umts cs call drop analysis guide book-libre

UMTS CS Call Drop Analysis

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Contents

Chapter 1 .........................................................................1

I ntroduction ....................................................................1

Chapter 2 .........................................................................3

Definition .........................................................................3

Definit ion of Call Drop from Drive Test Aspect ..................... 3

Definit ion of Call Drop at OMC Side .................................... 3

Chapter 3 .........................................................................5

Call Drop Analysis............................................................5

Call Drop Reasons............................................................ 5

Call Drops Caused by Poor Coverage ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Call Drop Caused by Neighbor Cells.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Call Drop Caused by I nterference ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Call Failure Caused by Two Cells Using the Same PSC .... . . . . . . . . . . . . 8

Call Drops Caused by Engineering Causes ... . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Call Drops Caused by 2G/ 3G I nteroperabilit y .. . . . . . . . . . . . . . . . . . . . . . . . . 12

Call Drops Caused by the System .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Analyzing Call Drops by DT ............................................. 13

Analyzing Call Drops by Traffic Stat ist ics........................... 15

Procedure of KPI Analysis .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Basic Methods to Analyze KPIs ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

KPI Analysis Tools.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Radio Param eters Involved During Opt im izat ion ................ 20

Radio Param eters Related with CS Call Drops ... . . . . . . . . . . . . . . . . . . . . . . 20

Tim er and Counter Related with Call Drop ... . . . . . . . . . . . . . . . . . . . . . . . . . . . 22



Confident ial and Proprietary Informat ion of ZTE CORPORATI ON 1

C h a p t e r 1

Introduction

This docum ent is com piled to guide the network opt im izat ion engineers to solve the call drop problem , to reduce the call drop rate, and to im prove the qualit y of the network. I t also int roduces ways to evaluate, test , analyze and solve the call drop problem . In addit ion, it also includes som e typical cases. I n the actual network opt im izat ion act iv it ies, handover and call drop are strongly related. In m ost cases, handover failure would lead to call drops. For this kind of call drops, you m ay refer to the guidebook for call drops caused by handover. This document m ainly focuses on call drops which are not caused by handover failures.




C h a p t e r 2

Definition

Definition of Call Drop from Drive Test Aspect Air interface signaling at the UE side: Call drops refer to call releases caused by Not Norm al Clearing, Not Norm al, or Unspecified when the m essage on the air interface sat isfy ing any of the following three condit ions:

n The UE receives any BCH inform at ion (system inform at ion) .

n The call is released for Not Norm al and the UE receives the RRC Release inform at ion.

n The UE receives CC Disconnect , CC Release Com plete, and CC Release inform at ion.

Signaling recorded at the RNC side: Call drops refer to call releases when the RNC has sent the I u Release Request to the CN through the Iu interface, or when the RNC has sent the RAB Release Request inform at ion to the CN through the user panel.

Definition of Call Drop at OMC Side The definit ion of call drop in a broad sense contains the call drop rates at both the CN and UTRAN sides. Since the network opt im izat ion focuses on the call drop rate at the UTRAN side, this docum ent only focuses on the KPI analysis at the UTRAN side.

The KPI s at the UTRAN side refers to the num ber of released RABs of different services t riggered by the RNC. Two aspects are involved: (1) After the RAB is established, the RNC sends the RAB RELEASE REQUEST inform at ion to the CN. (2) After the RAB is established, the RNC sends the I U RELEASE REQUEST to the CN, and then it receives the I U RELEASE COMMAND from the CN. The stat ist ics can be collected based on specific services.





Meanwhile the t raffic stat ist ics also im ply reasons that the RNC t r iggers the release of the RABs of different services.

The call drop rate can be calculated by the following formula:

%*SuccessCSRABSetup

iggedByRNCCSRabrelTrCDRCS 100_

=

CSRabrelTriggedByRNC contains the num ber of RABs included in RAB RELEASE REQUEST for CS services and that included in I U RELEASE REQUEST for CS services.

%*SuccessPSRABSetup

iggedByRNCPSRabrelTrCDRPS 100_

=

RabrelTriggedByRNC contains the num ber of RABs included in RAB RELEASE REQUEST for PS services and that included in I U RELEASE REQUEST for PS services.

I t should be specified that the RNC t raffic stat ist ics calculates the t im es of call drops through the signaling at the Iu interface, and counts the num ber of RAB RELEASE REQUEST and the num ber of IU RELEASE REQUEST init iated by the RNC. While call drops in the drive test aspect em phasizes the inform at ion at the air interface and non-access st ratum and their cause value. I t is different from call drops at the OMC side.




