owo300070 wcdma paging problem analysis issue 1

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Confidential Information of Huawei. No Spreading Without Permission

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2 types of paging is reviewed in the section 1.

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For CN originated paging: In order to request UTRAN connect to UE, CN initiates the paging procedure,

transmits paging message to the UTRAN through Iu interface, and UTRAN transmits the paging message from CN to UE through the paging procedure on Uu interface, which will make the UE initiate a signaling connection setup process with the CN.

For UTRAN originated paging: UE state transition: In order to trigger UE in the CELL_PCH or URA_PCH state to

carry out state transition (for example, transition to the CELL_FACH state), the UTRAN will perform a paging process. Meanwhile, the UE will initiate a cell update or URA update process, as a reply to the paging.

When the cell system message is updated: When system messages change, the UTRAN will trigger paging process in order to inform UE in the idle, CELL_PCH or URA_PCH state to carry out the system message update, so that the UE can read the updated system message.

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Paging type 1: 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 If UE is in CELL_PCH or URA_PCH state, the UTRAN transmits the paging

information in PAGING TYPE 1 message to UE. After received paging message, UE performs a cell update procedure to transit state to CELL_FACH.

As shown in the above figure, the CN initiates paging in a location area (LA), which is covered by two RNCs. After receiving a paging message, the RNC searches all the cells corresponding to the LAI, and then calculates the paging time, at which it will send the PAGING TYPE 1 message to these cells through the PCCH.

Paging type 1 is applicable for both CN initiated paging and UTRAN initiated paging.

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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

As shown in the above figure, if the UE is in the CELL_DCH or CELL_FACH state, the UTRAN will immediately transmit PAGING TYPE 2 message to the paged UE on DCCH channel.

Paging type 2 is only applied for CN initiated paging.

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Two types of paging are reviewed in the section 1.

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CN_PAGE_IDLE_UE_SUCC & UTRAN_PAGE1_SUCC which could be used to analyze the paging performance of one RNC should be more attended.

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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.

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RAN10/11/12 Counters related to paging:

Each of the counter is introduced in the following pages.

Counter Description

VS.RANAP.Paging.Att.IdleUE Number of CN Pagings to Idle UE for RNCVS.RANAP.Paging.Succ.IdleUE Number of Successful CN Pagings to Idle UE for RNCVS.UTRAN.Paging1.Att Number of UTRAN-Originated PAGING TYPE 1 Messages for RNCVS.UTRAN.SuccPage1 Number of Successful Responses to PAGING TYPE 1 Messages for RNC

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The measurement is triggered at point B as shown in the figure. When the RNC receives a PAGING message from the CN, and if the paged UE is in idle mode, the RNC sends a PAGING TYPE 1 message to the cell in the paged area. In this case, the RNC measures this counter.

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The measurement is triggered at point C as shown in in the figure. The RNC measures this counter when it receives a RRC CONNECTION REQUEST message from an UE with one of the following causes: "Terminating Conversational Call", "Terminating Streaming Call", "Terminating Interactive Call", "Terminating Background Call", "Terminating High Priority Signaling", "Terminating Low Priority Signaling, "Terminating cause unknown".

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The measurement is triggered at point A as shown in the figure. The RNC measures this counter when it initiates and sends a PAGING TYPE 1 message to an UE in CELL_PCH or URA_PCH state.

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The measurement is triggered at point B as shown in the figure. When the RNC receives a CELL UPDATE message with the cause of "paging response", it measures this counter.

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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.

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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.

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Time info required To know the paging failure occurred whether in the busy hour or non-busy hour.

If most of the paging failed in the busy hour, the operator should focus on the paging congestion.

Place info required To know the paging failure occurred in the same area or not. If most of the paging

failed in the same area, the failure could be caused by weak coverage. UE type required

To check whether the failures are caused by the same UE type or not. If most of the paging failure are caused by the same UE type, it could be caused by the UE itself.

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There are several cases for the paging problems related to the above aspects in section 2.5.

Some background knowledge is introduced first in the following pages, e.g. DRX, NP, and channel power allocation of PICH and PCH.

