocation of paging channels

Existence of coverage dead zones

UE performance problems

2.5 Paging Problems Optimization

The paging problem optimization methods are described in detail in Section 3.

2.6 Optimization Verification

After network is optimized and adjusted, the optimization results should be

verified. The frequently-used verifying methods are available as follows: have it run

and check relevant paging traffic statistics, check whether there are paging alarms,

collect subscriber complaint information, and carry out UE called dialing test on site.

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;

Subscriber complaint: Whether there is any subscriber complaint in a

certain period;

Dialing test: Test UE called success rate when traffic is at the peak and in

the place where subscriber complaint occurs. During dialing test, it is

unnecessary to through-connect UE, just listen to ring back tone and

prompt tone.

3 Typical Paging Problem Analysis

3.1 Too Large Paging Area Planned

3.1.1 Problem Analysis

In general, CN pages target UE in one paging area (location area or RA), which

is called intra-office paging. For CS domain service, CN identifies and pages UE with

location area. In protocols, location area is defined as the area where mobile

terminals can move freely without updating VLR. A location area may contain one or

more cells. For PS domain service, CN identifies and pages UE with routing area.

RA is defined as the area where mobile terminals can move freely without updating

SGSN in a specified operation mode. A RA may contain one or more cells. The

relationship between location area and RA is specified in GSM, namely, RA can be

equal to location area in size, or only the subset of a certain location area.

If paging area is planned too large, the same paging message of a network

paging mobile station may be sent in multi cells, which will overload the paging

channel as well as increasing signaling flow at the Iub interface. If the paging

channel of a cell is overloaded in a certain period, the paging message sent to UE in

the cell will be caused lost and the power-on subscriber within the service area can

not be paged (The subscriber is out of service).

On the contrary, if paging area is planned too small, subscribers will update

locations frequently while moving, which will increase signaling flow of the system.

When network is just put into service, PS paging traffic is not heavy, RA may be

planned larger, it can be planned as per n=1. With the development of network, the

demands for PS paging traffic are increasing, in this case RA can be planned

smaller consequently. However, too small RA may cause more paging area update

events of subscribers while moving, and that signaling overhead on network side

becomes larger. Moreover, frequent location update will influence the standby time

of UEs.



The paging area maximum value is determined by the paging channel capacity.

For estimation methods of paging area capacity, please refer to【2】, for paging

mixture capacity in typical environments in【2】, please refer to Table 4:

Table 4 Calculaiton Result of Paging Area When Cn Id Adopting IMSI

n Mmix

RA LA

1 3 1.738624042 0.002592996 133 133

2 3 1.741614489 0.003616996 95 190

3 3 1.74665366 0.004640996 74 222

4 3 1.744911078 0.005664996 61 244

5 3 1.745096566 0.006688996 51 255

6 3 1.745372494 0.007712996 44 264

7 3 1.745680526 0.008736996 39 273

8 3 1.745680526 0.009760996 35 280

9 3 1.746014764 0.010784996 32 288

10 3 1.746024614 0.011808996 29 290

Meaning of each field in the table:

n is the proportion of location area size to RA size, LA=nRA;

Mmix is the UEs paged quantity by each TTI of PCH;

H is traffic supported by a specified paging module in a DRX cycle length

period;

is the identification of the supported by a specified

paging module in an RA during a DRX cycle length period.

For instance, n=2, LA=190 means 190 cells in a location area. If the actual

location area of network is larger than 190, it is necessary to divide the location area.

It is recommended by GSM network optimization experiences that the location

area capacity of each local network is larger than 300 carrier frequencies and the

location area with paging traffic exceeding 50,000 in busy hour is divided.

3.1.2 Optimization Measures

Divide location areas where network capacity or paging traffic is bigger than a

specified threshold, which can reduce paging message flow efficiently.