C h a p t e r 3

Call Drop Analysis

Many reasons m ay lead to the call drop problem, and call drop is an expression of the deep network problem s. This chapter focuses on the call drop reasons, comm only-used call drop analysis methods, and m ain call-drop opt im izat ion inst rum ents.

Call Drop Reasons

Call Drops Caused by Poor Coverage

I n the definit ion of network coverage, the requirements of effect ive coverage for a certain sam pling point is that it s RSCP and Ec/ I o should be bet ter than the specified threshold. In this sect ion, bad coverage is represented by poor RSCP value. Note that coverage at cell edges is a special case. Coverage at cell edges would have bad RSCP value and excellent Ec/ I o owing to lit t le cell num ber, but st ill the coverage in these cell edges is defined as bad coverage.

I n UMTS network, init iat ion and m aintenance of different services would have different requirem ents on coverage. Table 1 list s the reference values.

T AB L E 1 RSC P AN D EC / IO T H R E S H O L D F O R D I F F E R ENT S E R V I C E S

Service Type RSCP [ dBm ] Ec/ I o [ dB]

AMR12.2K -105 -13

CS64K -100 -11

PS384K -95 -10

HSDPA -90 -8

The coverage condit ion at the UL and DL of the network can be est im ated through the power of the dedicated channels for the UL and DL before call drops, which can be perform ed through the following m ethods.





I f t he UL TX power before the call drop has reached the m axim um value and the UL BLER is bad, or it is found out through the single user t racing record at the RNC that the NodeB has reported RL failure, then the call drop is caused by bad UL coverage. I f t he DL TX power before the call drop has reached the m axim um value and the DL BLER is bad, then the call drop is caused by bad DL coverage.

For the coverage opt im izat ion method, see the WCDMA Radio Network Opt imizat ion Guide.

Call Drop Caused by Neighbor Cells

Missed neighbor cell

Neighbor cell opt im izat ion is an im portant link of radio network opt im izat ion. I f certain cells should be included but excluded from the neighbor cell list of one cell, then call drop would happen and the interference in the network would also increase and system capacit y would be im pacted. Therefore, neighbor cell opt im izat ion is an im portant part of the engineering opt im izat ion.

I t is easy to est im ate whether the cell is configured with any neighbor cell, and you can playback the call drop data, perform NCOS analysis, and analyze the scanner data.

n Use ZTE CNA to playback the call drop data. I f the blue pillar ( represent ing the detected set ) in the histogram of the pilot signals is the longest , then the missed neighbor cell problem exists.

n Use the autom at ic analysis tool of ZTE NCOS, it would study the handover data of the network, and autom at ically add the m issed neighbor cell. For details, see the operat ion guide of NCOS.

n During the drive test , the UE would acquire the neighbor cell list from the NodeB, and the scanner would scan the 512 PSCs and record the Ec/ I o. I f one of the PSCS is not included in the neighbor cell list , and it s pilot st rength is st ronger than the threshold, and the phenom enon lasts for a few seconds, then the m issed neighbor cell problem exists.

Rem oval of key neighbor cells caused by com binat ion of m acro diversit y

Assign the priority of the neighbor cell when perform ing the init ial neighbor cell planning, then opt im ize the priorit y and num ber of neighbor cells periodically with NCOS as the t raffic volum e increases.

Unt im ely update of the external cell inform at ion

Check the external cells of the RNC periodically, and ensure the cells in the list are correct .



Chapter 3 Call Drop Analysis


Call Drop Caused by Interference

Dist inguish the UL and DL interferences.

The interferences from the UL and DL would both lead to call drop. Generally, when the CPI CH RSCP of the act ive set is large than -85dBm , and the com prehensive Ec/ I o is lower than -13dB, call drop occurs, then the call drop is caused by the DL interference. Note that when the handover is not t im ely, the RSCP of the serving cell m ay be good, but the Ec/ I o is bad. However, the RSCP and Ec/ I o of the m onitored set are both excellent under this condit ion. When the UL RTWP is 10dB higher than the norm al value, which is -107~ -105, and the interference durat ion is 2s or 3s longer, call drop m ay happen and the problem m ust be solved.