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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 ＝ MAX(2k, PBP) frames K is the “CN domain specific DRX cycle length coefficient”, which is broadcasted

in SIB1. The typical value is 6. PBP is Paging Block Periodicity , which is 1 for FDD mode The paging period should be 640ms if K is 6

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The Page Indicator Channel (PICH) is a fixed rate (30kbps, SF=256) physical channel used by the NodeB to inform a UE (or a group of UEs) that a paging information will soon be transmitted on the PCH. Thus, the mobile only decodes the S-CCPCH when it is informed to do so by the PICH. This enables to do other processing and to save the mobiles’ battery.

The PICH carries Paging Indicators (PI), which are user specific and calculated by higher layers. It is always associated with the S-CCPCH to which the PCH is mapped.

The frame structure of the PICH is illustrated above. It is 10 ms long, and always contains 300 bits (SF=256). 288 of these bits are used to carry paging indicators, while the remaining 12 are not formally part of the PICH and shall not be transmitted. That part of the frame (last 12 bits) is reserved for possible future use.

In order not to waste radio resources, several PIs are multiplexed in time on the PICH. Depending on the configuration of the cell, 18, 36, 72 or 144 paging indicators can be multiplexed on one PICH radio frame. Thus, the number of bits reserved for each PI depends of the number of PIs per radio frame. For example, if there is 72 PIs in one radio frame, there will be 4 (288/72) consecutive bits for each PI. These bits are all identical. If the PI in a certain frame is “1”, it is an indication that the UE associated with that PI should read the corresponding frame of the S-CCPCH.

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The Secondary Common Control Physical Channel (S-CCPCH) is used to carry the FACH and PCH transport channels. Unlike the P-CCPCH, it is not broadcasted continuously. It is only transmitted when there is a PCH or FACH information to transmit. At the mobile side, the mobile only decodes the S-CCPCH when it expects a useful message on the PCH or FACH.

A UE will expect a message on the PCH after indication from the PICH (page indicator channel), and it will expect a message on the FACH after it has transmitted something on the RACH.

The FACH and the PCH can be mapped on the same or on separate S-CCPCHs. If they are mapped on the same S-CCPCH, TFCI bits have to be sent to support multiple transport formats

The figure above shows the frame structure of the S-CCPCH. There are 18 different slot formats determining the exact number of data, pilot and TFCI bits. The data bits correspond to the PCH and/or FACH bits coming from the transport sub-layer. Pilot bit are typically used when beamforming techniques are used.

The SF ranges from 4 to 256. The channelization code is assigned by the RRC layer as is the scrambling code, and they are fixed during the communication. They are sent on the BCCH so that every UE can decode the channel.

As said before, FACH can be used to carry user data. The difference with the dedicated channel is that it cannot use fast power control, nor soft handover. The advantage is that it is a fast access channel.

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Paging SFN formula: Paging Occasion (CELL SFN) = {(IMSI mod K) 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 K 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)

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SFN is the paging occasion of the UE As shown in the followed figure, the UE needs to monitor the frames (paging occasions)

indicated by the red dots, and then decodes the qth PI of this frame. (The qth PI is obtained based on the above formula).

If the UE has no IMSI, for instance when making an emergency call without USIM, the UE shall use as default numbers, IMSI = 0 and DRX cycle length = 256 (2.56 s), in the formulas above.

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DRXCYCLELENCOEF Parameter name: DRX cycle length coefficient Value range: 6~9 . Content: This parameter is broadcasted on SIB1. This parameter is used when a

UE is in idle mode. Recommended value: 6. Set this parameter through ADD CNDOMAIN, query it through LST CNDOMAIN,

and modify it through MOD CNDOMAIN. PICHMODE

Parameter name: PICH mode Value range: V18, V36, V72, V144 . Physical value range: 18, 36, 72, 144 . Content: Indicating the number of PIs contained in each frame on the PICH. Recommended value: V36 . Set this parameter through ADD PICH, query it through LST PICH.

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MACCPAGEREPEAT Parameter name: Number of page re-TX Number of retransmissions of paging message Value range: 0~2 . Content: If the number of retransmissions of paging message exceeds this

parameter value, retransmissions stop. Recommended value: 1. Set this parameter through SET DPUCFGDATA, query it through LST

DPUCFGDATA.