3.2 Improper Setting of CN Paging retransmission Times and Interval


In order to guarantee the paging success rate, CN will retransmission paging

message at the IU interface. CN paging retransmission times and interval can be

H RAPerDRXCycleLengthInNErlang

RAPerDRXCycleLengthInNErlang



configured. Because paging is initiated specific to location area, paging

retransmission will increase paging traffic. Especially in case of public downlink

channel congestion at the air interface, paging traffic will double due to paging

repeat, which will waste downlink channel resources seriously. CN paging

retransmission configuration should be in accordance with UTRAN.

CN paging retransmission is the retransmission paging in case that UE has not

responded at the first time, which can increase paging success rate and connection

rate. But from data in GSM network, paging retransmission success rate is low,

especially at the second time or even more, which contributes a little to paging

success rate and connection rate. Experience data indicates that: Generally, the

success rate of the first paging is approaching to 86%, and the retransmission

paging rate is 1.8%. Herein, retransmission paging success is mainly the second

paging success, while success rate of the third paging is estimated less than 0.2%

calculated by the decreasing pattern, and the third paging (the second repeat) has

little influence on paging success rate【6】. The paging mode of WCDMA is similar

with that of GSM, from online statistics, frequent CN paging contributes little to the

paging success rate; contrarily, it increases the system load.

It is no good for CN paging interval to be too short. CN sends paging messages

to RNC through the IU interface, and RNC calculates the paging time according to

IMSI attached by the paging message and arranges a delivery at the corresponding

paging time of the nearest paging cycle. Currently, RNC of our company also adopts

the paging retransmission mode, the default is repeating once and the

retransmission interval is a paging cycle. From analysis above, the maximum interval

between the time that RNC receives CN paging and the time that message is

delivered to the air interface is a paging cycle. If RNC retransmits once, CN paging

interval should be longer than two paging cycles. The paging cycle realized by our

company is 2.56s. If CN paging interval is bigger than 2.56s and less than 5.12,

when CN retransmitted paging reaches, and RNC has not finished repeat paging,

RNC will arrange paging in the following paging cycle, Thus only three paging

messages are sent in fact, not four expected, which causes paging message loss


CN paging retransmission configuration should be in accordance with UTRAN.

When UTRAN retransmits paging once, it is suggested for CN configuration to

retransmit once (totally twice) at the interval longer than two paging cycles. If the

paging cycle is 2.56s, it is suggested that CN paging retransmission interval be 5s

and configuration be 5s with the consideration of paging loss during Iu/Iub flow

control 【4】. Reduce paging repeat times and increase paging interval, keep the time



of no paging responses unchanged, interval of reporting prompt tones when there is

no paging response will not be influenced.

CN paging interval and repeat times can be configured through software

parameters.

3.3 Improper Setting of UTRAN Paging Retransmission Times and Interval


In order to reduce paging message flow at the Iu interface and increase the

probability that UE receives paging message, UTRAN can retransmit paging

messages. UTRAN paging retransmission configuration should be in accordance

with CN paging retransmission.

Paging is delivered at the fixed time (a paging cycle), and UTRAN paging

interval is the integer multiple of a paging cycle, one paging cycle in general.

Therefore, we can adjust the UTRAN paging repeat interval by adjusting DRC

paging cycle coefficient k.

UTRAN paging repeat times should not be too many; otherwise, adding paging

repeats at Iu interface, the paging channel load at UU interface will increase rapidly.

In addition, UTRAN is realized on MACC layer, and MACC does not identify the

specific PRC message, MACC will go on repeating paging even if UE replies paging

response message. Too many UTRAN paging repeat times will cause the

unnecessary system overhead and message flow at UU interface.


It is reasonable for paging retransmission times to keep the current default

configuration. It is also reasonable to adjust paging retransmission times on RNC

maintenance console using MML command SET WFMRCFGDATA.