Two reasons m ay cause DL interferences, which are pilot pollut ion and missed neighbor cell. The m issed neighbor cell has already been discussed in the above part and would not be repeated here. In the pilot pollut ion area, signals of mult iple cells exist , the RSCP of these cells is good, while Ec/ I o is bad, the UE would frequent ly reselect the neighbor cell or perform the handover, and the incoming and outcom ing of calls can hardly reach the UE. Generally, three factors would lead to pilot pollut ion in the network.

n Overshoot ing of sites at a high locat ion

n NodeBs in r ing-shaped dist ribut ion

n Wave-guide effect , large reflectors, and some other effects that m ay cause the distort ion of signals.

The t ypical feature of DL call drops is that the RNC sends the Act ive Set Update message, while the UE cannot receive it , then the call is dropped for RL Failure.

You can judge whether the UL interferences exist by the Average RTWP and Max RTWP on the OMC-R. For an idle cell, the Average RTWP is about -105dBm ; for a cell carrying 50% of UL load, the Average RTWP is around -102dBm . I f the Average RTWP of an idle cell exceeds -100dBm , we can believe that UL interferences exist . The UL interferences m ake the UL TX power of the cell in connected m ode increase, and then an excessively high BLER is generated. Then call drop happens. During handover, the newly-added link is out of sync for UL interferences, which further leads to failed handovers and call drops. The UL interference m ay be int ra-RAT or inter-RAT interferences. In m ost cases, the UL interferences are inter-RAT interferences.

When DL interference exists, the UL TX power is very sm all or the UL BLER m ay converge, however, when the DL TX power of the UE reaches the m aximum value, the DL BLER does not converge. I f UL interferences exist , the same problem would





insist . Thus, in actual analysis, this m ethod can be used to dist inguish whether interferences exist .

For m ethods to invest igate the interferences, see the UMTS I nterference I nvest igat ion Guidebook .

Call Failure Caused by Two Cells Using the Same PSC

Scenario One

F I G U R E 1 SC E N AR I O O N E T H AT MAY C AU S E T H E S AME PSC P R O B L E M

Cell A and Cell B (source cell) are configured as neighbor cell for each other, however, the geographical distance between Cell A and Cell B is huge. Cell A and Cell C has the sam e PSC, and Cell C and Cell B (source cell) is very close, however, Cell C and Cell B are not configured as neighbor cells for each other.

Under this situat ion, the UE detects signals from Cell C and sends Event 1A request to be soft handed over to Cell C. The PSC in the Event 1A request is 123. After receiving the Event 1A request , the RNC checks from the neighbor cell list of Cell B (source cell) for cells with PSC of 123, then it finds Cell A. Then the RNC t ries to build the radio link on Cell A. The RNC inst ructs the UE to add Cell A to it s act ive set . Then, the update of the act ive set t im es out for the cell measured by the UE is different from the cell where the radio link is built .





Scenario Two

F I G U R E 2 SC E N AR I O T WO T H AT MAY C AU S E T H E S A ME PSC P R O B L E M

I n this scenario, the UE has established the radio link with two cells, Cell B and Cell C. Cell A is the neighbor cell of Cell B, and Cell D is the neighbor cell of Cell C, and these two cells have the sam e PSC. When the UE is in soft handover state, the RNC would com bine the neighbor cell lists of Cell B and Cell C, then the same PSC problem would happen.

Scenario Three

F I G U R E 3 SC E N AR I O T H R E E T H AT MAY C AU S E T H E S A ME PSC P R O B L E M

Cell B and Cell D are not configured as neighbor cell for each other, however, these two cells are both included in the act ive set owing to the third-party handover am ong Cell B, Cell C, and Cell D. Cell A is the neighbor cell of Cell B, and Cell E is the neighbor cell of Cell D, and these two cells have the sam e PSC. The RNC would combine the neighbor cells of Cell B, Cell C, and Cell D in the act ive set , then the same PSC problem m ay occur.





Call Drops Caused by Engineering Causes

Reversely-connected antenna

You can check whether the diversity is reversely connected by the PSC dist r ibut ion figure of the dr ive test data. For the connect ion of the diversity, the PMS can be used to m easure the cell perform ance. The antenna would only generate power when UEs t ry to access the network, and the m easured value of the power equals to the dem odulat ion power. You can check the rat io of two antennas, if the power of one antenna is lower than the other one in a long period of t im e, then the diversit y must be reversely connected.

The balance level checking of two antennas in whole network can be im plem ented by OMCB measurement. However, you need to m anually process the acquired data.

An excessive VSWR

You can check the VSWR of the current site at the RNC SDR. I f t he VSWR is large than or equals to 1.4, then it m ust be adjusted.