WCDMA Power Control and Relevant Parameters N-37

MAXTXPOWER Parameter name: Max transmit power of cell Value Range: 0 to 500 Physical Value Range: 0dBm to 50 dBm, step 0.1dB The recommended value is 430, namely 43dBm Content: The sum of the maximum transmit power of all DL channels in a cell. Set this parameter through ADD CELLSETUP, query it through LST CELL and modify it through

MOD CELL

PCPICHPOWER Parameter name: PCPICH transmit power Value Range: -100 to 500 Physical Value Range: -10dBm to 50 dBm, step 0.1dB The recommended value is 330, namely 33dBm Content: This parameter should be set based on the actual environment and the downlink coverage

should be guaranteed firstly. If PCPICH transmit power is configured too great, the cell capacity will be decreased, for power resources is occupied by common channel and the interference to traffic channels is also increased.

Set this parameter through ADD PCPICH, query it through LST PCPICH and modify it through MOD CELL



MAXFACHPOWER Parameter name: Max transmit power of FACH Value range : -350 to 150 Physical Value Range：-35 to 15 dB, step 0.1dB The recommended value is 10, namely 1dB Content: The offset between the FACH transmit power and PCPICH transmit

power. Set this parameter through ADD FACH, query it through LST FACH, and modify

it through MOD SCCPCH

PCHPOWER Parameter name: PCH transmit power Value Range：-350 to 150 Physical Value Range：-35 to 15 dB, step 0.1dB The recommended value is -20, namely -2dB Content: The offset between the PCH transmit power and PCPICH transmit

power. Set this parameter through ADD PCH, query it through LST PCH, and modify it

through MOD SCCPCH



PICHPOWEROFFSET Parameter name: PICH power offset Value Range：-10 to 5 Physical Value Range：-10 to 5 dB , step 1dB The default value of this parameter is -7, namely -7dB Content: The offset between the PICH transmit power and PCPICH transmit

power. Set this parameter through ADD CHPWROFFSET, query it through LST

CHPWROFFSET, and modify it through MOD PICHPWROFFSET

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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. We’ll detail the solution in the following pages. 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 receivesthe wrong message. According to the specific subscriber complaints, if problems exist oncalled UE only (prompt tone “the subscriber you dialed is out of service”), because thereason is probably that the power allocation of PICH and PCH is too low or there areproblems with UE performance; if problems exist in calling and called UE, the reason isprobably that signal coverage dead zones exist in the area.

UE fails in responding after receiving paging messages. The problem is related toaccess failure, for solutions, please refer to “access problem optimization”.

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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.

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n is the proportion of location area size to RA size, LA=nRA; Mmix is the UEs paged quantity by each TTI of PCH; λ*H is traffic supported by a specified paging module in a DRX cycle length period; ErlangperDRXCycleLengthInNRA is the identification of the Erlang 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.

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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. 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.

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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 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. 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.

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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. (So that RNC already complete two times of transmission before the CN retransmitted paging reaches)

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 as expected (2 repeated transmission of 2 paging from CN), which causes paging message loss

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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 (means adjust the paging cycle time, which hence affect the UTRAN paging repeat interval)

UTRAN paging repeat times should not set to too many; otherwise, by 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 UDPUCFGDATA.

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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.

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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.

By default setting of Huawei, K is 6 in both CN-specific DRX cycle length coefficient and UTRAN-specific DRX cycle length coefficient.

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

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.

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.

Since 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.

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Generally, the following rules should be followed: 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 during lu/luB paging flow control;

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 during lu/luB paging flow control; (Configuration recommended)

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Number of PI in SIB5

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Np value too small, the corresponding IMSI number in each group will be larger, it means more users will be monitoring the same PI, means more UE will be called up to read the PI increases, not good in saving UE power.

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IMSI ( International Mobile Subscriber Identity ) : A unique number associated with all GSM and UMTS network mobile phone users. It is stored in the SIM inside the phone and is sent by the phone to the network.

P-TMSI ( Packet Temporary Mobile Subscriber Identity ) : Used as a shorter, more private, mobile identifier. Identifies the system that assigned it, and not directly the MS. Packet-TMSI is allocated by the SGSN and used for GPRS access only.

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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.

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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

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owo300070 wcdma paging problem analysis issue 1

Documents

paging information

paging type

paging process

paging procedure

paging time

types of paging

utran originated paging

cn originated paging