3.4 CN Adopts Whole-Network Paging


Considering improving call connection rate, CN side can be configured with

whole-network paging, which bears the characteristics that paging has overridden

the concept of location area, and is initiated specific to all UTRANs suspended under

whole CN. In this case, paging traffic becomes larger, especially when multiple

location areas are suspended under CN, the location areas with smaller capacity will

be overloaded, and can not recover for a long time.




Global paging should be avoided in CN. From experiences on GSM network,

the whole-network paging of CN is the important cause of overload of location area

with smaller capacity; meanwhile, CN global paging contributes little to the

improvement of connection rate of long-distance incoming call.

CN global paging is useful only when UE location area is recorded as failure by

NLR, which hardly occurs, however, once it occurs, it means serious fault occurs;

even more CN global paging does not work【6】.

3.5 Improper Setting of DRX Paging Cycle Coefficient


When UE is in IDLE and PCH state, UE will utilize the discontinuous reception

technology “Discontinuous Reception (DRX)” to reduce power overhead. According

to Protocol TS25.304, paging cycle length (DRX cycle length) = MAX (2K，PBP).

where, K refers to DRX cycle length coefficient, PBP refers to Paging Block

Periodicity which is applied only in TDD mode, for FDD, PBP=1, so, DRX cycle

length = 2K.

DRX paging cycle coefficient of UE has three sources: system message, CN

paging message and UU interface signaling of UTRAN, while the processing way of

CS and PS is different.

For PS domain, DRX paging cycle coefficient is obtained by the negotiation

between UE and SGSN through NAS layer message (attach process), and the

negotiation data is adopted as standard whether UE is in IDLE or connection state,

however, paging coefficient of CS domain is adopted if negotiation fails.

For CS domain, the minimum values of system message and CN paging

message are taken as DRX paging cycle coefficient if UE is in IDLE state. And the

minimum values of system message, CN paging message and signaling at UU

interface of UTRAN are taken as DRX paging cycle coefficient if UE is in connection

state.

Settings of DRX paging cycle coefficient K should be considered with the

following factors:

1. DRX paging cycle coefficient K determines the DRX cycle length, the larger

the value of K is, the longer the DRX cycle is. Meanwhile, UE power

consumption will be lower, however, the value of K makes UE paging cycle

longer. Namely, UE paging response time becomes longer. If K is set too

small, paging cycle will become smaller, and UE processing paging

overhead and power consumption will increase. In the protocol, the value is



within the range of 2 ~12, the current value of our company is 8, and the

paging cycle is 2.56 seconds.

2. According to the current coding mode of PCH, a TTI can transmit only 240

bits, if IMSI paging is utilized, only 3 UEs can be paged at the same paging

moment; if TMSI paging is utilized, only 5 can be paged at the same paging

moment. And if the number of UE paging at the same paging moment

exceeds the processing capacity of system, it will cause paging loss and

call loss. If K is set too small, the paging cycle will become shorter, the

probability of the same calculated UE paging moment will increase,

consequently, the probability of paging loss will increase.

3. Because UTRAN paging retransmission interval is a paging cycle, when

setting DRX paging cycle coefficient K, we should consider the

retransmission interval of UTRAN paging, and the K value should be set in

coordination with CN paging retransmission.


The lower limit of value K should be set with the consideration of saving UE

power consumption and reducing the probability of the same UE paging moment,

and the upper limit of value K should be set with the consideration of the repeat

interval of UTRAN paging. During the actual setting process, the value K should be a

value within 2 ~12, i.e., 5, 6, 7 and 8, and the corresponding paging cycles are

320ms, 640ms, 1280ms and 2560ms. Suppose that the CN paging repeat time is 1

(totally broadcasting 2 times), and the interval is 2s, so how should the UTRAN

repeat times and the intervals be set? Generally, the following rules should be

followed:

1. If the UTRAN retransmission time is 0 (no retransmission of UTRAN), the

DRX cycle of UTRAN shall be smaller than CN retransmission interval (2s) ,namely,

the value K at this time should be set to 7 or 8 (1.28s,2.56s), the setting to be 8 is

considered with the paging loss【4】during lu/luB paging flow control;