Mult i-band antenna problem

I n the network of som e cit ies, m ult i-band antennas exist . The operator usually refuses to adjust the param eters of the m ult i-band antenna for fearing of affect ing the subscribers of the exist ing 2G network. Then pilot pollut ion or overshoot ing m ay occur. To solve this problem , you should t ry to persuade the operator to change the antenna, so that 2G and 3G networks can have separate antennas. I f t hese antennas cannot be changed, then the specific environm ent must be carefully studied before taking any act ions. You can opt im ize the neighbor cells to avoid call drops.

Leakage of signals from indoor dist ribut ion system

I n m ost cit ies, call drops caused by signal leakage from indoor dist r ibut ion system exist . You should persuade the operator to reconst ruct the indoor dist ribut ion system . Or, the indoor dist r ibut ion system can be m erged to the whole network, which can be done by opt im izing of the coverage of the am bient outdoor cells and addit ion of neighbor cells.

Call drop caused by unsteady t ransmission

As the bot tom level of t ransmission medium , E1 would report the loss of E1 elect rical signals and recept ion failures at the rem ote end. Meanwhile, several E1s would be bound together as a group, and then E1 would report the fault of I MA group in non-operat ing m ode.

The following table list s several E1 fault s that m ust be handled and the related handling suggest ions.





T AB L E 2 C O MMO N E 1 F AU L T S AN D H AN D L I N G SU G G E S T I O N S

Fault Causes Solut ions

Lost of E1 elect r ical signals

The RX end detects no line circuit pulse or cannot detect logic 1 within cont inuous periods, then the LOS alarm is reported. This alarm is generally caused by the RX fault of the E1/ T1 or broken lines, then the E1/ T1 cannot detect the signals from the remote end.

1. Check whether the SA board is secure, and whether the E1 adapter is slack.

2. Check whether the pins of the adapter are dam aged.

3. Check whether the joint connector of the E1 cable is dam aged, and whether the joint connector is securely connected with the E1 cable.

4. Check whether the cabling of the E1 cable sat isfies the engineering specificat ion, whether the E1 cable bears any external force.

5. Use the E1 self- loop cable to recycle the line, if the alarm is cleared, then check the E1 cable at the peer end.

Rem ote recept ion failure of the E1

I t indicates the E1/ T1 remote alarm . This alarm indicates the abnormal recept ions at the rem ote end. The rem ote end inserts the RAI indicator bit to the signals and then sends it to the local end, and the local end reports the alarm after detect ing the alarm . The rem ote recept ion error is reported.

1. The TX line is faulty or broken. Check whether the TX line is correct ly connected. For details, see the Handling suggest ions for the LOS Alarm .

2. Check whether the frame st ructures of the E1 fram e at the local end and rem ote m atch. The E1 fram e at both ends m ust both work at dual- frame or m ult i- frame m ode.

3. Check for error codes at the TX line.

E1 frame out of sync

The first bit of slot 0 of both E1 and T1 carries the synchronous clock signals, which inform the RX end of the start of one fram e. I f the RX ends of the E1 and T1 are out of sync, then data fram es would be lost and the LOF alarm is reported.

1. Whether E1 and T1 work at the same state.

2. Check whether E1 frames are of the same m odes (dual- fram e/ m ult i- fram e).

3. Check whether the im pedance modes of E1/ T1 m atches.

4. Check for interferences from digital devices around E1/ T1.

5. Check whether the clock signals are normal.





Fault Causes Solut ions

SSCOP link error

This alarm is caused by that the SSCOP signaling link is unsuccessfully established or the SSCOP signaling from the remote end is not received within a certain period. Then the SSCOP link would be broken off, and this alarm is reported.

See the handling suggest ions for E1 faults.

I MA group in non-operat ing m ode

After the I MA group is successfully configured, if I MA remains in non-operat ing m ode for over 1s, then this alarm is reported.

See the handling suggest ions for E1 faults.

Current ly, som e sites are configured with I P t ransm ission. Therefore, the alarm of "Lack Ethernet electr ical signals" also should be handled on site.

Call Drops Caused by 2G/3G Interoperability

Opt im izat ion of 2G neighbor cells configured for 3G cells

I f the 2G cells are congested, or interfered, then the success rate of 3G -> 2G handovers is low. During the neighbor cell opt im izat ion, this kind of neighbor cells must be rem oved from the list .

Param eters m ust be refined based on different scenarios.

To im prove 3G-> 2G handover success rate, the param eters m ust be detailed planned based on different scenarios.

Com pat ibilit y of UEs

The 2G-> 3G handovers of som e cells are slow. This is because som e smuggled 3G handsets have some diff icult ies in support ing the 2G network.

2G/ 3G data synchronizat ion

To support 2G/ 3G handovers, the 2G/ 3G cells m ust be configured as the neighbor cells for each other first ly. I f the cell inform at ion is updated t im ely, then the handover would fail and cell reselect ion cannot be perform ed. Therefore, the data of 2G/ 3G network should be synchronized t im ely.





Call Drops Caused by the System

I f the alarm is not caused by the causes listed in the above sect ion, then it m ay be caused by the system . You need to check the alarm inform at ion of the equipment and system logs to further analyze reasons that cause call drops. For exam ple, an abnorm al NodeB would lead to the synchronizat ion failures, which would lead to frequent rem oval and addit ion of radio links, and then call drops m ay happen; call drops caused by poor DL signals m ay be because of abnorm al RF m odule, and call drops caused by that the UE fails to report the measurem ent report Event 1A.

I t should be noted that in m any foreign count r ies, the TX environm ent is bad and unstable. Therefore, influences of call drops caused by TX problem are huge.

Analyzing Call Drops by DT The following figure describes the flow chart for using DT and CQT to test call drops.





F I G U R E 4 FL O W C H AR T T O T E ST C AL L D R O P S B Y DT

Call drop data

The call drop data refers to the CNT test data and RNC signaling t racing data.

Call drop spots

Use CNA to analyze the call drops to acquire the locat ion where call drops happen. Then acquire the following data: pilot data collected before and after call drops, act ive set and m onitoring set inform at ion collected by the cell phone, and signaling flow.

Stability of the prim ary serving cell

The stabilit y of the prim ary serving cell refers to its changes. I f t he prim ary serving cell is stable, then analyze RSCP and Ec/ I o. I f the pr im ary serving cell changes frequent ly, then the handover param eters should be changed to avoid the ping-ping effect .

RSCP and Ec/ I o of the pr im ary serving cell

Check the RSCP and Ec/ I o of the opt im al cell, and then

n When the RSCP is bad, the coverage is poor.





n When the RSCP is norm al, while the Ec/ I o is bad, pilot pollut ion or DL interference exists.

n When RSCP and Ec/ I o are both norm al, if cells in the act ive set of the UE are not the opt im al cells (which can be checked through playback of data) , then the call drops m ay be caused by missed neighbor cell or unt im ely handovers; if cells in the act ive set of the UE are the opt imal cells, then call drops m ay be caused by UL interferences or abnorm al call drops.

Reproducing of problems with DT

Since you cannot collect all necessary inform at ion by one DT, then m ult iple DTs shall be performed to collect sufficient data. In addit ion, mult iple DTs can also help to ascertain whether the call drop is random or always happens at the sam e spot . Generally, if call drops always happen at the sam e spot , this problem must be solved, or if call drops happen random ly, mult iple DTs must be perform ed to find inner reasons.

Analyzing Call Drops by Traffic Statistics When analyzing the t raffic stat ist ics, check the call drops index on the RNC first ly to learn the operat ing status of the whole network. Meanwhile, a cell-by-cell analysis can be perform ed to acquire the detailed call drop indexes of each cell. During the analysis, the t raffic stat ist ic analyzing tool can be used to analyze the call drop situat ions of different services and the possible causes.

Acquire data about cells with abnorm al KPI s through the t raffic stat ist ics. I f KPIs of these cells used to be norm al, then the abnorm al KPIs m ay be brought by software version, hardware, t ransmission, antenna, or data, then you can check these aspects based on the alarm s. I f no obvious abnorm al cells exist , t he stat ist ics can be classified based on the carr ier in each sector, then cells with poor KPIs can be screened out . Further analyze the t raffic stat ist ics of these cells, such as analyzing m ore related KPIs, such as analyzing data at a shorter interval, or analyzing KPI s that are m ore likely to cause call drops, such as handover. Meanwhile, you can analyze the reasons for call drops based on system logs. During the analysis, you should consider the effect of other KPI s when focusing on a certain KPI . I t should be specified that the result of t raffic stat ist ics is m eaningful only when the t raffic volum e reaches a certain am ount . For exam ple, a 50% of call drop rate does not m ean that the network is bad. This value is m eaningful only when the calling num ber, succeed calling num ber, call drop t im es all m ake stat ist ical significances.