2. If the UTRAN retransmission time is 1, the DRX cycle of UTRAN shall be

smaller than half of the CN retransmission interval (1s) , namely, the value K at this

time should be set to 6 or 7 (0.64,1.28s), the setting to be 7 is considered with the

paging loss【4】during lu/luB paging flow control; (Configuration recommended)

3.6 Improper setting of Np value




Np refers to PI paging indication number delivered in a certain frame by paging

indication channel PICH, the value of which is within the range of (18, 36, 72, 144),

which is indicated by Number of PI per frame in System Message 5. UE will receive

PICH frame at the specified paging time and find the corresponding PI indicator bit

(the qth PI). Only when the corresponding PI indicator bit is effective, UE

demodulates the corresponding S-CCPCH frame.

NpNp

SFNSFNSFNSFNPIq mod144

144mod512/64/8/18

Where, PI = DRX index mod NP = (IMSI div 8192) mod NP, SFN refers to UE paging

moment.

Significance of Np in the actual network: IMSI is divided into Np groups by this

parameter, and all IMSIs in each group adopt the same PI. Influences of Np value on

network: If Np value is too small, the corresponding UE number in each group will be

larger. For each IMSI, if the probability that PI indication occurs increases, times that

IMSI is waken up will increase, which is no good in saving UE power. If Np value is

too large, the corresponding IMSI number in each group will be smaller. For each

IMSI, if the probability that PI indication occurs decreases, times that IMSI is waken

up will decrease. However, if Np is larger, the bit number of each PI will decrease

and PICH demodulation performance of UE will be required higher.


Np value is within the range of (18, 36, 72, 144), which should be determined

according to the current network subscriber quantity (subscriber traffic). Generally,

the value should be larger when subscribers are of large amount, and the value

should be smaller when there are fewer subscribers. In the actual network, the

intermediate 36 or 72 can be selected.

3.7 UE Identifier Utilized by CN Paging


Network can page multi UEs at the air interface at the same paging time using

paging type 1 message. Because of PCH capacity restriction, UE quantity is closely

related to UE identifiers used in paging messages, namely, UE identifier influences

the paging channel capacity. Compared with CN paging, the probability that UTRAN

initiate paging is much smaller. Only the case that CN initiates paging is considered:

When UE is in IDLE state, CN can only page three UEs at the same paging time

using IMSI;



When UE is in IDLE state, CN can only page five UEs at the same paging time

using TMSI or PTMSI;

When UE is in CELL_PCH or URA_PCH state, no matter what paging identifier

CN uses, UTRAN transfers UE identifier to U-RNTI and pages, it can only page five

UEs at the same paging time【2】.

From analysis above, CN can increase PCH capacity using UE temporary

identifiers “TMSI and PTMSI”.


CN optimization pages by using UE temporary identifier TMSI and PTMSI, and it

can be adjusted with software parameters.

3.8 UTRAN Should Activate IMSI ATTACH and DETACH Functions


After UE is powered on and registered successfully, MSC/VLR will set the

subscriber state as ATTACH state. IMSI DETACH means that after mobile

subscribers power off, MS initiates a DETACH flow and MSC/VLR sets the

subscriber state as IMSI detach. Generally, HLR will not be informed by this flow. In

DETACH state, if the MS is called, CN will directly inform the calling party that the

MS is powered off, which skips of the process of sending useless paging messages

to UE.

As shown in Figure 3 , UE determines whether IMSI ATTACH and DETACH

processes can be adopted through receiving the system message 1. Gsm MAP IE

consists of two octets, one is T3212, and bit1 of the other octet is ATT identifier. “0”

means that the network does not allow UE to adopt IMSI ATTACH and DETACH

processes, “1” refers to the admission of【1】. In Figure 3 , “0a 01” means that T3212

is 60 minutes, UE is admitted to adopt IMSI ATTACH and DETACH processes.