Procedure of KPI Analysis

The comm only used KPI analysis m ethod is the TOP cell m ethod, which m eans the top cells will be screened out according to certain index, then these top cells are opt im ized and then the top cells are selected again. After several repet it ions, the related KPI can be speedily converged. At the init ial stage of network const ruct ion, there are few subscribers in the network. At this stage, the KPIs of m any cells m ight be unstable, such as call drop rate. You can collect the data in seven days or longer periods, then select the top cell and then perform the opt im izat ion. For exam ple, opt im izat ion of call drop rate of CS services. When select ing top cells, you can select the cell with call drop num bers exceeding the specified threshold, and then arrange the pr iority based on the call drop rate.

The procedures of top cell select ion are the sam e as the procedures of handling input inform at ion from other team of engineers (complains or single site acceptance), and are shown in the following figure.

F I G U R E 5 FL O W C H AR T F O R T O P C E L L S E L E CT I O N





Basic Methods to Analyze KPIs

Speedily Collecting the Field Data

To locate the problem , you have collect data from m any different spots between the UE and the pdn server. While, speedy and accurate collect ion of the field data is essent ial to locate and solve the problem and to im prove the KPIs. Data collect ion can be divided into m ult iple layers.

Collect ing UE log, RNC signaling, KPI data, alarms, abnorm al probes, and packet captured at the I ub interface

NodeB and RNC debug log

Som e comm on skills are required to collect data of the first layer, and the network opt im izat ion & m aintenance personnel can easily m aster these skills. At present , m ost field quest ions can be located through the data analysis at this layer. Collect ion of the debug log of the second layer should be perform ed or rem otely supported by the relat ive R&D engineers. Data at this layer can help to solve som e deep layer problems.

The following chapter focuses on the data collect ion tool and m ethod for the first layer data, and only gives a brief int roduct ion to that of the second layer.

Health Check of Sites

For sites where alarm s are reported, you should first perform the health check for the site, which m ainly covers the following aspects:

n Alarm s

n Frequent ly added or rem oved com m on t ransport channels

n UL & DL power

n Radio link restore

n Balance level between two antennas

n Stat ist ics of service failures

The RL restore rate is shown by the NodeB cell m easurem ent recorded by PMC as shown in the following figure, and is accum ulated since the establishm ent of cells. I f t he RL restore rate of a cell is lower than 80% , the cell is t reated as abnorm al, and the possible causes are as follows:

n UL interferences

n I nsufficient cell radius or overshoot ing

n Reuse of the same PSC

n Abnorm al UL RF channel





For these possible causes, you m ay check them com bining other m easurement result s and data analysis.

KPI Analysis Tools

Signal Trace

This tool t races signaling of RNC, you can t race the signaling at the Iu, I ur, Iub, and Uu interfaces, TNL signaling, and RNL signaling through this tool. The m ost comm only used method to check the KPI s is to t race the RNL signaling. This tool is very useful, and can t race the signaling on the basis of cell ( t race signaling of m ult iple UEs) and I MSI ( t race signaling of one UE) .

I t should be em phasized that signaling t racing by cells can only t race the UE that init iates the call from this cell. The UE can be t raced as long as it rem ains in the sam e RNC, even if it is handed over to other cells. However, if a UE init iates the call from other cells and then is handed over this call, and it s call drop happens in this cell, it cannot be t raced. Therefore, when you t race the signaling of a cell with high call-drop rate, the signaling of cells in close handover relat ion with this cell should also be t raced, then the result would be m ore com prehensive.

The RNC R&D engineers also develop a RNC signaling t racing tool, WinSigAn, which can m ark the call drop spots m ore clearly.

RNC Association Log

This tool helps to record the context of the abnorm al system flow, and then the context would be counted and analyzed to locate the network problem .

I t is usually used when the system is abnorm al and no RNC signaling is t raced. I t can help to locate the problem by the t im e when the system except ion happens. The except ion can be queried on the basis of I MSI and CELL I D.

NodeB LMT

Besides all funct ions of OMCB, NodeB Local Maintenance Terminal (LMT) can also provide detailed cell and UE inform at ion.

The NodeB LMT consists of EOMS, EFMS, DMS, and PMS.