During actual network operation, UTRAN should activate IMSI ATTACH and

DETACH functions.

Table 5 IMSI ATTACH and DETACH Identifier

8 7 6 5 4 3 2 1 T3212 octet 1

Spare ATT octet 2



Figure 3 System Message 1 Resolution


Check and modify IMSI attachment and detachment admission indication by

using the Command “LST CNDOMAIN and MOD CNDOMAIN” on RNC

maintenance console.

3.9 Too Low Power allocation of Paging Channels


Phenomenon: Subscriber complaints are centralized in some certain areas,

when UE is called, the prompt tone “the subscriber is out of service” will occur, while

UE can work normally while calling; the print message of paging decoding failure can

be seen at the UE daemon, which is the problem of the over-low paging channel

power allocation.

Paging channels include PICH and PCH. Too low PICH power will lead to UE

resolution PI indication error. Resolution of PI value from “0” to “1” will cause false

alarm and UE power waste; resolution of P1 value from “1” to “0” will cause paging

miss and loss. If PCH power is too low, UE will fail in decoding the paging

messages.

The current baseline power allocation: PICH is -3dB and PCH is -2dB, which are

both relative to pilot channels power.




Improve the power allocation between PICH and PCH properly. Check and

modify PICH power by using MML Command LST CHPWROFFSET and MOD

PICHPWROFFSET on RNC maintenance console, check and modify PCH power by

using Command LST SCCPCH and MOD SCCPCH.

3.10 Existence of Coverage Dead Zones


Phenomenon: Subscriber complaints are centralized in a certain area, UE fails

both in calling and being called, the signal strength shown in UE panel is low, and

RSCP and EC/IO of pilot signals tested with the drive test equipment are lower than

indexes required for UE normal access, by which coverage dead zone is identified in

this area.


Carry out coverage optimization, for specific optimization measures, please

refer to relevant Guidance.

3.11 UE Performance Problems


UEs of different subscribers may be made by different manufacturers, the

receiving performance and demodulation performance of different UEs are also

different. From GSM network experiences, some paging problems are located in

UEs themselves.

Sort out and analyze subscriber complaint materials. If the called UE is of the

same type, problem may exist in UEs.


Take a called verification test on UEs from different manufacturers, if conditions

permit, compare UE receiving performance parameters and demodulation

performance parameters, including testing RSCP and EC/IO of pilot signals received

at the same place and learn about the service demodulation thresholds of UEs.

4 Outstanding Problems

Currently, there are mainly the following outstanding problems existing:

1. Relationship between paging retransmission times and interval of CN and

UTRAN is complicated. In case of PCH capacity, paging success rate and paging



loss rate specified, it is necessary to make simulation analysis on how to work out

the best paging retransmission strategy.

2. The upper limit of location area division is closely related to the service model

and the traffic model of location area. Currently, the theoretical value can be worked

out by using paging area planning tool. Since 3G service model and traffic model are

very complicated, some problems still exist: how big is the difference between

preconditions of paging area planning tool and actual cases, how is the agreement

of the theoretical value and the actual value, which require further theoretical study

and experiences from the network optimization practices.



List of references:

[1] 3GPP R1999 25_series, 2002/09

[2] Miao Jiashy, WCDMA RNP Paging Area Planning Guidance, 2003/03

[3] Chen Qi, Dong Yan, Zhou Xinjie, Zhi Bo, WCDMA RNP Network Optimization Method Soft

Subject Research Report, 2003/08

[4] Zhang Jiayi, WCDMA Paging Strategy Study, 2003/12

[5] Li Peng, Guidance that Subscribers Are out of Service, 2001/11

[6] Huawei GSM Paging Tackling Data, 2003/06

[7] Jiao Anqiang, WCDMA RNP Special Technologies Key Signaling Process Analysis Paging

Process, 2003/11

1. wcdma rno paging problem analysis guidance-20041101-a-1.0

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