NodeB Exception Probe

I n the field of the WCDMA comm ercial network, this tool can effect ively help to m onitor the operat ing status of the NodeB. Different m odules of the NodeB would record the inform at ion when except ions happen, thus facilit at ing the locat ion of problems. However, specialized knowledge is required. You have to understand the funct ions and interfaces of different boards. I f t he field engineers cannot analyze the report , t hey can sim ply send these data to the R&D engineers.

The except ion probe reported by different NodeBs can be saved on different OMCB servers based on the RNC they belong to. Then, this tool would download the file from different OMCB FTP, and then analyze them .

CTS

CTS is the tool for the CN, and it can be used to perform deep signaling by I MSI . Unlike SignalTrace, which is applicable to the signaling t racing within one RNC, CTS can perform the signaling t racing across the RNC border, Therefore, it is applicable to the signaling t racing of VI Ps.

CTS can t race the interact ive signaling am ong different NEs within the CN, and can t race the signaling at the Iu and Uu interfaces, and this is called deep t racing. The working principles of CTS is as follows: First establish an I MSI task on CTS server, and then sent this I MSI task to the CN, which is further sent to different m odules through the arranged interfaces, then each m odule collects the signaling related to I MSI , and then the collected signaling is t ransm it ted back to the CTS server through the CN. The above interfaces are all private interfaces, thus this tool only work on ZTE CN and RNC.

UE Log

DT is an im portant m eans to analyze KPIs. Many problem s, signaling t racing at the network side and t racing of problem s which are hard to be located, can be finally located after com bining the UE logs. The comm only used DT software is QXDM/ APEX(QCAT) , CNT/ CNA, and TEMS.





Radio Parameters Involved During Optimization

Radio Parameters Related with CS Call Drops

Time To Trigger

Time To Trigger is the interval between the m om ent that the events (1A, 1B, 1C, and 1D) are m onitored and the m oment that the events are reported. The set t ing of TTT would influence t im ely handover.

The adjustm ent of handover param eters should first ensure that this cell is overlapped by other cells, then you can adjust the related radio param eters to ensure that the t im e that the UE passes the handover area is longer than the handover delay of the whole system , thus ensuring the cont inuity of the services. The other is to ensure that the handover area ascertained by the signals and radio param eters cannot be too large to avoid the increase of handover overhead and reduct ion of resource ut ilizat ion rat io.

For areas where the signals m ay change great ly, the t r igger t im e of Event 1A must be reduced, and that of Event 1B must be increased. Meanwhile, the CI O of the corresponding neighbor cells should be adjusted so that Event 1A can happen earlier and Event 1B would happen later, thus ensuring successful handovers.

For highways, the cells are sparsely dist ributed. I f t he vehicles dr ive too quickly and cannot access the new cell in t ime, call drops would happen. The opt im izat ion is the sam e as that for the opt im izat ion for st reet corners in dense urban, which is to m ake cells with good signals join the act ive set speedily to ensure cont inuit y of services.

For the adjustment of the related param eters, a whole new set of param eters must be assigned to the target cell.

Cell Individual Offset

The sum of the value of Cell I ndividual Offset (CI O) and the actually m easured value is used in the evaluat ion of the events of the UE. The UE would use the or iginal m easurement value of this cell plus the CI O as the m easurem ent result for the int ra- frequency handover judgm ent . CI O can help to ascertain the cell edge.





The larger this param eter is set , the easier the soft handover will be, and m ore UEs will be in soft handover state. However, m ore resources are consum ed. This sm aller is param eter is set , the m ore difficult t he soft handover is.

CI O is valid only for the neighbor cell. For Event 1A, the CI O can be set in the neighbor cell; for Event 1B, the CI O can be set in the cell t o be rem oved. The formula is as follows:

Form ula of Event 1A t riggering:

),2/(10)1(1010 111

aaBest

N

i

iNewNew HRLogMWMLogWCIOLogMA

+

+

=

MNew is the m easurem ent of the to-be-evaluated cells that has entered the report range.

Mi is the m ean m easurem ent result of cells (exclude the best cell) in an act ive set .

NA is the current cell num ber (exclude the best cell) in the act ive set .

MBest is the measurem ent result of the opt im al cell in the act ive set .

W is the weight proport ion of the best cell t o the rest cells in the act ive set .

R1a is the report ing range of Event 1A.

H1a is the report ing hysteresis of Event 1A.

Form ula of Event 1B t riggering:

Mnew is the m easurem ent of the to-be-evaluated cells that has entered the report range.

Mi is the m ean m easurem ent result of cells (exclude the best cell) in an act ive set .

NA is the current cell num ber (exclude the best cell) in the act ive set .

MBest is the measurem ent result of the opt im al cell in the act ive set .

W is the weight proport ion of the best cell t o the rest cells in the act ive set .

R1bis the report ing range of Event 1B.

H1b is the report ing hysteresis of Event 1B.

Start/Stop Threshold for Compressed Mode

Com pressed m ode is frequent ly used during inter- frequency and inter-RAT handovers. The com pressed m ode is started before

1 1

1

10 10 (1 ) 10 ( /2),AN

Old Old i Best b b

i

LogM CIO W Log M W LogM R H=

+ + +





t he handover, and the system can use the t ime slot brought by com pressed m ode to perform the signal qualit y test on the inter-frequency or inter-RAT neighbor cells. I n the current system , the com pressed m ode is started through Event 2D, and stopped through Event 2F. The m easurem ent value of RSCP or Ec/ I o can be selected. Current ly, the default value is RSCP.

Generally, the quality and other related inform at ion of the target cell ( inter-frequency or inter-RAT) m ust be acquired for the com pressed m ode. Meanwhile, the m oving of the UE would lead to the deter iorate of the qualit y of the cell, t herefore, the start threshold of the com pressed m ode should sat isfy the condit ion that the UE can finish the m easurem ent of the target cell and report for handover before call drops happens. For the stop threshold, it should help to avoid the frequent start or stop of com pressed m ode.

I n dense urban, the cont inuous coverage of the 3G should be ensured, thus avoiding unnecessary inter-RAT handovers and increase of system load. For edges of the 3G network and highways, the UEs should be handed over to the 2G network before the heavy fading. Under this condit ion, the t r igger threshold of Event 2D should be raised so that the UE can init iate the com pressed m ode as early as possible.

Maximum DL TX power of the Radio Link

I f large am ounts of call drops happen due to coverage causes, then the m axim um DL TX power of the services can be increased appropriately. However, this is at the risk that the UEs at cell edges m ay consum e too much power, and then affect the other UEs, and reduce the DL capacity of the system. For cells with a great deal of access failures caused by excessive load, this param eter can be set to a sm all value.

Inter-Frequency/Inter-RAT Handover Threshold

The UE can be handed over to the inter-RAT/ frequency neighbor cells when the m easured value of the signals from these cells is higher than the threshold. This parameter can be set com bining the start threshold of the com pressed m ode. I f t his param eter is configured with a lit t le value, then the handover can be t r iggered early. I f t his param eter is configured with a large value, then the handover will be prolonged.

Timer and Counter Related with Call Drop

The following table list s the t im er and counter related to the UE.





T AB L E 3 T I ME R AN D C O U N T E R R E L AT E D T O T H E UE

Nam e Descript ion Value Range

Default Value

T312

Connected T312 in connected m ode, and indicates the t ime that UE waits from the synchronizat ion indicator from L1 when it starts to establish the DPCCH.

(1..15)s 1s

N312

Connected T312 in connected m ode, and indicates the num ber of synchronizat ion indicator that the UE received from L1 before the DPCCH is established.

(1, 2, 4, 10, 20, 50, 100, 200, 400, 600, 800, 1000)

1

T313 I ndicates the wait ing t im e of the UE in CELL_DCH state after the DPCCH channel is established.

(0..15)s 3s

N313 I ndicates the number of m axim um num ber of out of sync indicators that the UE receives from L1.

(1, 2, 4, 10, 20, 50, 100, 200)

20

T314 Start : When the criteria for radio link failure are fulfilled. The t imer is started if radio bearer(s) that are associated with T314 exist or if only RRC connect ion exists only to the CS domain.

(0, 2, 4, 6, 8, 12, 16, 20)s

4s

T315 Start : When the criteria for radio link failure are fulfilled. The t imer is started if radio bearer(s) that are associated with T314 exist or if only RRC connect ion exists only to the CS domain.

(0,10, 30, 60, 180, 600, 1200, 1800)s

30s

N315 I ndicates the maxim um number of synchronizat ion indicators that the UE received from L1 after T313 is act ivated.

(1, 2, 4, 10, 20, 50, 100, 200, 400, 600, 800, 1000)

1

T309 I ndicates the wait ing t im e of the UE after sends the inter-RAT handover requests.

(1..8)s 3s



umts cs call drop analysis guide book-libre

Documents

drop problem

drop rate

drop reasons

pdffactory pro trial

definit ion

addit ion

im izat ion act

im izat ion engineers